Honghao YUE, Yifei YANG, Yifan LU, Fei YANG, Jun WU, Qi RUAN,Zongquan DENG

State Key Laboratory of Robotics and System, Harbin Institute of Technology, Harbin 150001, China

KEYWORDS Connection;Low-shock;Non-pyrotechnic;Separation;Space

Abstract Space missions have become diversified in recent years,where connection and separation devices play a crucial role as key components of various spacecraft.Traditional pyrotechnic devices have the advantages of large carrying capacity, rapid motion and functional reliability. However,their shortcomings such as great release shock, poor safety, unrepeatability and other prominent defects make them unsuitable for new generation spacecraft such as microsatellites to separate at low shock or lock repeatedly, etc. Therefore, it is necessary to develop space non-pyrotechnic low-shock connection and separation devices (SNLD) which are required for advanced aerospace missions.In this paper,the progress of the research on space non-pyrotechnic low-shock connection and separation technology (SNLT) is summarized and reviewed. Proceed from the principle of reducing shock for non-pyrotechnic devices, present studies are classified from the perspective of actuating technology and systematic designing methods.For non-pyrotechnic actuating techniques,according to different driving sources, the separation devices are classified into several main categories: electric, magnetic, gas and thermal actuating devices. The actuation principle and application prospect of separation techniques are introduced and the working process, dimension and mechanical properties of typical devices are compared and evaluated. For the systematic designing method,the common mechanism types of SNLDs are summarized according to the designing concept of reducing shock. Then connection configurations are classified according to the structural

1. Introduction

Space connection and separation device could be called separation device in short,with the main functions of connecting(or locking), unlocking (or releasing) and separating (or ejecting)during the launch and on-orbit operation of missiles, rockets and other spacecraft. Space connection and separation device ensures reliable locking between the load and the separated body during launch and flight. When the spacecraft is launching into trajectory or returning to the earth, the controlling system issues an actuation command to effectively release and separate loads or certain invalid structures from the connecting platform. As shown in Fig. 1,1–3separation device is of great significance widely used for successful space missions,such as satellite-rocket separation, stage separation, the ejection of the fairing, the deployment of space mechanisms and other occasions.1–4According to the actuation method for unlocking and separation, separation devices could be divided into pyrotechnic devices and non-pyrotechnic devices, and the former has been applied further more generally up to now.Pyrotechnic separation devices, such as explosive bolts,pyrotechnic nuts, steel ball projectile locks, wedge projectile locks,etc.,mainly act relying on energy generated by gunpowder combustion or explosion. Moening et al. summarized the characteristics of pyrotechnic devices.5,6In detail, they have the superiorities of great technical stability, high density of energy transfer, small trigger energy, fast response speed,diverse types, and mature technology.However, the pyrotechnic device could only be actuated once,so that the reliability of space products cannot be verified by ground test to search about the cause of the fault.More seriously,the shortcomings such as high actuating risk, huge shock and serious pollution appear prominent. According to a research report published by NASA in 1985,the shock and vibration caused by the actuation of initiating explosive devices directly gave rise to failures among the counted 85 flight accidents.4

In recent years,in order to adapt to particular and complicated missions, some space payloads often strictly limit the process of separation. Therefore, new demands for separation devices are brought up, such as:

1) Position-sensitive devices, such as gravitational wave detectors, often consider the distance of the tested masses as sensors to convert the changing distance into the desired signal, which is strict with the positional accuracy of the released loads.7

Fig. 1 Common operating situations of separation devices.1–3

2) For large-diameter membrane mirrors,space deployable mechanisms, etc., structural vibrations and excessive dynamic response caused by a large deflection ought to be strictly prevented by accurately controlling the separation momentum and vibratory shock.8,9

3) Small payloads such as Micro-satellites and Nanosatellites are widely used in complex tasks such as reconnaissance, earth observation, etc. due to their low cost,fast response, flexibility, high functional density and short development time. Separation devices are asked for low manufacturing costs, lightweight and small structure size, meanwhile, altitude and orbit accuracy must be tightly controlled.10,11

4) Several large payloads, such as deployable antennas,space mechanical arms, spacecraft doors, docking space stations and space capture devices often need to operate repeatedly,which calls for locking and separating multiple times.12

Since pyrotechnic separation devices cannot meet the above-mentioned diversified space missions, non-pyrotechnic separation devices have been gradually researched enthusiastically so as to be used in a number of space missions.According to a research report by the Langley Research Center(LRC)of NASA, non-pyrotechnic connection and separation devices gave up the weighty ignition system, reduce or avoid harmful gas emissions, which make tests easier. They ensure the safety of the system for a low shock, equaling generally about 300 g to 3000 g or even lower. Furthermore, since non-pyrotechnic separation devices are mostly used flexibly and repeatedly, it is convenient for multiple verification tests.13

In summary, developing adequate space non-pyrotechnic low-shock connection and separation devices (SNLDs) is of important basis to ensure the implementation of space missions such as launching, deep space exploration, scientific experiments, space capture and on-orbit services, etc. Review articles of space non-pyrotechnics low-shock connection and separation technology (SNLT) is helpful for readers to understand the research progress and the latest researches within a short time.They will help readers fully realize existing research information to improve study efficiency and inspire aerospace researchers for the structural design of mechanisms, offering an entry and breakthrough for related exploration. Fig. 212–22intuitively reflects several published review articles specializing in space connection and separation technology. Based on the types of repetitive locking, Yang et al.12introduced the structural principle and characteristics of mechanical, electromagnetic and other locking technology. Lucy et al.13introduced the operation schemes and performances of various non-pyrotechnic separation devices based on R&D institutions and product series. Fosness et al.14summarized low-shock non-pyrotechnic separation efforts that are being managed by the Air Force Research Laboratory Space Vehicles Directorate. From the perspective of actuating modes, references15–18summarized the working types and actuating mechanisms of new non-pyrotechnic connection and separation devices based on shape memory alloy, paraffin, etc. The application of shape memory alloy in space missions such as separation of stars and arrows, compression and release of solar panels were comprehensively investigated.19,20Yang et al.21introduced the working principle and performance of fuse-type, motor-driving and memory alloy-driving unlocking devices based on clamp-band.Up to now,there has not been a summary of the categories and characteristics of nonpyrotechnic actuation modes, the design concept of lowshock mechanism, the characteristics of connection configuration and the systematic design concept of SNLDs.Based on all these problems mentioned, this work initially introduces the research status of SNLDs in detail from two perspectives of actuation technology and systematic designing methods.Fig. 3 illustrates the main contents of the article, involving the working principle, the actuating techniques, and the systematic designing methods of SNLD.

Fig. 2 Review articles on SNLT.12–22

Fig. 3 Working principle, actuating techniques and systematic designing methods of SNLD.

2. Principle of shock reduction of SNLD

The separation shock is usually caused by the instantaneous movement of structural parts,components,subsystems or systems. Severe shock may cause the failure of some components or the whole system, so it becomes an important factor that must be considered during the designing of spacecraft.23To reduce separation shock,its source should be analyzed in principle. Pyrotechnic separation device generally includes the actuator (i.e., a detonating assembly), the main charge, the mechanical assembly and sometimes an assembly for transporting explosives. The mechanical component is composed of actuating parts and structural parts, which bear large loads and maintain high structural rigidity.When the initiator in the detonating assembly is electrified, the bridge rectifier or wire with large internal resistance generates amounts of heat,which ignites the main charge and generates heavy gas pressure.Then the gas pressure drives the actuating parts of the mechanical assembly to move quickly, unlocking and separating the structural parts. Because the actuating speed of pyrotechnic devices is about milliseconds, the relative speed between components so high to produce a huge impact force. Fig. 424is a schematic diagram of NASA Standard Initiator (NSI) driven pin puller.24In addition, explosive cables, bellows actuators,pyro-valves, spherical bolts, cable or bolt cutters, pyrotechnic separation nuts, etc., are also commonly used in pyrotechnic separation devices.25

Generally, the shock of space connection and separation device mainly comes from the following aspects:26

1) Shock wave and stress wave produced by the explosion of energy storage materials.

Fig. 4 NASA Standard Initiator (NSI) driven pin puller.24

2) Propagation of stress wave and structural resonance phenomenon when the preload of the preloaded element is released.

3) Stress wave and structural resonance phenomenon caused by striking between elements.However, in non-pyrotechnic devices, there is no detonating assembly and main charge. They actuate relying on nonpyrotechnic actuators and specific mechanisms. Therefore,the principle of reducing shock is based on two aspects: actuating techniques and systematic designing methods.On the one hand, considering the actuating technique, there is no explosive energy released intensely when the actuator acts, which is a lengthy process. Then the load is released and separated slowly without a strong shock force brought by gunpowder.On the other hand, considering the systematic design, special mechanisms are organized by various connection configurations. In this way, the direction of the internal energy of the mechanisms is changed, and the shock and vibration are weakened.

3. Research on the actuating technique of SNLD

According to the source of the energy to unlock and separate,SNLDs could be divided into electric actuating devices, magnetic actuating devices,gas actuating devices and thermal actuating devices,etc.Here,the working principles and mechanical properties of some typical devices will be respectively discussed.As used herein,‘‘a(chǎn)ctuating”refers to the whole process from the connected state to the disconnected state, or just meaning the process of unlocking or releasing.

3.1. Electric actuating device

Electric actuating separation device directly uses electrical energy as an actuation source of unlocking and separating,usually making the motor a functional element. In addition,materials whose physical properties change with electricity,such as epoxy (EP), piezoelectric ceramics, etc., could also serve as actuating elements for separation.

3.1.1. Actuating principle of electric

Fig.5 SN9700 series cross-sectional view‘‘Fastened Position”.27

As easy to control and adopt,electricity can be utilized in various forms. For example, motors, invented according to the relationship between electricity and magnetism,convert electrical energy into mechanical energy to output torque and angular displacement. Besides, the physical properties of materials and the shape of objects are likely to change owing to electrical phenomena such as piezoelectric effects, ionization, etc.,thereby releasing the initial connection between components.

3.1.2. Motor actuating device

Hi-Shear Technology Corporation designed an SN9700 series separation device, which is locked shown in Fig. 5.27In this state,the cam drive ring keeps the roller bearings in the locked position,and the roller restricts the locking ring axially.When the motor operates, the output shaft rotates to drive the connected cam drive ring to rotate and drive the roller bearings,releasing the locking ring axially. Thus, the restoring wave spring drives the locking ring to move axially, relieving the radial constraint on the threaded segments to release the bolt.Only 28 volts DC is needed,the separation nuts can be started at 15 Watts and could be reset 25 times.27

Wang designed a separation device using redundant motors based on the two-section segmented nut, of which the locked state is shown in Fig. 6.28It relies on a combination of releasing clamps, linkages, pin shafts, bearing rods, balls, etc., to bear capacity. In the beginning, the rotating disk keeps the balls stunk in the bearing rod from axial movement. The two releasing clamps are screwed with the connecting bolt. When the system gives commands to separate,the motor is energized with the drive rod rotating the rotating disk.Then the balls are pressed into the groove of the rotating disk by the releasing spring, and the spring drives the linkages later and disengages the release clamp from the bolt. The device actuates redundantly using two motors under the force of 10 kN in about 47 ms with the release shock near 29.7 g.28

3.1.3. Electrically releasable epoxy bond

Air Force Research Laboratory (AFRL) and EIC Laboratories jointly developed an electrically releasable epoxy bond named ElectRelease, as shown schematically in Fig. 7.29The two-part solid epoxy forms a high-strength adhesive on the surface of the metal to achieve the bonding strength exceeding 21 GPa. When the release command is issued, the power supplies a low-voltage current to the metal substrates. After electrification, the bonding force is weakened and the metal substrates are peeled away from the epoxy. ElectRelease not only takes up a small bonding area for locking but also releases within 60 seconds for almost without heat and gaseous or liquid waste.29

3.1.4. Piezoelectric ceramic actuating device

Piezoelectric ceramics are smart materials that can contribute to the interconversion of mechanical stress and electricity.For piezoelectric ceramic actuators, an excitation electric field could be imposed on the medium to polarize it to generate mechanical deformation, outputting force and displacement.30The phenomenon of this process is called the inverse piezoelectric effect of the material, see Fig. 8.

Fig. 6 Separation device using redundant motors.28

Fig. 7 Schematic of an electrically releasable epoxy bond (ElectRelease).29

Fig. 8 Schematic of inverse piezoelectric effect.

Wang developed an unlocking separable structure based on piezoelectric ceramics,and its principle is shown in Fig.9.31At the connection status,the notched bolt connects the two bands distributed on the left and right,and the bolts pass through the piezoelectric ceramics. To release the device, a high-frequency voltage is applied to the piezoelectric ceramics, and the piezoelectric ceramics generate an alternating load, which causes the bolt with stress fatigue to break under the action of the alternating pre-tightening. Then, the two clamp bands are unlocked and separated under the drive of the compression spring between them.To improve the reliability of separation,the mechanism adopts dual redundancy, that is, three piezoelectric ceramic plates are overlapped and connected in series as the first redundancy, and two unlocking mechanisms are equipped as the second. The mechanism can be applied to the separation of stars and arrows with an axial tension–compression load of above 10 kN and an impact response of below 338 g, proven by tests.31Cao et al. designed a piezoelectric ceramic actuating connection and separation device, which is similar in principle and structure to the above-mentioned.32

3.1.5. Prospects for application

Fig. 10 Fixing device for the assembly and quick release of objects.37

Compared with others,the electric actuating separation device upon the energy source more directly with high utilization rate.However, there exist the following problems to be solved in practice:1)Though the motor actuating separation device performs excellently, it is rarely used currently. The performance of the motor should be further improved to better the performance of devices, such as developing subminiature motors to reduce the volume and mass of the device. And innovative mechanisms would be developed and mechanical efficiency would be improved.2)Epoxy,piezoelectric ceramics and other electrically sensitive materials have been used because of their small size, high sensitivity, etc. In order to meet the requirements of various space missions, resembling smart materials,such as electrostrictive materials, electrorheological materials,etc. should be developed energetically for new actuation methods.

3.2. Magnetic actuating device

Though electromagnet is small in size, it could output large force and has a long service life, high dynamic performance and strong adaptability to space environments. In a word,electromagnet has the basic conditions for being an actuator of SNLD.33–36

3.2.1. Actuating principle of electromagnet

The electromagnet is a device that converts electrical energy into magnetic energy based on the magnetic effect of current,generally composed of a coil, a core, an attracted slider, an armature or a magnetic panel, etc. According to the position and connectivity of the attracted slider, the coil and the core,common miniature electromagnets are classified into push–pull electromagnets, sucker electromagnets, rotary electromagnets,flapper electromagnets, tubular electromagnets, etc. Based on different principles, electromagnets can respectively output force or torque, linear displacement or angular displacement,etc., so as to actuate in various forms.

3.2.2. Push-pull electromagnet actuating device

Fig. 11 Non-pyrotechnic point-type separation device.39

The iron core inside the push–pull electromagnet is magnetized to generate attractive force,which drives the connected objects to move, thereby taking away the structural limit. Valembois et al. designed a quick-separating device upon an integrated segmented nut, as shown in Fig. 10.37The integrated segmented nut with longitudinal slots is wound with a multiturn metal link to connect with the bolt with screws. When the magnetic attraction retracts the finger, the blade cut the elastic metal link driven by the torsion spring, loosening the segmented nut and releasing the holding element.37Using a similar principle, Duforet et al. designed a resettable ultralow shock actuator (RULSA). RULSA has a nominal axial load with a release impact of less than 500 g within 100 milliseconds. The segmented nut is tightly wound by a spiral spring ribbon,of which the two ends respectively hook the longitudinal slots of the nut and the retractable pin of the electromagnet. When the electromagnet is energized, the pin is withdrawn so that the spiral spring ribbon expands due to the release of elastic energy. Then the nut pre-tightening force is released smoothly to disengage the screw.38

Ding et al. designed a non-pyrotechnic point-type separation device (Fig. 1139). In the connected status, the balls and the cylindrical pin limit the separation spindle respectively in the axial and circumferential directions. To actuate, the push–pull electromagnet attracts the cylindrical pin to disengage from the separating spindle which rotates under the action of the torsion spring then. Under the preload of the separating-end compression spring, the balls are pushed out of the spindle which is bounced towards the capturing-end by the spring, removing the separating-end from the capturing-end.39

3.2.3. Sucker electromagnet actuating device

The sucker electromagnet attracts the adjacent components relying on its magnetic conductive panel without moving itself.Its outer shell can shield magnetism. Zhang et al. developed a pendulum rod unlocking and separation device for microsatellite. The device is composed of a self-hold sucker electromagnet mechanism and a pendulum rod mechanism,as shown in Fig.12.40In the locked status,the electromagnet attracts the block so that the connected pendulum rod(2)restricts the pendulum rod (1) from rotating around the bracket of the adaptor. Thus, the bearing of the rod (1) holds down the satellite tightly.When receiving the unlocking signal,the electromagnet loses the attraction to the block, so that the pendulum rod (2)rotates under the action of the tension spring(2)to release the pendulum rod (1). Then the pendulum rod (1) rotates under the action of the tension spring (1), and the bearing unlocks the satellite.40

Han et al. of proposed an electromagnetic locking device based on a self-locking wedge for magnetically suspended flywheel, as shown in Fig. 13.41In the locked status, the electromagnet attracts the sucker to push the shaft to move horizontally, contrary to the elastic force of the compression spring and the friction force of the surfaces. As a result, the locking column holds down the rotor of the magnetically suspended flywheel based on the wedge surface.To unlock the flywheel, the electromagnet is demagnetized so that the shaft retracts in the opposite direction under the action of the compression spring.Thereby,the locking column deviates from the magnetic levitation flywheel rotor by the elastic force of the extension spring.41

3.2.4. Rotary electromagnet actuating device

The working principle of a rotary electromagnet is similar to that of a small motor. According to the self-locking principle of vortex coil spring, Jiang opened up a hold-on and release device, see Fig. 14.42The pre- tightened vortex coil spring holds the segmented nut tightly and makes it engaged with the bearing bolt. The spring is connected in hook-type with the groove of the segmented nut at the inner end and with the groove of the rotary disk at the outer end.The rotary disk with the bearing inside is prevented from rotating by the pin,and the pin is prevented from moving axially by the rotary valve connected with the rotary electromagnet.When the electromagnet is energized to drive the valve to rotate,the rotating disk will loosen the pin under the action of the spring. The elastic energy of the scroll spring slowly dissipates to drive the disk to rotate. Under the action of internal stress, the segmented nut slides down along the slope and separates along the radial direction. In the end, the bearing bolt is released. It has been proved that the device can withstand a load above 30 kN and can be unlocked under a voltage above 16 volts within 100 milliseconds.42He et al. recommended an electromagnet actuating non-pyrotechnic separation device based on vortex coil spring, similarly to the above in structure and working principle. In order to effectively reduce the release impact,the internal friction of the spring is released slowly, making the elastic force work slowly as well.43

Fig. 12 Pendulum rod unlocking and separation device for micro-satellite.40

Fig. 13 Locking/unlocking device configuration of magnetically suspended flywheel.41

3.2.5. Prospects for application

Magnetic actuating separation device has the advantages of small size and fast actuation, leaving the following problems to be solved: 1) The magnetism of the separation device may interfere with the electronic components and precision instruments inside the spacecraft, adversely affecting the normal operation of the load after released.Therefore,magnetic isolation should be carried out. 2) Actuation methods of the magnetic separation device are relatively simple for the electromagnet is currently the most commonly used actuation mechanism.It can be considered to apply smart materials sensitive to magnetism, such as magnetostrictive materials, magnetorheological fluids, etc.

3.3. Gas actuating device

Compared with solid and liquid, gas has the advantages of flexible movement, negligible mass, adjustable output force and controllable flow. It can do work to the external world as the actuation source of the separation device. Generally,the gas actuating separation device equipped with a gassupplying device regards the cylinder as the core mechanism.

3.3.1. Pneumatic separation mechanism based on cylinder

Sang developed a pneumatic locking and unlocking mechanism, mainly composed of cylinder, slider, base, locking bolt,etc., see Fig. 15.44In the connected status, the locking bolt presses the load to the base. Meanwhile, the threaded part of the bolt is embedded in the small through-hole of the slider which withstands the pre-tightening force transmitted by the bolt.At the beginning of the actuation,compressed gas is carried out through the pipe mouth and enters the cylinder to push the slider, and the tightening bolt enters the large through-hole from the small one. Since the diameter, D, of the large through-hole is slightly larger than the diameter of the polished rod of the bolt, so the bolt can be left from the slider.Finally,the pneumatic ejection-and-pushing component works to separate the load subsequently.The mechanism bears an axial force above 10 kN and can be unlocked within 30 milliseconds by testing.44

Fig. 14 The hold-on release device based on vortex coil spring self-locking principle.42

Fig. 15 Locking and unlocking device by gas.44

Fig. 16 Pneumatic unlocking and separation device.45

Cui proposed a pneumatic unlocking and separation device,as shown in Fig. 16.45In the connected status, the internal pressure of the cylinder balance with the pre-tightening force of the separation spring.Restricted by the cylinder,the locking pin buckles the annular groove of the satellite adapter to connect it with the platform adapter. During the progress of unlocking, the cylinder is inflated to move downwards against the preload of the spring until the locking pin is released.Later on, the piston of the cylinder pushes out the satellite adapter,separating the satellite from the platform. The device has the advantages of low shock and high-accuracy control and would be unlocked within about 0.5 seconds in case of the gas source pressure of 0.1 MPa.45

3.3.2. Actuating principle of cylinder

Cylinder is a device that converts the pressure energy of compressed gas into mechanical energy.Two-way single-rod cylinder is generally composed of the cylinder block,piston, piston rod, end cover, seal, etc., which works as shown in Fig. 17.When actuated,the first cavity is inflated,then the second cavity exhausts so that the piston moves due to the pressure difference and the piston rod outputs force and displacement outwards.

3.3.3. Prospects for application

Fig. 17 Two-way action of single-rod cylinder actuation process.

Fig. 18 Principles of thermal actuating.

Gas actuating separation device is of widely potential application on accounts of its advantages, such as low shock, large output force, adjustable flow and strong flexibility in harsh environments. However, there are the following technical problems to be determined: 1) The larger volume of the cylinder and the equipped large gas-supplying device enhance additional demands for the propulsion system.In order to increase the utilization rate of gas, it is necessary to reduce the gas as much as possible utilizing structural design and optimization.2)For the gas tightness of the cylinder and the friction between the piston and the inner wall interact with each other, a detailed theoretical analysis is required to prevent discharging gas. 3) Given many technologies are immature about the cylinder-type separation device,further research should be carried out in terms of reliability design, engineering compatibility, etc.

3.4. Thermal actuating device

Compared with other energies, thermal energy is dissipated more slowly, which greatly reduces the actuating shock of the separation device. Therefore, in recent years, various heat-sensitive materials, such as shape memory alloys, have attracted more and more attention and have been widely used in SNLT.

3.4.1. Principle of thermal actuating

Physical or chemical properties may change after heating.Based on the physical changes caused by heat, the principles of thermal actuating are shown in Fig. 18. Additionally, care has to be taken that some low-ignition-point materials may burn themselves after heating, which is uncontrollable and may cause damage to other components. Therefore, heating power and duration must be strictly controlled, and the temperature must be monitored.

3.4.2. Shape memory alloy actuating device

As an excellent smart material, shape memory alloy (SMA)46deforms under the influence of external force or magnetic field after at low temperature and can get back into shape by heating.This process attributed to the crystal phase transformation that occurs inside the alloy is called shape memory effect(SME). The basic types and working processes of SME are shown in Fig. 19, where Asrepresents the start temperature of austenite transformation,Afrepresents the end temperature of austenite transformation and Mfrepresents the end temperature of martensite transformation. SMA commonly serves as various forms including wire, spring, pipe, cylinder, rod, etc.,and the performance of them are qualitatively compared in Fig.20(a).In order to visually compare the application of different states of SMA in SNLT, Fig. 20 (b) summarizes representative achievements cited in this section. It can be seen that the SMA wire actuator has been widely used because of easy heating, low power required, small space occupation,and flexible design.47(1) SMA wire actuating separation device

Fig. 19 Three types of shape memory effect (SME).

Fig. 20 Performance and application of SMA in different forms.

SMA wire has the advantages of small space occupation,large output, flexible structure design, and various motion forms, etc. Here typical structural forms of separation devices based on SMA wire are introduced below.

1) SMA wire actuating rotary separation device

Va′zquez et al. developed a Non-Explosive Hold-Down Release Actuator (NEHRA), as shown in Fig. 21.48A helical torsion spring is placed around the segmented nut, one end of which is fixed on the housing and the other end is fixed on the wheel. After the spring is preloaded, it restrains the nut to make it closed. When the SMA wire is energized and heated, the wheel rotates due to the contraction force of SMA against the elastic force of the torsion spring. Then the steel balls enter the groove, and the segmented nut releases the bolt thanks to its own radial pre-tightening force.The actuator can be reset manually by reversing the crown,as shown in Fig. 21 (b). To ensure the reliability of separation, NEHRA actuates redundantly as the two SMA wires are connected to different electrical circuits, and one of them could be heated to actuate. NEHRA can work with a current of between 1.5 to 5 A and can be actuated with a current of 2.75 A in about 1 second.48

Christiansen et al. of Starsys Research Corporation developed a fast-acting non-pyrotechnic separation nut called Qwknut which works as shown in Fig. 22.49The rollers between the outer race and the segmented nut squeeze the nut radially and lock the bolt.When energized, the SMA wire retracts the latch arm to release the toggles. Subsequently, the outer race rotates under the action of the compassion spring,and the rollers enter the groove of the race, thus losing the radial constraint on the segmented nut to release the bolt.Qwknut converts sliding friction into rolling friction through rollers, so that it reduces power consumption and shortens the actuation time. The maximum impact during actuation achieves 60 g, and the allowable operating temperature varies from-65 ℃to 90 ℃.When the starting current is 3.5 A and in-40 °C between 60 °C, the device will be released in less than 100 milliseconds.49Based on the segmented nut, Jiang, Wang et al. developed an SMA wire actuating rotary lock/release device, similar to Qwknut in structure and working principle.50,51

Fig. 21 NEHRA and the reset process.48

Fig. 22 Latch geometry and operation of Qwknut.49

Fig. 23 SMA wire actuating separation device based on flywheel nut.52,53

In addition,Starsys developed a Fast-Acting,shockless separation nut (FASSN), consisting of a Nitinol actuator, latching mechanism, rotating flywheel nut, long-lead thread,housing,bolt and bolt catcher,see Fig.23(a).52In the connection state, the flywheel nut is restrained in the circumferential direction and is engaged with the bolt through the nonlatching threads. When the SMA wire is energized, the actuator works to release the internal latching mechanism, so that the flywheel nut in the circumferential direction rotates under its own preload. Finally, the preloaded spring drives the bolt gradually to enter the bolt catcher. According to the test,FASSN could withstand a maximum load of 62.27 kN and could be actuated in less than 50 milliseconds with a maximum impact of 400 g.52Peng et al. developed a large-load and lowshock release device similar to FASSN in principle,see Fig.23(b).53Two swing arms are used in the actuator to increase the actuation force of the SMA wire.The load-bearing capacity of the device is above 100 kN,and with a current of 5 A,it can be unlocked within 220 ms with a shock of below 500 g.53–55Yang et al.designed an SMA wire actuating connection and separation mechanism based on double swing arms and flywheel nut.The device is locked and unlocked through the screwing and disengagement of non-latching thread pairs between two bearing screws of different rotation directions and the flywheel nut.56.

2) SMA wire actuating linear separation device

Yoo et al. of Korea Advanced Institute of Science and Technology (KAIST) opened up a non-explosive release actuator using SMA wire, which works as shown in Fig. 24.57In the connected state, the rollers restrain the segmented nut to engage it with the bolt. The actuation process is divided into several steps. First of all, the SMA wires are energized to pull the trigger block upwards, and the rollers enter the groove of the trigger block to release it. Then, the locking sleeve moves upwards under the action of the compression spring, and the rollers enter the recess of the segmented nut. In the end, the nut segments are separated radially and the bolt is released.The device can withstand a preload of 15 kN and release within 50 milliseconds, accompanied by a shock acceleration below 350 g.57Gardi designed a simpler steel-lock. During the actuation,the SMA wires pull the sleeve to release the steel balls, thus freeing the plug.58

Yan et al.59–63has also developed a variety of SMA wire actuating separation devices and conducted related experimental studies,most of which used segmented nut,steel balls,locking hook and other structures. For example, the compact,quick-response SMA separation device add structures such as insulated pulleys, thus increasing the actuation stroke of the SMA wire and reducing friction. The structure of the device is shown in Fig. 25.60Later, Yan, et al. respectively made certain improvements for this device.60,61

Fig. 24 Release device of KAIST.57

Fig. 25 Compact, quick-response SMA separation device.60

Fig. 26 Working principle of release mechanism.64

3) SMA wire multi-stage actuating separation device

Fig. 27 Configuration of the locking device.66

In order to lift the reliability of separation,some devices are actuated by multi-stage triggers. For instance, Lee et al.designed a non-explosive separation actuator(NEA)based on SMA with a redundant function, which works as shown in Fig. 26.64The release pin and the steel balls are combined to bear the load, and the SMA wire 1 and 2 are respectively wound on the two actuators. In order to unlock the device,the SMA wire 1 is heated to drive the primary actuator to rotate, and the primary actuator is moved downward by the compression spring 1. Then, the release pin is moved upward by the compression spring 3, squeezing the lower balls into the grooves of the primary actuator so as to release the pin.If the primary actuator fails, the secondary actuator works upon the principle similar to the former.The device could bear a capacity of up to 500 N, and could be released in 761 milliseconds primarily or in 768 milliseconds secondarily, with an impact acceleration below 30 g.64In addition, in a new resettable hold down and release actuator(REACT),steel balls are combined to squeeze each other to improve the bearing capacity of the device.65

4) SMA wire actuating self-resettable separation device

Zhang et al.recommended a novel SMA actuated resettable locking device for magnetic bearing reaction wheel, using two wire actuators to lock and unlock. The working process is shown in Fig. 27.66During the locking process, the trigger SMA wire is energized to shrink,which drives the trigger block to move up and out of the groove of the dual-slope block.Then,the dual-slope block moves to the right under the action of the locking spring,so that the Y-shaped clamp is pushed up along the inclined surface to make the reaction wheel edge tightly held-down.When the unlocking SMA wire is energized,it shrinks and drives the dual-slope block to move to the left.Under the action of the safety spring, the trigger block enters the groove of the dual-slope block, and the connection between the Y-shaped clamp and the wheel disappears. The device could be reset automatically and it could run 76 times repeatedly by testing.66Under 7 V power supply, the wheel would be locked and unlocked respectively in 0.9 seconds and 5.6 seconds. The improved device is provided with two SMA wires, which improves the reliability of actuation.67

Liu et al. exploited a linear-bidirectional-offset unlocking mechanism based on SMA wire and compression spring, as shown in Fig.28.68When the SMA wire is energized and contracted,it pulls the sleeve and drives the slider to move against the spring, so that the slider is separated from the anchor bracket. After the energization is stopped, the SMA wire is cooled with the slider and the sleeve returning to its original position under the action of the spring.68

(2) SMA spring actuating separation device

Fig. 28 Linear-bidirectional-offset unlocking mechanism.68

Fig. 29 SMA spring actuating Low Force Nut (LFN).69

Lockheed Martin Astronautics (LMT) developed an SMA spring actuating low force nut (LFN) provided in Fig. 29.69At first, the SMA actuation spring is heated and stretches to push the plunger down. Then the ball in the sleeve enters the groove on the plunger, releasing the sleeve. Under the action of the steel actuation spring, the sleeve moves upward until a certain height position. Finally. due to the segment separator,the nut segments are released radially to separate the bolt.69

Tak et al. put forward an ultimate load and release time controllable non-explosive separation device using an SMA spring. In the locked state shown in Fig. 30 (a),70the aluminum deformation module causes the attached block to be held-down in the housing by the blocker. When the stretched spring-type SMA actuator is heated to shrink to its original shape, the aluminum deformation module is plastically deformed. Then the block is disengaged from the blocker and ejected under the action of the release spring (Fig. 30(b)70). Through a prototype model, the device could stand 1.51 kN of ultimate load. Under the preload of 150 N and the working power of 30 W, the device could be actuated in 55 seconds with a shock measuring 11.09 g.70.

(3) SMA pipe/cylinder/rod actuating separation device

Mckinnis of NASA proposed an SMA pipe actuating separation device based on the segmented nut, as shown in Fig. 31.71At the beginning, the locking structure tightens the segmented nut defined by the shear pin and the piston structure.During the unlocking process,the resistive element inside the SMA pipe is energized to heat the SMA pipe,and stretch it to push the piston structure upward to cut the pin. When the piston moves to a certain position, the radial constraint of the nut segments disappears, and the separation bolt is released from the separation platform.71

Fig. 31 SMA pipe actuating separation device based on segmented nut.71

TiNi Aerospace has exploited amounts of SMA actuating separation devices. Some products, such as Frangibolt(Fig. 3272), have been applied in mass production. By heating the pre-compressed SMA pipe actuator, the notched bolt is stretched and broken at the notch, so that the load can be released from the separation plane.This series of products currently developed can withstand and release loads up to about 30 kN to 40 kN, and start up within 30 seconds to 80 seconds.72,73

LMT has developed a Two-Stage Nut (TSN) actuated by SMA cylinder and spring, as shown in Fig. 33.69To operate the TSN, the SMA cylinder is heated to extend, compressing the collet supported by a belleville washer at the bottom,and gradually releasing the pre-tightening force of the nut. If any two of the three SMA springs, positioned tangentially between the three nut segments, are actuated, the bolt can be separated from the nut. The device is unlocked with a small actuation force within 30 milliseconds, accompanied by a slight shock due to the slow-releasing internal energy of the washer and the segmented nut.69

Fig. 30 Ultimate load and release time controllable separation device using a SMA spring.70

Fig. 32 Schematic illustration of Frangibolt device and heater location.72

Fig. 35 STAR Nut baseline design.75

Hi Shear Technology Corporation has produced the SN9600 series of separation nuts which work as shown in Fig. 34.74In the locked state, the ring restricts the threaded segments radially. When the nitinol column is heated to contract, the pre-tightening load of the loaded bolt gradually decreases until the force of the driving spring is greater than the friction between the segments and the ring. As a result,the ring is lifted by the spring and the segments are separated from the bolt. After the column has cooled down, the nut can be reset by pressing down the ring.The nitinol nut requires 125 watts for 15 to 30 seconds without any shock output virtually.74

(4) SNLD based on other shape memory materials

Apart from SMA, some shape memory polymer materials have been employed in separation devices. Starsys Research Corporation (SRC) and Composite Technology Development(CTD)jointly developed a shockless thermally actuated release nut(STAR Nut,see Fig.3575)using elastic memory composite material (EMC). At low temperature, EMC can easily be folded into a certain shape, and after heated to a certain temperature, it will return to its original state. The STAR Nut actuates based on two EMC actuating cylinders, and the preload bolt is rigidly connected with the inner one. The nut is screwed down so that the inner cylinder and the outer cylinder are tightly combined at the tapered surface. If any one of the two actuator cylinders is heated to restore the cylindrical shape, they can be separated and unlock the load axially.75

3.4.3. Paraffin actuating device

Paraffin is an amorphous mixture of hydrocarbons without a certain melting point. For the temperature and volume of paraffin change slowly when melting, it can continuously produce force and displacement. Therefore, paraffin can be used as the actuating element of SNLD.

(1) Common paraffin actuator based on an elastic squeeze boot

As the core component of the paraffin separation device,the paraffin actuator contains a certain amount of highvolume expansibility paraffin whose surface is in close contact with the heating element. Tibbitts et al. developed an Internally Heated High Output Paraffin actuator(IH-HOP, see Fig. 3676). During operation, the heating elements are energized to heat the expandable paraffin in the sealed cavity.The paraffin gradually transforms from solid to liquid and expands with the body expansion rate up to 15%. Then the paraffin produces sufficient hydrostatic pressure to squeeze the elastomer squeeze boot, which transmits the pressure to the actuator rod. Until the paraffin in the cavity is completely melted, the actuator rod reaches the maximum stroke and is separated from the elastomer squeeze boot. The weight of the paraffin actuator is less than 30 g, and it could generate 4 kN force with a stroke of 10 cm.76

Fig. 34 SN9600 separation nut.74

Fig. 36 Internally Heated High Output Paraffin (IH-HOP) actuator.76

(2) Improved paraffin actuator

Zhang et al.of made improvements on the aforementioned scheme based on elastic squeeze boot, and developed a large load space-used paraffin actuator. The working process is shown in Fig. 37.77Compared with IH-HOP, this device has many advantages. For example, the pressure generated after the paraffin is melted and expanded directly acts on the piston,which is converted into the driving force for separation,thus improving the utilization rate of liquid pressure. In addition, the friction force mainly exists between the housing and the seals, avoiding large direct friction between the paraffin and the elastic squeeze boot.To reset the actuator,the reset spring acts along with the gradual solidification of the paraffin.77

As shown in Fig. 38,78Zhang et al. designed a quick-reset non-pyrotechnical paraffin actuator that combined SMA with paraffin. There are several obvious advantages of this kind of actuator. First of all, the strain gauge can monitor the pressure of the paraffin to prevent leakage. Secondly,the displacement sensor monitors the displacement of the driving rod, so as to control the actuator more accurately.Finally, the SMA spring can be heated during the recovery process and generate pressure. The pressure works together with the elastic force of the metal bellows to further shorten the reset time.78

(3) Separation device based on other polymers

Some high-molecular polymers will change significantly after heating, so they are also used in SNLT. Meng designed an unlocking and separation device for nanosatellite,as shown in Fig. 39.79The resistance wire is installed inside the highmolecular polymer. When the resistance wire is energized,the high-molecular polymer is heated and changes of state,and the spring drives the sleeve to move downward. When the steel balls deviate from the clamp, the aircraft adapter is separated from the sleeve,and then the adapter is pushed away by the spring.79

Fig. 37 Scheme principle of large load space-used paraffin actuator.77

Fig. 38 A quick-reset non-pyrotechnical paraffin actuator using SMA spring.78

3.4.4. Liquid metal actuating device

Shang et al.proposed a locking/unlocking device based on liquid metal, see Fig. 40.80Firstly, the liquid metal with a low melting point is heated and melted, and then poured into the designed cavity consisting of two parts. After the liquid metal cools and solidifies,the two blocks are fixedly connected.If the liquid metal is reheated, it will melt and separate the two blocks. When the radius of the cavity filled with liquid metal is 4 mm, the locking force of the device can be greater than 963 N. If 100 watts of power is input to the heating rod, the device can be unlocked in 3 seconds.80Russell et al. also proposed an unlocking device based on low melting point alloy.When the alloy solidifies, the two parts of the robot joint can be locked together, and when the alloy melts, the joint shaft can move axially or rotate around the shaft. In this way, the robot can be reconfigured and assembled.81

3.4.5. HMAs actuating device

Yang et al. introduced a separation device based on hot-melt adhesives (HMAs), and its structure is shown in Fig. 41(a).82The two parts are joined together by HMAs, connected similarly to thread engagement. For actuation, the temperature of the heating rod is set as the melting point of the adhesive so that the two parts can be unlocked and then quickly separated by the spring.82According to the mechanism of physical connection and disconnection of special robots,Wang et al. elaborated on the three operation steps of how HMAs actuator deploys, namely supply, connection and disconnection, see Fig. 41 (b).83

Fig. 40 Locking/Unlocking device based on liquid metal.80

3.4.6. Thermal Knife actuating device

Thermal Knife refers to the mechanism used to fuse the rope.The key component of Thermal Kinfe is the electrothermal element,such as heating plate, heating tube, electrothermal wire,etc. The electrothermal element is usually pressed to maintain contact with the preloaded rope.When receiving the actuation signal, the electrothermal element is heated and transfers the heat to the rope fiber, so that the fiber reaches the melting point or breaks under the action of the pretension force due to the reduction of tensile strength. Then the compression force is gradually released. Since the rope fibers are to eroded gradually, almost no impact occurs. In a word, the Thermal Knife device is an ideal low-shock separator. In some fusetype release devices, low-melting metal wire instead of rope is restrained to lock and fused to separate the load, which are also regarded as Thermal Knife actuating devices.

(1)Thermal Knife actuating separation device based on the heating plate

Fig. 39 Unlocking and separation device for nanosatellite.79

Fig. 41 HMAs device and operations.82,83

Fokker Space developed a multipurpose hold-down and release mechanism(MHRM,Fig.4284)actuated by a Thermal Knife. In the locked state, the Dyneema cord is tightly bound to the assembled reels, and the heater element is closely contacted with the outer fibers of the cord under the action of the driving spring. When the Thermal Knife is energized, the heater element slowly melts the rope, weakens the fiber and shrinks the cross-section. Finally, the load-bearing capacity of the rope declines until it breaks under the action of pretension,thus separating the reels.84Through testing,MHRM can bear the load of 5.5 kN, and release in about 5 seconds to 20 seconds,accompanied by a release shock of 5.12 grms.85At present, MHRM has been applied for many space tasks such as solar arrays.86In MHRM, two Thermal Knives are redundantly placed, as shown in Fig. 42 (b). The Thermal Knife is mainly composed of High-temperature co-fired ceramics(HTCC)plate coated with conductive slurry,electrode,driving spring, and retraction rope that prevents the two knives from interfering with each other. Stewart and Hair of reduced the scraping effect of the rope on the Thermal Knife element through a special way the rope goes through,extending the life of the HTCC.Furthermore,they also studied several practical problems including how to measure the pretension of the rope accurately and how to prevent relaxation.87Duke Space redesigned Kevlar cable with barrel and reshaped spike to improve the robustness in hot environment.88

There are many ways to make the electrothermal element feed into the rope continuously along with the fusing process.Airbus Defence and Space Netherlands B.V., which has been committed to the research of space mechanism and installations, designed and developed a non-explosive low shock(NELS)hold down and release system(HDRS).The Engineering Model (1) is shown in Fig. 43.89When the Thermal Knife works, the torsion spring-hinge mechanism drives the heater plate to‘‘cut”the restraint cable to release the load.If the preload of the rope is 15 kN,the system would be actuated along with a maximum horizontal impact of 2 g.89

In order to solve some practical problems of locking and releasing, Guo et al. proposed a new type of multi-bar compaction and release device using a flexible cable and Thermal Knife, as shown in Fig. 44.90In the locked state, the Kevlar rope binds the rods of the mechanical arm, and the tension of the rope can be adjusted by rotating the preloaded nut.After broken,the rope would be recycled into the rotating rolling wheel driven by the stored energy of the scroll spring in the rolling-up component. In this way, the broken rope is kept away from the motion trajectory of the mechanical arm.90Li et al. designed a large load-bearing hold-down and release device based on Thermal Knife, in which the same principle is applied to recover the rope to avoid hooking the load.91

Zhang et al. designed an unlocking and releasing device(Fig. 4592) where the Thermal Knife is embedded in the bolt connection component.The heating plate is a fan-shaped electrothermal element including the basal plate made of stainless steel coated with the heating coating. The guiding column is covered with a compression spring driving the Thermal Knife to feed during the release process. Initially, the rope tightly binds the bottom housing and the top housing together.When the heating element is energized, the coating is heated and quickly transfers the heat to the rope through the basal plate.Until the wire is blown, the top housing is separated from the bottom housing under the action of the separation spring.92

Fig. 42 MHRM.84

Fig. 43 Cross-section of NELS.89

(2) Thermal Knife actuating separation device based on a heating pipe or rod

Cao et al. designed a new Thermal Knife restraint and release device composed of electrothermal elements, support of Thermal Knife, inhibiting device, rope, etc., as shown in Fig.46(a).93The electrothermal element is a heating pipe fixed on the support of a Thermal Knife. The structure of the two Thermal Knives is shown in Fig. 46 (b),93and they are arranged orthogonally and symmetrically. The rope locks the spacecraft structure with a certain pre-tightening force, and its two ends are connected by pinched ‘‘8-type” aluminum joints. Once the heating pipe is heated by an external power supply, some of the fibers in contact with the pipe begin to break. During this period, the tensile stress in the rope and the prestress of the structure accelerate the breaking progress.Eventually, the spacecraft structure deploys attributed to elastic components.93.

Choi et al. has been manufactured a high-preload and low-impact compact separation device based on wire thermal cutting for a small satellite, and verified its functional reliability. Ni-Cr wire is wrapped around the three holders to make the rigid balls restrain and lock the release pin. In order to separate the pin, the electric rod is energized to melt the clamped Ni-Cr wire. At the same time, the three holders and the rigid balls move slowly in the radial direction with the release pin moving out under the preload of the expandable mechanism. The working process is shown in Fig. 47.94The device is compact with a mass of only 125 g, and can be separated within 550 ms under a preload of 1000 kgf, accompanied by an impact of 32.17 g.94Zhao et al. designed a nonpyrotechnic fusing connection and separation device, which combines the three-segment nut with the rope. The tension of the restraint rope could be controlled through a ratchet mechanism. Given an electrical signal, the fuse cutter is heated to melt the rope, and the segmented nut is separated under the action of the spring between the conical cap and each segment.95

(3)Thermal Knife actuating separation device based on the heating wire

Fig. 44 Main components of the multi-bar compaction and release mechanism.90

Fig. 45 A bolt device used for unlocking and releasing based on Thermal Knife.92

Fig. 46 New Thermal Knife restraint and release device.93

Xuan et al. developed a small fuse holding-down and release mechanism (Fig. 4896). In the locked state, the rope passing through the resistance wire presses the panel part,etc. on the satellite platform. The tension of the rope can be adjusted by the preloaded bolt, and the adjusting nut can be turned to ensure close contact between the resistance wire and the rope. With the resistance wire energized, the rope would fuse at a certain temperature. Electric heating elements use resistance wire instead of a heating plate to avoid brittle fracture. A redundant design is adopted to improve the reliability of unlocking. The test confirmed that the mechanism required power of 6.25 W within a fusing time of about 6 s.96Zhang et al.designed a Kevlar wire burn-off device used in the hold-down and release mechanism for a rotor of precision mechanism, which could be released under the energized voltage between 3.5 V and 4.2 V within 4 s.97

Fig. 47 Wire thermal cutting based non-explosive separation device.94

Fig. 48 Small-scale fuse holding-down and release mechanism.96

Ren et al.designed a Thermal Knife unlocking device based on carbon fiber,as shown in Fig.49.Carbon fiber is used as an electrothermal element. The electrical resistance of carbon fiber could be changed by changing the knitting method of the fiber. The carbon fiber is wound on the rope in multiple turns to keep the Kevlar rope in close contact with it.98.

Tang et al. designed a miniature fuse cutter for aerospace(Fig. 5099). Two resistance wires are installed in the grooves of the Thermal Knife bases, and are connected to the cable connector. A compression spring for driving the resistance wires is arranged between the base and Thermal Knife bases.During actuation,the rope passes through the resistance wires and maintains contact with them. Then the rope is fused, and the resistance wire is electrified to release the load.The mass of the device is less than 50 g. At room temperature, the cutter could fuse the rope within 20 s and can actuate repeatedly for more than 20 times.99

Cooper,et al.of G&H Technology designed a spool assembly used in electro mechanical structural separation devices,as shown in Fig. 51 (a).100In the locked state, the wrapping wire is wound around the spool halves and is hooked on the lowmelting hot wire. When the device is energized to heat the low-melting hot wire to reduce the strength until it is broken under the pulling force of the winding part, thus leaving the wrapping wire free. Therefore, the circumferential stress of the wrapping wire is released and the restraint on the two spool halves disappears. Then, the shaft is separated from the spool in a short time under the axial load.100Holt et al.improved the structure of the spool(Fig.51(b)101),which eliminates the gap between the two halves,makes the whole device compact, and reduces the manufacturing cost.101

3.4.7. Magnesium alloy band actuating device

Magnesium alloy has the advantages of low density, high specific strength,strong resistance to vibration and shock,corrosion resistance, etc., and has a basic load-bearing capacity.The working process of the locking and separating device based on magnesium alloy is shown in Fig. 52. In the locked state,the magnesium alloy band prevents the pulling rod from rotating,causing it to press part A on the support ring.When a high-temperature gas flow passed the space where the magnesium alloy belt is located, the band drops rapidly in strength until it breaks. Finally, part A is released. Fu et al. analyzed the actuation process and verified that the alloy is actuated under the action of high temperature, high-speed gas and engine thrust. If the thickness of the magnesium alloy band is 3 mm, it would fracture within about 9 ms.102

3.4.8. Prospects for application

Fig. 49 Thermal Knife unlocking device based on carbon fiber.98

Fig. 50 Miniature fuse cutter for aerospace.99

Fig. 51 Electro mechanical spool.100,101

Slow heat transference makes the thermal actuating separation device work gradually,which is suitable for occasions required strictly for shock. However, there are the following technical problems to be solved: 1) Due to the alternation of high and low temperature in space environments, some devices with low actuation temperatures may have the risk of selfunlocking. Therefore, it is necessary to prevent self-unlocking through thermal protection and improving material properties.2) There are few studies on the constitutive relationship of some materials at the structural level,or the process of physical and chemical changes is rather complicated, making simulation analysis troublesome.Hence,experiments are used to verify the activation reliability of the thermal actuator, whereas,increasing the research cost.

3.5.Performance analysis of common non-pyrotechnic actuation methods

The actuation source categories of SNLDs are summarized in Fig. 53, and here the basic performance of typical devices is listed in Table 1.Taking Table 1 as a reference and considering many examples,general characteristics of the separation device based on various actuation sources are summarized in Table 2,from the aspects of structure size, bearing capacity, shock,repeatability, output force, actuation time, etc. It should be emphasized that the analysis in Table 2 only shows the general qualitative characteristics of the separation device based on various actuation methods. However, the characteristics of a particular device depend on its own structure, working principle and system settings. In a word, non-pyrotechnic actuation methods are generally superior to pyrotechnic actuation methods in some aspects,but each actuation method has disadvantages besides advantages. For researchers, it is necessary to accurately determine which actuation method is more conducive to the task according to the requirements, and make further efforts in innovative design and improvement of the structure.

Fig. 52 Device for joining and separating based on magnesium alloy.102

4. Research on the systematic designing methods of SNLD

The performance of SNLD is determined by its actuating source and structure. Therefore, the functional characteristics depend on the characteristics of components, the movement way of the mechanism, the connection configuration, etc.These elements are summarized as systematic design methods.From the point of the systematic design, the design concept and functional characteristics of SNLD will be described below.

Fig. 54 Low-shock mechanisms commonly used in SNLD.

4.1. Research on mechanism design of SNLD

Some special mechanisms are used in common SNLDs to realize connecting and separating functions with certain characteristics. These mechanisms are based on a steel ball, locking hook or compression bar, notched bolt, segmented nut, flywheel nut, etc., as shown in Fig. 54. The working process of these special mechanisms have been described in detail in chapter 3.According to the design concept of reducing shock,these mechanisms could be divided into low-shock ones based on energy flow and low-shock ones based on miniaturization.

4.1.1. Low-shock mechanism design based on energy flow

Considering the energy flow direction of the mechanical system,the outward transmission of impact force can be reduced by changing the characteristics and ways of energy transmission and transformation. As a commonly used structure for connection and separation devices, the segmented nut(Fig. 54 (a)) is generally composed of a bolt, several inner nut segments, limit sleeves, etc. (sometimes with elements between the nut segments and the sleeve). A reliable threaded connection is formed between the inner nut segments with the bolt based on the tightening of the limit sleeve.When the actuator drives the limit sleeve to rotate, the threaded connection disappears, and the bolt is released. Because the attenuation of a pre-tightening force of segmented nut is gradual, the release of strain energy is also gradual, which avoids large impact force. Besides, the segmented nut mechanism has the advantages of simple structure, small volume, large bearing capacity, small actuating displacement and fast actuating speed, etc. Flywheel nut is a special flywheel with threads,and its working process is shown in Fig. 54 (b). The flywheel nut and the load-bearing bolt are engaged by non-selflocking threads, and the nut is prevented from rotating. Then to be unlocked,the flywheel is released and rotates to separate from the bolt under the action of its own preload.By combining with non-self-locking thread, flywheel nut can bear a large load. The strain energy of the released bolt is converted into the rotational kinetic energy of the flywheel nut. In this way,the energy is converted and released gradually. This energy flow mode effectively prolongs the energy-releasing progress and greatly reduces shock.

4.1.2. Low-shock mechanism design based on miniaturization

In recent years,with the development of electronic equipment,sensors and miniaturized instruments, miniaturized payloads(such as microsatellites) have been gradually used in scientific research, commercial.103These small loads are more sensitive to shock and vibration under the same force because of their small mass, small volume and small inertia. Considering the configuration characteristics of the separation device, the separation device should also meet the requirement of miniaturization, so as to adapt to the small load. In recent years,miniaturized devices have been widely researched because of the limited space of spacecraft. For example, the spacing of some unfolding mechanisms is narrow, which can provide a small installation area for the separation device.Small separation devices can meet new design requirements of light spacecraft and the low launch cost. Devices based on steel balls additionally consist of locking components,limit sleeves,fixed structures,etc.,where the working principle is shown in Fig.54(c). Initially, the fixed structure is connected to the separated platform and the locking component is connected to the load.The steel balls restrict the clamping element from axially under the action of the limiting sleeve.When the actuator works,the limit sleeve moves to release the steel balls,and the load is separated from the locking component.Steel balls are widely used in SNLDs because of many advantages. For example, steel balls are small in size and flexible to design mechanisms,which can bear forces in all directions. Moreover, they can convert sliding friction into rolling friction, and need less actuating force.Notched bolt is a kind of structure that applies the principle of stress concentration into unlocking, see Fig. 54 (d).Slotted near the pressed surface, the bolt is used to reliably connect the load and the body. When the actuator pulls the bolt, the stress concentration at the notch breaks the bolt and separates the parts. Notched bolts are mainly actuated by SMA tube and piezoelectric ceramic plate, and could produce small displacement. In addition, some locking hooks,claws, compression bars, etc., which are used for mechanical locking, have also been adopted on many occasions, such as docking of space stations, locking and separating of pallets,and opening and closing of shuttle doors. In order to bear a large load, some force-increasing and force-reducing mechanisms, such as wedge mechanism, screw mechanism, lever mechanism, etc., are often used to improve the load-bearing capacity and increase the output force of the actuator. Upon the existing research, Fig. 55 visually shows the application of the above special mechanisms in SNLDs. It could be seen that the segmented nuts and steel balls provide sufficient design methods and innovative ideas for the design of SNLDs and have been widely used.

Table 1 Properties of typical SNLDs based on various actuation modes.

Table 2 General characteristics of SNLDs based on various actuation modes.

4.2. Research on connection configurations of SNLD

The connection configurations of SNLD refer to the connection form between the load and the separation platform.According to the structural forms of connection, the connection configurations of SNLD are commonly classified into single-point, multi-point, banded and box-storage mode, etc.How to select the adaptive one among various configurations often depends on the type of load and requirements of the specific tasks. Whether the transmission path is reasonable and the spatial arrangement is appropriate determines whether the connecting platform can bear the requested load. The design of connection configurations involves the layout of connection points,that is,the corresponding locking and releasing position on the connecting platform and the load.And the following problems should be considered in the design process:1)A tight and reliable connection state meeting the requirements of modal,response,strength and rigidity.2)Enough space for the SNLD, and the operation not interfering with the subsequent functions of the load.3)No many counterclaims putting forward to the design of the load and the connection platform.To solve the above problems,it is necessary to design the number and position of the connection points according to space environmental conditions to obtain the best layout position.

4.2.1. Single-point configuration

The single-point connection configuration means that there is only one connection point in SNLD, where the device is locked, unlocked and separated. Li et al. designed a release mechanism for a small-light folding wing, as shown in Fig. 56.104QWKNUT is embedded in the missile body, and the release bolt is directly connected with the folding wing by a single-point configuration. When the wing needs to be deployed, the SMA wire actuates to let the release bolt separate from the missile body under the action of the bolt’s own preload and the initial prestress of the wing. Therefore, the wing is rapidly deployed. Since the size of QWKNUT is only equal to 66 × 42 × 22 mm3, the space of the connection and separation device occupied in the missile body is greatly saved.104

4.2.2. Multi-point configuration

Multi-point connection configuration means that there are multiple connection points in SNLD, and only one actuator is used in common. The locking force is distributed to each point, and would be released at the same time once the device is actuated. Therefore, SNLD by the multi-point connection configuration is also called the linkage connection and separation device.From the perspective of the number of connection points, multi-point connection configuration can be divided into two-point, three-point, four-point configuration and so on.

(1) Two-point connection configuration

Zhang designed an unlocking and separation device for micro satellites, as shown in Fig. 57.105The satellite body,on which two connecting rods pass through the adapter and the substrate, is placed on the adapter. In addition, the compression nuts are pressed against the end faces of the two forks and tightened, so that the satellite is connected in two-point configuration. As soon as the sucker electromagnet is energized to actuate,the pullrod is separated from the electromagnet actuation mechanism (Fig. 57 (a)) and moves along the track of the pushing block under the action of the compression spring. Eventually, the forks break away from the connecting rods against the friction with the compression nuts (Fig. 57(b)), and the satellite is separated from the adapter under the action of four separating springs.105.

(2) Three-point connection configuration

Gai et al.of designed a lock and release device for satelliterocket separation,as shown in Fig.58.106Three locking mechanisms, composed of a lock and release oscillating bar, an Lshaped plate jaw and a connecting screw,hold-down the satellite in three-point configuration. When the push/pull electromagnet is energized to extract the iron core from the rotary roller, the roller rotates under the action of the releasing torsion spring,and drives the oscillating bar to rotate.As a result,the oscillating bar separates from the claw, and the claw rotates under the action of the preload of the preloaded compression spring to free the connecting screw,thus releasing the satellite fixedly connected to the screw.106

(3) Four-point connection configuration

Han et al.developed a linkage-type connection and separation device for micro-nano satellite, mainly composed of four ejection-guide mechanisms, four reinforcement-preload mechanisms and one trigger-release mechanism, as shown in Fig. 59.107The reinforcement-preload mechanism is distributed on the two sides of the satellite, and the footings of the satellite are pressed by the pressing levers in four holding-down points. The ejection-guide mechanism is installed on the four corners of the satellite to support, separate and guide the satellite. The trigger-release mechanism is mainly composed of Kevlar rope and an actuator similar to Frangibolt based on SMA pipe. At the beginning of the actuation, the notched bolt in the actuator is broken and releases the Kevlar rope, lifting the restraint on the pressing levers.Then, the pressing levers rotate under the action of the torsional springs to release the footings of the satellite, so that the satellite is separated from the fixed base board owing to the compression springs of the ejection-guide mechanism.107In some devices, though the load is connected in multiple points, each point corresponds to an independent locking mechanism and actuator without interference on each other.Due to the poor synchronization performance in separation,most of these devices have been replaced by linkage-type devices in multi-point configuration.

4.2.3. Banded configuration

Fig. 55 Number of SNLDs based on some special mechanisms.

Fig. 56 Release mechanism for a small-light folding wing.104

Banded connection and separation device,one of the common mechanism forms in satellite-rocket separation,108,109includes an upper framework attached to the spacecraft interface, a lower framework attached to the payload adapter, elastic metal belt, tensioner, separation actuator. In the early years,explosive bolts were mainly used to separate the two frameworks. In the early years, the banded connection and separation device mainly used explosive bolts to release the strain energy of the elastic metal belt.Then the two frameworks with flanges are unlocked and separated under the action of the separating spring. The working process is shown in Fig. 60.3Although these devices have the advantages of high connection reliability and rigidity, their disadvantages such as large separation shock, non-repeatability, and poor safety outweigh the advantages in some missions. Therefore, in recent years, the traditional banded devices have been transformed into new types of non-pyrotechnic devices by replacing pyrotechnics with non-pyrotechnic actuation methods.

(1) Low-shock banded connection and separation device based on FASSN.

Dowen et al. of developed a reusable, low-shock clamp band separation system based on FASSN, as shown in Fig. 61.110During the launching process, the flywheel is restrained by the locking arm along the circumferential direction,and the system keeps locked.After the SMA wire is energized and contracted, the release arm rotates and disconnects from the lock arm, then the lock arm rotates and releases the flywheel. Then, the flywheel rotates under its own preload and the reverse driving torque generated by the high-lead thread. Finally, the bolt is released and the two frameworks are unlocked.110

(2) V-band release mechanism based on fuse link release device

Air Force Research Laboratory (AFRL) has funded NEA Electronics to develop a V-band release mechanism (VBRM)based on a fuse release device, as shown in Fig. 62.111Two 9101 fuse release mechanisms (Fig. 62 (a)) are adopted redundantly so that the reliability of separation is ensured. The two 9101 release mechanisms are bolted to transfer the preload to the two bands, which can be separated after any one of the release mechanisms actuates.111.

(3) Motor actuating latch-type banded device

Planetary Systems Corporation of the US has developed a motorized light band (MLB, Fig. 63.112). The device is an inner-ring type banded device. In the locked state, the hinged leaf assembly is installed on the lower ring,and the inner hoop ring is in an outwardly tensioned state to fasten the hinge blade assembly with the upper ring. When released, the inner hoop ring returns to its original shape and shrinks inward, the hinged leaf assembly is separated from the upper ring, and the upper ring is separated from the lower ring under the action of the separating spring.The deformation process of the inner hoop ring is actuated by a mechanism composed of a set of gears, ball screws, sliders and connecting rods driven by a motor, and the action time is about 0.065 s.112MLB has been applied in tasks such as SHERPA113–115.

Fig. 57 Unlocking and separation device for micro satellite.105

Fig. 58 Lock and release device used for satellite-rocket separation.106

Walter et al. designed a motor actuating latch-type separation device as shown in Fig. 64,116which is composed of a motor driving mechanism and a locking mechanism, etc. The locking mechanism includes several latches on the lower structure and their corresponding bearings on the upper structure.The number of latches and bearings required depends on the structural dimensions of the connected parts. The latches are driven by the motor mechanism to compress and free the bearings, so as to realize the connection and separation of the structures. In this way, multiple connection forces are applied and released simultaneously.The device is suitable for the connection and separation of large components and occasions that need sealed.116

Fig. 59 Linkage-type connection and separation device for micro-nano satellite.107

Fig. 60 Working progress of banded connection and separation device.3

4.2.4. Box-storage configuration

In recent years, small satellites with a mass of less than 50 kg have gradually been widely exploited because of their advantages of low cost,short launching period,high standardization and modularity, which bring huge economic benefits.117In order to be delivered into orbit at one time,a few tiny satellites are often put into a containment box. This configuration of connection is called box-storage configuration. Here several typical box-storage connection and separation devices will be introduced below.

(1) The P-POD

The Poly Picosatellite Orbital Deployer (P-POD) is a kind of device jointly developed by Cal Poly and Stanford University.The P-POD118is used to store CubeSats with a mass of no more than 1 kg and a volume no more than 10 cm3. Several versions,such as P-POD Mk.I,Mk.II,and Mk.III,have been developed successively,some of which are shown in Fig.65.119–121The P-POD mainly consists of a box with springs and a door controlled by a release mechanism. One device can deploy and launch up to three CubeSats at once. The original version, P-POD Mk. I, relies on a burn wire deployment system to release the satellites. After receiving the signal from the launch vehicle, the rope cutter assembly is energized and melts the Vectran rope in about 30 s, thus the door is opened and the satellite is pushed out by the spring.119The burning power is supplied by the Mk.I itself,and no additional energy is required except for the trigger signal. The P-POD Mk. II uses the QWKNUT, a non-pyrotechnic segmented nut, to release the satellites faster.Furthermore,to ensure the reliability of separation,the Mk.II is furnished with a switch that can determine the angle at which the door opens.The P-POD Mk.III is equipped with larger access ports on both sides. Larger spring plungers are used that are convenient for satellite integration. In addition, the structure of the door and bracket has been improved to bear enough shear force.120

Fig. 62 V-band release mechanism based on fuse link release device.111

Fig. 63 Mark II MLB.112

(2) XPOD separation system

The Space Flight Laboratory (SFL) of University of Toronto Institute for Aerospace Studies (UTIAS) has developed the XPOD family of separation systems, as shown in Fig. 66.122XPOD Single and XPOD Triple can accommodate satellites with a cross-section of 10×10 cm that meets the Cal Poly CubeSat standard.The latest developed XPOD GNB and XPOD DUO are used to accommodate satellites with a crosssection of 20 × 20 cm and weighing up to 7.5 kg and 14 kg respectively. The structure of these two versions is semienclosed so as to install accessories. In the initial state, the satellite is fixed to the XPOD. After the release mechanism at the top is actuated, the main spring at the bottom pushes the satellites away.122

(3) Picosatellite deployer with controllable release function

Yang et al. designed a picosatellite deployer based on an SMA spring releasing mechanism. As shown in Fig. 67,123the deployer is mainly composed of a box,a box cover,a guide frame,guide rods,driving springs,a releasing mechanism,and a releasing modular for box cover (RMBC). The releasing modular for the box cover is a kind of SMA actuator, and the release device is a hook-type unlocker based on SMA spring.When the release signal is received,the RMBC actuates to open the box cover. At this time, the releasing mechanism placed at the bottom of the box is unlocked,and the outermost satellite is ejected under the action of the driving spring.When the lock hook is quickly reset under the action of spring 1, it intercepts the satellite that comes up as a substitute, so as to wait for the next separation mission. Each satellite is controllably separated, and they have the same initial separation velocity.123

(4) Magnetic actuating separation mechanism for Star of Aoxiang

Fig. 64 Motor actuating latch-type banded device.116

Fig. 65 P-POD.119–121

Northwestern Polytechnical University (NWPU) independently developed Star of AOXiang, a tiny satellite.124In the locked state, the CubeSat is confined in the box by the door.An electromagnetic unlocking device is placed on the deployer(Fig. 68125), which is composed of an electromagnet, a mandrel, a returning spring, a rolling bearing and a linear bearing assembly. When the deployer receives the separation signal,the electromagnetic device unlocks and opens the door, and the CubeSat slides along the guide rail under the force of the separation spring to push the door to rotate. After the door reaches a certain angle,it is locked with a spring pin to prevent it from rebounding, or interfering with and colliding with the CubeSat.125,126

4.2.5. Characteristics and applications of common connection

configurations

Here the general advantages,limitations and common applications of the common connection configurations of SNLDs are listed in Table 3.

5. Research prospect

5.1. Key technical problems to be solved

Judging from the development status, the research of SNLT has made great progress,and a variety of devices have formed a mature product spectrum series.Although some technologies are still being researched, fruitful results have been achieved.SNLDs effectively overcome the inherent defects of pyrotechnic devices. They have the advantages of low shock, no pollution, repeatability, etc., and have been widely used at present.However, as human participation in space activities, such as satellite formations, space exploration and manned landings on the moon,etc.,has been developing rapidly,higher requirements have been put forward for SNLDs. In order to cope with the harsh space environment and adapt to complex tasks,there are the following technical problems that need to be solved urgently in the field of SNLT.

Fig. 66 XPOD family of separation systems.122

Fig. 67 Picosatellite deployer based on SMA spring.123

5.1.1. Study on constitutive relation of actuating source

There are various types of non-pyrotechnic actuation sources,and the constitutive relationships between different actuation sources are quite different.Accurately comprehending the constitutive relationship of the material or structure of the actuation source attributes to accurately evaluating the feasibility of the connection and separation scheme and controlling the actuation process of the device. Specifically, the following factors need considering.

(1) The adaptability to the space environment

Space environment127generally refers to the space area where spacecraft acts and works,and the main types are shown in Fig. 69. A harsh space environment brings great challenges to spacecraft.As a result,improving the adaptability of SNLD to the environment is an important prerequisite to ensure the reliability of connection and separation. In order to evaluate and improve adaptability, it is necessary to solve the constitutive relationship of the actuation source. On the one hand,some scholars have conducted related research. For example,considering that SMA is too sensitive to the hightemperature environment of space, high-temperature SMA with higher working temperature has been developed and applied.128However, the technology of high-temperature SMA has just started, and it is not widely used. On the other hand,a complex environment has extremely high requirements for ground-based simulation tests, which greatly increases the cost of the experiment and makes it more difficult.

(2) Characteristic analysis and test verification

In a very complicated force environment, SNLD is often affected by spatial vibration and shock.Many devices are actuated under multi-physics couplings, such as thermal–mechanical coupling, electro-magnetic coupling, fluid-thermal–mechanical coupling, etc. The mechanical properties of connecting and separating could be verified by targeted simulation tests on the ground in various ways. However, it is difficult to reflect the complex stress environment and actuation process.In addition,the time and the cost of repeated tests are also difficult to control. Therefore, the following problems must be solved. 1) A systematic theoretical system can be constructed,which is applicable to analyze and emulate the performance of devices under complex working conditions and actuation processes. For different devices, a constitutive model of the driving source can be established, and the numerical simulation of the separation process can be put forward.Then,according to the test results, the models and methods are optimized appropriately. 2) A reasonable and effective quantitative evaluation method of reliability can be formulated combining with technical indicators.In this way,the sensitivity and influence of design parameters on the bearing capacity and separation characteristics could be evaluated. Furthermore, the evaluation method could provide technical references for verification tests. 3) The real physical field environment should be considered in the stress state under various loads,and theoretical and simulation analysis should be carried out. 4) Appropriate experimental facilities and workbenches should be developed for a certain SNLD by adopting correct experimental and evaluation methods. This measure is mainly aimed at overcoming the shortcomings of the general experimental systems and accurately reflecting the influence of input on the output, including displacement, shock, temperature, time, speed,etc.

Fig. 68 ‘‘Star of Aoxiang” separation mechanism.125

Table 3 Comparison of various connection configurations.

5.1.2. Optimal coupling of multiple performances

(1) Contradictions between multi-target performance

For most SNLDs,shock,response speed,and load-bearing characteristics are all important technical indicators.However,due to the mutual restriction of design conditions,it is difficult to combine various performances optimally according to the existing technology.

1) The contradiction of low shock and fast response is difficult to manage. From the perspective of energy release, a variety of means have been adopted to lower shock such as delaying the process of energy release, converting compaction energy into other forms of energy and releasing it continuously, and using some buffer structure, etc. These methods effectively reduce the shock, but extend the separation time.On the contrary,fast actuation progress often increases the relative speed between the moving components and the shock.

Fig. 69 Type of space environment.

2)It is difficult to achieve‘‘Strong connection”and‘‘Weak unlocking”129at the same time. ‘‘Strong connection” means that the device can carry a large load, and ‘‘weak unlocking”means that the device only needs a small actuation force to unlock and separate. Some heavy-duty devices are actuated under a force that is several times the connection force, with high input power and great impact.

Therefore, we need to adopt a variety of methods to optimize the overall performance of the device. For example, the structure and the force transmission path of SNLD should be optimized,so that the mechanical response of concentrated loads is contained and the magnitude of the vibration response is restrained. Force-increasing or force-reducing mechanisms could be used appropriately to compact the structure and make the device work reliably.

(2) Difficult to separate synchronously

After separation, the attitude of the load should be strictly defined in some space missions,such as the satellite-rocket separation, the pallets and containers of cargo spacecraft, etc.However, the inherent working mechanism of some SNLDs affects the synchronization,which may have an adverse impact on the attitude after separation.For example,the movement of the internal parts of the device is difficult to control. Due to differences in the accuracy of manufacturing and assembly, it is difficult to release the connection force at each point together. If multiple separation devices act on one load, they are much harder to control. For researchers, it is necessary to shorten the actuation time as much as possible through applicable design and control methods. In addition, linkage mechanisms can be employed under the premise of fully considering the problems of manufacturing and assembly.Advanced control methods,such as the reinforcement learning method, could also be used to improve the controllability of the device and the synchronization of separation.

5.1.3. The reliability study of SNLT

Research on the reliability of space connection and separation technology is an arduous system engineering. In recent years,researchers have proposed many theories and methods for analyzing, evaluating and testing the reliability of spacecraft mechanisms. These methods can be used to evaluate the reliability of part of the separation device or the entire system, but most of them are applied to pyrotechnics.130–135The pyrotechnic device has a large connection and carrying capacity, and the strong impact generated by the burning of a large number of explosives ensures reliable separation. In contrast,since the impact force of non-pyrotechnic separation devices is smaller,it is particularly important to improve the reliability of separation.Existing SNLDs often use redundant design,safety factor method, and special structural design methods to improve functional reliability. For example, the stress concentration can be maximized by studying the shape of the notched bolt.Lubricant can be applied to the relative moving surface of the mechanism to prevent the moving pair from jamming.However,most of these measures are quantitative and qualitative researches specifically aimed at a certain device from the design concept. At present, there is still a lack of universal and systematic reliability research theories and methods. It is an important technical problem how to design, verify and accurately evaluate the functional reliability of new products based on the existing reliability research methods, combined with the specific working mechanism and test conditions.

5.1.4. Standardization and modularization of SNLT

The impact and explosion process, dynamics, and structural parameter design of SNLD have been researched extensively for a long time.For example,there are relevant technical standards for the design of explosive bolts and other devices.In the early years, ESA put forward specific requirements for pyrotechnic devices in spacecraft in terms of design, analysis,verification, manufacturing, operation, and safety.136In contrast, the research of non-pyrotechnic devices started late and the number of the types is large,there is no unified technical standard yet. Technical standards should involve power supply parameters, dimensions, interface types, vibration and shock, and load-bearing capacity, etc. The formulation of relevant technical standards can enable researchers to make full use of existing mature technologies, shorten the device development cycle and save costs. The pertinence, reliability and consistency of the device design will also be guaranteed.

5.2. Future research trend

While the progress of the aerospace industry,new technologies and standards related to big data,new energy,new technology,and new materials are emerging one after another. Human beings are increasingly participating in the latest exploration activities, such as multi-satellite with one arrow, deep space exploration, commercial aerospace and on-orbit operation.In this context, SNLT shows the following latest research trends.

5.2.1. Intelligent repeatable locking and releasing technology

In recent years, there have been more and more occasions where spacecraft need to be repeatedly locked and released.For example, a round trip of aircraft, docking of space stations, separation of rocket stages, repeated opening and closing of spacecraft doors, deployment of solar wings and antennas, etc.2,137In order to be quickly assembled and separated with high reliability, intelligent and repeatable locking technology has become a hot area of research.Not only should an intelligent repeatable locking device meet the needs of general connection and separation tasks, but also it should generally have the following characteristics138,139: 1) The device should be lightweight and connected quickly and reliably with high rigidity,high bending and torsion resistance.2)The locking process could be guided, self-adaptive, and could be corrected in displacement and angle certainly. 3) The device could be connected flexibly, controlled compliantly, loaded elastically and buffered to protect itself. 4) The locking force is controllable,and the energy of the butt collision can be converted into a small space. 5) The device is quickly actuated,connected and separated.

5.2.2. Development and application of actuators based on smart materials

As a new type of material developed in the 1990 s,intelligent or smart material is a functional system that is organically combined with multiple material components.140Several common types are shown in Fig. 70. Different from traditional materials, smart materials can actively couple mechanical deformation with changes of temperature, current and magnetic field,and transform mechanical response and non-mechanical response. As smart materials can integrate functions of bearing, sensing, and actuation within a small structure, they have been widely used in aerospace.141–143For example,since SMA has the advantages of low price,large deformation and various shapes, etc., it is extensively applied in SNLT. Piezoelectric material is good at mass,volume,rigidity,stability of vibration and responsiveness,which can be used as a separation actuator in the driving mechanism of a variant aircraft. Speed, force and displacement output of the smart material actuator could be accurately controlled by combining modern design means with electronic technology and a feedback control system.Smart materials have played an important role in the development of SNLT. In the future, more smart materials will be employed with high-performance and diversified connection and separation functions.

Fig. 70 Common types of smart materials.

5.2.3. Research on modular and integrated interface

To improve the flexibility of spacecraft to adapt to space payloads,various countries have carried out relevant research on a modular design. Modular design means that the connecting platform or load of spacecraft is composed of standard replaceable modules with different functions, which makes large spacecraft assembled rapidly and makes faulty modules replaced conveniently. Modular interfaces can improve work efficiency and reduce production costs, and could be used in many tasks,such as modular robots,reconfigurable manipulators,satellites on-orbit and off-orbit,on-orbit refueling of fuel,and transportation of goods between space and earth.144–146Modular design requires SNLD to reduce the difficulty of load installation, improve the adaptability of the platform to load,and facilitate the rapid assembly, disassembly, expansion and combination of serialized spacecraft. In addition, the mechanical interface is integrated with the electrical interface,the gas–liquid interface and the information interface based on selflocking and controllable separation. Power and information should be transmitted reliably by floating and over-travel design. The positions of various interfaces should be arranged reasonably to avoid occupying too much space inside the payload.

5.2.4. Extension and transfer of SNLT

The crossover of technologies have injected vitality into aerospace,and the aerospace industry is gradually going the way of internationalization and commercialization. Because of the advantages of low shock, non-pollution, low manufacturing cost, repeatability, etc., SNLD will no longer be used only in space. For example, modern warfare aims at improving maneuverability and precision, which leads to the emergence and development of ammunition released by non-pyrotechnic devices, underwater weapons separated with low noise, intelligent folding wing aircraft, etc.147,148It can be predicted that the technological achievements of SNLT will gradually expand into more fields such as navigation, agriculture, resource exploration and so on.

6. Conclusion

Based on the new requirements of space missions for lowshock and repeatable connection and separation devices, the paper summarizes and elaborates the research progress of SNLT.

(1) Research on the principle of shock reduction. By the huge gas pressure generated by the combustion of initiating explosive devices,the actuating parts are driven to move quickly. Then the initiating explosive devices can be quickly unlocked. In this process, severe impact and stress waves, and large shocking force between components cause violent shock effects. SNLDs reduce separation shock in two ways. As for actuating source,there is no severe energy released due to the burning of initiating explosive devices during the actuating process.For the systematic design,the separation shock of SNLDs can be weakened by ingenious low-shock mechanisms and appropriate connection configuration.

(2) Research on actuation technology. According to the sources of actuation, SNLDs are mainly classified into electric, magnetic, gas, and thermal actuating devices.Among them,the electric actuating devices have a direct energy source, high utilization rates and could respond quickly. Magnetic actuating devices mainly use electromagnets as actuators. They have advantages of miniaturization and quick actuation, but the interaction between magnetic effect and environment must be considered. Gas actuating devices mostly use air cylinders as actuators, which have the advantages of adjustable flow rate, large output force and adaptability in harsh environments. There are many kinds of thermal actuating devices,of which devices based on SMA are the most common. This kind of device makes use of the slow release process of heat energy to reduce the separation shock,but its adaptability to complex temperature environments should be improved.

(3) Research on systematic designing methods. From the perspective of mechanism design, energy is transferred and transformed inside the mechanism during the operation progress of certain devices. If energy is released slowly, the separation shock would be effectively reduced. Some miniaturized mechanisms are designed to meet the technical requirements of large load, low unlocking force, etc. Considering the structural form of connection, the connection configurations are mainly divided into single-point type, multi-point type, banded type and box-storage type,etc.These configurations are suitable for different types of loads respectively, and should be chosen according to the requirements of application occasions.

Although the SNLT has made fruitful achievements, there are still some technical difficulties to be studied in many fields.In order to accurately control the working process of SNLDs in complex space environments, it is necessary to study the constitutive relation of actuating source and establish the system theory. It is often impossible to achieve the best performance index, so multi-objective coupling characteristics should be considered comprehensively. In addition, the reliability and standardization of SNLT are also urgently needed to be studied. In the future, researchers should develop highly intelligent releasable locking devices with lightweight,high tolerance, self-adaptation, high reliability and weak impact.Based on intelligent materials, structures and systems, more and more separation devices will appear which are suitable for diverse needs. In order to improve the matching between load and connecting platform, integrated, quick loading and unloading interface capable of transmitting machinery, electricity, liquid (gas), etc. will be developed. This will facilitate quick connection and separation tasks,such as on-orbit assembly. As a universal key technology, the research results of SNLT will be applied to navigation, aviation and other fields.In a word, if researchers could seize accurate technical requirements and innovate the design concept, SNLT can create greater application value.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This study is supported by State Key Laboratory of Robotics and System (HIT), Heilongjiang Touyan Team and the Programme of Introducing Talents of Discipline to Universities(No. B07018).