L.K.Gite,A.Anandaraj,R.S.Deodhar,D.K.Joshi,K.M.Rajan

Armament Research Development Establishment(ARDE),Defence Research Development Organization,Ministry of Defence,Government of India,Pashan, Pune 411 021,Maharashtra,India

Effect of firing conditions&release height on terminalperformance of submunitions and conditions for optimum height of release

L.K.Gite*,A.Anandaraj,R.S.Deodhar,D.K.Joshi,K.M.Rajan

Armament Research Development Establishment(ARDE),Defence Research Development Organization,Ministry of Defence,Government of India,Pashan, Pune 411 021,Maharashtra,India

A R T I C L E I N F O

Article history:

DPICM

Submunitions

Forward throw

Height of release/burst

Ejection velocity

Firing altitudes

Firing conditions

Point mass model 6 DOF

Submunitions should exhibit optimum terminal performance at target end when released from certain pre-determined height.Selection of an optimum height of release of the submunitions depends on the terminal parameters like forward throw,remaining velocity,impact angle and flight time.In this paper, the effects of initial firing conditions and height of release on terminal performance of submunitions discussed in detail.For different height of release,the relation between range and forward throw is also established&validated for a number of firing altitude and rocket con figurations.

?2017 Published by Elsevier Ltd.This is an open access article under the CC BY-NC-ND license(http:// creativecommons.org/licenses/by-nc-nd/4.0/).

1.Introduction

The family of scatter-able submunitions adds new dimension to the munition warfare.Submunitions can force the enemy in kill zones,change their direction of attack and spend time in clearing/ breeching operation.Submunitions have always played a major role in denying battle field&obstacle in mobility to armed forces and very effective to provide tactical advantage to the commander in the field.To gain tacticaladvantage,the submunition fields have to be pre-selected and laid.Laying ofsubmunition fields manually is a time consuming and manpower intensive operation.An artillery rocket with Dual Purpose Improvised Conventional Munition (DPICM)warhead contains around 220 submunitions.The rocket with DPICMwarhead follows the same ballistic trajectory similar to the rocket with a conventional high explosive warhead upto a predetermined height wherein it an electronic-time fuze is initiated to release the submunitions.An ejection mechanism eject submunitions from the warhead.A complete salvo firing of such rockets can lay around 2600 submunitions in the targetarea,in less than a minute.This provides high maneuverability with rapid, flexible means of delaying,harassing,paralyzing,canalizing or wearing down the enemy forces in both offensive and defensive operations[1,2].

Except for a very short duration after release,the submunition follows a steep trajectory until it reaches the designated target or ground.The trajectory of submunitions mainly depends on height, velocity and the flight path angle of the rocket at the point of release.Further,it also depends on the shape,size,mass,ejection velocity of the submunition,the aerodynamics of parachute,firing altitude and prevailing meteorologicalconditions[3,4].

The submunition trajectory will not be exactly vertical from burst point.The submunition willmove forward due to the release velocity,release angle and ejection velocity.This additionaltravelin down range of the submunition is called the forward throw(see Fig.1).The forward throw of the submunitions is an essential parameter while computing the fire parameters using either Firing Tables or Fire Prediction Software.This paper describes the effectof height of release,firing altitude and rocket con figuration on theterminal parameters like impact velocity,impact angle,flight time and forward throw of the submunition at different map range. Achievable minimum and maximum ranges,near vertical impact angle,minimum impact velocity and time of flight are the parameters which govern the selection ofan optimum height ofrelease of the submunitions.This paper establishes the relation of forward throw with ranges for given rocket con figuration,firing altitude and optimum height of release.

2.Simulation matrix

A six degree of freedom(6-DOF)Trajectory Model used to simulate the rocket trajectory from launch to height of release (HoR)and a Point Mass(PM)Trajectory Modelused to simulate the trajectory ofa single submunition from the pointofits release from the rocket,at a given release height to the target/ground impact.6 DOF trajectory model computes positions and velocities of the rocket up to given height of release which serves as initial conditions to PM along with ejection conditions and aerodynamics of submunitions.The point mass model simulates the trajectory of a submunition and forward throw and impact parameters of submunition are computed(see Fig.2).

The simulation of submunition trajectory is carried out for following different cases and their combinations,as discussed in the subsequent paragraphs:

2.1.Height of release

Height of release of the submunitions is a crucial parameter determining the proper functioning of the submunition at an impact.The following six heights of release are selected for the study.

(a)250 m.

(b)500 m.

(c)750 m.

(d)1000 m.

(e)1250 m.

(f)1500 m.

2.2.Firing altitude

The following firing altitudes for both launcher and the target are considered for simulation to take into account of the meteorological effects.

(a)0 m(Mean Sea Level-MSL).

(b)2000m.

(c)4000 m.

(d)6000 m.

2.3.Effect of braking ring

Effect of two types of braking rings small and big,on the terminal performance of the submunitions is studied at different ranges along with rocketwithout any braking ring.A brake ring is a simple metallic annular ring attached to the ogive nose of the rocket to increase the drag that reduces the range to achieve steeper angle of descent.

3.Submunition trajectory inputs

The rocket is fired from a launcher and at a pre-designated height,the fuze initiates the warhead wherein the warhead casing splits into three petals with the help of a flexible linear shaped charge(FLSC)system.The submunitions are also given an initial ejection velocity with the help of an ejection mechanism,further aided by rocket's roll.After ejection,the ribbon attached to the submunition gets deployed which causes substantial reduction in the velocity ofsubmunition,and further helps in attaining a steady state velocity.

3.1.Physical&initial inputs

Various inputs and initialconditions ofsubmunition considered for the simulation are given below:

a)Mass=0.23 kg.

b)Diameter=25 mm.

c)Length=50 mm.

d)Stabilization Mechanism=Ribbon.

e)Ejection Velocity in Lateral direction=4 m/s.

f)Ejection Velocity in Forward direction=50 m/s.

It is assumed that ejection velocities are constants at allrelease conditions(see Fig.3).

3.2.Other inputs

Other inputs that play a signi ficant role in the submunition trajectory are trajectory elements computed by 6-DOF ofthe rocket at desired height of release and ejection velocity.The CD-Mach pro file of DPICM with ribbon is shown in Fig.4.All simulations were carried out under ICAO Std.Meteorological conditions.The summary of the input conditions given in Table 1.

4.Analysis of input to submunition trajectory from 6-DOF

The artillery rocket considered for this study can be fired in three different con figurations i.e.,without brake ring(WBR),with small brake ring(SBR)and with big brake ring(BBR)to achieve optimal angle of impact.

The remaining velocity of rocket at predetermined height of release of submunitions for mean sea level conditions and at high altitude conditions are shown in Figs.5 and 6.The average rocket velocity at the time it releases the submunitions is~350 m/s at sea level,for ranges between 20 km and 40 km.At high altitude conditions,the average rocket velocity at release is~500 m/s for ranges between 40 km and 70 km.

5.Simulation results

The submunition trajectory is terminated as it reaches the firing altitude at the target end.The trajectory chart of submunitions, impact velocity,impact angle and time flight of submunition for different conditions are discussed in subsequent sections.

5.1.Trajectories of submunitions

The trajectory of submunition is simulated using Point Mass model.The trajectory envelope of the submunitions for different release angles and height of release are shown in Figs.9 and 10.

5.2.Impact velocity of submunitions

The submunitions are released with velocity around 350 m/s at sea level conditions and 500 m/s at 6000 m firing altitude.The submunitions start descending and stabilizes after some distance due to the drag offered by the ribbon.The velocity pro file of submunition for sea level is shown Fig.11.It is observed that velocity for the submunitions at time of landing is between 50 and 70 m/s which is suf ficiently safe for its structure.

5.3.Time of flight of submunitions

Time of flight is also another vital terminal parameter which is considered during the design,to decide the optimum height of release of the submunitions.The time of flight of submunition should be less than~20s to avoid air burst due to self-destruction mode.Self-destruction mode is necessary as per international moratorium on anti-personal submunition to nullify undesirable consequence to friendly troops and civilians[5].Based on this constraint,the maximum release height of the submunitions wherein the time of flight would remain less than 20s is approximately 1250 m(see Table 3).

5.4.Impact angle

This is an importantparameter,as this submunition is meantfor top attack to achieve enhanced shape charge performance.The minimum height of release of the submunition should be not less than 1000 m to ensure an impact angle of 70°for better terminal performance(see Fig.12).

5.5.Achievable ranges

The rocket system should be capable ofdeploying submunitions at all operating ranges.The minimum range achieved using the rocket with big brake ring(BBR)and rocket without brake ring gives maximum range.Apex of trajectory for minimum range will decide the lower limit of height of release.The achievable ranges analysis is carried out for Mean Sea Level,2000m,4000 m and 6000 m altitude conditions for different height of release.It is observed thatthe minimum range for the deploymentof DPICMfor 1000 m heightofrelease is around 11.2 km atsea leveland 13 km at 6000 m altitude.The maximum range for 1000 m height ofrelease is 37 km at sea level conditions and 67 km at 6000 m altitude conditions.The lower height of release gives more advantages for minimum and maximum range coverage.

5.6.Relationship between release angle and forward throw

Forward Throw ofthe submunitions is inversely proportionalto its release angle and it is linear.When release angle is less,the trajectory is shallow and hence,the forward throw is more and as the release angle increases,the trajectory becomes steeper and the forward throw decreases,irrespective of submunitions and rocket con figurations(see Fig.13).

5.7.Relationship between range and forward throw

The forward throw is higher at minimum ranges than the maximum range because at minimum ranges the release angle is very shallow and for maximum ranges the release angle is steeper. Moreover,the horizontalvelocity components for lower ranges will be higher than the maximum ranges at release point.(see Figs.14 and 15).

5.8.Numericalexpression of forward throw&trial results

Forward throw can be expressed as a function ofrange or firing angle for given set of conditions.For fire prediction,the correction in range due to forward throw is required to be carried out.In fire prediction process,map range is the main input to compute firing angle and hence it is necessary to build the forward throw relation with respect to map range instead of firing angle to cater its correction.A third degree polynomial expression in term of range (x)is fitted.The sample of polynomial coef ficients is shown in Table 4.

Forward Throw(x)=a3x3+a2x2+a1x+a0

The forward throw for a desired range,at firing altitude 1500 m &at height of release 1000 m,can obtained by interpolation after evaluating case A&B.Similarly,forward throw for heightofrelease 1100 m at firing altitude 4000 m can obtained by interpolation after evaluating case C&D.The polynomial of degree 3 chosen to keep the error minimum within±10 m in map range.The same is implemented in fire prediction software,to predict fire parameters during dynamic trials.The accuracy of the conventional highexplosive rocket system is less than 1.5%of range for all ballistic variations.Explicit and exact forward measurement of throw is dif ficult,but overall accuracy including forward throw and other ballistic variations achieved was less than 1.5%of range for sample map ranges and with different brake ring con figurations at different firing altitude.The precise functionality of the submunitions and the accuracy level of high explosive warhead is upheld for submunition type warhead at height of release 1000 m indicates the simulated forward throw and impact parameters are in order.

Table 1 Input variable and their sources.

Table 2 Impact velocity of munitions.

Table 3 Time of flight of submunitions.

Table 4 Coef ficients of polynomial for wbr.

6.Conclusions

In this study,a generic hybrid trajectory model is developed to optimize the submunition parameters as well as its release conditions.The model being generic,it can be used for any type of submunition with minimum modi fications.Extensive simulations carried out using this model for DPICM warhead,indicated that, heightofrelease of the submunitions is an important parameter to achieve safe landing and optimum performance ofsubmunitions at the target end.Height of release is optimized based on the higher impact angle and lower time of flight.At the same time,selected height of release give maximum achievable range coverage.This study shows for designed DPICM con figuration,1000 m height of release of the submunitions have better terminalperformance.

Similarly,the forward throw of the submunitions depends mainly on the release conditions,submunition aerodynamics and atmospheric conditions.Shallow trajectories have more forward throw than the steeper trajectories and forward throw increases with increase in height of release.Further,due to atmospheric conditions,forward throw athigher altitude is more as compared to sea level.The polynomial relation between forward throw and range is established for a given rocketcon figuration,heightof firing altitude and height of release.This relation is used in firing tables and fire prediction software and validated in field trials.

Acknowledgement

The authors sincerely acknowledge the support and encouragement of Warhead Group,HR Council Members and Director ARDE for preparing and publishing this work.

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18 January 2017

*Corresponding author.

E-mail addresses:lkgite@arde.drdo.in (L.K.Gite),aranadraj@arde.drdo.in (A.Anandaraj),rsdeodhar@arde.drdo.in (R.S.Deodhar),dkjoshi@arde.drdo.in (D.K.Joshi),kmrajan@arde.drdo.in(K.M.Rajan).

Peer review under responsibility of China Ordnance Society.

http://dx.doi.org/10.1016/j.dt.2017.04.005

2214-9147/?2017 Published by Elsevier Ltd.This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Received in revised form 5 April 2017

Accepted 24 April 2017

Available online 27 April 2017