同濟(jì)大學(xué)建筑與城市規(guī)劃學(xué)院

朱蔚然 ZHU Weiran/ 譯

王祥 WANG Xiang/ 校

3D打印早已開始在各個(gè)方面徹底改變著地球上的設(shè)計(jì)，諸如現(xiàn)在我們家中的衣服、鞋和家具，甚至房屋本身，都可以被3D打印。但在太空中怎樣進(jìn)行3D打印呢？該技術(shù)對(duì)航天工業(yè)又可能產(chǎn)生怎樣的影響？也許，我們至少能確定三個(gè)需要考慮的方面：太空建筑的建造、零部件的生產(chǎn)以及食物的準(zhǔn)備。在每個(gè)方面，3D打印都具有潛在優(yōu)勢(shì)，但單從建筑學(xué)的角度來看，建筑物的建造顯然最為重要。其原因之一便是成本——從地球向太空運(yùn)輸建筑材料確實(shí)過于昂貴，也許將一塊普通的磚運(yùn)到月球就要花費(fèi)200萬美元；另一個(gè)因素則是安全——使用3D打印技術(shù)進(jìn)行建造，可讓機(jī)器人在人類入住之前便建造好棲息地與基礎(chǔ)設(shè)施（圖1），從而降低建筑工人遭受宇宙輻射的風(fēng)險(xiǎn)。

1 太空3D打印的材料選擇和技術(shù)難題

考慮到將材料運(yùn)輸至太空的成本問題，美國(guó)國(guó)家航空航天局（NASA）及歐洲航天局（ESA）都遵循一種就地資源利用（ISRIU）的政策，即在現(xiàn)場(chǎng)盡可能充分地利用所有材料。例如在月球上進(jìn)行3D打印時(shí)，月表土自然會(huì)成為建筑材料的首選——這是一種覆蓋月球表面的細(xì)粉狀類石墨物質(zhì)（圖2）。同樣的，在火星上也有火星表層土，富含鐵礦藏且可被開采[1]。

在月球3D打印中，存在著許多明顯的問題。首先，月球受到極大溫差的影響，其溫度范圍能從白天的123℃（253℉）降至夜間的-233℃（-387℉），而月表在日光曝曬區(qū)域與陰影區(qū)域之間也存在著顯著的溫差，這些因素有可能導(dǎo)致在施工過程中出現(xiàn)結(jié)構(gòu)養(yǎng)護(hù)及其他方面的問題（圖3）；其次，月球上的一天大約等于地球上的十四天，這意味著一個(gè)月球日中沒有日光照射的時(shí)間會(huì)很長(zhǎng)，而這將不利于使用太陽能作為工程作業(yè)的主要能量來源1；第三是其他方面考慮，如真空中的操作問題以及隕石、輻射與光強(qiáng)帶來的挑戰(zhàn)，這些都將使問題進(jìn)一步復(fù)雜化；第四，尚且不論月球上是否存有水源，水源的量也還是一個(gè)未知數(shù)；最后，如果沒有強(qiáng)大的維護(hù)支持系統(tǒng)，任何機(jī)器人系統(tǒng)想要運(yùn)行，都必須具有100%的可靠性。

然而，在月球上進(jìn)行建造也有一些顯著的優(yōu)勢(shì)。月球上的重力減小意味著屈曲力也隨之減小，并且因?yàn)樵虑驔]有大氣層，月表也就沒有風(fēng)和雨。沒有風(fēng)意味著不用考慮側(cè)向力，沒有雨意味著無需在惡劣天氣下被迫停止施工，同時(shí)月球上還沒有地震[2]。但總體來說，在月球上進(jìn)行建造是弊大于利的。

盡管在某些方面火星的條件更接近于地球，比如火星上的一天和地球的一天幾乎相等，且不同于月球，火星和地球一樣是有季節(jié)之分的。火星還擁有稀薄的大氣層，雖然其大氣的95%都是二氧化碳。此外，火星上的重力比月球大，約為地球上的38%。但是火星同月球一樣有著類似的問題，條件仍舊惡劣，如輻射及寒冷的氣候，其表面溫度可從20℃（68℉）降至-153℃（-243℉）。而且與月球不同的是，火星會(huì)遭受沙塵暴和風(fēng)速高達(dá)30m/s（100ft/s）的強(qiáng)風(fēng)。

1 火星冰屋提案（利用一層外部ETFE 膜防止3D 打印冰殼在火星大氣中融化）

2 兩種太空3D打印的提案

在太空3D打印這一領(lǐng)域，已有兩個(gè)機(jī)構(gòu)成功競(jìng)得贊助權(quán)，以研究在月球和火星上進(jìn)行建筑3D打印的潛力。其中之一便是歐洲航天局，他們利用D-Shape工藝在月球上進(jìn)行建筑3D打印。這是一種使用光固化技術(shù)的大型3D打印機(jī)，其逐層的打印工藝可將沙子與無機(jī)粘合劑結(jié)合在一起，創(chuàng)造出類似石頭般的物質(zhì)。該研究團(tuán)隊(duì)是由D-Shape工藝的發(fā)明者、工程師恩里科·迪尼（Enrico Dini）、福斯特建筑事務(wù)所的建筑師、太空顧問阿爾塔公司（Alta SpA）以及圣安娜高等研究院機(jī)器人實(shí)驗(yàn)室（PERCRO）的科學(xué)家們組成。

另一個(gè)機(jī)構(gòu)是美國(guó)國(guó)家航空航天局，他們計(jì)劃利用輪廓成型工藝（Contour Crafting）在月球和火星上進(jìn)行建筑3D打印。這是一種層積打印技術(shù)，可以通過計(jì)算機(jī)控制的噴嘴擠出混凝土進(jìn)行打印。該研究團(tuán)隊(duì)全部由來自于南加州大學(xué)（USC）的研究學(xué)者組成，其中包括輪廓成型工藝的發(fā)明者比赫洛克·霍什內(nèi)維斯（Behrokh Khoshnevis）、建筑師尼爾·里奇、結(jié)構(gòu)工程師安德爾斯·卡爾森（Anders Carlson）以及太空建筑師馬杜·唐加盧（Madhu Thangavelu）。

2.1 D-Shape工藝

D-Shape 3D打印技術(shù)就像是一臺(tái)巨型Z-Corp2打印機(jī)，該系統(tǒng)是由恩里科·迪尼開發(fā)的，旨在增加3D打印的尺度以便低成本地打印具有建筑規(guī)模的物體。迪尼最初嘗試使用環(huán)氧樹脂或聚氨酯樹脂作為粘合劑沉積至各種石材或非石材的粉末中，但由于樹脂易燃且有毒，他很快便放棄了這一做法。此外，樹脂的添加還要求在打印中使用高維護(hù)性的噴嘴，而且會(huì)產(chǎn)生具有低彈性模量的聚合物，導(dǎo)致最終成型物體的形變。

因此迪尼另外使用了一種氯酸鹽基、低粘度且具有高表面張力的液體材料。這種材料在添加金屬氧化物催化劑后，將具有非凡的網(wǎng)狀性[3]，同時(shí)還很便宜且只需一個(gè)低維護(hù)性的噴嘴。它的凝固速度也更快，并具有更高的拉伸性能，催化劑中含有金屬氧化物，所以顆粒狀物質(zhì)在催化反應(yīng)中不是惰性的，而是積極深入地參與了反應(yīng)。通過這種方法獲得的材料不是普通的混凝土——一種惰性顆粒略微粘結(jié)的抗拉強(qiáng)度差的材料，而是一種類似礦物、具有牢固的微晶結(jié)構(gòu)并有著很高的硬度和抗拉強(qiáng)度的材料[3]。

月球項(xiàng)目的合作提出了一種使用月表土以及迪尼研發(fā)的專有“墨水”進(jìn)行打印可居住建筑物的想法。歐洲航天局的提議是首先部署一個(gè)充氣系統(tǒng)以支持最初的外殼打印活動(dòng)，隨后將其移除并在其內(nèi)插入二次充氣系統(tǒng)來提供一個(gè)密閉加壓的內(nèi)部環(huán)境。因?yàn)榇蛴〕鰜淼耐鈿ぶ鹘Y(jié)構(gòu)在第一個(gè)充氣密閉空間的內(nèi)側(cè)，無法提供側(cè)推力的支撐，所以二次充氣系統(tǒng)是絕對(duì)必要的3。外殼將采用蜂窩狀結(jié)構(gòu)以減少打印所需的“墨水”，同時(shí)保持其結(jié)構(gòu)的完整性。最終，打印出的外殼可防止來自輻射與隕石的危害，而內(nèi)側(cè)的充氣膜將提供一個(gè)密閉加壓的居住環(huán)境（圖4）。

2 根據(jù)D-Shape 工藝提出的月球3D 打印提案：利用月表土進(jìn)行建造

3 根據(jù)D-Shape 工藝提出的月球3D 打印提案：具有大尺寸厚度的外部保護(hù)層

4 歐洲航天局提出的利用D-Shape 工藝月球表面基地設(shè)計(jì)方案

D-Shape的優(yōu)點(diǎn)之一是它可在表土上進(jìn)行打印工作，且這層表土能夠支撐正在打印的任何結(jié)構(gòu)，這讓打印扁拱成為現(xiàn)實(shí)。然而它的缺點(diǎn)之一是其“墨水”需要從地球上運(yùn)出，盡管嘗試使用輕質(zhì)的蜂窩狀結(jié)構(gòu)，在減少打印量的同時(shí)保持結(jié)構(gòu)系統(tǒng)的有效深度，但這樣做依舊非常昂貴。

D-Shape的主要缺點(diǎn)是使用時(shí)必須在真空條件下注入液體。盡管輪廓成型工藝中也有挑戰(zhàn)，但D-Shape面對(duì)的這個(gè)問題更加嚴(yán)峻，因?yàn)樵谡婵罩斜盟褪遣豢赡艿?，所以D-Shape需要另一種沉淀聚合的方式。這個(gè)問題也早有爭(zhēng)論：雖然這種液體可以很快浸入月表土，但月球上的真空環(huán)境無疑會(huì)給D-Shape技術(shù)帶來相當(dāng)大的問題。

2.2 輪廓成型工藝

輪廓成型工藝是比赫洛克·霍什內(nèi)維斯教授發(fā)明的一種數(shù)控建造工藝，使用計(jì)算機(jī)控制的噴嘴擠出混凝土并進(jìn)行層級(jí)打印，可由電腦模型直接生產(chǎn)構(gòu)件。重要的是，該工藝還設(shè)置了一個(gè)泥刀跟隨噴嘴運(yùn)動(dòng)，以對(duì)擠出材料進(jìn)行表面抹平（圖5，6）。該材料是一種快硬水泥，其強(qiáng)度足以在擠出后立即自支撐并逐漸強(qiáng)化。

輪廓成型工藝中不再有模板的需求，而在傳統(tǒng)的混凝土施工中，使用模板不僅需要耗時(shí)且費(fèi)錢的二次施工，且模板往往在使用后就被丟棄，這樣的施工過程并不具有環(huán)境可持續(xù)性。相比之下，只要解決了建造技術(shù)的初始成本問題，輪廓成型工藝便是一種相對(duì)快速、環(huán)境可持續(xù)且價(jià)格低廉的建造方式。

5 美國(guó)國(guó)家航空航天局提出的利用輪廓成型工藝的月面機(jī)器人3D打印技術(shù)

6 輪廓成型工藝的月面機(jī)器人登陸平臺(tái)

輪廓成型工藝傾向于一種特定的建構(gòu)邏輯，即基于重力思維并可在垂直方向上進(jìn)行微小的增量操作?！埃ㄝ喞尚凸に嚕┬枰撤N‘哥特式’的建構(gòu)邏輯，比如它更適用于起拱相對(duì)較大的圓拱，并避免用于扁拱的建造中；它會(huì)創(chuàng)新性地使用層級(jí)打印技術(shù)來建造更多形式的建筑物。而傳統(tǒng)的建造方式會(huì)使用砌塊，比如讓表層砌塊的鋪砌路徑與底層砌塊的路徑形成一定角度，來層層砌筑圓拱。[4]”因此，與D-Shape相比，輪廓成型工藝不太適合打印扁拱，但仍是可行的——扁拱必須在平面上進(jìn)行水平打印后，再使用機(jī)器人將其吊裝到垂直方向上對(duì)應(yīng)的位置。輪廓成型工藝的材料可能還需要在抗拉性能上進(jìn)行額外的強(qiáng)化，比如將金屬材料自動(dòng)添加到聚合物中或聚合物被噴嘴擠出所形成的纖維中。

美國(guó)國(guó)家航空航天局與歐洲航天局兩個(gè)提案的關(guān)鍵區(qū)別在于，歐洲航天局旨在為人類創(chuàng)造一個(gè)加壓的居住環(huán)境；而美國(guó)國(guó)家航空航天局只針對(duì)基礎(chǔ)設(shè)施的建設(shè)給出了提案，如著陸墊、道路、未加壓的庇護(hù)所及防爆墻等，這與美國(guó)國(guó)家航空航天局當(dāng)前的政策是一致的，也就是將完工的可居住建筑物送入太空，并利用當(dāng)?shù)刭Y源來建設(shè)基礎(chǔ)設(shè)施。美國(guó)國(guó)家航空航天局資助的研究項(xiàng)目探索了輪廓成型工藝在月球和火星上的使用，其中兩項(xiàng)技術(shù)已經(jīng)被采用，一種是使用安裝在全地形六足“地外探索者”（ATHLETE）月球車上的輪廓成型工藝機(jī)器人，通過噴嘴擠壓出混凝土；另一種正在探索的是燒結(jié)方法，可用于制造具有更高耐受性的砌塊來建設(shè)著陸墊與道路。

考慮到月球上幾乎沒有水，而水是混凝土建筑中的傳統(tǒng)粘合劑，輪廓成型工藝系統(tǒng)將使用硫作為新的粘合劑。雖然月球上有硫，但仍需開采，而出于多種技術(shù)原因，在月球上采礦的整體過程都具有挑戰(zhàn)性。另外，因?yàn)閺牡厍蛏系娜魏挝恢枚伎梢钥吹皆虑虮砻?，所以還應(yīng)注意不要破壞其外形——除非在地球無法觀看的那一面進(jìn)行開采4。同時(shí)，考慮到硫在120℃（248℉）的溫度下會(huì)熔化，在月球上作業(yè)還應(yīng)注意確保讓摻入了硫的月表土不要暴露在極端的溫度下。

總而言之，D-Shape與輪廓成型工藝各有利弊，但目前兩家機(jī)構(gòu)都缺乏在地球上的原型設(shè)計(jì)。而如果沒有在地球上進(jìn)行充分的實(shí)驗(yàn)和測(cè)試，那么將任何機(jī)器人建造技術(shù)運(yùn)送到月球上都沒有意義，因?yàn)榫S護(hù)將成為一個(gè)關(guān)鍵問題，所以這兩個(gè)系統(tǒng)都必須具有100%的可靠性才能在地外環(huán)境中部署。

3 3D打印技術(shù)在太空中的其他應(yīng)用

3 D 打印在零部件制造方面具有可觀的優(yōu)勢(shì)——在太空中按需打印零部件當(dāng)然比隨船運(yùn)送或由補(bǔ)給船運(yùn)送更好。但是，在太空中進(jìn)行3D打印的真正問題是重力太小?？紤]到大多數(shù)地面3D打印技術(shù)都是在粉床平臺(tái)上進(jìn)行，在太空中就必須開發(fā)一種替代技術(shù)。一家致力于解決這些難題的公司——“太空制造 （Made In Space）”，生產(chǎn)出了可用于國(guó)際空間站的3D打印機(jī)。其中，一種3D塑料打印的原型機(jī)正在空間站中進(jìn)行安裝，另一種用于打印金屬的原型機(jī)不久后也將投入使用。

通過3D打印，包裝食品在品種、風(fēng)味、形式和質(zhì)地上也可以更具吸引力，美國(guó)國(guó)家航空航天局目前正在探索在太空中進(jìn)行食品3D打印的方法[5]。深入太空的任務(wù)，如火星任務(wù)的關(guān)鍵問題之一是確保為宇航員提供健康的飲食結(jié)構(gòu)來滿足他們的營(yíng)養(yǎng)需求，同時(shí)還需包含足夠的食品多樣性?？紤]到冷藏和冷凍需要大量能源，當(dāng)前的政策是僅提供單獨(dú)包裝的耐貯存食品，但加工技術(shù)本身會(huì)降低本就微量的營(yíng)養(yǎng)素[5]。然而即便帶上這些預(yù)先準(zhǔn)備好的食物，在飛往火星的旅途中也可能使食品超出保質(zhì)期，因此必須找到替代方法來準(zhǔn)備食物。而3D打印的優(yōu)點(diǎn)則在于，只要提供多種原料，就可實(shí)現(xiàn)食品生產(chǎn)的可定制化。

3D printing has begun to revolutionize all aspects of design on Earth.Much of the contents of our homes– our clothes,shoes and furniture– is now being 3D printed,and even the home itself.But what about 3D printing in Space? What impact might the technique have on the space industry? We could perhaps identify three distinct areas:the construction of space structures,fabrication of spare parts and preparation of food.In each case,3D printing offers potential advantages.From an architectural perspective,however,the construction of structures is clearly the most significant,and one of the reasons for this is cost.It is simply too expensive to transport building materials from Earth; for example,it could cost up to$2 million to transport an ordinary brick to the Moon.Safety is another factor; the use of 3D-printed fabrication technologies would allow habitats and infrastructure to be constructed by robots ahead of human presence,reducing the risk of radiation exposure for construction workers.

Given the cost of delivering materials to space,both NASA and the European Space Agency (ESA) pursue a policy of in-situ resource utilisation (ISRU),which–in plain language– means making the most of materials available on site.In terms of printing on the Moon,the natural choice of construction material is lunar regolith,the fine,powdery,graphite-like substance that coats its surface.Likewise on Mars is Martian regolith,which is rich in iron deposits that could also be mined[1].

Printing on the Moon poses a number of obvious problems.Firstly,the Moon is subject to an extreme range of temperatures,varying from 123°C (253°F) during the day to–233°C (–387°F) at night.Moreover,there is a significant temperature difference between stark daylight and shadow.These can cause problems in terms of curing and other construction processes.Secondly,the length of a lunar day– approximately 14 times the length of a terrestrial day– means that there will be significant periods with no sunlight,which is inconvenient if solar power is to be the primary source of energy1.Thirdly are other issues such as the problems of operating in a vacuum,and the challenges presented by meteorites,radiation and light intensity,which complicate matters still further.Fourthly,it is still not clear how much water– if any– exists on the Moon.And finally,any robotic system will need to be 100 per cent reliable if it is to operate without a robust maintenance support system.

There are,however,a few significant advantages to building on the Moon.The reduced gravity means that buckling forces are less,and because there is no atmosphere there is no wind or rain.The lack of wind means that there are no lateral forces to contend with,and the lack of rain means that construction does not need to be halted in inclement weather.The Moon is also a seismically quiet environment[2].In general,though,the disadvantages of building on the Moon outweigh the advantages.

Mars poses similar problems,although in some respects conditions are closer to those on Earth.The length of a Martian day,for example,is almost the same as an earth day,and Mars– unlike the Moon– also has seasons.In addition Mars has a slight atmosphere,although it consists of 95 per cent carbon dioxide,and a gravitational force of around 38 per cent of that of the Earth,greater than that on the Moon.But conditions still remain hostile,with similar problems with radiation,and a severely cold climate where temperatures vary between 20°C and–153°C(68°F and–243°F).And,unlike the Moon,Mars suffers from dust storms and gusts of wind of up to 30 metres (100 feet) per second.

There are two rival consortia that have been sponsored to conduct research on the potential for 3D printing structures on the Moon and Mars.One is sponsored by the ESA and exploits the potential of D-Shape(a large-scale 3D printer that uses stereolithography,a layer by layer printing process,to bind sand with an inorganic binder to create stone-like objects) in order to print structures on the Moon.The ESA consortium is made up engineer Enrico Dini (the inventor of D-Shape),architects Foster+Partners,space consultants Alta SpA and research scientists at the Laboratorio di Robotica Percettiva (PERCRO) of the Scuola Superiore Sant’Anna.

The other is sponsored by NASA and exploits the potential of the Contour Crafting (CC) process (a layered printing technology that extrudes concrete through a computer-controlled nozzle) to print structures on the Moon and Mars.The NASA consortium consists of mechanical engineer Behrokh Khoshnevis (the inventor of CC),myself as architect,structural engineer Anders Carlson and space architect Madhu Thangavelu,all from the University of Southern California (USC).

D-Shape

The D-Shape 3D printing technology effectively operates as a giant Z Corp printer4.The system was developed by Enrico Dini to increase the scale of 3D printing operations in order to print objects the size of buildings at a low cost.Dini experimented initially in the use of epoxy or polyurethane resins that were used as a form of binder and deposited into various forms of stone dust or powder.However,he soon abandoned this because of the flammability and toxicity of the resins.Moreover,the resins also required a high-maintenance nozzle,and had the added disadvantage of producing a conglomerate with a low elasticity modulus leading to deformation in the final object.

As a result,Dini bean using an alternative,a ‘chlorate based,low viscosity,high superficial tension liquid with extraordinary reticulate properties if added to metallic oxides used as a catalyzer[3]’.This had the added benefit of being cheap and requiring only a low-maintenance nozzle.It also sets faster and has higher-tensile properties:‘The catalyst contains metal oxides.This way,the granular material is not inert during the catalytic reaction,and instead it is actively and deeply involved in the reaction.Therefore,the material obtained through this method is not an ordinary concrete material,ie a poor tensionresistant material in which inert granules are slightly bound together; instead it is a mineral-like material,which demonstrates a high level of hardness and a high tensile strength,due to tough microcrystalline structure[3].’

The collaboration on the Moon project posits the ideas of printing habitable structures using regolith and Dini’s proprietary ‘ink’.The ESA proposal deploys an initial inflatable system to support the initial printing activities.This would then be removed and a secondary inflatable system inserted to provide the pressurised interior.Given that the main printed structure would struggle to contain the thrust of a pressurized internal volume,the secondary inflatable system is an absolute necessity3.The shell of the structure would have a honeycomb structure in order to reduce the amount of ink needed for printing yet retain its structural integrity.As a result,the outer printed shell would provide protection from radiation and meteorites,while the inner inflated membrane would provide the pressurised container for habitation.

One of the advantages of D-Shape is that it prints on a bed of regolith that serves to support whatever structure is being printed.This allows shallow arches to be printed.By contrast,one of the disadvantages is that its ink needs to be transported from Earth,which would be costly,despite attempts to reduce the amount of printing using honeycomb construction that maintains the effective depth of the structural system while reducing the mass involved.

However,the main disadvantage of D-Shape is that of having to inject fluids in a vacuum.Although there are also challenges with Contour Crafting,in that pumping is impossible in a vacuum,thereby necessitating an alternative method of depositing aggregate,those faced by D-Shape are more severe.The argument has been made that the fluid can embed itself quickly in the regolith,but the vacuum of the Moon will undoubtedly cause considerable problems for the D-Shape technique.

Contour Crafting

Contour Crafting is a digitally controlled construction process invented by Professor Behrokh Khoshnevis that fabricates components directly from computer models,using layered fabrication technology by extruding concrete through a computer-guided nozzle.Importantly,the technique involves the use of a trowel that follows the nozzle and smoothes out the surface of the extruded material.The material used is a form of rapid-hardening cement that gains sufficient strength to be self-supporting almost immediately after extrusion,although it does not gain its full strength until later.

CC successfully obviates the need for any formwork or shuttering.In traditional concrete construction,not only does formwork entail a costly and time-consuming secondary process of construction,it is also often discarded after use,rendering concrete construction environmentally unsustainable.By comparison,CC is a relatively rapid,environmentally sustainable and cheap method of construction,once the initial cost of the fabrication technology has been accommodated.

CC favors a particular tectonic logic of construction based on a gravitational logic and involving slight incremental deviations from the vertical:‘[CC]entails either a certain “gothic” logic of construction that,for example,relies on relatively steep vaults and avoids the use of shallow arches,or an inventive use of techniques of layering that allow a wider range of forms to be assembled.Traditional construction methods for building rounded abode vaults using bricks,for example,deploy an initial skin of brickwork whose courses are set at an angle to the base of the intended vault.[4]’ Compared to D-Shape,CC is therefore less suitable for printing shallow arches.It is still possible to print shallow arches using the CC technique,although the arches would have to be printed horizontally on a flat surface and then robotically hoisted into their vertical position.CC may also require additional tensile reinforcement.This can take the form of metal ties or cleats inserted robotically into the aggregate or fibers extruded with the aggregate.

One of the key differences between the NASA and ESA proposals is that the ESA aims to create pressurized habitats for human occupation,whereas the NASA project is only for infrastructural elements,such as landing pads,roads,unpressurised shelters and blast walls.This is in line with NASA’s current policy of sending ready-made habitats into space,reserving ISRU activities for infrastructural elements.In the NASA-funded research project exploring the use of CC on the Moon and Mars,two techniques have been pursued.One employs a CC robot mounted on an All-Terrain Hex-Legged Extra-Terrestrial Explorer(ATHLETE) lunar rover extruding concrete through a nozzle.The other technique being explored is a form of sintering,used to create tiles with greater tolerances for landing pads and roads.

Given that there is little or no water on the Moon,the CC system relies on sulfur as a binding agent,as opposed to water,which is the traditional binder in concrete construction.Sulfur is present on the Moon,but it would still need to be mined.The whole process of mining on the Moon is challenging for a number of technical reasons,and there is also the fact that its surface is visible from any point on Earth,and care should be taken to avoid disfiguring it– unless the mining is performed on the side that is never seen from Earth4.Care should also be taken to ensure that the sulphur-based regolith is not exposed to extreme temperatures,given that sulfur melts at 120°C(248°F).

In short,both D-Shape and CC have advantages and disadvantages.It is fair to say,however,that at present both suffer from insufficient prototyping in a terrestrial context.It makes little sense to send any robotic fabrication technology to the Moon that has not been tried and tested fully on Earth,as maintenance would be a key issue.Both systems would need to be 100 per cent reliable to be deployed in an extraterrestrial context.

Other Applications in Space

3D printing offers considerable benefits in terms of spare parts.It makes more sense to be able to print spare parts on demand in Space than to bring them on board,or have them delivered by a supply ship.However,the real problem for 3D printing in Space is the lack of gravity.Given that most terrestrial 3D printing technologies depend on a bed of powder in which the object ‘floats’ as it is being printed,an alternative technique must be developed.One company pursuing these challenges is Made In Space,which has produced a 3D printer to be used on the International Space Station:an initial prototype for 3D printing plastics is currently being deployed and a second for printing metals will follow shortly.

Prepackaged food could also be made more appealing in terms of variety,flavor,form and texture with the use of 3D printing,and NASA is currently exploring ways of 3D printing food in Space[5].One of the key concerns with missions into deep space,such as missions to Mars,is to ensure that astronauts are able to have a healthy diet that meets their nutritional needs and contains enough variety.Given that refrigeration and freezing require significant amounts of energy,current policy is to only provide shelf-stable foods that are individually prepackaged.Technologies used for processing can also degrade micronutrients[5].However,flights to Mars are likely to exceed the shelf life of even these pre-prepared foods,and alternative ways of preparing foods must be found.3D printing offers the advantage of customizing food production,offering variety and a range of possible ingredients.

注釋

1 一種能確保太陽能持續(xù)供應(yīng)的方法是，將太陽能電池板安裝在塔柱上的任意一根桿件上，使其高度足以確保太陽能板持續(xù)暴露在陽光下。

2 Z Corporation（通?？s寫為Z-Corp）是一種快速成型工藝，其主要采用噴墨式打印頭在粉床上移動(dòng)，選擇性地沉積液體粘合劑。

3 傳統(tǒng)的地面技術(shù)會(huì)施加更大的負(fù)載來控制壓力，而此方法在月球上效果不佳。因?yàn)樵虑蛏系囊κ堑厍蛏系?/6，所以要產(chǎn)生這樣的推力需要6 倍于地球上的重量。

4 地球上看不見的那面通常稱為月球的“暗面”，盡管它受到的陽光照射和其可見面一樣多。