同济大学建筑与城市规划学院

朱蔚然 ZHU Weiran/ 译

王祥 WANG Xiang/ 校

3D打印早已开始在各个方面彻底改变着地球上的设计，诸如现在我们家中的衣服、鞋和家具，甚至房屋本身，都可以被3D打印。但在太空中怎样进行3D打印呢？该技术对航天工业又可能产生怎样的影响？也许，我们至少能确定三个需要考虑的方面：太空建筑的建造、零部件的生产以及食物的准备。在每个方面，3D打印都具有潜在优势，但单从建筑学的角度来看，建筑物的建造显然最为重要。其原因之一便是成本——从地球向太空运输建筑材料确实过于昂贵，也许将一块普通的砖运到月球就要花费200万美元；另一个因素则是安全——使用3D打印技术进行建造，可让机器人在人类入住之前便建造好栖息地与基础设施（图1），从而降低建筑工人遭受宇宙辐射的风险。

1 太空3D打印的材料选择和技术难题

考虑到将材料运输至太空的成本问题，美国国家航空航天局（NASA）及欧洲航天局（ESA）都遵循一种就地资源利用（ISRIU）的政策，即在现场尽可能充分地利用所有材料。例如在月球上进行3D打印时，月表土自然会成为建筑材料的首选——这是一种覆盖月球表面的细粉状类石墨物质（图2）。同样的，在火星上也有火星表层土，富含铁矿藏且可被开采[1]。

在月球3D打印中，存在着许多明显的问题。首先，月球受到极大温差的影响，其温度范围能从白天的123℃（253℉）降至夜间的-233℃（-387℉），而月表在日光曝晒区域与阴影区域之间也存在着显著的温差，这些因素有可能导致在施工过程中出现结构养护及其他方面的问题（图3）；其次，月球上的一天大约等于地球上的十四天，这意味着一个月球日中没有日光照射的时间会很长，而这将不利于使用太阳能作为工程作业的主要能量来源1；第三是其他方面考虑，如真空中的操作问题以及陨石、辐射与光强带来的挑战，这些都将使问题进一步复杂化；第四，尚且不论月球上是否存有水源，水源的量也还是一个未知数；最后，如果没有强大的维护支持系统，任何机器人系统想要运行，都必须具有100%的可靠性。

然而，在月球上进行建造也有一些显著的优势。月球上的重力减小意味着屈曲力也随之减小，并且因为月球没有大气层，月表也就没有风和雨。没有风意味着不用考虑侧向力，没有雨意味着无需在恶劣天气下被迫停止施工，同时月球上还没有地震[2]。但总体来说，在月球上进行建造是弊大于利的。

尽管在某些方面火星的条件更接近于地球，比如火星上的一天和地球的一天几乎相等，且不同于月球，火星和地球一样是有季节之分的。火星还拥有稀薄的大气层，虽然其大气的95%都是二氧化碳。此外，火星上的重力比月球大，约为地球上的38%。但是火星同月球一样有着类似的问题，条件仍旧恶劣，如辐射及寒冷的气候，其表面温度可从20℃（68℉）降至-153℃（-243℉）。而且与月球不同的是，火星会遭受沙尘暴和风速高达30m/s（100ft/s）的强风。

1 火星冰屋提案（利用一层外部ETFE 膜防止3D 打印冰壳在火星大气中融化）

2 两种太空3D打印的提案

在太空3D打印这一领域，已有两个机构成功竞得赞助权，以研究在月球和火星上进行建筑3D打印的潜力。其中之一便是欧洲航天局，他们利用D-Shape工艺在月球上进行建筑3D打印。这是一种使用光固化技术的大型3D打印机，其逐层的打印工艺可将沙子与无机粘合剂结合在一起，创造出类似石头般的物质。该研究团队是由D-Shape工艺的发明者、工程师恩里科·迪尼（Enrico Dini）、福斯特建筑事务所的建筑师、太空顾问阿尔塔公司（Alta SpA）以及圣安娜高等研究院机器人实验室（PERCRO）的科学家们组成。

另一个机构是美国国家航空航天局，他们计划利用轮廓成型工艺（Contour Crafting）在月球和火星上进行建筑3D打印。这是一种层积打印技术，可以通过计算机控制的喷嘴挤出混凝土进行打印。该研究团队全部由来自于南加州大学（USC）的研究学者组成，其中包括轮廓成型工艺的发明者比赫洛克·霍什内维斯（Behrokh Khoshnevis）、建筑师尼尔·里奇、结构工程师安德尔斯·卡尔森（Anders Carlson）以及太空建筑师马杜·唐加卢（Madhu Thangavelu）。

2.1 D-Shape工艺

D-Shape 3D打印技术就像是一台巨型Z-Corp2打印机，该系统是由恩里科·迪尼开发的，旨在增加3D打印的尺度以便低成本地打印具有建筑规模的物体。迪尼最初尝试使用环氧树脂或聚氨酯树脂作为粘合剂沉积至各种石材或非石材的粉末中，但由于树脂易燃且有毒，他很快便放弃了这一做法。此外，树脂的添加还要求在打印中使用高维护性的喷嘴，而且会产生具有低弹性模量的聚合物，导致最终成型物体的形变。

因此迪尼另外使用了一种氯酸盐基、低粘度且具有高表面张力的液体材料。这种材料在添加金属氧化物催化剂后，将具有非凡的网状性[3]，同时还很便宜且只需一个低维护性的喷嘴。它的凝固速度也更快，并具有更高的拉伸性能，催化剂中含有金属氧化物，所以颗粒状物质在催化反应中不是惰性的，而是积极深入地参与了反应。通过这种方法获得的材料不是普通的混凝土——一种惰性颗粒略微粘结的抗拉强度差的材料，而是一种类似矿物、具有牢固的微晶结构并有着很高的硬度和抗拉强度的材料[3]。

月球项目的合作提出了一种使用月表土以及迪尼研发的专有“墨水”进行打印可居住建筑物的想法。欧洲航天局的提议是首先部署一个充气系统以支持最初的外壳打印活动，随后将其移除并在其内插入二次充气系统来提供一个密闭加压的内部环境。因为打印出来的外壳主结构在第一个充气密闭空间的内侧，无法提供侧推力的支撑，所以二次充气系统是绝对必要的3。外壳将采用蜂窝状结构以减少打印所需的“墨水”，同时保持其结构的完整性。最终，打印出的外壳可防止来自辐射与陨石的危害，而内侧的充气膜将提供一个密闭加压的居住环境（图4）。

2 根据D-Shape 工艺提出的月球3D 打印提案：利用月表土进行建造

3 根据D-Shape 工艺提出的月球3D 打印提案：具有大尺寸厚度的外部保护层

4 欧洲航天局提出的利用D-Shape 工艺月球表面基地设计方案

D-Shape的优点之一是它可在表土上进行打印工作，且这层表土能够支撑正在打印的任何结构，这让打印扁拱成为现实。然而它的缺点之一是其“墨水”需要从地球上运出，尽管尝试使用轻质的蜂窝状结构，在减少打印量的同时保持结构系统的有效深度，但这样做依旧非常昂贵。

D-Shape的主要缺点是使用时必须在真空条件下注入液体。尽管轮廓成型工艺中也有挑战，但D-Shape面对的这个问题更加严峻，因为在真空中泵送是不可能的，所以D-Shape需要另一种沉淀聚合的方式。这个问题也早有争论：虽然这种液体可以很快浸入月表土，但月球上的真空环境无疑会给D-Shape技术带来相当大的问题。

2.2 轮廓成型工艺

轮廓成型工艺是比赫洛克·霍什内维斯教授发明的一种数控建造工艺，使用计算机控制的喷嘴挤出混凝土并进行层级打印，可由电脑模型直接生产构件。重要的是，该工艺还设置了一个泥刀跟随喷嘴运动，以对挤出材料进行表面抹平（图5，6）。该材料是一种快硬水泥，其强度足以在挤出后立即自支撑并逐渐强化。

轮廓成型工艺中不再有模板的需求，而在传统的混凝土施工中，使用模板不仅需要耗时且费钱的二次施工，且模板往往在使用后就被丢弃，这样的施工过程并不具有环境可持续性。相比之下，只要解决了建造技术的初始成本问题，轮廓成型工艺便是一种相对快速、环境可持续且价格低廉的建造方式。

5 美国国家航空航天局提出的利用轮廓成型工艺的月面机器人3D打印技术

6 轮廓成型工艺的月面机器人登陆平台

轮廓成型工艺倾向于一种特定的建构逻辑，即基于重力思维并可在垂直方向上进行微小的增量操作。“（轮廓成型工艺）需要某种‘哥特式’的建构逻辑，比如它更适用于起拱相对较大的圆拱，并避免用于扁拱的建造中；它会创新性地使用层级打印技术来建造更多形式的建筑物。而传统的建造方式会使用砌块，比如让表层砌块的铺砌路径与底层砌块的路径形成一定角度，来层层砌筑圆拱。[4]”因此，与D-Shape相比，轮廓成型工艺不太适合打印扁拱，但仍是可行的——扁拱必须在平面上进行水平打印后，再使用机器人将其吊装到垂直方向上对应的位置。轮廓成型工艺的材料可能还需要在抗拉性能上进行额外的强化，比如将金属材料自动添加到聚合物中或聚合物被喷嘴挤出所形成的纤维中。

美国国家航空航天局与欧洲航天局两个提案的关键区别在于，欧洲航天局旨在为人类创造一个加压的居住环境；而美国国家航空航天局只针对基础设施的建设给出了提案，如着陆垫、道路、未加压的庇护所及防爆墙等，这与美国国家航空航天局当前的政策是一致的，也就是将完工的可居住建筑物送入太空，并利用当地资源来建设基础设施。美国国家航空航天局资助的研究项目探索了轮廓成型工艺在月球和火星上的使用，其中两项技术已经被采用，一种是使用安装在全地形六足“地外探索者”（ATHLETE）月球车上的轮廓成型工艺机器人，通过喷嘴挤压出混凝土；另一种正在探索的是烧结方法，可用于制造具有更高耐受性的砌块来建设着陆垫与道路。

考虑到月球上几乎没有水，而水是混凝土建筑中的传统粘合剂，轮廓成型工艺系统将使用硫作为新的粘合剂。虽然月球上有硫，但仍需开采，而出于多种技术原因，在月球上采矿的整体过程都具有挑战性。另外，因为从地球上的任何位置都可以看到月球表面，所以还应注意不要破坏其外形——除非在地球无法观看的那一面进行开采4。同时，考虑到硫在120℃（248℉）的温度下会熔化，在月球上作业还应注意确保让掺入了硫的月表土不要暴露在极端的温度下。

总而言之，D-Shape与轮廓成型工艺各有利弊，但目前两家机构都缺乏在地球上的原型设计。而如果没有在地球上进行充分的实验和测试，那么将任何机器人建造技术运送到月球上都没有意义，因为维护将成为一个关键问题，所以这两个系统都必须具有100%的可靠性才能在地外环境中部署。

3 3D打印技术在太空中的其他应用

3 D 打印在零部件制造方面具有可观的优势——在太空中按需打印零部件当然比随船运送或由补给船运送更好。但是，在太空中进行3D打印的真正问题是重力太小。考虑到大多数地面3D打印技术都是在粉床平台上进行，在太空中就必须开发一种替代技术。一家致力于解决这些难题的公司——“太空制造 （Made In Space）”，生产出了可用于国际空间站的3D打印机。其中，一种3D塑料打印的原型机正在空间站中进行安装，另一种用于打印金属的原型机不久后也将投入使用。

通过3D打印，包装食品在品种、风味、形式和质地上也可以更具吸引力，美国国家航空航天局目前正在探索在太空中进行食品3D打印的方法[5]。深入太空的任务，如火星任务的关键问题之一是确保为宇航员提供健康的饮食结构来满足他们的营养需求，同时还需包含足够的食品多样性。考虑到冷藏和冷冻需要大量能源，当前的政策是仅提供单独包装的耐贮存食品，但加工技术本身会降低本就微量的营养素[5]。然而即便带上这些预先准备好的食物，在飞往火星的旅途中也可能使食品超出保质期，因此必须找到替代方法来准备食物。而3D打印的优点则在于，只要提供多种原料，就可实现食品生产的可定制化。

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.

注释

1 一种能确保太阳能持续供应的方法是，将太阳能电池板安装在塔柱上的任意一根杆件上，使其高度足以确保太阳能板持续暴露在阳光下。

2 Z Corporation（通常缩写为Z-Corp）是一种快速成型工艺，其主要采用喷墨式打印头在粉床上移动，选择性地沉积液体粘合剂。

3 传统的地面技术会施加更大的负载来控制压力，而此方法在月球上效果不佳。因为月球上的引力是地球上的1/6，所以要产生这样的推力需要6 倍于地球上的重量。

4 地球上看不见的那面通常称为月球的“暗面”，尽管它受到的阳光照射和其可见面一样多。