3D Printing in Space (2014)

1

Introduction

The Air Force and NASA have jointly asked the National Research Council (NRC) to conduct a study exploring the possibilities presented by new approaches to manufacturing space hardware, and especially to address the promise of additive manufacturing, specifically in space.

THE POTENTIAL OF ADDITIVE MANUFACTURING IN SPACE

This report identifies the benefits, gaps between current and desired capabilities, and technology development paths for additive manufacturing’s use in space systems. Although the report is aimed at in-space additive manufacturing and the benefits to space and non-space products it might bring, it will also make clear that space systems will have long-term benefits from and dependence on terrestrial additive manufacturing as well. The platforms the committee examined included ground-based test beds, the International Space Station (ISS), and human-tended platforms (both internal and external to the platform), free flyers (e.g., satellites), and non-terrestrial planetary-based platforms (e.g., in situ resource utilization and habitats).

In addressing its charge (Box 1.1), the committee explored the missions and space operating environments of the Air Force and NASA, assessing the applicability of additive manufacturing approaches, and identifying promising and potential results.

It is natural for NASA and the Air Force to explore opportunities for using additive manufacturing technologies in space where additive manufacturing could offer the potential to

• Reduce launch vehicle volumes as compared to an equivalent spacecraft and
• Enable tailoring of launch vehicle systems that deliver materials to orbit.

Both factors may contribute to improving launch economics. In addition, additive manufacturing in space could also

• Enable the design and manufacture of new materials and novel parts that have never been created before, potentially creating space-only parts that function well in zero gravity but not in a terrestrial environment;
• Transform operations and logistics planning via the ability to launch broad categories of materials that can be manufactured in situ into a range of parts with a wide variety of functionality; and

• Perhaps even transform the trade space when developing space hardware and robotic systems such that functional, small spacecraft can be fully manufactured in space to suit the needs of specific owners.

The committee has also identified a number of obstacles to achieving these desired outcomes, including a lack of clarity on mission scenarios that would drive the development of appropriate additive manufacturing technologies toward addressing real NASA and Air Force challenges, obsession with a new and novel technology without a clear eye toward potential costs, and lack of understanding of technical limitations and performance criteria.

To examine the possibilities for Air Force and NASA missions, the committee discussed the feasibility of the concept of space-based, additive manufacturing of space hardware (including, but not limited to, a fully functional

small spacecraft). Where techniques are not yet mature or do not exist, the committee tried to identify the science and technology gaps between current additive manufacturing capabilities and the capabilities required to enable a space-based additive manufacturing concept.

The overall pace of implementation of additive manufacturing technologies will depend on the extent to which new engineering and testing protocols can be developed, evaluated and approved by professional organizations and the engineering and management communities in the aerospace industry and governments. New designs based on the unique materials, structures and manufacturing processes of additive manufacturing will need to prove their durability and safety for the applications in which they are targeted. Inclusion of additive manufacturing into all aspects of space operations may well extend over several decades, during which the various techniques for additive manufacturing hardware production are studied, tested, evaluated, and validated in a myriad of ways.

Additive manufacturing is already mature for a limited number of aircraft components and space-oriented components that could be manufactured on the ground. Yet the application of additive manufacturing in space is not feasible today, except for very limited and experimental purposes. Because of this, the Air Force and NASA will have to begin considering research and development (R&D) strategies to guide their investments wisely.

DIFFERENT USERS, DIFFERENT REQUIREMENTS, OVERLAPPING TECHNOLOGIES

For NASA, high-quality work in space science and human exploration within an acceptable cost budget is most important. NASA’s domains of interest include Earth, the Moon and other solar system objects, the disciplines of astrophysics and heliophysics, as well as human exploration of the surfaces of the Moon, asteroids and Mars. NASA is interested in additive manufacturing, seeking both to fulfill their responsibilities for advancing aeronautic and other technologies as well as finding cost effective ways to conduct scientific and exploration missions.

The Air Force has special responsibilities for understanding and taking advantage of additive manufacturing in the context of its responsibility to operate and sustain a fleet of approximately 55 spacecraft in five separate constellations, defined as follows:

• Protected communications (AEHF, Milstar);
• Wideband communications (WGS, DSCS);
• Missile warning (SBIRS, DSP);
• Position, navigation, and timing (GPS-III, GPS-II);
• Space situational awareness (SBSS, GSSAP); and
• Weather information (DMSP, future systems).¹

In addition, a core Air Force mission is space superiority, and the Air Force operates many space- and ground-based systems to accomplish that mission. It is also actively developing new systems. Cost and speed of innovation are critical to maintain competitive advantage over potential adversaries, and additive manufacturing may be a critical technology to do that.

The Air Force Research Laboratory’s (AFRL’s) interest in in situ maintenance, repair, and production of Earth-orbiting space systems is a logical consideration related to reducing the costs of building and launching spacecraft built in ground-based additive manufacturing facilities; learning if additive manufacturing in space provides an effective means to lower cost satellites built in space; and determining if additive manufacturing can provide a means for space-based maintenance and repair to extend the lifetimes of satellites once they are in use.

With respect to reducing costs, aerospace companies are pursuing projects aimed at better understanding the value of additive manufacturing as a way to lower the costs of tooling and as a production tool for manufacturing key components of aircraft and spacecraft. Examples include key structural elements of high-performance fighter

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¹ AEHF, Advanced Extremely High Frequency satellite; WGS, World Global Satcom; DSCS, Defense Satellite Communications System; SBIRS, Space-Based Infrared System; DSP, Defense Support Program; GPS, Global Positioning System; SBSS, Space Based Space Surveillance; GSSAP, Geosynchronous Space Situational Awareness Program; DMSP, Defense Meteorological Support Program.

aircraft and rocket engine components. Some companies are actively pursuing new means of constructing batteries and electric power and communication wiring to be integrated with satellite structures.

Finally, because of the Air Force’s goal of seeking total system cost reduction, AFRL seeks advice on whether it may be possible to develop a means of using a build in space capability to undertake in situ satellite repair and/or maintenance involving bringing satellites back from their operational locations to a space-based repair and maintenance facility for repairs, upgrades, refueling, etc. This concept could result in a significant impact on the annual costs of the five constellations, depending on amortization of the costs involved, to create an additive manufacturing facility in low Earth orbit as well as its annual operating expenses, taken in the context of current program expenditures.²

A RECENT HISTORY OF ADDITIVE MANUFACTURING

Additive manufacturing—commonly referred to as “3D printing”—is a general term encompassing various manufacturing methodologies, using different constructive materials and additive processes, each of which has specific advantages and constraints. In 2009, ASTM International formed the ASTM committee F42 on Additive Manufacturing Technologies to develop standards for additive manufacturing.³ An important contribution of the ASTM F42 committee to date is a terminology standard that defines the different processes used to build three-dimensional parts from computer-aided design (CAD) files. Within this standard, there is a definition for “additive manufacturing,” which is “a process of joining materials to make objects from 3D model data, usually layer upon layer, as opposed to subtractive manufacturing methodologies.”⁴ The definition adopted by ASTM International is the definition used in this report.

Additive manufacturing technology dates back to the 1980s when its industrial applications were largely seen as rapid prototyping of newly designed parts. This manufacturing method is currently receiving broad attention in U.S. industry and research universities, in part as a result of the recent creation of America Makes, formerly known as the National Additive Manufacturing Innovation Institute.⁵

America Makes is the current government-led consortium addressing additive manufacturing issues. NASA and AFRL both support America Makes, and it can serve an important role coordinating actions and avoiding duplication of effort, possibly with the creation of a working group on in-space additive manufacturing. Nevertheless, NASA has its own missions and interests and will have to take the lead to advance these.

Similar developments are also taking place within the European Union (EU). The EU agenda began in 2010 with the adoption of the Additive Manufacturing (AM) Sub-Platform 2010.⁶ This work acted as a focal point for additive manufacturing development studies and resulted in the EU Strategic Research Agenda of 2013.⁷ The overarching purpose is to provide guidance and coordination across the entire EU, where additive manufacturing is seen as one of the key enablers for long-term European economic progress, including the aerospace industrial sector. Organizations now involved with additive manufacturing studies include Germany’s Fraunhofer Additive Manufacturing Alliance, focused on materials, technology, engineering, and quality; Universität Paderborn’s Direct Manufacturing Research Center in Nixdorf, Germany; Belgium’s Additive Manufacturing.be Network; and the multi-national European Additive Manufacturing Group, among many others.⁸

The European Space Agency (ESA) is actively exploring the role of additive manufacturing in space. In 2012, ESA conducted a study “Universal parts Fabricator-Replicator for Space Applications” that focused on using both

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² The Defense Advanced Research Projects Agency (DARPA) conducted in-space servicing experiments with its Orbital Express spacecraft in 2007 and is currently sponsoring the Phoenix program which has analogous goals. Phoenix is discussed later in the report.

³ ASTM International, formerly known as the American Society for Testing and Materials, is a globally recognized leader in the development and delivery of international voluntary consensus standards.

⁴ ASTM F2792-12A, ref.

⁵ See http://www.americamakes.us and http://www.ewi.org/additive-manufacturing-consortium/.

⁶ See AM Platform website at http://www.rm-platform.com/.

⁷ AM Sub-Platform, 2013 Additive Manufacturing: Strategic Research Agenda, Version 2, http://www.rm-platform.com/linkdoc/AM_SRA_FINAL-V2.pdf.

⁸ See European Powder Metallurgy Association, “European Additive Manufacturing Group (EAMG),” http://www.epma.com/europeanadditive-manufacturing-group, accessed March 11, 2014.

polymeric and metallic materials to develop replacement parts on the ISS. The Italian Space Agency (ASI) is funding a project to place a fused-deposition modeling (FDM) machine on the ISS.

The EU’s AMAZE (Additive Manufacturing Aiming Towards Zero Waste and Efficient Production of High-Tech Metal Products) project involves 28 industrial partners across Europe and includes in-space applications as a core area. ESA is also supporting research on in situ additive manufacturing of habitats on the lunar surface using technology similar to that being developed in the United States. These activities indicate that there is an opportunity for international cooperation for developing this technology.⁹

There has also been extensive exposure of additive manufacturing in the public media of North America and Europe and throughout the globe, where the use of this technology is heralded as the beginning of a new era in consumer and engineering manufacturing. Much of this attention is related to the technology intersections of lowcost computer-based computational capability; advances in ease of use of CAD software; low-cost, high-precision XYZ platforms and controllers; and a variety of additive manufacturing techniques. Technological progress in methods and materials supported by research in industry, government laboratories, and academia is being exported to a new consumer base, and excitement for the technology is rising as low-cost systems enable a more diverse end-user community to acquire the technology for personal and commercial uses. Additive manufacturing is also driving new developments in materials science, manufacturing technology, and perhaps most importantly, substantial changes in the creative design/development process for end users.

Since the introduction of the first working 3D printer in 1984 by Charles Hull of 3D Systems, additive manufacturing has become increasingly important for traditional, ground-based production of consumer and industrial products. The most comprehensive source on the state of the additive manufacturing industry and technology is contained in the annual “Wohlers Report” produced by Wohlers Associates.¹⁰ According to the Wohlers Report 2014, to date, 63 companies worldwide have manufactured more than 66,000 professional-grade additive manufacturing systems for eight principal industrial sectors. Of these eight sectors, the largest, at 21.8 percent, is consumer products and electronics, followed by motor vehicles at 18.6 percent, medical and dental uses at 16.4 percent, and industrial and business machines at 13.4 percent. Aerospace follows at 10.2 percent.

The dominant industrial user areas are determined by Wohlers¹¹ to be production of consumer products/electronics, followed by fabrication of parts for motor vehicles and machine parts for industrial and business equipment. The specific, dominant application areas for the products of these machines are given in Figure 1.1.

The production of functional parts for preproduction manufacturing activities, including presentation models, production of tooling components, patterns for metal castings, and patterns for prototype tooling, is 54 percent of the total use functions, and the creation of saleable, functional parts is 28 percent. The remaining 18 percent of uses are spread across four other areas. As these uses evolved and experience was gained in different industries, additive manufacturing products transitioned from industrial prototypes to applied parts used in engineering applications, thus driving the development of new and broader engineering standards for the additive manufacturing industry as a whole. Currently, terrestrial use of additive manufacturing in production has demonstrated some cost benefits for manufacturing very complex, end-item parts that do not lend themselves to linear or angular machining. Additive manufacturing is much too slow and expensive to compete with high-volume fabrication techniques such as injection molding, stamping, casting, or pressing when parts lend themselves to these conventional manufacturing processes.

According to recent industry reports, the sale of additive manufacturing machines for metal manufacturing in 2013 increased 76 percent over the previous year, and overall, the market for 3D-printing products and services grew to more than $3 billion in 2013, representing a growth of 35 percent over 2012. The primary buyers are in

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⁹ See Tommaso Ghidini, European Space Agency, “An Overview of Current AM Activities at the European Space Agency,” presentation to the 3D Printing and Additive Manufacturing-Industrial Applications Global Summit 2013, November 19-20, 2013, http://www.3d-printingadditive-manufacturing.com/media/downloads/52-d1-12-20-c-tommaso-ghidini-esa.pdf, and European Space Agency, “3D Printing for Space: The Additive Revolution,” October 16, 2013, http://www.esa.int/Our_Activities/Human_Spaceflight/Research/3D_printing_for_space_the_additive_revolution.

¹⁰ T.T. Wohlers, Wohlers Report 2014, 3D Printing and Additive Manufacturing State of the Industry, Annual Worldwide Progress Report, Wohlers Associates, Inc., Fort Collins, Colo., 2014.

¹¹ Ibid.

FIGURE 1.1 Uses of additive manufacturing systems, 2013. SOURCE: T.T. Wohlers, Wohlers Report 2014, 3D Printing and Additive Manufacturing State of the Industry, Annual Worldwide Progress Report, Wohlers Associates, Inc., Fort Collins, Colo., 2014. Courtesy of Wohlers Associates, Inc.

the medical (dental in particular) and aerospace industries, and with both prototyping and manufacturing uses. Aerospace companies especially are using 3D printers for testing and certification as they gear up for larger-scale manufacturing.¹²

Over the last quarter century, the United States led all other countries by a large margin (38 percent) of total industrial additive manufacturing systems installed. In the past 10 years, the United States led the world with 10 additive manufacturing companies, followed by Europe and Japan with 7 each, and China with 3. In more recent years, however, the geography of manufacturing and sales of additive manufacturing units has expanded and shifted. As of May 2013, 16 companies in Europe, 7 in China, 4 in the United States, and 2 in Japan produced and marketed additive manufacturing systems. Indeed, most companies that manufacture metal powder-bed fusion additive manufacturing systems are currently located outside the United States: 7 are in Europe, and 2 are in China.¹³

The total revenue from sales of additive manufacturing products and services (shown in millions of dollars) has been rising steadily since 1993 (Figure 1.2).

Market dynamics has successfully built the economic strength of additive manufacturing, supported by rapid commercial applications, company mergers and acquisitions, and investments via government interests. (A current perspective on the additive manufacturing industry and associated technologies can be found in the Wohlers Report 2014.¹⁴) A thorough discussion of the historical development of additive manufacturing technologies is provided by Gibson, et al.¹⁵

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¹² See Wohlers Associates, Inc., “Metal Additive Manufacturing Grows by Nearly 76% According to Wohlers Report 2014,” media release, May 21, 2014, http://wohlersassociates.com/press64.html, and Alex Knapp, “Sales of 3D Metal Printers Grew Over 75% in 2013,” Forbes. com, May 21, 2014, http://www.forbes.com/sites/alexknapp/2014/05/21/sales-of-3d-metal-printers-grew-over-75-in-2013/.

¹³ T. Wohlers, Tracking global growth in industrial-scale additive manufacturing, 3D Printing and Additive Manufacturing 1(1):2-3, 2014, doi:10.1089/3dp.2013.0004.

¹⁴ T.T. Wohlers, Wohlers Report 2014, 2014.

¹⁵ I. Gibson, D.W. Rosen, and B. Stucker, Chapter 2, “Development of Additive Manufacturing Technology,” in Additive Manufacturing Technologies, Springer Science+Business Media, 2010, doi:10.1007/978-1-4419-1120-9_2.

FIGURE 1.2 Global additive manufacturing revenues from products and services, 1993-2012 (vertical axis indicates millions of dollars and horizontal axis indicates years). Service revenues total $1.2 billion in 2012. The blue at the bottom indicates revenue from products sold, the red at the top indicates revenue from services sold. SOURCE: T.T. Wohlers, Wohlers Report 2014, 3D Printing and Additive Manufacturing State of the Industry, Annual Worldwide Progress Report, Wohlers Associates, Inc., Fort Collins, Colo., 2014. Courtesy of Wohlers Associates, Inc.

STANDARDS FOR ADDITIVE MANUFACTURING

ASTM International is a global entity involved in the development and publication of international, voluntary, consensus technical standards. More than 12,000 ASTM standards have been developed for enhanced product safety, quality, market access, trade, and consumer confidence. In addition, a separate organization, the International Organization Standardization (ISO), is involved in technical product standards. Hence, standards for additive manufacturing are now being jointly developed by Committee F42 of the ASTM¹⁶ and Technical Committee 261 of ISO through a first-ever agreement of its kind.¹⁷

The complexity of developing standards lies in the areas of terminology, processes and materials, test methods,

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¹⁶ See ASTM International, “Committee F42 on Additive Manufacturing Technologies,” http://www.astm.org/COMMITTEE/F42.htm, accessed March 11, 2014.

¹⁷ See Joint Plan for Additive Manufacturing Standards Development, ISO/TC 261 and ASTMF42, AM Standards Development Plan at http://www.astm.org/COMMITTEE/F42.htm.

and design and data formats. These in turn devolve into six different areas related to raw materials, processes and equipment, and finished parts. Figure 1.3 shows the standards structure for additive manufacturing.

To illustrate the complexity of what lies ahead for additive manufacturing, the following is a list of the high-priority standards that are current work items for ASTM/ISO and in-process for standards development:

• Qualification and certification methods,
• Design guidelines,
• Test methods for characteristics of raw materials,
• Test methods for mechanical properties of finished additive manufacturing parts,
• Material recycling (reuse) guidelines,
• Standard protocols for round-robin testing,
• Standard test artifacts, and
• Requirements for purchased additive manufacturing parts.

FIGURE 1.3 Structure of additive manufacturing (AM) standards for ASTM and ISO. SOURCE: Courtesy of ASTM Committee F42 on Additive Manufacturing Technologies, copyright ASTM International.

Broad-scale use of additive manufacturing within the fields of engineering and product development will not occur until these standards have been accepted by ASTM and ISO and accepted by the tens of thousands of manufacturing organizations worldwide. It is on this basis that additive manufacturing, as it exists today, is not a widely and fully accepted manufacturing process.

HARMONIZATION OF EXISTING TERMINOLOGY STANDARDS

The ASTM currently recognizes specific terminology for fundamental additive manufacturing. The seven categories given for additive manufacturing technologies under ASTM Standard F2792-12A are outlined below.

Vat Photopolymerization

In vat photopolymerization, a liquid photopolymer contained in a vat is selectively photocured using an energy source. Common vat photopolymerization technology includes a laser that scans a beam across a vat of photopolymer and another that projects the entire area image (area processing) onto the liquid surface using a light source and an image projection system. Common processes in this category include the first commercialized technology, stereolithography, known as SL or SLA (for stereolithography apparatus), and DLP for the area projection technology (because the area projection technology employs a Texas Instruments Digital Light Processing chip). There are numerous trademarked terms used in the additive manufacturing industry by equipment manufacturers for particular machines. It is not the intent of this report to provide an exhaustive list of these specific terms, although some are highlighted for reference.¹⁸

Material Extrusion

In material extrusion, 3D parts are constructed layer by layer using materials extruded through a nozzle or orifice that is placed in desired regions using some form of translation mechanism. One implementation of this technology selectively deposits thermoplastic material through a heated nozzle, much like a glue gun placed on stages that move the nozzle to selectively deposit the material. FDM is an example of a process in this category, which is widely used in prototyping shops.

Material Jetting

Material jetting selectively deposits droplets of material onto a platform. In common implementations, photopolymer and wax-like materials are used. Multi-jet modeling (MJM) and PolyJet are two commonly used names referring to particular machines that use material jetting. These systems typically use print heads with multiple nozzles that are capable of printing parts with multiple materials.

Binder Jetting

Binder jetting is similar to material jetting except the build material is not selectively deposited. Instead, a binder or glue material is selectively deposited onto a bed of particles contained in some type of container or vat. The particles are glued together in the regions where the binder is deposited. After binder deposition, a platform is moved downward a distance equal to one layer of thickness, and a new layer of particles is raked over the build container from a powder source. There can be more than one source of powder providing the new powder to the build container. This process was originally developed at the Massachusetts Institute of Technology, which was called 3D printing (but never trademarked), and licensed to several companies that continue to provide machines today.

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¹⁸ For more complete lists, the interested reader can refer to, for example, T.T. Wohlers, Wohlers Report 2014, 2014.

Powder Bed Fusion

Powder bed fusion represents a group of technologies that typically use either polymer or metal powders contained in a build container or vat. The material is selectively bound together using a scanning energy source, typically a laser or an electron beam. Parts are built in a build chamber on a platform that moves downward after each layer is fabricated. As each layer is completed, a new powder layer is raked to provide a thin and uniform layer of new powder over the previously fabricated layers. Common terminology for processes that employ this technique include selective laser sintering, laser sintering, selective laser melting, direct metal laser sintering, electron beam melting, and others.

Sheet Lamination

Sheet lamination printing technologies bond sheets of material together to form the 3D shapes. In the additive process, a sheet of material is bonded on top of a previous sheet using glue, ultrasonic consolidation, or some other method. Typically, sheet lamination processes require combining additive manufacturing with some form of subtractive manufacturing process such as cutting or machining. After depositing a sheet (or possibly multiple sheets), a cutting or milling mechanism is used to define the features of the layer(s). In the case of machining, an end mill can machine away unwanted material for that layer, or in another case, a knife can be used to cut out the desired features for the current layer. Subsequent layers are deposited, and the subtractive process is repeated, as required, for each layer. The first commercialized technology in this category was referred to as Laminated Object Manufacturing, although this company is no longer in business. Today, there is a paper-based technology on the market as well as one that uses thin metal tapes bonded together using a process called ultrasonic additive manufacturing.

Directed-Energy Deposition

In directed-energy deposition, three-dimensional shapes are constructed using lasers or electron beams directed at the build surface, with material fed into the build region to coincide with the incident energy source. A wire feed or powder feed system is used to deliver material into the build zone. Two common processes in this category include laser-engineered net shaping, which uses a laser with a powder feed system that enables more than one material to be deposited simultaneously, as well as direct manufacturing, which uses an electron beam and a wire feed system. Each of these processes have benefits and trade-offs when compared to one another, including material choice, build speed, layer thickness, surface quality, cost, and feasible part geometries, among others.

As for terrestrial additive manufacturing and manufacturing in general, issues like intellectual property, cybersecurity, counterfeit parts, and so on will have to be addressed by the community. Those working on space-based additive manufacturing will have to determine if there are any additional considerations unique to their field.

GROUND-BASED ADDITIVE MANUFACTURING FOR AEROSPACE USE

Additive manufacturing applications are advancing significantly for aerospace uses. Lockheed Martin used additively manufactured brackets for microwave communication parts for NASA’s Juno spacecraft launched in 2011, which is now under way toward Jupiter (Figures 1.4 and 1.5).

Other aerospace companies are exploring the use of additive manufacturing in spacecraft products or development projects in an effort to reduce time and costs. For example, Aerojet Rocketdyne recently used additive manufacturing to manufacture and successfully test a LOX/H2 rocket engine injector.¹⁹ The same company is also

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¹⁹ A video clip of the test firing of this rocket engine is at http://www.youtube.com/watch?feature=player_embedded&v=40R9GQjawTE.

FIGURE 1.4 The Juno spacecraft, which will orbit Jupiter in 2016, includes the first known additively manufactured space system component, made by Lockheed Martin. SOURCE: Courtesy of NASA/JPL.

FIGURE 1.5 Additively manufactured waveguide brackets (shown by red arrows) installed on the Jupiter Juno spacecraft during assembly. SOURCE: Courtesy of Lockheed Martin.

FIGURE 1.6 Examples of small aircraft produced partially or entirely with additively manufactured parts by Aurora Flight Sciences. The increasing sophistication of machines produced with additive manufacturing technology is relevant to the in-space production of parts and even entire small spacecraft. SOURCE: Courtesy of Aurora Flight Sciences.

offering for sale four different space-qualified thruster systems produced with additive manufacturing for CubeSats and other small satellites.²⁰

Other aerospace groups focusing on smaller constructed artifacts have made significant advances in using additive manufacturing for their products. In 2012 Aurora Flight Sciences of Manassas, Virginia, built and flew a thermoplastic drone system constructed via FDM using a commercial 3D printer. Aurora has produced and flown several 3D-printed aircraft to date. These aircraft, shown in Figure 1.6, have been used to prove-out aerodynamic designs before going into higher-volume production, as well as innovative structural arrangements not possible using traditional construction methods. Aurora has also flown aircraft with active sensors embedded into 3D-printed wings and printed heat exchangers using novel materials in the 3D-printing process. Similarly, the European company Airbus Group (formerly EADS), in collaboration with faculty and students at Leeds University, England, recently constructed a fully functional metallic prototype of the airframe and propulsion system for a drone aircraft. The prototype was created with a commercial additive manufacturing machine in a university laboratory, and a full flight-capable version is anticipated. In addition, the University of Sheffield is conducting similar work, and others are rumored to be investigating additively manufactured small unmanned aerial systems as well. These examples demonstrate that additive manufacturing is able to create complex geometrical shapes, many of which are impossible to produce with traditional casting or machining.

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²⁰ D. Schmuland, C. Carpenter, R. Masse, and J. Overly, “New Insights into Additive Manufacturing Processes: Enabling Low-Cost, High-Impulse Propulsion Systems,” 27th Annual AIAA/USU Conference on Small Satellites, AIAA Paper SSC13-VII-4, 2013, American Institute of Aeronautics and Astronautics, Reston, Va.

Another area where additive manufacturing holds intriguing promise is in the potential to create materials, and combinations of materials, in support of creating specific product material properties. A group at the Jet Propulsion Laboratory (JPL) is advancing the creation of objects with custom compositional gradients within metal structures, allowing engineers to design additively manufactured objects with localized, specific values of selected physical characteristics, such as an artifact’s hardness, rigidity, and/or electrical and thermal conductivities. These manufacturing opportunities present the possibility of previously unattainable material combinations.²¹ New technically sophisticated parts tailored for performance under various structural load and temperature conditions are emerging. An example of this new capability, a gradient-metallic alloy mirror assembly developed at JPL, is shown in Figure 1.7.

These unique mechanical parts are illustrative of the extraordinary opportunities available to enhance capabilities of equipment exposed to the harsh and complex physical conditions of space.

In the 2000s, the Defense Advanced Research Projects Agency launched its Direct Write program to develop the means to fabricate various types of conformal integrated electronic components, such as power supplies, connectors, application-specific integrated circuits, and many different types of environmental sensors. This initiative and its follow-on activities have contributed to integrating electric and electronic devices and systems into additively manufactured structures.

It may soon be possible to produce inexpensive electronic circuits imbedded in or on the surface of larger structures using additive manufacturing or direct-write machines. Figure 1.8 illustrates recent printed conductor work of Simon Leigh and colleagues at Imperial College, London, under the aegis of the United Kingdom’s Research Centre in Nondestructive Evaluation.²²

Finally, additive manufacturing offers economic incentives during the manufacturing process as compared to typical (subtractive) manufacturing processes. From a raw materials perspective, additive manufacturing creates a minimum of manufacturing byproduct (waste). From a bill of materials perspective, depending on the material type and form, cost savings of more than 75 percent can accrue from using additive manufacturing rather than milling methods of material removal. Current demonstrations of additive manufacturing production of simple part types show improvement in speed of product creation by about 40 percent. However, currently, the variety of materials (metals and plastics) available to support additive manufacturing is only a small subset of those used in subtractive manufacturing.

Research is under way to develop more sophisticated hybrid additive manufacturing systems that combine additive manufacturing machines with direct-write machines and other manufacturing technologies to enable embedding of electronic components and circuitry in three dimensions during fabrication. Figure 1.9 demonstrates several examples of structures with integrated electronics fabricated via experimental additive manufacturing systems.

Recent innovations in the additive manufacturing industry involve building machines capable of adding additive manufacturing materials to preconstructed metallic or other substrates. This enables a considerable savings in construction time for specific items where a casting or other manufactured item is used as a foundation for a more complex surface put in place via additive manufacturing. Other, new machines include combinations of additive manufacturing and subtractive tools to speed up the development of prototype tools or other constructed objects.

While the many benefits and potential of additive manufacturing for ground-based aerospace applications are clear from the preceding discussion, there are also currently various disadvantages that must be considered in the application of this technology. These are predominantly materials and processing related. Disadvantages of additive manufacturing have been well documented and include such issues as cost of operation (equipment, maintenance, and materials); machine performance (size, speed, reliability, repeatability, and reproducibility of the produced parts); availability of materials. These areas are currently topics of much research, and methods to overcome them will be available in time.

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²¹ D.C. Hofmann, J.P.C. Borgonia, R.P. Dillon, E.J. Suh, J.L. Mulder, and P.B. Gardner, “Applications for Gradient Metal Alloys Fabricated Using Additive Manufacturing,” NASA Technical Brief, Jet Propulsion Laboratory, October 1, 2013, http://www.techbriefs.com/component/content/article/17446.

²² S.J. Leigh, R.J. Bradley, C.P. Purssell, D.R. Billson, and D.A. Hutchins, A simple, lowcost conductive composite material for 3D printing of electronic sensors, PLoS ONE 7(11): e49365, 2012, doi:10.1371/journal.pone.0049365.

FIGURE 1.7 (a) An isothermal slice of the Fe-Ni-Cr ternary phase diagram showing how different gradient compositions can be mapped. The lines represent composition gradients between 304L stainless steel and Invar 36, a simplified Inconel 625 alloy and a NiFeCr alloy. In some gradients, the path intersects brittle intermetallic phases, which can be avoided by changing the path to go through more desirable phases (the segmented green line). (b) An isogrid mirror fabricated using a 3D plastic printer and (c) the same part fabricated using laser-engineered net shaping (LENS) after some finish machining. The mirror surface is made of Invar 36 and the isogrid backing is a gradient alloy that transitions from Invar 36 to stainless steel. (d) A gradient alloy mirror assembly with a metal-coated glass mirror attached to the Invar side of the assembly using epoxy. The mirror assembly transitions into stainless steel at the base. (e) Test samples of a Ti-V gradient alloy being fabricated by LENS. (f) The compositions (as measured through electron dispersive XRD) of the gradient mirror assembly in (d) showing the transition between Invar and stainless steel. (g) A plot of hardness and thermal expansion across the gradient mirror assembly. The intermediate phases of the gradient have been designed to be soft austenite (as demonstrated by the decreased hardness). The controllable thermal expansion makes this part alluring for optics applications. One side of the gradient has a near-zero thermal expansion, while the other side matches steel. SOURCE: NASA, Tech Briefs, “Applications for Gradient Metal Alloys Fabricated Using Additive Manufacturing: A New Roadmap for Gradient Metals that Could Be Used in Cars, Optics, Aircraft, and Sporting Goods,” Jet Propulsion Laboratory, Pasadena, Calif., October 1, 2013, http://www.techbriefs.com/component/content/article/5ntb/tech-briefs/materials/17446. Images courtesy of NASA/Jet Propulsion Laboratory/California Institute of Technology.

FIGURE 1.8 High-resolution scanning electron microscope images show both the cut edge of the conducting material (panel a) and the ink-jet printed, electrically conducting materials (panel c). Panel (b) shows a drawn length of the conductor being used to light a 4 mm light-emitting diode (scale bar 5 mm) and panel (d) shows a 3D printer-constructed chess piece being used as a conductive link (scale bar 10 mm). SOURCE: S.J. Leigh, R.J. Bradley, C.P. Purssell, D.R. Billson, and D.A. Hutchins, A simple, low-cost conductive composite material for 3D printing of electronic sensors, PLoS ONE 7(11): e49365, 2012, doi:10.1371/journal.pone.0049365.

ADDITIVE MANUFACTURING CONSTRUCTION OF SPACECRAFT ON EARTH

At the present time, an active area of additive manufacturing for space applications is the development of CubeSats. First conceived and developed in the late 1990s by faculty and students at California State University, San Luis Obispo, and Stanford University, the first CubeSat was launched into low Earth orbit (LEO) in 2003 aboard a Russian rocket.²³ Today, more than 100 of these small (the basic unit [U] size is a base of 10 cm × 10 cm and a height of some multiple of 10 cm, upgradable to 60 cm) satellites have now been placed in LEO for the needs of a wide variety of technical applications. Figure 1.10 shows the increasing popularity of CubeSats among a range of private, government, and public organizations worldwide. Information about specific CubeSat missions, those already flown and in the queue for future flights, is available on the Internet. In addition to those flown previously, more than 90 CubeSats were launched during 2013, including the release of 28 platforms from the Minotaur I ORS-3 launch of November 2013 and 12 platforms from the NROL-39 Atlas V GEMSat launch of December 2011.²⁴ There was a significant increase in the number of commercial CubeSats launched in 2013 as well, indicating a transition from government and R&D activities to a much broader user community. Although additive manufacturing’s benefits can accrue to any size satellite, the low cost and relative simplicity of CubeSats makes them ideal for experimentation with this new technology.

CubeSats were originally built using traditional spacecraft technologies. More recently, many are being built with a wide range of components and external structures produced with additive manufacturing materials. The

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²³ Debra Werner, “Profile: Jordi Puig-Suari, Co-Founder of Tyvak Nano-Satellite Systems LLC,” Space News, August 13, 2012.

²⁴ These CubeSats were provided through NASA’s Educational Launch of Nanosatellites (ELaNa) program and the National Reconnaissance Office’s Mission Integration Directorate.

overarching goal is to simplify construction and mass of all on-board equipment, including power, communication, propulsion, thermal control, attitude control, digital systems, and instrumentation to the greatest extent possible.

One such CubeSat, Montana State University’s (MSU’s) PrintSat, is a good example of the advantages of additive manufacturing. This satellite was built to demonstrate the utility of using additive manufacturing for space structures and mechanisms. The satellite structure is printed from Windform XT 2.0, a polyamide-based carbon-filled material used for demanding terrestrial applications. The payload elements, which are not additively manufactured, will include a single-chip hybrid radiation micro dosimeter, a loads cell, and a surface resistivity sensor to measure the surface resistivity of the satellite’s nickel plating. Laboratory measurements will be made on an identical structure to enable evaluation of changes expected while PrintSat is in orbit. Full telemetry with data, housekeeping, and Global Positioning System-determined attitude and location will be enabled. The overall system was designed by faculty and students of MSU’s Space Science and Engineering Laboratory using CADCAM and other engineering tools. Implementation of the designs was done by CRP-USA at their U.S. production facility. Complete flight testing of PrintSat was successfully done to NASA standards at MSU, and the flight-ready package was sent to Sandia National Laboratories for launch aboard a small launch vehicle known as Super Strypi as part of Sandia National Laboratories’ Operationally Responsive Space-4 program. Figure 1.11 gives views of PrintSat’s key features.

ADDITIVE MANUFACTURING CONSTRUCTION IN SPACE

NASA, the Air Force, other government agencies, universities, nonprofit organizations and commercial aerospace firms are now exploring various forms of additive manufacturing for designing, constructing, and operating individual components, subsystems, and entire systems for a wide range of autonomous space systems. In addition, additive manufacturing applications may influence and benefit human-related systems or facilities operated temporarily or permanently in space or, prospectively, on or near planetary bodies such as the moons of Earth or Mars, asteroids, or Mars. Such applications may emerge in the future and will likely benefit from the application of the new additive manufacturing processes suiting the rigorous environmental and functional constraints associated with space conditions. They will also emerge as professional confidence is gained in the use of new engineering standards.

A BRIEF HISTORY OF SPACE-BASED CONSTRUCTION

Construction in space dates to the earliest days of the space program. Early concepts involved the integration of large, complex components such as space station modules, rather than the manufacturing of parts or components in space. For most of the history of the space program, space operations required either launching fully integrated spacecraft or connecting components in orbit either robotically or with human assistance. Even simple structures and objects were entirely manufactured on the ground and launched into space and connected by conventional methods. Although the Soviet space program conducted on-orbit welding experiments, and in-space welding has

FIGURE 1.9 Example structures with embedded electronics fabricated via additive manufacturing combined with direct-write and other manufacturing technologies. (A) Magnetic flux sensor system with curved surfaces and modern miniaturized electronic components (microcontroller, conductive ink interconnect, light-emitting diodes, Hall effect sensors, and power supply connector) printed via stereolithography and direct print technologies; (B) Novelty six-sided gaming die with microprocessor and accelerometer manufactured via stereolithography and direct print technologies; (C) CubeSat module produced by stereolithography and direct print technologies (top) as well as fused-deposition modeling (FDM), CNC routing, and direct print technologies (bottom); (D) CubeSat 1-unit (1u) housing produced by FDM incorporating solar cells and signal and power buses into a polycarbonate substrate (top) as well as a surface antenna embedded via ultrasonic wire embedding technology (bottom). SOURCE: (A, C, and D) W.M. Keck Center for 3D Innovation, University of Texas, El Paso; (B) E. MacDonald, R. Salas, D. Espalin, M. Perez, E. Aguilera, D. Muse, R.B. Wicker, 3D printing for the rapid prototyping of structural electronics, IEEE Access 2:234-242, 2014, doi:10.1109/ACCESS.2014.2311810.

FIGURE 1.10 CubeSat launches by year, 2000 to July 2014. The number of CubeSats manifested per year are sorted by the sponsoring organization’s country of origin. These numbers are higher than the number of operational CubeSats, because some suffered launch failures, deployment failures, and so on. NOTE: *, launches as of July 14, 2014. SOURCE: Data were compiled from M. Swartwout, The first one hundred CubeSats: A statistical look, Journal of Small Satellites 2(2):213-233, 2013, Appendix A (2000-2012) and G. Krebs, Gunter’s Space Page, “CubeSat,” http://space.skyrocket.de/doc_sat/cubesat.htm, accessed July 14, 2014 (2013-2014).

been studied by at least one U.S. aerospace contractor, neither NASA nor other space programs chose to employ supporting technologies, such as welding for space construction, and the ISS was largely an assembly and integration project with all manufacturing performed on the ground.

However, there have been several proposals for in-space manufacturing of construction materials. In the mid-1970s, under NASA contract, Grumman Aerospace built a Space Fabrication Demonstration System, also known as a “Beam Builder (B2),” that was capable of assembling triangular cross-section aluminum truss structures. This device was tested at NASA Marshall Space Flight Center (MSFC) (Figures 1.12 and 1.13).

In addition to the Beam Builder, NASA considered the possible use of in situ materials, such as lunar regolith or martian soil, for the construction of structures. But no substantial work has been done on this subject.

A BRIEF HISTORY OF ADDITIVE MANUFACTURING ABOARD THE ISS

Prior to the development of the ISS, there was a detailed study and experimental testing of additive manufacturing for space applications. This was the work begun in 1999 by K.G. Cooper and M.R. Griffin, employees

FIGURE 1.11 (A) The principal components of the PrintSat spacecraft, (B) overview of the PrintSat external structure, and (C) space environment effects surface sensors and test solar cells. PrintSat is scheduled for launch to low Earth orbit in the first quarter of 2015 aboard a Super Strypi rocket. PrintSat is the product of a consortium led by Montana State University and implemented in Windform XT 2.0 carbon-fiber-reinforced composite material. Additive manufacturing printing was done by CRP-USA (see http://www.crp-usa.net/). SOURCE: Courtesy of the Space Science and Engineering Laboratory at Montana State University on behalf of the PrintSat team.

of NASA MSFC.²⁵ In 2000, the NRC published a report that referred to “direct manufacturing” (another term sometimes used for additive manufacturing) and stated, “In remote locations such as the Moon or Mars, direct fabrication from computer numerical control programs could be used to produce items on location, reducing reliance on spare parts inventories.”²⁶

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²⁵ K.G. Cooper and M.R. Griffin, Microgravity Manufacturing Via Fused Deposition, NASA/TM-2003-212636, Marshall Space Flight Center, Huntsville, Ala., July 2003.

²⁶ The report also noted that the concept of a “universal, compact machine shop with a very broad capability that might even extend to repairing itself would be included in the spacecraft or at the base” originated in 1987, but was now becoming conceivable because of additive manufacturing (NRC, Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies, National Academy Press, Washington, D.C., 2000, pp. 99-100).

FIGURE 1.12 Grumman“Beam Builder” machine tested at Marshall Space Flight Center in the late 1970s. The machine used three rolls of rolled aluminum that it bent and then welded with cross braces (the cross braces were stored in the orange cartridges visible on the side). Grumman followed this work by studying a machine that made beams out of composite materials. SOURCE: Courtesy of NASA.

FIGURE 1.13 Artist concept of the Grumman “Beam Builder (B2)” device in space. Although a mockup of the Beam Builder was built and tested in the underwater extra-vehicular activity (EVA) simulator at Marshall Space Flight Center, the work stopped by the early 1980s, and NASA focused on on-orbit assembly of completed parts rather than in-space manufacturing. SOURCE: Courtesy of NASA.

FIGURE 1.14 A 1999 test aboard a KC-135 “Vomit Comet” of an original fused-deposition modeling device. These flight tests were conducted by Cooper and Griffin. This early work, sponsored by NASA’s Marshall Space Flight Center, was the forerunner to the upcoming International Space Station experiments. SOURCE: Courtesy of NASA.

Cooper and Griffin foresaw, discussed, evaluated, and anticipated the value of additive manufacturing in microgravity in space enabled by CAD tools and FDM, available at that time via a plastic build material and a 3D printer developed by Stratasys, Inc. Cooper and Griffin wrote, “This task [a report of their work] demonstrated the benefits of the FDM technique to quickly and inexpensively produce replacement components or repair broken hardware in a Space Shuttle or International Space Station (ISS) environment.”²⁷

With NASA MSFC funding, Cooper and Griffin conducted laboratory experiments and many low-gravity KC-135 low-gravity aircraft experiments. These demonstrated the capability of then-existing FDM equipment to fabricate small test articles in a microgravity environment. Their original device is shown in Figure 1.14. Cooper and Griffin then developed a hardware implementation plan for using FDM for further experiments aboard the ISS, whose initial operations were still in the future. Based on the results of their experiments, they proposed using an ISS FDM device with a 10 cm × 10 cm × 10 cm working volume, a total instrument mass of approximately 45-65 kg with a physical envelope of 0.45 m × 0.5 m × 0.6 m using a peak power of 300 W with air cooling of 150 W.²⁸ With respect to operations, they stated, “Direct ground control for the input of configuration files and real-time adjustments to the hardware operational parameters would be advantageous to maximizing science return and reducing crew loading.”²⁹

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²⁷ Cooper and Griffin, Microgravity Manufacturing Via Fused Depposition, 2003.

²⁸ Ibid.

²⁹ Ibid.

FIGURE 1.15 3D printer developed by Made In Space, Inc., for testing aboard the International Space Station undergoing microgravity testing aboard a parabolic aircraft in 2013. SOURCE: Courtesy of Made In Space, Inc.

In 2010, the start-up company Made In Space, Inc., of Moffett Field, California, was founded and secured funds to build, test and continue the microgravity flight-testing of custom-built and commercially available extrusion additive manufacturing machines with the assistance of NASA’s Flight Opportunities Program. Two aircraft flight campaigns in 2011 and 2013 verified and extended the earlier work by Cooper and Griffin which led the company to develop their own space-qualified 3D printer with a NASA technology readiness level of 6, shown in Figure 1.15. This device is scheduled for testing aboard the ISS in late 2014.

CONCLUSION

Additive manufacturing holds the potential to extend manufacturing capabilities to physical scales currently unobtainable with current spaceflight hardware construction practices. Manufacturing in space may make possible the construction of structures—and possibly entire subsystems—that are fully optimized to the zero-gravity environment, yielding volume-to-mass efficiencies that go well beyond what is attainable at present. The impact of this technology may revolutionize approaches to design.