The extraordinary advantages of a weightless laboratory have fascinated scientists for decades. That’s because observing phenomena and processes in microgravity conditions can help prepare the field for deep-space human explorations and provide knowledge to improve the quality of life on Earth. Microgravity offers an ideal environment to explore the basics of many types of scientific research and can hold the key to unlocking the full potential of 3D printing. Without the distortion experienced on Earth, investigators can gain insight into the inner workings of physical and biological systems, leading to the advancement of additive manufacturing (AM) technology in orbit.
For years, researchers have performed studies in microgravity. Before the National Space Laboratory of the International Space Station (ISS) became a platform for research in orbit, space agencies relied solely on other means. These included drop towers, suborbital space flights, artificial microgravity simulators, and, particularly, on parabolic flights on Earth that can be tuned to allow for zero gravity or reduced gravity levels like those found on the surface of the Moon or Mars. Many of these innovative and viable options have been around since the 1950s, but are only limited to short continuous spans of microgravity that can only last seconds.
The only permanently occupied microgravity laboratory aboard the ISS has allowed researchers and space crew to carry out hundreds of hours of experiments, proving theories and revealing previously unexplained phenomena. The have even taken the first steps toward realizing the requirements for an on-demand, microgravity 3D printing station off Earth.
The ISS provides a valuable starting point to propel the potential of 3D printing technology for space. According to ISS Chief Scientist Kirt Costello, “in space, you don’t have buoyancy-driven convection. Hot things don’t rise over colder things, so, a lot of times, that can lead to discoveries when you’re doing material science, especially involved around melting or processing of materials. So, there have been advances where people look at new materials in space and how to form stronger and more advantageous materials using microgravity as one of the factors.”
With extremely limited availability of Earth-based logistics support, 3D printing capabilities could become one of the most important technologies in space. Spaceflight missions today require the National Aeronautics and Space Administration (NASA) to send up more than 7,000 pounds of spare parts to the ISS every year. While another 29,000 pounds of spaceflight hardware spares are stored aboard the station and 39,000 await on the ground, ready to fly if needed.
This logistics system might work well for a spacecraft that is orbiting 250 miles above Earth, but for future missions to the Moon, Mars, and beyond, this is simply not viable. It takes about three days for a spacecraft to reach the Moon and, at a cost of $10,000 per pound, any Moon colony would become a very expensive undertaking very quickly. Space crew will need to make their own spare parts, tools, and materials. Therefore, enabling on-demand manufacturing with common raw materials is essential, with some researchers exploring the use of a variety of recycled, onboard waste material as feedstock.
First In-space 3D Printers
Up until now, the space station has received several 3D printing experiments and 3D printing system platforms. In the fall of 2014, NASA and Made In Space (MIS) executed the first demonstration of on-orbit manufacturing using a fused filament fabrication (FFF) printer as part of the 3D Printing in Zero-G Technology Demonstration Mission. Once installed in the ISS’ Microgravity Science Glovebox – a sealed facility for investigations – the printer was set to work immediately, even churning out the very first wrench 3D printed on the ISS by Commander Barry “Butch” Wilmore.
The primary objective of the mission was to prove critical operational functions of the printer, as well as evaluate the impact of microgravity on material outcomes with the FFF process by manufacturing mechanical property test articles and functional tools, and, finally, to demonstrate remote commanding. According to NASA’s analysis, after a four-year comparative research experiment in which the space agency fabricated tools and other objects both onboard the ISS and in simulated microgravity using 3D printers on Earth, all objects performed equally well. In fact, in the published study, the researchers noted no significant microgravity effects on material outcomes in the physics-based model of the FFF process.
The 3D printing mission successfully demonstrated the first step toward manufacturing in space. However, MIS kept advancing its vision of off-Earth manufacturing by launching another commercial printing facility. This time, the company’s Additive Manufacturing Facility (AMF) reached orbit in March 2016 and has already additively manufactured more than 100 individual parts for a variety of commercial and private customers, utilizing three different polymers: ABS plastic, green polyethylene bioplastic, and space-capable plastic PEI/PC.
Leveraging 3D printing for space developments is the ultimate use for the technology. On Earth, AM competes with older, more established manufacturing platforms. In space, 3D printing can become the first, most reliable, and cost-effective production platform in an entirely new commercial dimension. 3D printing in space is an enabling technology that is crucial to human exploration beyond the low Earth orbit (LEO) environment. With some modification to the key systems, MIS was able to demonstrate that AM with extrusion-based machines functions similarly in microgravity as it does on the ground, allowing for a full proof of concept.
Nonetheless, based on MIS’s experience, using polymer feedstocks for in-orbit AM does not show substantial differences in the end product versus terrestrial production. While this is good to predict what can be produced in orbit, scientists consider that the lack of structural differences using polymer feedstocks may also limit the potential for material performance advantages for in-space AM.
Alternatively, this is not the case for metals. Metal AM in microgravity changes product microstructure and porosity. An expert panel discussing the advantages and limitations of in-space manufacturing during the 2020 virtual Additive Manufacturing in Space Workshop, organized by the Center for the Advancement of Science in Space (CASIS) – manager of the ISS U.S. National Laboratory – determined that, in microgravity, the lack of convection-driven mixing in the melt pool impacts elemental mixing/homogeneity of composition during deposition, as well as cooling rates. Therefore, studies should consider metal-wire (such as for directed energy deposition) or polymer-filament systems, along with “mixed-media” products, such as fiber-reinforced plastics or metal-wire-reinforced ceramics.
“If we want to establish a sustainable presence off Earth, we need to come up with new materials or we need to adapt the old materials to be used and reduced in a microgravity environment, with special attention paid to the environment,” explained NASA Project Manager Jennifer Edmunson. “So, if we want something that survives on the lunar surface, it has to be prepared to deal with the thermoswings, radiation, micrometeor impact and electrostatically charged lunar surface. If the new materials can survive in the lunar environment, they most likely will be able to thrive pretty well on Earth.”
Experts believe that novel approaches to in-space production are necessary where terrestrial systems do not readily translate to microgravity conditions. Opening up new options for feedstocks, including soft materials like elastomers, foams, and rubbers; low-viscosity inks; new polymer options like longer cure time thermosets, filled polymer systems, continuous fiber reinforcement, and semi-crystalline polymers. Moreover, there is a need to study how in-situ materials will translate into suitable AM feedstocks, especially for in-situ resource utilization in other planetary bodies, where space crews can expect to find regolith-type materials that could have the potential to become 3D printing materials.
Although the ISS National Lab is an ideal environment to explore the possibilities of 3D printing in microgravity, there are still many unanswered questions. Up until now, collaborative and innovative approaches between space agencies and private companies have been enabling AM to thrive in space. As human habitation initiatives expand, primarily aiming at exploring more of the lunar surface than ever before, this unique orbiting laboratory will be essential to understand how humans can live sustainably in orbit for long periods of time.
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