• Fire safety—NASA should develop improved methods for screening materials in terms of flammability and fire suppression in space environments. (AP6)
• Combustion processes—NASA should conduct droplet-phase, gas-phase, and solid combustion experiments in reduced gravity with longer durations, larger scales, new fuels (e.g., biofuels and synthetic fuels), and practical aerospace materials relevant to future missions. (AP7)
• Numerical simulation of combustion—NASA should develop and validate detailed single-phase and multiphase numerical combustion models to relate reduced-gravity and Earth-gravity tests, to interpret data from experiments and missions, and to facilitate experimental and design activities. (AP8)
• Materials synthesis and processing and the control of microstructure and properties—NASA should support research in reduced gravity on the development of materials synthesis and processing and the control of microstructures to improve the properties of existing and new materials on the ground. (AP9)
• Advanced materials—NASA should support research to develop new and advanced materials that would enable operations in increasingly harsh space environments and reduce the cost of human exploration. (AP10)
• In situ resource utilization—NASA should support fundamental and applied research to develop technologies that would facilitate the extraction, synthesis, and processing of minerals, metals, and other materials that are available on extraterrestrial surfaces. (AP11)
Currently, NASA has ground-based laboratories, drop towers, aircraft, the space shuttle, and the ISS for studying reduced-gravity phenomena. Reduced-gravity test durations range from several seconds to days.
The ISS creates significant possibilities for experimentation because of its microgravity environment and the ability to vent to an infinite high vacuum. The combustion integrated rack on the ISS has two inserts, the MDCA (Multi-user Droplet Combustion Assembly) and ACME (Advanced Combustion via Microgravity Experiments). The combustion integrated rack is currently configured for two sets of investigations using MDCA: the Flame Extinguishment Experiment (FLEX), which focuses on the efficiency of fire suppressants in microgravity; and FLEX-2, which focuses on more fundamental science issues. The fluids integrated rack is a multipurpose facility for fluid physics research in space. In addition, the microgravity glove box can be used for a wealth of experiments in the combustion, fluids, and materials areas. The Space Dynamically Responding Ultrasonic Matrix System (SpaceDRUMS) has recently been installed on the ISS to provide a facility for the containerless processing of materials using acoustic levitation.
Reduced-gravity experimental platforms are currently limited to aircraft (test duration up to 30 s with 10–2 g-jitter) and NASA’s newly designed centrifuge in drop towers (test duration up to 5 s). The latter produces a very clean artificial gravitational level from 0 to 1 g, but it is subject to the Coriolis force and gravity gradients due to its small size. No reduced-gravity platforms capable of supporting longer test durations are currently available.
Other than the now-canceled ISS centrifuge, future possibilities include a rotating free-flyer (with or without a tether). This could include an emptied cargo vessel for long-duration experiments. For example, when cargo is delivered to the ISS, it comes in relatively large vessels (with volumes up to about 40 m3), which are emptied and then ejected back into Earth’s atmosphere where they burn up on re-entry. Before they are destroyed, however, these vessels could be used for relatively large-scale microgravity experiments lasting up to 2 h. The possibility of using these vessels for experiments is a potentially important opportunity for both basic science research and research in support of the exploration mission. For example, empty cargo vessels could be used for fire safety tests to assess whether the properties of a combustion system scale to larger sizes. The absence of g-jitter created by equipment and astronauts also makes them an ideal platform for crystal growth experiments that are particularly sensitive to vibrations.