Guidelines for use of the random positioning machine as a reduced-gravity analog.

With expanding spaceflight mission plans to the Moon, Mars, and beyond, investigating biological phenomena in reduced-gravity environments is a critical need. Ground-based spaceflight-analog platforms, such as the two-frame Random Positioning Machine (RPM), represent an alternative to operationally challenging spaceflight-based experiments. We developed a kinematic model to define the acceleration and gravitational level experienced by a sample at the center of an RPM. In comparing this to the RPM's performance with accelerometers and motion capture sensors, we validated the RPM and defined optimized performance parameters for time-averaged microgravity and reduced-gravity simulation (rotation rate: outer frame > 30 deg/s, inner frame > 20 deg/s slower; sample size: <10-15 cm from the center depending on rotation rate; experimental duration: >25 min). We also present the validated designs for an RPM capable of simulating microgravity, lunar gravity, Martian gravity, and hypergravity simultaneously. Finally, we identify "pole bias" as a critical limitation for two-frame RPMs: when the outer frame is aligned vertically, which occurs twice during each rotation, the gravity vector is aligned with a sample and generates two poles. At higher rotation rates, this significantly reduces the fidelity of an RPM's reduced-gravity simulation, and we thus introduce the design and validation for a novel three-frame RPM that can mitigate pole bias. We validated the RPM platform, optimized experimental design guidelines, and identified novel RPM designs for microgravity and reduced-gravity investigations.