Investigation of the Unique Stability Characteristics of the NASA Maraia Reentry Vehicle

NASA's Maraia Capsule, currently in development, is a small, autonomous re-entry vehicle designed to return to Earth from the International Space Station (ISS). It will act as a flight test bed for various capsule systems and provide on-demand sample return for the ISS. NASA Johnson Space Center tasked the United States Air Force Academy Department of Aeronautics to define the baseline aerodynamic characteristics of the capsule in subsonic flight. This baseline characterization included lift, drag and moment data for the capsule at Mach 0.3 and Mach 0.45 between 0 and 28 degrees angle of attack. A shifting center of gravity study, which is the focus of this paper, was also performed. Although the capsule was stable in pitch for all angles of attack tested with the NASA-provided moment reference center (MRC) location, the shifting center of gravity study concluded that shifting the MRC aft had little to no effect on pitch stability up to 16 degrees angle of attack. This was quite an unexpected result. After 16 degrees angle of attack, the farthest aft MRCs became more stable for a brief range of angles of attack before becoming unstable. Beyond approximately 24 degrees, normal stability characteristics returned as farther aft MRCs demonstrated less stability. Similar results were found through computational fluid dynamics (CFD) methods using NASA's OVERFLOW code. The MRC study also provided a recommended center of gravity envelope of 0 to 9 inches aft of the heat shield for the full scale Maraia which would provide stable characteristics within the angle of attack range evaluated. The counterintuitive trends of the shifting MRC study were further investigated in terms of surface pressures. The data was obtained using a 1:4.44 scale model in the U.S. Air Force Academy's subsonic wind tunnel. This model contained 16 pressure ports at various locations along the shoulder, backshell, and base of the Maraia model, and the model was rotated to obtain a full pressure mapping of the Maraia surface. Both Mach 0.3 and 0.45 speeds revealed very similar trends. All lower shoulder ports experienced a drop in surface pressures at 16 and 18 degrees angle of attack with various levels of increasing pressures beyond this attitude based on their relative location. Lower backshell ports experienced a drop in surface pressures at 16 degrees angle of attack as well. Ports near the base of the capsule were shown to have a subsequent increase in pressure immediately after this drop, while ports nearer the heat shield demonstrated a delay in their pressure increase to as late as 24 degrees angle of attack. Upper backshell port pressures and base port pressures remained essentially constant throughout the alpha sweeps. These wake characteristics were visualized with Fieldview software as well as the U.S. Air Force Academy's water tunnel. The wake region around the bottom of the capsule was shown to significantly influence the aerodynamic characteristics of Maraia in subsonic flight and was correlated to the unexpected stability trends. This analysis will be important to NASA for determination of center of gravity location envelope limits and when considering potential modifications to the Maraia capsule geometry, especially in the lower shoulder and backshell region.