Feasibility Study of a Magnetic Suspension System for Testing the Dynamic Stability of Blunt Bodies in NASA GRC’s 225 square centimeter Supersonic Wind Tunnel

The dynamic stability of blunt bodies during planetary entry is difficult to quantify as computational methods have yet to demonstrate accurate predictive capabilities and experimental methods cannot explicitly measure damping derivatives. Ballistic range testing has been used in the past to determine dynamic behavior of blunt body vehicles by firing test models down a test range from a gun at low supersonic Mach numbers. Photographs are taken of the test model during flight to monitor the capsule's position and angle. A 6-degree of freedom simulation is then fit to the data points captured by the photos and the capsule aerodynamics are returned. This method is an imperfect test set-up as the uniformity of conditions throughout the test range and the initial release state is variable. In addition, the primary test range used to capture data for the MSL entry vehicle, Eglin Air Force Base, was recently decommissioned. The aim of this project is to conduct a feasibility study in order to determine design requirements for a magnetic suspension system in the NASA GRC (GRC) 225 square centimeter Supersonic Wind Tunnel. This much-needed alternate method has the potential to measure damping coefficients more accurately than traditional ballistic range testing and wind tunnel tests with stings. In the proposed experimental set-up, a magnetic suspension system will balance the aerodynamic, gravitational and magnetic forces so the free-to-oscillate model is held in the test section. High-speed cameras will capture the model's position and angle of attack over time and a trajectory will be fit to these data points, much like ballistic range testing. Although magnetic suspension has been used in the past, the proposed system will be a new, innovative implementation of this technology. Blunt body models have never been tested using magnetic suspension and a completely new positioning system will have to be designed, making this project a challenging design problem. The blunt body test models will comprise a non-magnetic material surrounding a spherical iron core. The geometry of the test model itself places a size limitation on the spherical iron core so the suspension system will have to be powerful enough to overcome this constraint. Initial design questions regarding system implementation will be analyzed and answered by assessing problems in the wind tunnel operation, the magnetic suspension system and the design of the model. These problems will be evaluated using MATLAB for simulation purposes and wind tunnel tests. The main design questions to be answered include determining the lowest possible operating dynamic pressure so minimize the force on the magnet through a wind tunnel test, defining the allowable size of the model to avoid blockage but to provide enough magnetic force, and finally resolving the physical constraints of necessary visual and maintenance access despite a potentially massive magnetic suspension system surrounding the tunnel. This project is important because a deep understanding of the behavior of capsules during entry is necessary for trajectory analysis as well as the safety of robotic or human missions. Magnetic suspension would allow for inexpensive testing of blunt body capsules so that dynamic aerodynamics coefficients can be determined to an improved or similar degree of accuracy as ballistic range testing. This project would serve to answer design questions that would be used to create an extremely beneficial modeling tool in the GRC 225 square centimeter tunnel as well as open the door to impactful innovations in magnetic suspension. More »