Aerodynamic thrust vectoring for attitude control of a vertically thrusting jet engine.

摘要

NASA's long range vision for space exploration includes human and robotic missions to extraterrestrial bodies including the moon, asteroids and the martian surface. All feasible extraterrestrial landing sites in the solar system are smaller and have gravitational fields of lesser strength than Earth's gravity field. Thus, a need exists for evaluating autonomous and human-piloted landing techniques in these reduced-gravity situations. A small-scale, free-flying, reduced-gravity simulation vehicle was designed by a group of senior mechanical engineering students with the help of faculty and graduate student advisors at Utah State University during the 2009--2010 academic year. The design reproduces many of the capabilities of NASA's 1960s era lunar landing research vehicle using small, inexpensive modern digital avionics instead of the large, expensive analog technology available at that time. The final vehicle design consists of an outer maneuvering platform and an inner gravity offset platform. The two platforms are connected through a set of concentric gimbals which allow them to move in tandem through lateral, vertical, and yawing motions, while remaining independent of each other in rolling and pitching motions. A small radio-controlled jet engine was used on the inner platform to offset a fraction of Earth's gravity (5/6th for lunar simulations), allowing the outer platform to act as though it is flying in a reduced-gravity environment. Imperative to the stability of the vehicle and fidelity of the simulation, the jet engine must remain in a vertical orientation to not contribute to lateral motions. To this end, a thrust vectoring mechanism was designed and built that, together with a suite of sensors and a closed loop control algorithm, enables precise orientation control of the jet engine. Detailed designs for the thrust vectoring mechanisms and control avionics are presented. The thrust vectoring mechanism uses thin airfoils, mounted directly behind the nozzle, to deflect the engine's exhaust plume. Both pitch and yaw control can be generated. The thrust vectoring airfoil sections were sized using the two-dimensional airfoil section compressible-flow CFD code, XFOIL, developed at the Massachusetts Institute of Technology. Because of the high exhaust temperatures of the nozzle plume, viscous calculations derived from XFOIL were considered to be inaccurate. XFOIL was run in inviscid flow mode and viscosity adjustments were calculated using a Utah State University-developed compressible skin friction code. A series of ground tests were conducted to demonstrate the thrust vectoring system's ability to control the orientation of the jet engine. Detailed test results are presented.