Development of a Two-Fault Tolerant Cold Gas Propulsion System and Air Bearing Testbed for Application to a Spacecraft-Inspection CubeSat

Abstract

The design of a cold gas propulsion system intended to demonstrate the feasibility of a two-fault tolerant system architecture is presented. The objective of this design is to protect against a jet fail-on type of malfunction which could endanger a spacecraft or its human occupants in the event of a collision. The design reference mission is a close-proximity external inspection task of an in-flight spacecraft, and we seek to demonstrate continued controllability in the event that up to two separate valve failures occur. Fault-tolerance was achieved by implementing a series of in-line valves and fixed-volume plenums for each thruster to limit the amount of uncontrolled impulse in a valve fail-open scenario. A computer simulation was developed to predict the atypical thrust profile expected in this type of system architecture, and an air bearing-based test platform was constructed to test a 3 degree-of-freedom analog propulsion system prototype. This testbed utilizes computer vision-based position tracking to provide Kalman filter-based estimates of the velocity and acceleration states of the spacecraft analog. A wireless user interface was developed to provide user control of the spacecraft analog, control over individual valve states to simulate valve fail-open failure modes, and to relay pressure and thrust data to validate thruster design. Strong emphasis is placed on utilizing commercially available products, 3D printed pressure vessels, and in-house manufacturing to develop the system in the hopes of disseminating the knowledge gained from this work to aid student teams and other institutions in developing their own satellite testbeds. The results from static testing indicate good agreement with the theory-based simulation predictions, and adverse motion induced by different failure modes was consistently bound to a low level even in two-fault failure scenarios. The most significant adverse motion was induced by two failures which occurred on the same thruster, as well as uncontrolled gravitational forces due to imperfections in the air bearing table. Recommendations are provided for future iterations of the propulsion system and corresponding air bearing testbed.