Measuring and Mitigating Roman Space Telescope Reaction Wheel Imbalance Forces and Torques

The Roman Space Telescope (RST) uses a Reaction Wheel Isolation System (RWIS) to reduce reaction wheel (RW) vibration transmitted to the spacecraft bus and into the optical payload. Mechanical shorts such as thermal straps and electrical harnesses that cross from one side of the isolator to the other can degrade the isolator’s performance and adversely impact observatory jitter. Validating the behavior and influence of mechanical shorts, and ultimately the final, as-isolated RW exported disturbance, necessitates a test to be performed at the RW assembly level including the RWIS, thermal straps, and RW harnesses.

RWIS transmissibility testing was conducted with and without the parasitic harness and thermal strap loads to (a) characterize their effects on the RWIS’s isolation performance, (b) facilitate thermal strap design modifications to minimize impact to system, (c) evaluate overall system performance, and (d) generate correlated models of thermal straps and RW harnesses for prediction of on-orbit jitter performance. A novel gravity offload system was designed to support the test article while measuring accelerometer responses at key locations and RW-induced forces and moments at the interface to the spacecraft bus.

A finite element model of the test configuration comprising the gravity offload system, RW, RWIS, harness, and thermal straps is anchored by tap and dynamometer data through tuning of key parameters in the model. Although correlation of the test configuration model proved to be challenging due to the complexity of the gravity offload system and its dynamic coupling to modes of test article at higher frequencies, thermal strap and RW harness model tuning has benefitted from data within the RWs’ operating speed range where less coupling between the offload system and test article is present. Correlated models are subsequently incorporated into the observatory model to improve predictions for on-orbit jitter.