How to burn up a reaction wheel in the atmosphere

A re-entering satellite poses a very small risk of casualty on the ground. To make sure the risks aren’t higher, either a controlled reentry is required or designs that demise upon reentry. This consideration becomes more important as the number of satellites in space increases and increases at an accelerated pace with future constellations. 

A recent TDE study wanted to further investigate the break-up processes of one of the parts of a satellite – a reaction wheel – during re-entry, with the ultimate goal of having a fully demisable reaction wheel at the release altitude of 78km.

Various designs were analysed to optimise their demisability and during the first simulations the ball bearing unit was identified as a key element (i.e. demising late) during the demise process in previous studies. With the ball bearing unit being the key piece of technology in a reaction wheel, any design changes were carefully considered against heritage requirements.

Seven cases were considered and computer simultions were run to model how the ball bearing unit might react in different mechanical and thermal settings.

The activity discovered that a complete demise of an reaction wheel with 68Nms requires a combination of: an aluminium flywheel, an aluminium resupply/“oil chamber”, an aluminium upper threaded ring (to achieve a release of the central shaft from the ball bearing unit) and an aluminium rotor flange.

Surprisingly, the activity found two unexpected results. Firstly, merely changing the material of the oil chamber from steel to aluminium was the final step to achieving complete demisability of the reaction wheel and the. Secondly, using thermite was more complex than initially expected, and the amount and placement of thermite to support demisability during re-entry became a difficult optimisation problem.