Speaker
Description
Recent studies have highlighted reaction wheels as critical elements which are likely to survive re-entry and contribute to casualty risk. The major element of concern is the flywheel which is often constructed of stainless steel in larger reaction wheels, although the survival of the steel ball bearing unit due to its shielded location is also an issue. Baseline simulations with the SAM tool using a seven-component compound model are consistent with these studies.
Wind tunnel tests have been performed on an engineering model of a 120mm diameter reaction wheel with a steel flywheel and ball bearing unit. An initial test was performed at low heat flux and the failure of the aluminium housing was observed. Running at higher heat flux on the surviving steel flywheel and ball bearing unit parts shows a heat flux profile which is dependent on the local heating. The parts were tested at heat fluxes higher than would be expected in re-entry and demise was not observed, supporting the suggestion that these objects can be a casualty risk. Rebuilds of the heat fluxes on the surfaces have been performed, and an energy balance using data from the demisable materials studies has shown consistent results.
A wide variety of potential design-for-demise techniques have been simulated using SAM to assess the reduction in the casualty area produced. Each assessment has been performed using 1000 runs accounting for uncertainties in the release conditions and the aerothermodynamic heating. Improved demisability is most clearly seen with changes to the flywheel material. Copper is identified as a viable alternative with demise performance to close to that of aluminium but the capability to maintain the flywheel inertia due to its higher density.
Finally, a small benefit from a spoked flywheel is seen using the inclination-based Modified Lees approach for the aerothermal heating. However, if the local length-scale SAM heating model is used, which is much more in line with the test observations, significantly higher heat fluxes are seen on the spoked model. A reduction of under 10% in the casualty area is seen for the standard wheel, but more than 20% for the spoked wheel. In this case the casualty area is still driven by the steel flywheel, and the demise probability of the ball bearing unit is almost doubled. Allied to a change in the flywheel material, this design shows potential.
