Aeolus spacecraft will provide a global observation of three-dimensional wind fields over the Earth's surface. Observations will be performed with ALADIN instrument which uses the active Doppler Wind Lidar method. This puts very high requirements on contamination of mirrors and the Transmit/Receive Optics (TRO). An operation of spacecraft in orbit creates an own ambient atmosphere due to outgassing, thruster firings, leakage etc. Density, pressure, and species composition of this atmosphere differ significantly from hard vacuum properties in orbit. It is impossible to model this in ground facilities and only a numerical simulation can provide a contamination prediction. Main goals of the paper are a numerical analysis of an interaction of outgassing and free-stream flow and prediction of the ALADIN optics contamination.
The mirrors and TRO locate inside a baffle and there is no direct light connecting the mirrors and solar arrays. Therefore, the mirrors can be contaminated by material outgassing only due to collisions between the species and with free-stream flow. Computations are performed with the direct simulation Monte Carlo method. The method is de facto a main tool of rarefied gas dynamics and allows one to obtain a correct solution for the case under consideration. There is a large difference in spatial scales, which makes the analysis difficult. The computational domain has to be large enough (hundreds meters) to catch properly backscattering fluxes and the spatial resolution has to be fine enough to resolve a small size of a hole (a diameter of 60 mm) in
the primary mirror. Therefore, the analysis was performed in the following two steps.
I. Computation of three-dimensional flow around Aeolus spacecraft to obtain mass fluxes at the entrance of the baffle.
II. Three-dimensional computation of a flow inside the baffle using results of step I.
Computations were performed for few time points with different outgassing species. Relationships between mass fluxes on the optics and outgassing rates were obtained to calculate a mass load over the mission.
The full-length paper will present also
- a comparison with Scialdone analytical model for backscattering of outgassing species and
- an analysis of outgassing from an internal side of the baffle.
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