Mars Entry Simulation in the Arc Heated Facility L2K

Kurzfassung

Keeping the total vehicle weight as low as possible, i.e. maximising the pay load, is one of the main objectives of Martian missions. Therefore mainly the Aerocapture technique instead of propulsion systems is used to decelerate the vehicle during the entry phase. But, this configuration increases the requirements on the thermal protection systems of the re-entry vehicle. There is lack of reliable data of on the behaviour of potential TPS materials in the Martian atmosphere, which consists mainly of CO2 (97%) and N2 (3%) and produces further gas species like CO, O, N, NO, etc. in a shock layer or similar high enthalpy areas of the flow around the vehicle. The characterisation and qualification of TPS materials or components of these vehicles can only be performed in well calibrated long duration high enthalpy facilities. The arc heated facility L2K of DLR with an electrical power of 1.4 MW has been intensively used for European space programmes like HERMES, MSTP, X-38/TETRA, etc.. It used to be operated with air, nitrogen and argon as working gas. Recently the gas supply system of the facility has been upgraded in such a way, that the Martian atmosphere can be simulated with a simultaneous injection of CO2 and N2. In addition the performance of the vacuum system has been improved by adding a further roots pump, in order to guarantee a homogeneous hypersonic flow field even during injection of additional noble gas to suppress possible chemical reactions between CO and other species downstream of the test chamber. This measure allows a reliable operation of the facility in terms of the safety of personal and equipment. Comparative tests using the TPS basic material SiC and a cold wall heat flux sensor have been performed in Martian and air atmosphere. In order to achieve the same heat flux rate, i.e. surface temperature, at the same stagnation pressure higher specific enthalpy levels had to be set in the Martian atmosphere compared to air. As shown in Figure 1 a completely different surface temperature development was measured in air and Martian atmosphere at the same gas mass flow rate and reservoir pressure. It is correlated to the differences in the oxidation process of the model surface leading to a different development of the surface properties like catalycity and emissivity. The nozzle flow and the flow field around the model has been computed using CFD codes including chemical and thermal non-equilibrium processes.