Plasma Wind Tunnel Investigation of European Ablators in Nitrogen/Methane Using Emission Spectroscopy

For atmospheric reentries at high enthalpies ablative heat shield materials are used, such as those for probes entering the atmosphere of Saturn’s moon Titan, such as Cassini-Huygens in December, 2004. The characterization of such materials in a nitrogen/methane atmosphere is of interest. A European ablative material, AQ60, has been investigated in plasma wind tunnel tests at the IRS plasma wind tunnel PWK1 using the magnetoplasma dynamic generator RD5 as plasma source in a nitrogen/methane atmosphere. The dimensions of the samples are 45？mm in length with a diameter of 39？mm. The actual ablator has a thickness of 40？mm. The ablator is mounted on an aluminium substructure. The experiments were conducted at two different heat flux regimes, 1.4？MW/m2 and 0.3？MW/m2. In this paper, results of emission spectroscopy at these plasma conditions in terms of plasma species’ temperatures will be presented, including the investigation of the free-stream species, N2 and , and the major erosion product C2, at a wavelength range around 500？nm–600？nm. 1. Introduction For atmospheric entries concerning celestial bodies such as Saturn’s moon Titan, a suitable shielding of the atmospheric entry vehicle from the high heat fluxes generated during such entries is required. For such high speed entries, ablative materials are usually selected as heat shield materials [1, 2]. Thus, on the one hand, the characteristics of the ablators chosen need to be determined. On the other hand, the processes occurring during ablation are currently not well known and understood. Here, basic investigations into the processes in the boundary layer are required in order to facilitate a better understanding thereof and to enable the choice of and improving a suitable heat shield material. Optical plasma investigation such as emission spectroscopy is well suited to this task, as it enables the identification of the predominant plasma species over the spatial region of interest, providing a great amount of spectral information within the restricted measurement interval of less than 50？s, as well as their temporal progression. In this paper, therefore, emission spectroscopic results of four representative measurements at two heat flux regimes and two measurement distances to the probe surface each will be presented. These measurements are both resolved in time and space in the vertical direction perpendicular to the plasma jet axis. Results are presented in terms of plasma species vibration and rotation temperatures obtained fitting a numerically obtained spectrum to the measured one. A temporal as