RECOMBINATION OF ATOMIC OXYGEN ON ZIRCONIUM BASED MATERIALS FOR HYPERSONIC ATMOSPHERIC RE ENTRY
Catalytic recombination of dissociated atmospheric oxygen molecules on the surface of thermal protection systems (TPS) located on nose and leading edges can be a significant additional heat source during the atmospheric re entry of a space vehicle. Typically, TPS of re entry vehicles are designed considering the constituent materials as fully catalytic, a strongly conservative assumption that can lead to an over sizing of the total mass required for the TPS. However, mass, performance, and cost considerations in future re usable re entry vehicles require a more sensible design approach based upon a more realistic assumption of partial catalycity. Ground test simulation under atmospheric re entry conditions is therefore necessary to characterize and select base materials for TPS. Very high temperature and low pressure air plasma are reproduced within the MESOX facility out of equilibrium plasma with a degree of dissociation of oxygen reaching 70 for an earth entry phase. In this paper, experimental results are given on the catalytic properties of zirconium based ceramics that represent a promising class of new TPS materials for the manufacturing of slender shaped hot structures in hypersonic re entry vehicles. Two different zirconium based compounds, produced by hot pressing and machined into their final shape by mean of electrical discharge machining or, alternatively, by diamond tooling were tested in the temperature range 900 2500 K. Three kinds of zirconia different by their sintering additives were also tested in the same temperature range. The effects of the crystalline structure of zirconia materials and the one of surface machining for the zirconium based ceramics are underlined and new experimental results on the recombination coefficient of atomic oxygen are obtained at high temperature on this class of new TPS. 1. MESOX SOLAR FACILITY AND DETERMINATION OF THE RECOMBINATION COEFFICIENT The MESOX facility (Fig. 1) is described in details elsewhere [1, 2] here only its main features are outlined. The ceramic sample (25 mm diameter and 3 mm thickness) is put in a plasma reactor consisting of a silica tube (quartz) 50 cm length and 5 cm diameter with CaF2 viewports. The air plasma is generated by a 300 W microwave discharge surrounding the sample. A regulator, a gauge and a vacuum pump allow the precise control of the total pressure during experiments. The airflow is fixed at 4 l/h and is coming from to the top of the reactor and then pumped down. The sample is placed on a sample holder at the centre of the plasma discharge. The reactor is positioned in such a way to let the sample be at 25 mm above the theoretical focus of the 6 kW solar furnace equipped with a variable opening shutter. Due to this shutter, the available incident concentrated solar flux can reach 5 MW.m. Fig. 1: MESOX solar facility and image of air plasma The spectroscopic bench used for Optical Emission Spectroscopy (OES) is composed of an optical sampling system including a lens and a mirror, and a monochromator (spectrometer Triax 550 Jobin Yvon) equipped with an Optical Multichannel Analyzer (OMA) The microwave discharge is imaged by the silica lens (magnification 0.1) on the slit entrance of the monochromator. The 55 cm focal length monochromator working with a 1200 grooves/mm grating (also available 1800 and 2400 grooves/mm gratings) and a 100 mm width slit allows a spectral resolution of 0.2 nm. The dispersed light is analyzed by means of the CCD matrix (1024 x 128) of the OMA detector. Each of the 128 lines of the matrix gives information on the relative atomic oxygen concentration at different distances from the surface of the sample with a spatial resolution around 270 mm. A spectral analysis over the 128 lines is performed very quickly after the solar radiation breaking. The total duration of a scan is 200 ms. Therefore all the spectral and spatial needed information are taken simultaneously allowing a good accuracy. The recombination coefficientg is the ratio of the flux of recombined atoms to the total flux of atoms impinging the surface: