Optical measurements for a superorbital expansion tube

The applicability of optical techniques to studies of flows in a superorbital expansion tube has been investigated. Emission studies of flows of air over a cylinder at enthalpies around 14 MJ/kg and 93 MJ/kg have been performed. They showed the presence of a range of impurities at the moderate enthalpy condition. At the high enthalpy condition radiation occurred mostly from silicon and ionised species. The strength of this radiation indicates that only optical techniques that can use efficient spectral filtering and aperturing may be used to study these flows. One such technique, holographic interferometry, has been utilised to study the shock shape and flow density around a number of models. For flow over a cylinder, a significant degree of ionisation was observed behind the bow shock. To quantify both the total density and the electron concentration, a two wavelength holographic interferometry technique has been developed. A single holographic plate was used to simultaneously record holograms at two separate wavelengths. Reconstruction of the beams generated two interferograms that were analysed to obtain measurements of conditions along the stagnation streamline.