Ionizing Nitrogen and Air Flows in a Superorbital Expansion Tube

Abstract

Vehicles on interplanetary missions enter atmospheres at very high (superorbital) velocities. The kinetic energy of the craft causes dissociation of molecules and ionization of all of the species present in the gas. Ground-based testing of such conditions in nitrogen has been performed in a new superorbital expansion tube, X2, using two optical techniques. Emission measurements identified sources of visible radiation that may influence optical measurements and can also contribute to radiative heat transfer. As well, the emission spectra from Stark-broadened hydrogen lines were used to measure electron concentrations between the bow shock and the body. Two-wavelength holographic interferometry was used to provide two-dimensional density and electron concentration profiles of the flow. Peak electron levels of around (4.0 +/- 0.6) x 10(16) cm(-3) were observed that agreed well with equilibrium estimates, A gradual increase in electron population after the shock was observed, reaching a maximum of about one-quarter of the distance from the body to the shock along the stagnation streamline. Thereafter the concentration decreased because of the influence of the body. Comparisons were also made between Rows over different sized cylinders and between air and nitrogen Rows.