I. Introduction

After more than 40 years, high-enthalpy facilities suitable for aerodynamic testing are still mostly based on the shock tunnel principle.1 Recent use of these facilities involves the development of space planes and reentry vehicles for studying the complex aerothermochemistry associated with flight at high velocities. The effects of thermal and chemical relaxation in air become important for flight velocities greater than approximately 4 km/s. In addition, high-enthalpy, aerodynamic-impulse facilities are not only used to study high-temperature effects but are also suitable for generating high Mach number, high Reynolds number flows to investigate viscosity-dominated effects at low-enthalpy conditions.

In aiming at higher-flow velocities, that is, higher stagnation enthalpies, it became obvious quickly that a technological barrier exists that would be very hard to overcome with conventional shock tubes. Therefore, Stalker modified the conventional shock tunnel to a free-piston shock tunnel.2 In these facilities, the driver gas is compressed by a heavy piston accelerated to nearly sonic speed. Between the piston and the main diaphragm, high values of temperature and pressure are achieved to generate a strong shockwave propagating along the driven section of the shock tunnel. Before the diaphragm opens, the piston is...