I. Introduction

Flow at high Mach numbers, in the "hypersonic" regime, behaves quite differently from that at lower supersonic Mach numbers. The characteristics of hypersonic flowfields have been the topic of many texts; more detailed information may be found in Refs. 1 5. These characteristics pose challenging design constraints on facility designers and result in the implementation of ingenious techniques to acquire and validate information, which will be ultimately used to prove future hypersonic flight vehicles.

Mathematically, the linearized equations of supersonic flow are no longer adequate when the Mach number exceeds aproximately 5. Additionally, there are certain physical phenomena that become increasingly important to properly characterize flows in this speed regime. Additionally, a separate term, hypervelocity, for labeling flight at very high Mach numbers appears justified. It is used typically for velocities greater than 3 km/s or approximately Mach 9 in atmospheric flight. At this boundary, oxygen molecules in air start to dissociate; at higher speeds, further dissociation of oxygen and nitrogen molecules produces chemical reactions beyond which ionization occurs. These boundaries, in the stagnation region of an equilibrium airflow, are shown in Fig. 1. For speeds greater than 4 km/s and altitudes less than 40 km, chemical equilibrium exists because the free stream density is high. At higher altitudes, a relatively...