Approximating the Material Stresses and System Requirements for Hypersonic Flight

The flow fields around a flight vehicle at hypersonic speeds are markedly different from those at both subsonic and supersonic speeds. The hypersonic regime is not at a discrete speed but evolves over a continuum as speed increases, but is generally defined as a speed of Mach 5 or higher. The flow fields around a vehicle cause the resultant high temperatures and high heat fluxes for the flight system/materials during hypersonic flight, and these stressors in hypersonic flight are called “The Heat Barrier.” Real gas effects come into play at hypersonic speeds due to these high temperatures, and include vibrational excitation, dissociation, chemical reactions, and ionization. The underlying cause of this change in the flow fields is that the pressure waves created by the body moving through the atmosphere can only travel at the speed of sound.

It is these pressure waves that travel ahead of the body and shape the flow field far from the body of a subsonic vehicle. At hypersonic speeds, the pressure waves have virtually no time to propagate away from the flight body, creating a condition where the pressure waves stack up very close to the flight body. These stacked up pressure waves form a bow shock very close to the flight body, where viscous heating occurs due to shear forces. These high-temperature gases close to the flight body surface lead to the high heat fluxes and high surface temperatures found in hypersonic flight.