Allen was interested in the full range of aerodynamics research, and made contributions to the study of subsonic, transonic, supersonic and hypersonic flow.

The peculiarity in hypersonic flows are as follows:

Finally, the increased temperature of hypersonic flows mean that real gas effects become important.

Additionally, rarefied hypersonic flows (usually defined as those with a Knudsen number above 0.1) do not follow the Navier-Stokes equations.

Professor Hayes followed his work on supersonics with impressive studies of hypersonic flow, said to begin at five times the speed of sound.

For example, while the behavior of hypersonic flow is understood, building a scramjet aircraft to fly at hypersonic speeds has seen very limited success.

Transonic, supersonic, and hypersonic flows are all compressible.

As they lack mechanical compressors, scramjets require the high kinetic energy of a hypersonic flow to compress the incoming air to operational conditions.

In the 80s we installed pressure sensors in the nose cap of the space shuttle to study hypersonic flow.

A hypersonic flow may be characterized by certain things that can no longer be discounted in analysis.