The intrinsic limitations of a conventional hypersonic wind tunnel can be overcome, potentially, if heat is added to the supersonic flow downstream of the throat. The fluid mechanical issues which arise if radiant heat is absorbed downstream of the throat are discussed. In particular, predictions based on an axisymmetric model which includes ray-tracing and energy absorption along rays, real gas thermodynamics and the full unsteady Navier-Stokes equations with a boundary layer turbulence model are discussed in terms of the underlying physics of the flow. Issues of interest which affect flow quality, throat survivability and heat transfer include the refraction of rays, spatial and temporal departures from idealized one-dimensional behavior, transient and boundary layer phenomena, etc. Similarly, electron beam and other sources of heating are discussed. Comparison between prediction and preliminary supersonic flow experiments, in one case at a Reynolds number of 1010/meter using air at 3000 atmospheres, are being made and their connection with proof-of-principle experiments discussed. The work is being pursued with Professor Richard Miles at Princeton and colleagues at DOE National Laboratories.