At this speed, the air surrounding the vehicle undergoes extreme compression and, as a result, heats to thousands of degrees. In fact, during the reentry phase of space flight, the air surrounding the vehicle must be regarded as a chemically reacting and radiating mixture of atoms, molecules, and ions. Manned space vehicles must survive reentry with 100 percent certainty and, in the case of modern space planes, descend afterwards to a runway in a manner similar to a conventional airplane. This challenging goal must be attained economically, both with respect to the size and complexity of the vehicle itself, and with respect to the cost of the vehicle's design and construction. Therefore, the space vehicle design team must be aided by numerical simulation.
Hypersonic flows are essentially impossible to model reliably in a wind tunnel.
Moreover, wind tunnel experimentation is expensive for a given configuration and very difficult to apply to subtle design changes. Unfortunately, numerical simulation is also far from 100 percent reliable. What appear to be small changes in a given model for the physics can often lead to substantial changes in a computed solution. Moreover, one computed solution can vary from another one, given the same configuration, depending only on the particular numerical algorithm employed. For these reasons, it is desirable to compare numerical simulation to wind tunnel experimentation on certain standard test problems.