The achievable total enthalpy and the pressure level in a shock tunnel depend on its capability to generate strong shock waves. To produce a strong shock wave, high pressure and high sound speed are two key parameters for driver gases. There are various techniques to increase the driver gas sound speed, which are essentially different approaches in the way to raise the driver gas temperature. The first technique to increase the driver gas sound speed is by the use of a light gas, and the second one is by heating the light gas to a high temperature with gas heaters. The light-gas-heated shock tunnel is introduced in this chapter, and the electrical heaters are discussed in detail, including the relatively simple electrical resistance heaters and electric-arc heaters. Strictly speaking, the electric-arc heating is not a gasdynamic technique and it is not capable of completing flight-condition duplication for hypervelocity testing. However, it is selected because it can generate extremely high total enthalpies and is useful in certain applications.

The free-piston driver is a powerful technique to increase both the driver gas sound speed and pressure. Therefore, it is capable of generating high-enthalpy flows and offering high performance among various gasdynamic shock drivers. So far, it has been implemented in a number of major reflected-shock as well as shock-expansion wind tunnels around the world. The free-piston driver has the advantage that a high driver gas pressure is automatically generated in the same process. On the other hand, the driver is far more complex mechanically and requires operation-tuning in order to operate effectively. Moreover, its test time is short and the test flow is not steady because the piston motion is difficult to control. In this chapter, the basic concepts of the free-piston driver are discussed. The analytical theory that describes the piston dynamics and the method for tuned piston operation are presented. Examples of major free-piston-driven test facilities as well as their applications in hypersonic testing are also summarized.

In order to introduce hypersonic ground testing facilities, background information in hypersonics is presented to show to readers what we want to do, where we have been, and where we are going to go. These will provide with a good indication of the research needs that are called as hypersonic vehicle ground testing. It is of fundamental importance that a vehicle design must be carefully evaluated in ground test facilities before flight testing can proceed. Indeed, the development of hypersonic vehicles is related to the capability development of hypersonic ground testing facilities.