Performance considerations in the operation of free-piston driven hypersonic test facilities

There are two fundamental limitations to useful test time in a free-piston driven shock tunnel. One is the contamination of the test flow by driver gas (a limitation in all shock tunnels). The second is the ability to maintain a nearly constant stagnation pressure in the gas driving the nozzle flow. However, practical experience has shown that it is possible to manipulate the operating variables of a tunnel of a given design so as to trade off these two phenomena and obtain the optimum conditions for particular experimental requirements. While contamination remains an absolute limit to performance, both experimental results and theoretical investigations have shown that the requirement for “constant” pressure can be relaxed. With appropriate treatment of data, valid and useful tests can be carried out in a falling (or rising) pressure regime.