Theories and technologies for duplicating hypersonic flight conditions for ground testing

The development of aeronautics and astronautics, from its practical beginning with the Wright brothers’ first airplane on 17 December 1903, has been driven by one primary aim to fly faster, higher and cheaper. There is an exponential increase in both aircraft speed and altitude over the past 100 years, and now the era of the hypersonic flight is approaching [1]. With practical hypersonic airplanes, a two-hour flight to anywhere in the world will be possible and the spaceaccess expense will be cut by 99% with reusable Two-Stage-To-Orbit (TSTO) techniques. The impact of hypersonic flight on the world’s modern society could be revolutionary.Hypersonic flight is, and in the foreseeable future will be, the driver of national security, and civilian transportation and space access. After 50 years of hypersonic research, both manned and unmanned hypersonic flights have been successfully achieved; however, practical hypersonic flight with air-breathing propulsion is still far ahead of us [2–4]. Hypersonic gas dynamics is fundamentally different from subsonic and supersonic ones. Hypersonic flows are usually characterized by the presence of strong shocks and non-equilibrium gas dynamics and chemistry where gas molecular vibrations, O2-dissociations, N2-dissociations, and electronic excitation and ionizations take place as the flight speed increases from 1.5 km/s to 10 km/s.The accurate prediction of nonequilibrium chemically reacting gas flows is a critical issue for the design of any hypersonic vehicle. The ground-testing facility has been the main means for hypersonic vehicle development for decades because of limitation in physically modeling non-equilibrium chemically reacting gas flow, especially, for air-breathing airframe-integrated cruisers. The most difficult problem arising from the hypersonic testing is that the model scaling laws widely applied in subsonic and supersonic gas dynamics are not valid for hypersonic flows in which thermochemical reactions dominate—that is, the reaction scale remains unchanged when the test model is reduced in its size for wind-tunnel tests. Several types of hypersonic test facilities have been developed so far, such as shock tunnels, air-heated wind tunnels, arc-heated wind tunnels and combustion-based propulsion test facilities. These different facilities are used to address various aspects of the design problems associated with hypersonic flight, and there is no single groundbased facility capable of duplicating the hypersonic flight environment [5]. Nowadays, critical designs of hypersonic vehicles are usually validated with hypersonic flight tests [6]. The flight test is a more reliable tool than others on the ground, but quite expensive and time-consuming for new vehicle development [7]. If one looks back at the last century, supersonic flights had been achieved within 60 years from 1903 to Concorde beginning its commercial service across the Atlantic Ocean in 1967. However, practical hypersonic flight is still under exploration after 50 years of work since 22 September 1963, when Robert M. White flew the North American X-15 at a Mach number of 6.7.There are several technical challenges for hypersonic vehicle development, such as hypersonic propulsion, integrated aerothermal structure and advance groundtesting facilities.The lack of advanced test facilities that can reproduce true hypersonic flight conditions is considered to be the fundamental one.The existing test facilities are inadequate for required technology development, such as propulsion higher than Mach 8, thermal environment identification and large-scale, integrated thermal-structural testing [3–5]. To carry out a reliable ground test at hypersonic Mach numbers, there are four key parameters required for hypersonic test facilities. The first parameter is the test gas and it is the key issue to ensure correct thermochemistry in test flows. The second is the total flow pressure and temperature with which the altitude pressure and temperature, and the flight speed can be reproduced with the proper nozzle expansion. Consequently, the chemical-reactionprocess canbe simulated correctly, as well as the aerodynamic forces and momentum. The third is the nozzle size. In other words, the nozzle exit must be big enough to accommodate a large vehicle model so that the chemical-reaction time scale can be much smaller than that of the test gas flowing past the model. The last one is the test duration. For air-breathing hypersonic vehicles, the airframe is also part of the scramjet engine; therefore, the test