The Scramjet: A Solution for Hypersonic Aerodynamic Propulsion

Abstract

The challenge of achieving hypersonic velocities with an air -breathing engine has been the prospect since the late 1950’s. The space program and the Lunar landings and the introduction of the ballistic missiles seemed however to be the confirmation that the rocket technology was the means to reach high velocities and long ranges. Despite this, air -breathing propul sion has been research ed both in the academia and in different national establishments around the world. Although break -through events have been discerned they have not yet been realized in an operational hypersonic air -breathing propulsion system. This pa per will describe some of the reasons for this: The extreme temperatures and pressures generated by hypersonic speeds require some unique designs and processes that cannot be extrapolated from the supersonic flight technology. To achieve hypersonic speed s with an air -breathing engine one must probably consider the use of supersonic combustion. The combustion challenges are enormous when the very high internal velocities only permit ultra -short time for fuel injection, mixing and burning. In hypersonic a ir -breathing propulsion a problematic flight Mach number range exists, approximately between Mach 6 and 8, where neither subsonic or supersonic combustion processes seem appropriate. This zone, which could be denoted as the “Heat barrier” has some conceptu al similarities to the “Sound barrier” in aviation some 60 years ago. This “barrier” has to be avoided or else bridged by some means, for instance by using hybrid engines or by new innovative internal engine processes. As for the necessary R&D, there is a great discrepancy between of what should be done (theory) and what can be done (experiment). Many of the sub -processes are of interferential nature, at which the development of hypersonic air -breathing propulsion would benefit by simply bridging existing but isolated R&D efforts. But unfortunately most of the R&D and experimental data are classified and therefore difficult to reach. An attempt is here done to display certain notable, qualitative properties of supersonic mixing and combustion processes by a nalysis based on first principles of gasdynamics. Further, an account is given of experiments using windtunnel testing in assessing the supersonic mixing problem. Data are presented of the scramjet research performed in the Swedish academia in the 70’s an d 80’s. The outcome of this research did identify the bottleneck in the realisation of a scramjet engine residing mainly in the fuel -air preparation. Based on these findings some suggestions for improving certain aspects in this area are also presented.