The American Air Force has reported that a test of the unmanned hypersonic X-51A “Waverider” scramjet has failed. During the test flight the aircraft disconnected successfully from the the wing of a B-52 bomber but only 16 seconds later a defect in a control fin caused the “Waverider” to spiral out of control and eventually break up over the Pacific. The test aircraft was planned to reach a top speed of 7000 km/hr and hold Mach 6 for 300 seconds. This recent event continues the series of failed tests that have plagued the project since its first flight in May 2010. Of originally four prototypes the Pentagon now has only 1 test aircraft remaining. In 2004 the older “X-43” scramjet model reached air speeds of up to Mach 10 – equal to around 11,000 km/hr.
The Scramjet Technology
A scramjet, or supersonic combustion ramjet, is a development of the ramjet engine in which combustion takes places at supersonic rather than subsonic speeds. Both engine variants require high initial vehicle velocities in order to compress and decelerate the incoming air in a converging chamber. Since the airflow throughout the engine and especially the combustion process remains at supersonic air flow the scramjet can operate more efficiently at very high flight velocities.
The scramjet is solely comprised of a converging inlet, a fuel injection point and a converging nozzle. As the supersonic airflow is compressed the temperature of the fluid rises to such an extent that a simple injection of gaseous fuel is sufficient to combust the chemical with the atmospheric oxygen. The combustion process raises the enthalpy of the fluid such that an expansion throughout the divergent exhaust nozzle leads to incredible acceleration of the air and consequently thrust. The principle of expanding a high-enthalpy fluid to generate thrust is similar to standard turbofan and turbojet engines, only that a scramjet does not use multiple rotating compressor stages in the inlet. As they lack mechanical compressors operation of scramjets is limited to near-hypersonic velocities since the high kinetic energy of a hypersonic flow is required to compress the incoming air to operational conditions. Thus, a scramjet-powered vehicle must be accelerated to the required velocity by some other means of propulsion.
The elimination of all moving parts greatly reduces the complexity, weight and susceptibility to mechanical failure of the engine. Furthermore, in turbofans and turbojets the rotating compressors are driven by turbine stages located in the diverging nozzle. The turbine stages are powered by the accelerating exhaust gases and therefore reduce the available energy output. In turbofan and turbojet engines the energy output and thrust can be directly increased by raising the turbine entry temperature i.e. burning more fuel or guaranteeing a more efficient combustion process. Throughout the years turbine entry temperatures have approached the melting point of the turbine blade metals, thus increasing the risk of static and creep failure at the highly stressed turbine inner hub. In the past, solutions to this problem included using nickel-based superalloys, thermal barrier coatings, or casting the turbine blade as a single crystal in order to remove the deformation planes at the grain boundaries. Today almost all turbine blades also feature direct air film cooling around the blades. In this technique cooler air from the compressor stages is bled to and then through channels in the turbine blades and finally allowed to flow out through tiny holes on the turbine blade surface. Ultimately this bled air is then lost from doing any useful work as combusted air. Finally, as turbofan and turbojet engines approach Mach 1 there is an issue with the flow becoming supersonic at the tips of the rotating compressor blades. Any supersonic flow will terminate in a shockwave that will disturb the uniformity of the flow throughout the compressor and cause pressure surges. This will reduce the efficiency of the compressor or even cause single blades to break off.
Thus the higher efficiency and reduced complexity makes the scramjet a better solution for hypersonic propulsion. Currently the scramjet technology would facilitate sudden airborne attacks but is not yet suited for manned flight.
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