Scramjet
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A scramjet (supersonic combustion ramjet) is a variant of a ramjet airbreathing jet engine in which combustion takes place in supersonic airflow. As in ramjets, a scramjet relies on high vehicle speed to compress the incoming air forcefully before combustion (hence ramjet), but whereas a ramjet decelerates the air to subsonic velocities before combustion using shock cones, a scramjet has no shock cone and slows the airflow using shockwaves produced by its ignition source in place of a shock cone. This allows the scramjet to operate efficiently at extremely high speeds and does not require any moving parts.
The operating regime and design of the scramjet results in great practical and technical challenges in their implementation. Because they can only operate in hypersonic airflow (>Mach 5), the aircraft to which they are attached must attain a hypersonic airspeed before the scramjet can be enabled. Any craft moving at such velocities is subject to a great deal of aerodynamic heating, necessitating the use of specialized (and expensive) materials in the construction of the craft, particularly at the nose, leading edges of the wings, air intakes, and within the engine. The engine's fuel has only a few milliseconds of residence time to complete combustion, foregoing the use of complex hydrocarbon fuels (like kerosene) which take a long time to completely burn. Not only must the fuel be chemically simple, it must also have a favorable energy-to-weight ratio, and preferably a high thermal capacity (so it can be used as an airframe coolant prior to combustion). The scramjet's operating environment exceeds the specifications of traditional jet engine control equipment, thus providing an additional design challenge.
These challenges have hitherto prevented the widespread use of scramjets; they have so far mostly been demonstrated in research test articles, on experimental vehicles, and a handful of military aircraft. Proposed applications (if technical challenges were resolved) include high-speed intercontinental transport and space launch, to which they would be incredibly well-suited.