Hypersonic Flight

Scramjets

Scramjets are supersonic combustion ramjets. They are a variant of an air-breathing jet engine or ramjet. They use high speeds to force air to compress as well as decelerate before it is used in combustion. Inside a ramjet, the air is compressed and slowed to subsonic velocities while at the same time it moves at a supersonic speed. In contrast, the scramjet uses airflow that is supersonic so that its utilization of the compressed air is more efficient. Scramjets can operate at supersonic speeds. They have achieved speeds of Mach 12 and Mach 24 for unmanned flights.

The design of scramjets is similar to all jets. Propulsion (forward thrust) is achieved by gases expelled from the engine. The engine is fed air through an intake opening that is compressed and decelerated; this air is combined with a gaseous fuel to burn the atmospheric oxygen that is in the air received by the inlet. The burn produces heat, which energizes the gases to produce thrust through a diverging nozzle.

Scramjet designs are different from turbojet or turbofan jet engines. The typical jet engine uses a rotating fan-like component to compress air and the burn then heats it; or, it may use multiple rotating turbine engines to compress air. These numerous moving parts are more complicated, add weight to the aircraft, and are liable to failure. Scramjets, in contrast, lack moving parts; however, they must be accelerated to high speeds in order to achieve optimal cruising speeds. It would essentially be a scramjet in a rocket plane.

Boeing Company is a manufacturer of aircraft for both civilian and military purposes. It built the unmanned experimental Boeing X-51A scramjet in a consortium with Pratt and Whitney Rocketdyne. The X-51 is also known as the X-51 WaveRider because it uses its supersonic shockwaves to add lift. In order to achieve its hypersonic speeds, it was launched at flight altitude from a Boeing B-52 Strato-fortress over the Pacific Ocean on its first flight on May 26, 2010. It achieved speeds over Mach 5, or over a mile a second. The speed is sufficient for flight from New York to Los Angeles in less than 40 minutes.

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