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Artist drawing of the X-43A with a scramjet propulsion system.

Thrust is the force which moves any aircraft through the air. Thrust is generated by the propulsion system of the aircraft. Different propulsion systems develop thrust in different ways, but all thrust is generated through some application of Newton's third law of motion. For every action there is an equal and opposite reaction. In any propulsion system, a working fluid is accelerated by the system and the reaction to this acceleration produces a force on the system. A general derivation of the thrust equation shows that the amount of thrust generated depends on the mass flow through the engine and the exit velocity of the gas. Engineers use a thermodynamic analysis of the scramjet to predict thrust and fuel flow.

In the early 1900's some of the original ideas concerning ramjet propulsion were first developed in Europe. Thrust is produced by passing the hot exhaust from the combustion of a fuel through a nozzle. The nozzle accelerates the flow, and the reaction to this acceleration produces thrust. To maintain the flow through the nozzle, the combustion must occur at a pressure that is higher than the pressure at the nozzle exit. In a ramjet, the high pressure is produced by "ramming" external air into the combustor using the forward speed of the vehicle. The external air that is brought into the propulsion system becomes the working fluid, much like a turbojet engine. The combustion process in a ramjet occurs at subsonic speeds in the combustor. For a vehicle traveling supersonically the air entering the engine must be slowed to subsonic speeds by shock waves generated in the aircraft inlet. Much above Mach 5, the performance losses from the shock waves become so great that the engine can no longer produce net thrust.

In the 1960's an improved ramjet was proposed in which the combustion in the burner would occur supersonically. In the supersonic combustion ramjet, or scramjet, the losses associated with slowing the flow would be minimized and the engine could produce net thrust for a hypersonic vehicle. Tests were begun to design the supersonic burner and to better integrate the inlet and nozzle with the airframe. Because the scramjet uses external air for combustion, it is a more efficient propulsion system for flight within the atmosphere than a rocket, which must carry all of its oxygen. Scramjets are ideally suited for hypersonic flight within the atmosphere.

Shown above is an artist drawing of the X-43A scramjet-powered aircraft. This aircraft is un-manned and launched from a B-52 on the nose of a Pegasus rocket. The rocket powers the X-43 to near Mach 7 where the X-43 separates from the rocket and flies using the scramjet propulsion system. The X-43A successfully demonstrated scramjet propulsion for the first time in March, 2004.

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Editor: Nancy Hall
NASA Official: Nancy Hall
Last Updated: May 13 2021

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