Thrust is the force which moves an aircraft through the air. Thrust is generated by the propulsion system of the aircraft. Different types of engines develop thrust in different ways, although thrust is usually generated through some application of Newton's third law - action <-> reaction. A gas is accelerated by the propulsion system and the reaction to this acceleration produces a force on the engine. 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.
During and following World War II, there were a number of rocket powered aircraft built to explore high speed flight. The X-1 used to break the "sound barrier" and the X-15 were rocket powered airplanes. In a rocket engine stored fuel and stored oxidizer are mixed and exploded in a combustion chamber. The hot exhaust is then passed through a nozzle which accelerates the flow. For a rocket, the accelerated gas, or working fluid, is the hot exhaust; the surrounding atmosphere is not used. That's why a rocket will work in space, where there is no surrounding air, and a jet engine or propeller will not work. Jets and propellers rely on the atmosphere to provide the working fluid.
There are two main categories of rocket engines; liquid rockets and solid rockets. In a liquid rocket, the fuel and the source of oxygen (oxidizer) necessary for combustion are stored separately and pumped into the combustion chamber of the nozzle where burning occurs. In a solid rocket, the fuel and oxidizer are mixed together and packed into a solid cylinder. Under normal temperature conditions, the fuel and oxidizer will not burn; but they will burn when exposed to a source of heat. Some type of igniter is used to initiate the burning of a solid rocket motor at the end of the propellant facing the nozzle. Once the fuel starts to burn, hot exhaust gas is produced, which is used to propel the rocket, and a "flame front" is produced which moves into the propellant. Once the burning starts, it will proceed until all the propellant is burned. With a liquid rocket, you can stop the thrust by turning off the flow of fuel; but with a solid rocket, you would have to destroy the casing to stop the engine. Liquid rockets tend to be heavier and more complex because of the pumps, and you usually put the fuel in the rocket just before launch. A solid rocket is much easier to handle and can sit for years before firing.
On this slide, we show a picture of an X-15 rocket powered airplane at the upper left and a picture of a rocket engine test at the lower right. For the picture at the right, we only see the outside of the rocket nozzle, with the hot gas exiting out the bottom. The X-15 carried a single pilot to a height of more than 60 miles above the earth and flew more than six times the speed of sound nearly 30 years ago. These altitude and speed records are only exceeded today by the Space Shuttle. You can learn more about the X-15 by visiting the NASA Dryden picture gallery.(http://www.dfrc.nasa.gov/gallery/photo/NIX/x-series-aircraft.html)
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