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Picture of the rocket powered X-15 and a nozzle test.

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.

During and following World War II, there were a number of rocket- powered aircraft built to explore high speed flight. The X-1A, used to break the "sound barrier", and the X-15 were rocket-powered airplanes. In a rocket engine , fuel and a source of oxygen, called an oxidizer, are mixed and exploded in a combustion chamber. The combustion produces hot exhaust which is passed through a nozzle to accelerate the flow and produce thrust. For a rocket, the accelerated gas, or working fluid, is the hot exhaust produced during combustion. This is a different working fluid than you find in a turbine engine or a propeller powered aircraft. Turbine engines and propellers use air from the atmosphere as the working fluid, but rockets use the combustion exhaust gases. In outer space there is no atmosphere so turbines and propellers can not work there. This explains why a rocket works in space but a turbine engine or a propeller does not work.

There are two main categories of rocket engines; liquid rockets and solid rockets. In a liquid rocket, the propellants, the fuel and the oxidizer, are stored separately as liquids and are pumped into the combustion chamber of the nozzle where burning occurs. In a solid rocket, the propellants are mixed together and packed into a solid cylinder. Under normal temperature conditions, the propellants do not burn; but they will burn when exposed to a source of heat provided by an igniter. Once the burning starts, it proceeds until all the propellant is exhausted. With a liquid rocket, you can stop the thrust by turning off the flow of propellants; but with a solid rocket, you have to destroy the casing to stop the engine. Liquid rockets tend to be heavier and more complex because of the pumps and storage tanks. The propellants are loaded into 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 was powered by a liquid rocket engine and carried a single pilot to a height of more than 60 miles above the earth. The X-15 flew more than six times the speed of sound nearly 40 years ago. The speed record for a piloted aircraft is only exceeded today by the Space Shuttle. The altitude record is only topped by the Space Shuttle and the recent Space Ship 1, which also used rocket propulsion.

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

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