Ignoring the effects of friction may seem unfair. But, on the contrary, it makes our example closer to the case of a real rocket since friction-free conditions prevail in space. In fact, it is the absence of friction which dictates that rockets are the only means of space propulsion available, at least for the foreseeable future. A common mistake is to assume that rockets work by pushing against something, just as we move by pushing our feet against the ground, making use of the frictional force between two surfaces. But in space there is nothing to push against.
Our example of the sledge and pebbles also points out one of the biggest problems in spaceflight. This is the need for a rocket to carry its own reaction mass – the mass it has to expel in order to gain speed. In the case of a jet aircraft, for example, which also works by the principle of action and reaction, new reaction mass is continually obtained from the surrounding air. A jet engine takes in air as it moves along (aided, especially at low speeds, by fast-spinning turbine blades), heats and compresses this air by using it to burn propellant, and then allows the resulting exhaust gases to expand and escape as high-speed reaction mass at the rear. A rocket cannot do this. It must, with the interesting exception of the interstellar ramjet, be completely self-contained and have all of its reaction mass onboard from the outset. Unlike with an aircraft, this reaction mass also doubles as the propellant – the mixture of fuel and oxidizer which is burned in the combustion chamber of the rocket to release energy. The resulting hot exhaust gases are then allowed to escape from a nozzle and, in accordance with Newton's third law, provide a forward thrust.
Related category ROCKETRY TERMINOLOGY
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