Ioonos

Producing its own movement

Think of when you water plants with a garden hose. If the water pressure is very strong, then the hose pushes you backwards slightly. That is precisely the principal of the rocket engine. As there is no air in outer space, no tarmac or water to push against, as is the case with cars and boats, the propulsive force comes from the quantity of matter expelled. In the case of a rocket, the matter is the gas produced by the mixture of fuel* and oxidizer*, the two types of chemical product that are stored in the tanks, and together referred to as propellant*. A rocket’s thrust is proportional to the rate of flow of the gas and its speed of ejection.

Gas: a matter of strategy

As you can see, the question of the rate of flow and speed of ejection of the gas is of vital importance. It is sometimes difficult to achieve both at the same time. The relationship between flow and speed of ejection therefore has to optimised, depending on what one wants the rocket engine to accomplish. For example, at lift-off the rocket is very heavy because it is full of propellant (about 90% of its total mass). So massive thrust is necessary to get it off the ground. In this case, powder propellants are used, which enable substantial thrust but at a relatively low ejection speed. These are the famous boosters that you see on the side of Ariane 5. Once lift-off has been accomplished, extremely effective propellants consisting of hydrogen and oxygen are used, producing less thrust but at high ejection speeds. As a result, rather than one engine, a rocket has several engines.