Rocket fuel works on the basis of Newton’s Third Law of Motion, which states that ‘every action is accompanied by an equal and opposite reaction’. By ﬁring fuel out the back of a rocket, the force propels it upwards with acceleration equal to the force at which the fuel is expelled. It is almost identical to how a jet plane is able to ﬂy in the atmosphere. However, one difference is that jet planes use oxygen in the atmosphere to ignite their fuel, while a rocket must carry its own oxidiser.
There are two main types of rocket fuel used on modern rockets: liquid and solid. Liquid propellants separate fuel and oxidisers and the two are combined in a combustion chamber where they burn and are ﬁred out from the base of the rocket. While more complex than solid fuel, the ability to control the ﬂow of propellant means the engine can be throttled to a particular speed. Liquid fuels are further subcategorised into either petroleum, cryogens or hypergols. Petroleum is fuel derived from crude oil and hydrocarbons, cryogens are those stored at very low temperatures (such as liquid hydrogen), while hypergols are able to self-ignite on contact between the fuel and the oxidiser.
Solid rocket fuels are those in which the fuel and oxidiser compounds are already combined. Most use an aluminium powder as the fuel and an ammonium perchlorate as the oxidiser, while an iron powder is used as a catalyst for the reaction. All that’s required is a spark to start them burning. While they are much simpler than their liquid counterparts, they cannot be stopped once they have been ignited. For that reason, most modern rockets have hybrid engines, which use a combination of both solid and liquid fuel boosters. Solid fuels are generally used more for the initial launch sequence, when the speed needs to be at its maximum, whereas liquid fuels are used later so the speed can be adjusted to get the rocket’s payload on to the right trajectory.