Chemical rocket engines use a fuel (something to burn) and an oxidiser (something to react with the fuel). Together, they are referred to as the propellant.
As the propellant reacts inside a combustion chamber, the chemical reaction produces hot gases. It is the ejection of these rapidly expanding hot gases at high speed from the rocket nozzle that creates thrust.
The fuel and oxidiser can be stored as solids, liquids or a hybrid (a combination of solid and liquid).
Solid propellant rocket engines
In a solid fuel engine, the fuel and oxidiser are already mixed together and set as a solid inside the combustion chamber. This solid is called the propellant grain.
The rate at which the chemical reaction takes place depends on the type of fuel chosen and the surface area of the exposed grain. The inside length is normally hollow section to increase the amount of grain exposed and available to react. A star-shaped hollow section is often used to maintain a steady burn with even thrust.
The Space Shuttle has two solid rocket boosters (SRBs). These are the two big white rocket sections on the side of the Space Shuttle that produce the visible flames and smoke. The SRBs are the largest solid fuel engines ever used in a launch. Each SRB burns nearly 4,000 kg of propellant each second and ejects the resulting hot gases to produce a thrust of 12.5 mega newtons (MN).
Compare this with much smaller engines for model rockets that can be made to produce as little as 2 newtons (N) of thrust.
Liquid propellant rocket engines
Liquid propellant rocket engines use a liquid fuel (such as liquid hydrogen or kerosene) and liquid oxidiser (such as liquid oxygen). These are stored in separate tanks and then pumped into the combustion chamber as required. As they are sprayed into the combustion chamber through injection nozzles, they rapidly mix together and react before being ejected.
One advantage of a liquid fuel system is that the amount of thrust can be controlled. This is done by limiting how quickly the fuel is pumped into the combustion chamber.
The three main engines on the tail of the Space Shuttle orbiter are liquid fuel rocket engines. The external tank (ET) is the big orange tank and contains two separate storage tanks – one containing liquid hydrogen and one containing liquid oxygen.
The hydrogen and oxygen are pumped to the three main engines. They are sprayed into a combustion chamber where the hydrogen reacts with the oxygen to form gaseous water. It is the high-speed ejection of this gaseous water that produces the thrust.
Each main engine produces a thrust of 1.8 MN (1.8 million N). It does this by reacting 1,340 litres of propellant each second and ejecting the gaseous water at a speed of 3,560 m/s (12,800 km/h).
Hybrid propellant rocket engines
A hybrid propellant system has the fuel as a solid inside the combustion chamber. The liquid oxidiser is stored in a separate tank. The simplest hybrid system is to have the oxidiser under pressure in its tank. When a valve is opened, this oxidiser is released into the combustion chamber. It then reacts with the solid fuel before being ejected.
One example of a hybrid system is the Ātea-1 launched by Rocket Lab.