If you think of a 'rocket', you might think of a vehicle which flies people to space. But did you know that a rocket is also a type of engine? We use rocket engines to send humans, satellites and robots into space. This lets us explore space and send information around the globe.


Image Source: Giphy

So how does a rocket engine work?

You may have heard a character on TV or in a film say, "Engage thrusters!" Like with many engines, it is something called thrust which drives rockets along. You may have experienced thrust at home. Have you ever turned on a garden hose and felt it push back into your hand? This is an example of thrust. The backwards thrust is a result of the water which is pushed out of the hose. 

Rockets create thrust by burning fuel at high temperatures. This makes a huge explosion. The gas from the explosion escapes from the base of the rocket, down towards the ground. This results in the thrust that propels the rocket in the opposite direction, up to space! It even works in the vacuum of space where there is no air! Isaac Newton's '3rd Law of Motion' describes this process,


For a given force in one direction there is an equal and opposite force in the opposing direction. 


Falcon Heavy Rocket Launch
Image Credit: SpaceX

What kind of fuel does a rocket use?

There are two main types of rocket engines. Some use liquid fuel and others use solid fuel.

Most modern liquid fuel engines use liquid hydrogen as the fuel and liquid oxygen as the oxidizer. When lit, this mixture burns at very high temperatures, up to about 3000 °C. Rocket scientists on Earth can use remote controls to change the amount of thrust this type of engine makes. They can even stop and restart the engine while it is in flight. The big challenge with these engines is that hydrogen and oxygen stay as a liquid at very low temperatures. They must be kept at minus 150 °C and below! It is a huge challenge to keep fuel this cold on a spacecraft. It involves an area of science called 'cryogenics'.

Solid fuel engines carry a 'solid' block or brick of fuel. The fuel is a mix of powdered aluminium and an oxidizer, usually ammonium perchlorate. This type of rocket does not need to keep the fuel in super-cold tanks. The challenge with this fuel is that once it is lit, it must keep on burning until it is all used up. The small rockets fixed to the outside of the main rocket often have solid fuel engines. These are also known as booster rockets. Fireworks and model rockets use solid fuel.


What does a rocket engine contain?

Liquid fuel engines contain pumps which mix the fuel and oxidizer. The fuel burns in the combustion chamber, creating an explosion. The gas from the explosion is forced through a narrow part of the rocket called the throat. This increases the pressure and produces greater thrust. A nozzle directs the exhaust out of the base of the rocket.

Solid fuel rockets contain a solid block of fuel in a combustion chamber. An igniter lights the solid fuel mixed with oxidizer. Like in liquid fuel engines, the burned propellant is forced through a throat in the rocket. A nozzle directs the exhaust out of the base of the rocket.


When were rockets invented?

China was the first country to use rockets in the 1200s. These solid fuel rockets were used as fireworks and as weapons in war. Over the next 700 years, people around the world made powerful solid fuel rockets. They were still mostly used as fireworks or as weapons.

A Russian scientist in the early 1900s came up with the idea of liquid fuel engines. An American scientist was the first person to launch a first liquid fuel rocket in 1926. German scientists built powerful liquid fuel rockets to use as weapons during World War II. In 1957, Russia became the first nation to launch a space probe using a rocket.