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Fuel and oxygen

Rockets and landing · level 2, lesson 4 of 6

A question to start

Put a glass jar over a burning candle (with a grown-up). After a little while the flame goes out. Why? And if a fire needs air, how can a rocket engine burn in space?

The idea

A fire needs three things: fuel to burn, oxygen to burn it with, and heat to start it. The candle uses the oxygen in the air in the jar. When the oxygen is used up, the flame goes out.

In space there is no air, so there is no oxygen. A rocket has to carry its own. Most rockets have two big tanks: one of fuel and one of oxygen, kept so cold that it is a liquid (liquid oxygen, at −183 °C). Pumps push both into the engine, where they burn together and make very hot gas that rushes out of the nozzle.

Fuel Burned with Used in
Kerosene (like jet fuel) liquid oxygen many rockets that launch from Earth
Liquid hydrogen liquid oxygen the upper stages of some rockets; very light, very cold
Liquid methane liquid oxygen newer rockets; methane can be made on Mars
Solid fuel (fuel and oxygen-giver mixed) already inside boosters, fireworks, model rockets

Most of a rocket's weight is oxygen! For every kilogram of fuel, a kerosene engine burns more than two kilograms of oxygen.

On Mars. The air of Mars is almost all carbon dioxide, with hardly any oxygen, so you could not light a fire there. But carbon dioxide has oxygen locked in it. A machine called MOXIE, a test carried on the Perseverance rover, split carbon dioxide from the Mars air to make pure oxygen, a little at a time, between 2021 and 2023. Bigger machines like it could one day make oxygen for people to breathe and for rockets to fly home.

Worked examples

1. Why the candle goes out. The jar has only a little air, and the flame uses its oxygen. No oxygen, no fire.

2. A model rocket. A model rocket uses a small solid motor. The fuel and the oxygen-giving chemical are mixed together in it, so it can burn without air, just like a big rocket.

Try it

  1. What three things does a fire need?
  2. Why does a rocket carry oxygen?
  3. What is liquid oxygen?
  4. Could you light a campfire on Mars? Why or why not?
Answers
  1. Fuel, oxygen and heat.
  2. There is no air (and so no oxygen) in space for its fuel to burn with.
  3. Oxygen made so cold that it turns into a liquid, so a lot of it fits in a tank.
  4. No: the air of Mars has almost no oxygen. (A fire would need oxygen brought or made.)

Common mistake

"Rocket fuel burns by itself." Fuel can only burn with oxygen. The oxygen, which a rocket must carry, is just as important as the fuel, and usually heavier.

Where it leads