How rockets work, from Newton's third law to reaching orbit
On this page
Let go of a balloon you have blown up but not tied, and it shoots across the room. Air rushes out of the neck one way; the balloon goes the other. A rocket works on exactly the same principle, only with burning fuel instead of air, and with enough force to lift thousands of tonnes.
Newton's third law
Newton's third law says that when object A pushes on object B, B pushes back on A with a force of the same size in the opposite direction.
Inside a rocket engine, fuel burns in a chamber and produces a large volume of very hot gas. The engine forces that gas out of the nozzle at high speed. The engine pushes the gas downwards, so the gas pushes the engine, and the rocket bolted to it, upwards. That upward force is thrust.
A rocket does not need air to push against. It pushes against its own exhaust, which is why it works perfectly well in the vacuum of space.
Why rockets carry their own oxygen
Burning needs oxygen. A jet engine takes oxygen from the air it sucks in, but a rocket has to keep working where there is almost no air at all, so it carries its own supply. The substance that provides it is the oxidiser. Some common pairings:
- liquid hydrogen with liquid oxygen
- a refined kerosene, similar to jet fuel, with liquid oxygen
- solid propellant, a rubbery mix of fuel and oxidiser cast inside a casing, often used in boosters strapped to the sides
Liquid oxygen must be kept at about −183 °C. That is why you see frost on the tanks and clouds of vapour around a rocket on the pad.
Forces at lift-off
Two forces matter at the moment of launch: weight pulling down and thrust pushing up. The rocket rises only if thrust is greater than weight.
Worked example
A rocket has a mass of 500,000 kg and its engines give a thrust of 7,000,000 N. Take gravitational field strength as 10 N/kg.
- Weight = mass × gravitational field strength = 500,000 × 10 = 5,000,000 N.
- Resultant force = thrust − weight = 7,000,000 − 5,000,000 = 2,000,000 N upwards.
- Acceleration = resultant force ÷ mass = 2,000,000 ÷ 500,000 = 4 m/s².
Now follow what happens next. The engines burn hundreds of kilograms of propellant every second, so the mass keeps falling. If thrust stays about the same, the resultant force grows and so does the acceleration. That is why a launch looks slow for the first few seconds and then speeds up dramatically. (Air resistance acts once the rocket is moving; it is left out here to keep the numbers simple.)
Try one yourself. A model rocket of mass 0.5 kg produces 8 N of thrust. With 10 N/kg, its weight is 5 N, so the resultant force is 8 − 5 = 3 N upwards and its acceleration is 3 ÷ 0.5 = 6 m/s².
Stages: throwing away the empty parts
At launch, most of a rocket's mass is propellant. Once a tank is empty it is dead weight, and so are the engines that drained it. Large rockets are therefore built in stages stacked on top of each other. When the bottom stage runs dry it drops off, and the next stage's engines fire. The rocket no longer has to accelerate the empty tanks and heavy engines below.
The Saturn V that sent Apollo 11 to the Moon had three stages, and of a rocket about 110 metres tall, only the small command capsule came home. Our guide to Apollo 11 follows each piece.
Getting into orbit
A spacecraft in orbit has not escaped gravity. It is moving sideways so fast that, as it falls towards the Earth, the curved surface drops away beneath it at the same rate. It falls continuously but never gets any closer.
Newton pictured this with a cannon on a very tall mountain. A slow cannonball lands nearby; a faster one lands further away. Fire it fast enough and the ground curves away as quickly as the ball falls, so it goes all the way round.
For a low orbit about 400 km up, the speed needed is about 7.7 km per second, roughly 28,000 km/h. So rockets climb vertically only briefly. They soon tilt over and spend most of their effort building up sideways speed. Reaching low Earth orbit usually takes around eight to ten minutes.
Most launches head east, the direction the Earth spins. At the Equator the ground is already moving at about 1,670 km/h, a free boost that shrinks towards the poles. This is one reason Europe launches from Kourou in French Guiana, close to the Equator. Sites are also usually on a coast, so that rockets climb over sea rather than over towns.
To leave the Earth altogether, without further push, a spacecraft would need about 11.2 km per second, the escape velocity.
Timeline
| Date | Event |
|---|---|
| By the 1200s | Gunpowder rockets in use in China |
| 1903 | Konstantin Tsiolkovsky in Russia publishes the mathematics of rockets in space |
| 1926 | Robert Goddard launches the first liquid-fuelled rocket, in Massachusetts, USA |
| 1957 | The Soviet Union launches Sputnik 1, the first artificial satellite |
| 1961 | Yuri Gagarin becomes the first person in space |
| 1969 | A Saturn V launches Apollo 11 to the Moon |
| 2015 | A Falcon 9 first stage lands upright after launching a payload to orbit |
Recovering and reusing first stages, instead of letting them fall into the sea, has made launches cheaper. Some of the rockets that now carry crews and cargo to the International Space Station are reusable.
Four misconceptions
- "Rockets push against the air." They push against their own exhaust and work better in a vacuum.
- "There is no gravity in orbit." Gravity is what keeps a spacecraft in orbit. Astronauts float because they are falling along with their spacecraft.
- "Thrust is the resultant force." You must subtract weight (and drag, once moving).
- "Big rockets steer with fins." Large rockets mostly steer by swivelling their engines slightly, which tilts the direction of thrust.
For more practice on forces and motion, the physics topics on ModernScholar cover resultant force, acceleration and Newton's laws.
Activity: balloon rocket
You need: about 4 m of string, a drinking straw, sticky tape, a long balloon and a tape measure. Ask an adult to help tie the string tightly between two chairs.
- Thread the straw onto the string before tying the second end.
- Blow up the balloon but do not tie it. Pinch the neck closed.
- Tape the balloon under the straw, neck pointing towards one chair.
- Let go and measure how far it travels.
- Repeat three times with a full balloon and three times with a half-full one, and find the mean distance for each.
Write one sentence using the words thrust and third law to explain what you saw. Then predict what will happen if you tape a paper clip to the balloon to add mass, and test it.
Sources and further reading
- Beginner's Guide to Rockets – NASA Glenn Research Center – clear explanations of thrust, weight and rocket flight, written for students.
- Space Transportation – European Space Agency – ESA's launchers, including Ariane, and launches from Kourou.
