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how do rockets work?


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Rockets work by accelerating gas to very high speeds inside and then letting the gas escape from the back of the rocket.



Rockets work by accelerating gas to very high speeds inside and then letting the gas escape from the back of the rocket.


But how much gas must escape and how fast does it have to move to make a working rocket?

Sir Isaac Newton discovered that an object can only be accelerated if a force is applied to it. He came up with a simple equation to describe this:


Force = mass x acceleration

We need to know one more piece of 'rocket science' to make a rocket work. Newton described that too. He realised that if an object (let's call it A) applies a force to another object (called B), then object (B) automatically applies the same force to object (A).

Even though it's true, it's often difficult to accept because, in our everyday lives, other effects (like friction) usually complicate things. It's only at times when we can get rid of most friction (for example by skating on an ice rink) that we perhaps start to appreciate what a smart guy Newton was!

Every force has an equal and opposite force

What's that got to do with rockets? In the case of rockets, Newton is saying that


force on the gas


force on the rocket

(mass x acceleration) of gas


(mass x acceleration) of rocket


Ready to launch a rocket? Let's try it out with a few numbers.


Let's pretend we have a rocket with a mass of 1000 kg and that its engine can accelerate the exhaust gas at 500 m/s2. Sir Isaac Newton's equation tells us that


(mass x 500 m/s2) of gas = (1000 kg x acceleration) of rocket


How quickly will the rocket accelerate?
We don't know until we know how much gas (mass of gas) is being pushed out in the exhaust each second. Here's a rollover table of values which shows how the acceleration of the rocket changes as more and more gas is accelerated out into the exhaust.


How does the upward acceleration differ depending on the amount of exhaust gas pushed out?


Rollover the amounts and compare the results.

Image of a rocket
Compare the upward acceleration of the rocket to the mass of gas in it's exhaust

The big problem with getting rockets from Earth into space is GRAVITY. Gravity near the Earth's surface produces a force which accelerates everything downwards at a rate of 10 m/s2. From the rollover can you see how much fuel will have to be used each second in order to beat gravity and so make the rocket to begin to move upwards? [Rollover here]

Image of how a normal rocket compares to a multi-stage rocket

Goddard was also the first person to suggest the interesting idea of multi-stage rockets. If all the rocket fuel is held in one big container, that container has to be carried very high into space. However what if the rocket were made of smaller compartments fitted together? Once the fuel in the first compartment is finished, that compartment can be thrown away. The remaining rocket doesn't weigh so much and so less fuel is needed for it to carry on its journey. Modern rockets generally have 2 or 3 stages (as the containers are called) to make use of this idea.


Find out more about the rocket pioneer Robert Goddard, the first person to make a rocket engine that used liquid fuel rather than explosives.

How to build a rocketship

Find out more about satellites and rockets in:


What do satellites do?


Newton and gravity


Orbiting the Earth


Goddard: rocket pioneer


Race into space


Getting into orbit


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