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Visiting Canberra? Want to learn more about rockets? Join rocket scientist Dr Yuri Blashtov of the Kazakhstan Institute for Space Research (KISR) as he acknowledges Australian achievements in space and accepts the Questacon challenge to launch 10 rockets during his public performance. More...
As long ago as 1200 AD, firework-style rockets of bamboo and gunpowder had been invented by the Chinese and used in battle. Space rockets are a more modern adaptation of the same technology, and were first suggested in the late 1800s by Russian Konstantin Tsiolkovsky.
A single push can launch an object into the air as a projectile. Rockets are self propelled as they need to be accelerated by a continual force or thrust. Space rockets all work on the same principle, which was first noted by Isaac Newton around 300 years ago: If there is a force pushing in one direction, there is an equal (reaction) force pushing in the opposite direction.
Space rockets carry their own fuel and oxidiser. They burn fuel and produce large amounts of exhaust gases which escape at high speed out of the bottom of the rocket. The reaction force pushes the rocket in the opposite direction. Some burning fuel escapes with the exhaust gases and this explains the flame. Rockets do not move by ‘pushing’ against the air – this is why they can fly in space where there is no air.
Space Fuel
The payload – the cargo to be taken into space – is only a small part of a rocket. Nearly all of the mass of a space rocket is made up of containers full of fuel and oxidiser needed to provide the thrust for launch and control. Exhaust gases from the burning fuel escape at the base of the rocket and the rocket is pushed upwards.
Liquid fuel rockets store fuel and oxidiser in separate tanks and pump them into a combustion chamber where they mix together and explode. The liquids are often liquid hydrogen and liquid oxygen which burn to produce water vapour. Liquid fuel rockets tend to be heavy and complex because of the pumps, but fuel flow can be controlled.
Commercial rockets often have solid and liquid fuel systems. The shuttle launcher has two big solid rocket boosters as well as a large external fuel tank containing liquid fuel for the shuttle’s own engines.
Orbit and Beyond
A force of 9.81 newtons is needed to lift each kilogram of the rocket against Earth’s gravity. Additional force is needed to accelerate the rocket to a speed that is suitable for the purpose of the rocket! That’s about 8 km a second to go into orbit, or more than 11 km a second to escape Earth’s gravity and not be pulled into Earth’s orbit! Once that speed has been reached there is no need for any more thrust as the rocket will keep on doing what it’s doing – going the same way at the same speed and in the same direction until another force acts on it. (That’s Newton’s first law.) After this, force is needed to change direction or slow down the rocket.
Acceleration depends on the amount of force and the amount of mass. More force, more acceleration, more mass, less acceleration – that’s Newton’s Second Law. This also explains why the rockets seem to lift off really slowly – they are so very heavy – about 2 million kilograms all up for a shuttle launch; mainly fuel and fuel containers! As fuel gets used up the rockets get lighter, and the thrust then accelerates them faster and faster.