THE ENGINES THAT
CAME IN FROM THE COLD

Rocket Science

The basic principle
Rockets, like all objects, act according to Newton’s laws of motion. Rocket engines rely on the principle that to every action there is an equal and opposite reaction. What they do is push mass extremely quickly out one end, creating a force in the opposite direction. It’s exactly the same principle as blowing up a balloon and releasing it - air rushes out of the back and the balloon shoots forward. With a rocket engine, the escaping mass isn’t air, it’s the fast-moving, high-pressure exhaust gases created by burning rocket fuel in a confined space.

Rocket engines also work in space. This is possible because they push the rocket forward by throwing gas out of the back, not by pushing against the air behind them.

Lift-off
A rocket can only lift off if the reaction force created by the engines is large enough to overcome the pull of gravity on the rocket. So it takes a huge burn of fuel to lift a rocket off the launchpad, creating the enormous amounts of flame and steam which are such a spectacular feature of the launch.

The power of a rocket engine is known as its thrust.

Rocket engines
At their simplest, liquid-propellant rocket engines are just high-pressure blast chambers with liquid fuel and liquid oxygen pumped in at one end, and hot exhaust gases escaping from a nozzle at the other end.

The fuel and oxygen are pumped in at very high pressure, the fuel burns in the confined chamber and creates exhaust gases which are forced at high pressure and speed towards the exhaust nozzle. The cone-shaped nozzle accelerates the gases even further, so that they blast away from the rear of the engine at anything up 16,000 kilometres per hour.

Solid fuel rocket engines are simply cylinders packed with a propellant. The important thing is that the propellant burns rapidly, but does not explode.

The propellant must have a hollow tube drilled down its length. When the rocket is ignited, the fuel burns along the entire length of the tube, and the exhaust gases are forced out of the nozzle at the base of the rocket. The tube gets wider and wider until all the fuel has burned away

Rocket fuels
Most modern rockets use liquid fuels. The simplest fuel of all is liquid hydrogen. Fuels need oxygen to burn, so rockets carry tanks of liquid oxygen.

The hydrogen and oxygen burn to produce water vapour and a great deal of energy. It is this energy that gives the exhaust gases the high speed and high pressure that creates the propulsive force.

Rocket engines can also use more familiar fuels. Robert Goddard used gasoline (petrol) to fuel the first liquid-propellant rockets. The V2 rocket engine created by Werner Von Braun used ethanol (alcohol), and the latest RD180 uses kerosene (aviation fuel). When these fuels burn, they produce carbon dioxide gas as well as water vapour.

The Space Shuttle uses liquid hydrogen for its main engines, but the launch booster uses nearly 500 tonnes of solid propellant - a mixture containing aluminium (the fuel), ammonium perchlorate (the oxidiser) and a binder to hold it all together.

Burn of fuel
The mass of fuel burned per second is only a tiny fraction of the mass of the rocket itself. Because the exhaust gases are blasted out at high speed, they have a very large momentum (momentum equals the mass of something multiplied by its speed in the direction of movement).

Just before lift-off, the rocket has zero momentum. But once it starts moving Newton’s third law means the momentum of the much heavier rocket must be exactly equal and opposite to the momentum of the much lighter exhaust gases.

The rocket moves forward, but moves much more slowly than the exhaust gases blasted backwards from the motors.

Newton’s laws of motion
The first law essentially states that objects don’t move or change direction unless a large enough force is applied. So a rocket will sit on the launch-pad until the engines successfully fire up and apply the force needed for lift-off.

The second law links force with acceleration. By applying a constant force in a particular direction, an object such as a rocket will go faster and faster in that direction. And for a particular rocket, the bigger the force, the greater the acceleration from a standing start on the launchpad.

Newton’s third law explains why rockets work at all. At its simplest, it says ‘action and reaction are equal and opposite’. The action of the rocket engine (which accelerates the exhaust gases) is exactly equal and opposite to the reaction of the exhaust gases (which accelerates the rocket).

Gravity
Gravity is a force measured in units called Newtons. The Earth’s gravity pulls all objects towards it with a force of 9.81 Newtons for every kilogram of the object's mass. To hold a one kilogram object (such as a bag of sugar) at a steady height, an upward force of 9.81 Newtons needs to be applied. A greater force will cause the bag to move upwards.

Gravity on the Moon is only about a sixth of gravity on Earth, which is why astronauts can bound about on the lunar surface. This also explains why a relatively small force is needed to blast the landing craft from the lunar surface.

Thrust
Thrust is a force also measured in Newtons. Ten Newtons of thrust is just enough to lift a kilogram bag of sugar. A rocket engine like the RD180, with a thrust of 3.8 million Newtons, can lift 380 tonnes of rocket and payload off the launchpad.

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