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Rocket Engines

One of the greatest dreams of man is the exploration of the space. But the realisation of this dream is very difficult caused by problems like this:

The vacuum of space
Heat management problems
The difficulty of re-entry
Orbital mechanics
Micrometeorites and space debris
Cosmic and solar radiation
Restroom facilities in a weightless environment
And so on...

But the biggest problem of all is to get a spaceship off the ground. That is where rocket engines come in.

The Basics

Most engines use rotational energy to drive (gasoline engine, electric motor, gas turbines). Rocket engines are fundamentally different. They are reaction engines. The basic principle driving a rocket engine is the famous Newtonian principle: “to every action there is an equal and opposite reaction”. A rocket engine is throwing mass in one direction and benefiting from the reaction that occurs in the other direction as a result.

Let’s look at a few examples to get a better picture of this:

If you shoot a shotgun, you can feel a recoil. The shotgun is shooting a piece of metal in one direction – it’s throwing mass. The recoil you can feel is the reaction of this. If you were wearing roller skates when you shot the gun, then the gun would be acting like a react engine and you would react by rolling in the opposite direction.
When you blow up a balloon and let it go it flies all over the room. You have created a rocket engine. In this case, air molecules are thrown from the inside of the balloon. The balloon reacts in the opposite direction.

A rocket engine is throwing mass in the form of a high-pressure gas. The engine throws the mass of gas out in one direction to get a reaction in the opposite direction. The mass comes from the weight of the fuel that the rocket engine burns. The burning process accelerates the mass of fuel so that it comes out of the rocket nozzle at high speed. The fact that the fuel turns from a solid or liquid into a gas when it burns does not change its mass.

One of the problems rockets have is that the objects that the engine wants to throw actually weigh something, and the rocket has to carry that weight around. So you need more fuel to transport your fuel. That is why you have to have a huge rocket to get a tiny person into space – you have to carry a lot of fuel.

Solid-fuel Rocket Engines

Solid-fuel rocket engines were the first engines created by man. They were invented hundreds of years ago in China.

What you need for a solid-fuel rocket is something that burns very quickly but does not explode. The Solid fuel of rocket engines is like gunpowder, but with a specially mix of nitrate, carbon and sulphur. So it releases its power over a period of time.

The solid fuel is cylindrical, with a tube drilled down the middle. When you light the fuel, it burns along the wall of the tube. The inner channel can also be a star. The idea is to increase the surface area of the channel. This increases the burn area and therefore the thrust. As the fuel burns the shape evens into a circle. This gives the engine high initial thrust and lower thrust in the middle of the flight.

Solid-fuel rocket engines have three important advantages:

Simplicity
Low cost
Safety

They also have two disadvantages:

Thrust cannot be controlled
Once ignited, the engine cannot be stopped or restarted.

The disadvantages mean that solid-fuel rockets are useful for short-lifetime tasks like missiles or booster systems.

Liquid Propellant Rockets

In 1926, Robert Goddard tested the first liquid propellant rocket engine. His engine used gasoline and liquid oxygen.

The basic idea is simple. In most liquid propellant rocket engines, a fuel and an oxidiser are pumped into a combustion chamber. There they burn to create a high-pressure and high-velocity stream of hot gases. These gases flow through a nozzle which accelerates them further.

One of the big problems in a liquid propellant rocket engine is cooling the combustion chamber and nozzle. So the fuel, a cold liquified gas, is first circulated around the super-heated parts to cool them.

Another problem is that the pumps have to generate extremely high pressures to overcome the pressure that the burning fuel creates in the combustion chamber.

Other Possibilities

Rocket engines that don’t need to produce much thrust are very small. For example, some satellites use engines that blow nitrogen gas from a tank through a nozzle. Thrusters like these are used on shuttle’s manoeuvring systems.

New engine designs are trying to find ways to accelerate ions or atomic particles to extremely high speeds to create thrust.