Space travel and exploration is, and has been for the five decades, one of the hottest scientific topics on the planet. We live in an age where we have the tools to physically explore regions beyond Earth; tools that require huge skill to engineer. One such feat of engineering are the ones that simply give a spacecraft enough force to escape the Earth’s gravitational pull: Rockets.
So, how do they work?
We’ll start with the main feature that rockets provide for a spacecraft..
You may have heard in a sci-fi film “Engage thrusters!” or “Thrusters at maximum!”. This is one and the same with thrust in real spacecraft; the force which propels any aircraft through the air. There are many different kinds of propulsion systems which are used to exert this force – you may have encountered one yourself whilst turning on the garden hose too quickly and feeling it fire back into your hand – that is an example of water propulsion, where water was the ‘fuel’. Rockets operate in a similar way, it is a simple effect of Newton’s 3rd Law: For a given force applied in one direction there is an equal and opposite force applied in the opposing direction.
But what is the most effective form of fuel? Some fuel when burned undergoes more of a chemical reaction than others (flammability) due to the nature of the material and the elements which are its constituents. So to get the most out of rocket fuel (because rockets need a lot of it!) what kind do we need? To answer these questions, we need to have a look first at two main categories of rocket engines: liquid rockets and solid rockets.
Liquid Rocket Engines
Liquid rocket engines are the main part of a space shuttle’s source of thrust. For the liquid rocket engine to produce thrust, fuel and oxidizer (which are stored in separate chambers) undergo a chemical reaction when they are pumped into a combustion chamber. The combustion chamber essentially mixes the fuel oxidizer together and burns it, creating a huge explosion which is then controlled by the exhaust; the resultant explosion builds an extremely high pressure in the throat of the exhaust and this pressure force is then directed using the nozzle. Much like how water at high pressure races out of a hose pipe and sends it backwards in your hand, the rocket expels the high pressure explosion out of the exhaust and this propels the craft upwards.
The fuel used in liquid rocket engines is liquid hydrogen – the lightest element and burns very intensely (~3,000°C) and efficiently when combined with an oxidizer – in this case liquid oxygen. The use of this fuel is extremely efficient in terms of the amount of fuel to the amount of thrust generated from the chemical reaction when mixed and burned. The only problem, however, is that both have to be kept at cryogenic temperatures (~150°C and below) to keep them in liquid form….which in itself is an enormous challenge aboard a spacecraft.
Solid Rocket Engines
In terms of spacecraft, solid rocket engines are usually the smaller rockets fixed onto the outside of the main liquid rocket engine/s (often called boosters). Solid rocket engines, unlike liquid rocket engines, do not keep their fuel and oxidizer in separate chambers but have a ‘solid’ block of flammable fuel. The disadvantage of this is that once the fuel has been ignited all of it has to be used – it cannot be moderated like with liquid rocket engines. This is why several smaller solid rockets are placed on the outside of a main liquid rocket engine, so that they provide a strong, initial burst of thrust to produce a lot of upwards acceleration to escape Earth’s gravitational pull. When the solid fuel has been depleted and the rockets are no longer active they are released from the main rocket (one stage) to decrease the amount of weight of the whole spacecraft. With solid rocket engines the fuel is a mixture of powdered aluminium and ammonium perchlorate as the oxidiser - these do not have to kept at cryogenic temperatures.
The most powerful rockets ever made are still those which were used in the 1960s and 1970s to get astronauts to the Moon, but more are being developed now which aim to lower the cost of space travel, and eventually get people to Mars.
Test your knowledge with our interactive Rockets Quiz!