Three, two, one, liftoff! A spacecraft blasts from the launch pad, propelled by the massive thrust generated by its rockets. In a sense, each engine is exploding. Igniting and controlling these immense explosions is the key to space travel, but how is this achieved?
Before we get into the mechanics of individual types of rocket engines, we need to familiarise ourselves with the basic physics involved. The principle is very simple indeed. It’s not rocket science (except, of course, it is).
In an internal combustion engine – the type used in cars – gas is ignited and expands in a chamber, forcing a piston down. This then turns a crankshaft leading to the wheels. The explosive energy contained within the burning fuel is thus converted to mechanical energy.
In a rocket engine, the principle is similar – except for one key difference. Instead of the gases hitting a piston, they are forced directly out of a nozzle.
To create the explosion, you need two key elements: a fuel source and an oxidising agent. The fuel stores the chemical energy, released as it burns when ignited. But to help it ignite, an oxidiser such as oxygen, hydrogen peroxide or halogen is needed. These oxidisers effectively steal one or more electrons from the atoms comprising the fuel, a tiny but violent act that triggers an explosion.
In a rocket, the oxidiser and fuel are introduced to each other in a combustion chamber. Their explosive meeting creates huge amounts of high-temperature exhaust gas, which is under immense pressure.
These gases are then forced out of the nozzle at the base of the engine, generating thrust. The result is a perfect example of Newton’s Third Law: for every action, there is an equal and opposite reaction. As the exhaust gases are forced down, the engine and the spacecraft to which it is bolted are forced up. The narrower the nozzle, and the more pressure inside the chamber, the greater the thrust.
There are two main types of rocket used today.
If you’ve ever lit a firework or a hobby rocket, then you’ve seen the power of solid fuel rocketry. These are the simplest illustrations of the principles of rocket power and work on the chemical energy stored in solid propellants.
The most powerful solid rocket engines used to date were the ones strapped to the side of NASA’s space shuttles, known as solid rocket boosters. Generating about 11.6 million Newtons each, they helped propel the shuttle to a sufficient speed for its liquid fuel engines to kick in and finish the journey into orbit.
The fuel source for each solid rocket booster was a 450,000-kilogram solid core of aluminium powder – a form not dissimilar to the foil wrap in your kitchen. The oxidiser was oxygen derived from ammonium perchlorate, combined with iron oxide that served as a catalyst.
A hole was drilled all the way through the aluminium core. At ignition, the inside wall of this hole burned strongly, generating heat and pressure and therefore thrust. The hollowed-out section wasn’t actually a simple circle, but a 10-pointed star. This increased the surface area and again pumped up the Newtons.
Once all the solid fuel had burned away, the solid rocket boosters had done their job and were jettisoned from the shuttle, parachuting back to earth, where they were collected and prepped for reuse. The time from ignition to separation was just two minutes. In that time the craft had travelled 45 kilometres above the ground.
Liquid fuel engines are far more complicated and difficult to produce compared to their solid fuel counterparts.
Their fuels are stored in the form of liquid gas – for instance, liquid hydrogen, with liquid oxygen as the oxidiser – that require specialised cryogenic tanks to keep them at super low temperatures.
Liquid oxygen must be kept below -183 ºC, while the hydrogen needs to be colder still, at -252 ºC. This means that the fuels must be handled carefully and that the containment must be able to withstand pressure extremes.
There are four main parts of a liquid fuel rocket engine:
• The fuel tank, where the propellants such as liquid hydrogen, oxygen or kerosene are stored.
• The turbopump, which works in a similar way to turbochargers in a car. It forces fuel at high pressure towards the combustion chamber, to prevent the pressure inside the chamber forcing it back the way it came.
• Controlling valves, which regulate the flow of fuel into the chamber, thereby increasing or decreasing thrust.
• And the chamber itself, where the fuel and oxidiser are mixed, ignited and the resulting exhaust directed out of the nozzle.
Liquid fuel engines have distinct advantages over their solid counterparts. Not only can the thrust they produce be controlled, but they can also be shut down and restarted as needed.
This makes them ideal for spacecraft that have to make multiple burns throughout their flights, to correct course or to enter or leave orbit.
But this flexibility comes at a price. Liquid fuelled engines are significantly more complex and expensive to produce than their solid fuel counterparts. They also need a lot of fuel.
You can see this just by looking at the size of the orange liquid fuel tank on a space shuttle.
Fully stocked, the standard liquid fuel tank for the space shuttle missions weighed 760,000 kilograms. That’s a big tank, but rocket engines are very, very thirsty.
After the solid fuel engines had been emptied and jettisoned, they drained in just six minutes, the thrust pushing the craft from 4,828 to 27,358 kilometres per hour in the process.
Unlike the solid fuel tanks, the giant aluminium-copper alloy liquid tanks were never reused – mainly because they routinely smashed to pieces when they landed in the ocean.
Related reading: Reusable rockets explained