We use fuels almost every day, but the vast majority are fundamentally very similar. Diesel, petrol and ethanol are all liquids primarily made up of carbon and hydrogen, merely differing in the exact combinations of these atoms.
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The world of rocketry, by comparison, is far more diverse. There are two main families of rocket fuels, which vary in their engine design and chemical makeup. Choosing one over the other can lead to major changes in rocket performance, making this an important decision for all space missions.
All rockets work on a basic principle: ejecting material out one end at high speed will cause the rocket itself to be pushed in the opposite direction. Most systems achieve this through combustion, harnessing similar explosive forces to most common engines.
Solid fuel is the oldest and simplest form of rocket propellant. In the 13th century, Chinese soldiers first used gunpowder to help propel their arrows to greater distances. While ingredients and techniques have been refined, the fundamental process of solid-fuel rocketry remains the same.
Modern solid fuel contains two main components - a fuel, and a source of oxygen. These are mixed, along with other ingredients to hold them together, and placed in a reaction chamber. The resulting compound generally has a rubbery consistency, which means it can be shaped. All that's needed to launch one of these rockets is a spark; once ignited, the fuel will burn until completely used up. This means that solid-fuel reactions cannot be controlled in-flight, but the boosters are simple to build, store and use.
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Liquid-fuel rockets operate more like everyday engines. Oxygen and fuel are kept as liquids in separate tanks. From there, they are directed into a combustion chamber where the reaction occurs, propelling the rocket. Changing the rate at which this happens is relatively easy, and allows the rocket's flight to be controlled and altered in real-time. Liquid fuel also has a higher "specific impulse" than solid fuel; this means that the same mass of fuel will generate more thrust. This is particularly useful for space travel, where every possible kilogram should be saved.
Almost all modern rockets use one or both of these fuels. However, new systems are being invented across the world, including one in Canberra.
The Helicon Double Layer Thruster (HDLT) is an experimental rocket design being developed by Dr Christine Charles and Professor Rod Boswell at the Australian National University. This system can run on a wide range of raw materials, and works by converting these into plasma. Plasma is a state of matter (like solid, liquid and gas) in which electrons are stripped from their atoms, resulting in a kind of "charged gas".
The charged particles are accelerated by electromagnetic forces, and leave the rocket at much higher speeds than other fuel types. As a result, far less fuel is needed to create the same level of thrust. Because of this incredibly high specific impulse, the HDLT will be ideal for long-distance space travel.
None of these fuels is inherently better than the others; each is simply suited to different situations. It's the job of engineers to decide which works best for their particular mission - but that's only rocket science.
- Lachlan Reichstein recently completed a Bachelor of Science in Chemistry and Science Communication at ANU.
