There are two main types of fuel used to get rockets off Earth: solid and liquid. In the United States, NASA and private space agencies use both.
- Solid rockets are simple and reliable, like a Roman candle, and once ignited there’s no stopping them: they burn until they run out, and can’t be throttled to control thrust. Solid fuel is a composite typically consisting of a solid oxidizer (ie. ammonium nitrate, ammonium dinitramide, ammonium perchlorate, potassium nitrate) in a polymer binder (binding agent) mixed with energetic compounds (ie. HMX, RDX), metallic additives (ie. beryllium, aluminium), plasticizers, stabilizers, and burn rate modifiers (ie. copper oxide, iron oxide).
- Liquid rockets provide less raw thrust, but can be controlled, allowing astronauts to regulate the speed of a rocketship, and even close and open the propellant valves to turn the rocket off and on. Examples of liquid fuel include liquid oxygen (LOX); liquid hydrogen; or Dinitrogen tetroxide combined with hydrazine (N2H4), MMH, or UDMH.
Gas propellants are occasionally used in some applications, but they are largely impractical for space travel. Gel propellants have interested some physicists due to their low vapor pressure when compared to liquid propellants. This reduces the risk of explosion. Gel propellants behave like a solid propellant in storage and like a liquid propellant in use.
What Else Do Rockets Need Besides Fuel?
In order to get an object to space, you of course need fuel. You also need oxygen to burn, aerodynamic surfaces and gimbaling engines to steer, and somewhere for the “hot stuff” to come out to provide enough thrust.
Fuel and oxygen are mixed and ignited inside the rocket motor, and then the exploding, burning mixture expands and pours out the back of the rocket to create the thrust needed to propel it forward. As opposed to an airplane engine, which operates within the atmosphere and thus can take in air to combine with fuel for its combustion reaction, a rocket needs to be able to operate in the emptiness of space, where there’s no oxygen. Accordingly, rockets have to carry not just fuel, but also their own oxygen supply. When you look at a rocket on a launch pad, most of what you see is simply the propellant tanks—fuel and oxygen—needed to get to space.
How Has Rocket Fuel Changed Over Time?
There have been few changes in the fundamental chemistry of rocket fuel since the beginning of spaceflight, but there are designs in the works for more fuel-efficient rockets.
In order to improve their efficiency, rockets need to be less fuel-hungry, which means the fuel needs to come out the back as fast as possible to give the desired momentum, and achieve the same thrust. Ionized gas, propelled through a rocket nozzle using a magnetic accelerator, weighs substantially less than traditional rocket fuels. The ionized particles are pushed out the back of the rocket at an incredibly high velocity, which compensates for their small weight, or mass.
Ion propulsion works well for long, sustained propulsion, but because it creates a lower specific impulse, it so far only works on small satellites already in orbit and has not been scaled up for large spaceships. To do this will require a powerful energy source— perhaps nuclear, or something not yet invented.