How do spacecrafts propel through space where there is no oxygen for combustion?


How do spacecrafts propel through space where there is no oxygen for combustion?

In: 4

Three pieces:

1. There’s nothing to slow them down, so once they get going they can just coast forever. A spacecraft like Voyager hasn’t had propulsion in decades, but is still rapidly flying away from the solar system.

2. Not all space propulsion systems are based on burning things. Ultimately for a spacecraft propulsion system to work it needs to eject something with momentum. To do that takes energy. Combustion is convenient since you wind up with hot exhaust that has both momentum and energy, so it’s a nicely packaged way of getting lots of thrust in space, but there are other systems that are better in other settings. For example, one may use solar panels to generate electricity, then use that electricity to shoot some Xenon gas at *incredible* velocities. This winds up being a lot more efficient, and oxygen is never needed.

3. The rockets that do use oxygen bring it along with them. It’s effectively just another fuel–you get one tank for oxygen (typically liquefied for the sake of space) and one tank that contains something like Kerosene or liquefied hydrogen.

Some rockets have oxygen inside of them. Others use chemicals that don’t need oxygen to burn. Most of the fires we see/make on the ground or in the air use oxygen, because it’s easy, but it’s possible to make fire without it.

Rockets don’t use atmospheric oxygen, they carry all their propellant with them. This is why multiple stages are used, so you can drop the huge propellant tanks it took to get into orbit, and the extremely heavy engines needed to lift those tanks off the ground.

As for what you’re burning, you might be burning liquid oxygen and either kerosene or liquid hydrogen, but these are usually used early in flight, and discarded by the time the spacecraft starts its actual mission(these are relatively difficult to light, and cryogenic propellants don’t keep forever). For the long term propulsion, you usually use hypergolic bipropellants like hydrazine and N2O4, or in some newer satellites, ion drives that use solar power and noble gases.

Monopropellants like hydrogen peroxide are even simpler, but have less shelf life and performance.

How much you can accelerate depends on the mass of your rocket compared to the mass of your propellant, and the exhaust velocity.

If you have single stage a rocket with an exhaust velocity of 3000m/s , that is 90% propellant by mass, you can accelerate by 6.9 kilometers per second before you run out of fuel. This is called Δv (delta v, or change in velocity).

It takes about 9km/s of Δv to get into orbit.

The oxygen is carried in the fuel. Part of its chemical makeup.

Even at launch, rockets do not use atmospheric oxygen. They have no air intake, nothing is taken into the rocket to aid combustion.

Certain craft use hypergolic fuels, which ignite spontaneously when two chemicals are mixed, and unless I’m mistaken, these are not oxygen-based….

There’s an analogy with explosives, which will detonate on their own, without the need to take in oxygen first. The reaction either uses the oxygen in the chemical makeup, or uses a chemical reaction not based on oxygen.