I’ve never really understood the physics of this. Obviously it works somehow — I’m not a moonlanding denier or anything — but my (admittedly primitive) brain continues to insist that a rocket thruster needs something to push *against* in order to work.
So what is it pushing against if space is essentially a void?
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“every action has an equal and opposite reaction” is the rule at play here. Take note that it doesn’t say “in the presence of an atmosphere…”
When you’re standing near a person, and you suddenly push them away, what happens to you? If you are standing with your feet together, you’ll fall over as well. When you shove your friend, your body pushes on their body, but their body also pushes on your body, causing you to fall over as well.
This is exactly what a rocket does. The fuel is built so that it can sustain an ignition reaction even without an atmosphere, and that reaction causes it to expand. Normally it would expand outward, but this is constrained by the rocket nozzle to only expand in one direction. The expansion of the fuel is the action, and the reaction is a change in velocity to the rocket, in the opposite direction. The fuel pushes on the rocket, and the rocket pushes on the fuel.
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