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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Imagine that rocket fuel and its oxidizing agent, when combined and ignited, is like trillions of microscopic baseball pitchers all throwing baseballs in every direction.
Now you put these into a shaped nozzle so that all the baseballs exit the nozzle in roughly the same direction.
When a tiny pitcher throws a baseball, the ball moves away at high speed, but due to Newton’s laws of equal and opposite reaction, the pitcher is pushed slightly in the opposite direction.
So trillions of pitchers throwing their baseballs results in a lot of baseballs being ejected at very high speed out of the end of the nozzle, and the equal and opposite force pushes the pitchers the other way, against the nozzle and the rocket it is attached to, pushing the rocket in the opposite direction of the baseballs. The baseballs don’t themselves have to push against anything. It was the force of throwing the baseball out the back of the nozzle that caused the pitcher to be pushed in the opposite direction.
This keeps happening until all the baseballs are used up (solid fuel rocket) or until the rocket control system stops adding pitchers and baseballs into the nozzle (liquid fuel rocket).
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