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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They’re pushing against the rocket. Remember Newton’s 3rd Law of Motion: every action has an equal and opposite reaction. If you throw stuff out the back of the rocket, it pushes the rocket forward. That’s what a rocket does: it pushes some stuff very quickly out the back of the rocket.
Cool concept: Start with a stationary rocket and fire its engine so it’s moving. Then back way up and look at the whole system of the rocket and all of the exhaust that got it moving. You will see that the whole rocket + exhaust system is still holding still.
You would see a (relatively) slow moving and heavy rocket moving away from very fast-moving, lightweight rocket exhaust.
Imagine you have a row boat filled with bowling balls. If you throw a bowling ball out of the backwards out of the boat, Newton’s 3rd law says that there is an equal and opposite reaction which pushes you and the boat forwards.
In a space rocket, the bowling balls are atoms of fuel. You throw the fuel out of the back of the rocket. You get an equal and opposite reaction which pushes the rocket forward.
You know how cannons recoil because they shoot out the cannon ball so fast? Rockets work the same way only with exhaust gas. They react the fuel with stored oxygen and the released energy shoots it out the nozzle so fast that it pushes the rocket forward. If you sit on a wheeled office chair and fire an extinguisher you’ll go shooting backwards in much the same way.
Lets say I have an explosion in the middle of the vacuum of space. Stuff goes in all directions.
Now, lets say I put a shield on one side. Stuff only goes one way, the shield is blown the other way.
That shield is my rocket, but its directing its explosion in one specific direction, the rest of the rocket is being shoved the other way.
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