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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Rockets do not rely on pushing against something else in order to move as they use the principle of conservation of momentum to propel themselves forward based on Newton’s third law….every action has an equal and opposite reaction.
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.
They are sending the exhaust gasses out of the nozzle at extremely high speed. You know how if you jump out of a small boat, you push the boat the opposite direction of your jump? The same phenomena is happening here, just in a continuous manner as fuel is combusted and pushed overboard.
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.
It’s “against” the exhaust gasses.
Rockets work by speeding up and exhausting gas. They actually work better in space than in the atmosphere.
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.
The thrusters are pushing against the burnt propellant. Rocket goes one way, hot exhaust gas goes the other, momentum is conserved!
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.
Rockets don’t work by pushing against the world around them, they work by pushing against their own exhaust. The gasses expanding inside the combustion chamber and nozzle are pushing outward in all directions, and since nozzle is open on one end, they can only push against the engine in one direction, pushing it forward.
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