How does propulsion work in a vacuum?

Question

i.e take a rocket in space that is firing its engine, what is the propulsion propelling itself against?

Answer 1

Itself, essentially.

A rocket engine generates a rapidly expanding gas, whether that be a chemical decomposition or a violent combustion reaction. This rapidly expanding gas pushes on everything around it. In the case of a rocket engine it pushes on the sides of the engine, but because it pushes on all sides equally, and the sides push back evenly, there is no net effect. It also pushes on the ends of the engine, but because one end is open the gas can escape, and there is nothing to counter the pushing against the closed end. As a result the expanding gas, being forced out of one end, pushes the other end of the engine and hence moves the rocket.

There is a common misconception that rockets work somehow by pushing against the surrounding air, but this clearly cannot be the case, or else they would not operate in space.

Answer 2

Newtons law states that. For every action there is an equal and opposite reaction.
What this means is that the hot gasses are not pushing against anything, they are simply being thrown out of the back of the jet.
If you were to sit on the ice and throw a heavy object away from you, you would move in the opposite direction to that object, more slowly than the object as you would be heavier than that object.
Better still sit on the ice and point a fire mans hose across the ice, turn the water on and you will move across the ice in the opposite direction at quite a speed. This is a demonstration of how a jet works.
More practically get out your garden hose and turn it on , you will feel it pushing you backwards, this will work in a vacuum, now sit on a trolley, you will move backwards.
Another perfect example of Newtons law is the firing of a gun, have you seen the clip of the man who fires a burst from an AK47 and then falls flat on his back? This is caused by the same principle as the jet engine. In the case of the bullets they have small mass but extremely high velocity. As energy is mass times velocity, if the bullet goes twice as fast you get twice the energy. Exactly the same energy works in the opposite direction to push the gunman backwards, much to his suprise he falls over backwards.
In each of these cases nothing is being pushed against, it is action and reaction.
Did Isaac Newton invent the jet engine?

Answer 3

A rocket does not produce thrust by pushing against the air or anything else. It works by the conservation of momentum. Conservation of momentum says that the total momentum of a system will remain constant unless acted on by an outside force. So, consider the momentum of the rocket at rest as 0. Now, if we cause a small mass of propellant to exit the rocket at high velocity, that propellant has a finite amount of momentum in the direction which it is moving. In order to maintain 0 total momentum, the rocket must move (slowly, due to its greater mass) in the opposite direction.

Answer 4

First imagine a gas being introduced to a vacuum, or near vacuum. The gas would expand until pressure was equalised. Now imagine a spaceship being in the way.

A rocket motor operates by generating a large amount of gas which pushes out in all directions. If this happens between a large mass (a planet) and a smaller mass (a space ship) the smaller mass will move away from the larger mass. If it happens next to a single mass, it will still move away from where the gas is being generated.

(Hey! I'm answering questions on rocket science!)

Answer 5

Every action has an equal and opposite reaction.
So the act of shooting atoms transfers the same energy into moving the spacecraft forward.
It has nothing to do with pushing on anything.

Answer 6

It generates thrust (power) which causes momentum (movement) in the opposite direction.

Answer 7

newtons third law of motion

Answer 8

Well in space it is propelling itself agains the pull of the earth's atmosphere or closest rotating sphere.