How doe's a spacecraft propel itself, when there's nothing in space to push against?

Mass transfer.

The mechanism is one by Newton, not Einstein, and it is Newton's 3rd law.

This mechanism is the whole action-reaction thing and applied to the rocket is simply; as the fuel is burned and exhaust expelled the expulsion of the exhaust mass is what propels the rocket in the opposite direction.

Inside the rocket there is a gas chamber. Gas is burnt inside the chamber.

If there is no outlet, the pressure will be enormous so as to break the chamber and all parts of the chamber will be made to pieces which will fly of in all directions.

However, we are not allowing the chamber to explode. There is an outlet for the gas to come out of the chamber. The enormous pressure of the gas pushes the gas outside the chamber (rocket).

If there WERE NO OPENING, the pressure acts in both the front and back of the rocket equally. The rocket cannot move. (Of course, in all directions the pressure acts equally; but we consider the pressure acting along the direction of the opening alone)

Now since there is an opening at the back, there is an unbalanced force which acts on the rocket in the forward direction alone. Hence the rocket is pushed forward.

In terms of Newton’s third law, we can say that the rocket pushes the gas outside and hence the gas pushes the rocket forward.

A spacecraft propels itself not by "pushing against something" but by creating a force called thrust in the direction that it needs to travel in.

This done by expelling the right amount of gasses (ie: the resulting hot gasses from the combustion of the spacecraft's fuel) in the opposite direction. Newton's 3rd law of gravity states that for every action there is an equal but opposite reaction.

It's the same effect as when you let go of a balloon and the air comes out of it, the action of the air being expelled from the balloon creates a reaction in the balloon travelling in the opposite direction.

This effect does not need anything to "push against".

From Newton's 'Laws of Motion', "To every force there is an equal and opposite reaction".

Stand on solid ground and push against a brick wall. Your shoulders may move back but you feet will remain stationary because of the frictional force.

Repeat the experiment while on ice skates on a skating rink and you will move backwards. The frictional force between your feet and the rink is much smaller than on solid ground.

Explode rocket fuel in a chamber with but one open exit. The force of the exploding gases imparts a forward thrust on the rocket equal to the explosive force produced by the rapid combustion of the fuel. There is no force in front of the rocket to resist its forward motion.

Watch a jet plane and see the vapour trail left behind. The plane flies at high altitude to reduce the frictional force of the air preventing its forward motion.

Finally, push down on a helical spring with your maximum effort.

Hold that position as long as you can then lift your hand. The spring returns to its original length and you will have a mark on your hand, even a bruise. The more force you place on the spring the more resistance it will offer. See first paragraph.

You do not need to push against some thing to move forward. What every body else is confusing with is the frictional force.

Newtons third law of motion said that for every action there is equal and opposite reaction. The force produced by the emitted burned gases off the rocket shell provides it the forward momentum.

How ever there is no need for the rocket to fire on endlesly. The rocket trajecory is chosen out before lanching so that the rocket may use the gravitational pulls of the universal objects like planets and sun's foces to move foward in a tangential trajectory.

if the spacecraft exists, there is something in space to push against: the spacecraft.

rockets are "reaction engines." Ek (kinetic energy) = ((1/2)mv^2), where m= the mass of the ejectorate (products of combustion) and v= the speed at which the ejectorate is expelled. The force pressing against the ejectorate is applied not only to the ejectorate, but also (through the motor) to the spacecraft.

there's some good info at Wikipedia, too; you might want to start with http://en.wikipedia.org/wiki/Specific_impulse or http://en.wikipedia.org/wiki/Tsiolkovsky_rocket_eq... and do further research from there.

Spacecraft also "propel themselves" by partnering with celestial bodies (commonly: "the slingshot effect")

oh my god!The first answer changed the science upside down!!!lol!it is not the einstein theory it was the newtons thrid law that stated "action has a equal and opposite reaction."And coming to your question yes there is nothing in the space to push against,but do you ever thought of satellites revolving round the earth with out any force?Why becoz to escape from the earth`s gravity the object should have an escape velocity to escape from the influence of earth`s gravity.Since the spacecraft will raise above from the ground with a thrust force against the ground and gradually raise to overcome the gravity of earth it will attain the escape velocity and when it comes fully out from the earth boundary since there is no space it could not propel itself but we know it still as the velocity and acceleration with which it overcame the gravity of earth(escape velocity).got it?So due to there is no external force existing in the space and according to the newton`s first law"a body will continously in the state of rest or uniform acceleration when there is no external force".So there is no force on the craft acting to reduce its velocity.Therefore it will continue with the same acceleration in space and it doesn`t need any propelsion.I hope you understood.

There is something. It's the spacecraft itself. When the fuel mixture burns it expands in all directions so it is pushing the spacecraft forward at the same time the gasses are escaping out the rear of the exit nozzle.

It uses small manoevring rockets to move, using Newton's third law(... Not Einstein's) Every acton has an equal and opposite reaction. The rocket blast is directed in a chosen direction. this pushes the spacecraft in the opposite direction, just as the guy above described with the rowing boat and machine gun.

The space craft is a specially designed machine used by astronomers to travel in space.The combustor present in the rear engine combusts the fuel(nascent hydrogen+nascent oxygen) and the hot gases ejecting push the space craft forwards .The reason is that due to the newtons third law of motion:- there is always an equal and opposite reaction to every action.