Originally Posted By: paul
why do you need to throw something outside the ship?
why not just throw it out inside the ship.

the same reaction would occur.

and you still have your propellant.

Doesn't work that way - material thrown within the ship will encounter resistance with the air, hull, etc. This will generate a force equal to the force of the propellant, thus neutralizing the thrust of the propellant. Its the ol' opposite and equal reaction thingie - the movement of the propellant will "push" on the ship, but the interaction of the propellant with the ship will push back equally. Net effect - zero thrust.

These kinds of internal energy transfer system only work in places where there is friction to counter the unwanted "return" energy of the propellant. Basically, you can thrust in one direction using a lot of force quickly, pushing the object forward. You then recover your propellant slowly, so the force of the propellant moving in the "wrong" direction doesn't exceed the static friction holding you in place.

That doesn't work in space - no friction.

Originally Posted By: paul
you would only need a compressor and a nozzle that vectors
inside a compartment that has a lower pressure than the compressed propellant.

Doesn't work that way, for the reasons mentioned above. In space, the net thrust of this kind of system is zero. And even in places where it is possible, the ISP would suck.

Originally Posted By: paul
and you could use nitrogen , its safer you could actually breath it and its innert. so non flamable.

The propellant used is dictated by the engine driving the spacecraft. Nobel gases are preferred for ion-based engines, due to the relative ease of ionizing them and the lack of intermolecular bonds. If using chemical engines, you need something that burns. If using nuclear, you need a fissile/fussile fuel, etc, etc, etc.