Replies

The point seems pretty clear to me. On Earth, travel is usually performed by applying a mechanical force against an external mass - car wheels against the road, a propeller against air against air or water, your foot against the ground. All the travelling object needs is an appropriate form of mechanical linkage to push against the external mass.

In space, there's no external mass, so carrying propellant is the only way to get around.

What space travellers want is a way to get around with a solar powered motor.

I wish I found this thread earlier, but I was out to dinner.

Car wheels against the road: all that is happening is mechanical energy is being released in a way that moves the car forward. Wheel to road is just another example of clutch to flywheel or gear to gear, or gear to driveshaft.
The energy comes from chemical potential energy in the liquid hydrocarbon fuel being combusted and converted into heat and gas which pushes the piston.

With your foot and the ground, that is also merely transmission, the energy comes from metabolism and the processing of sugars.

The key point was said in an earlier post referencing Newton's law, "III. For every action there is an equal and opposite reaction." If you were floating free in space and had a bowling ball in your hand, and you threw it away with vigor, the ball would be set in straight line motion away from you, but you would be moving away from the point of release as well, in a straight line and in the opposite direction. You will have propelled yourself. And you would continue on in that line and the ball opposite forever until some force acted to stop you or change your directions.

Now take an amount of gas the total mass of which is equal to the ball, compress it, and release it suddenly while floating in space. You acceleration however brief, a better word would be impulse, would be the same as if you had thrown the ball. A rocket engine produces thrust that is as if you had a limitless supply of those impulses (until the fuel runs out) laid them end to end so there is no beginning or end, just a stream.