Accelerating, then decelerating, without expending energy or propellant in space?

If I understand it correctly, moving in the absence of forces like gravity and without friction doesn't require energy, only accelerating and decelerating does. So you could move a spaceship a very long distance with very low energy requirement if you just move it very slowly. Now spending more energy to achieve the same end result just in less time sounds a bit strange to me so I was wondering if there is some theoretical way around that.

For example, on the earth, there are bikes with brakes that wind up springs. After you brake, there is elastic energy in the spring and when you release the spring, it gets converted back to kinetic energy. Brake again and you moved fast and the bike is theoretically (no friction) in the same state as before, just in a different place and because it both accelerated and decelerated, I assume the earth didn't move, even minimally. So theoretically, the bike could do this infinitely.

Now, is something like this theoretically possible in space? For example, could the spaceship collect the propellant in a really long elastic bag which then snaps back and decelerates it again?

P.S.: I'm not a physicist so apologies if I didn't use some concepts correctly. I know energy cannot really be lost, what I mean with "expended" is that the spaceship has less of it.

posted over 7 years ago

Konrad Höffner‭

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