[LastMuel]

The thought was to re-collect the propellant for reuse in an environment where you cannot easily replenish it, once you have harnessed the energy from burning the components of that propellant.

I don't think that the boat experiment is a good analogue, though, as it seems the engine would be pushing with more force in the backward direction than could be harnessed in the forward direction.

It still seems like there could be a possible way to re-collect the water for further use as propellant in a vacuum once you have harnessed the energy from it's combustion. Essentially using solar to propel your craft and then reclaiming the raw propellant (water) so that it can be separated again.