Originally Posted By: paul

but the forces that the air will impart to the pipe during the settling process will not all be in the direction
that would slow the pipe.

Sure. But on average they will be in the same direction. It's easy to imagine a random cloud of gas that just disperses in all directions the same. But this isn't like that, the entire cloud of gas starts off moving in one specific direction. It doesn't just forget that it was moving backwards rather than forwards.

using pvc pipe and a couple of valves compressed air and water.

and a swimming pool in place of outer space.

Excellent idea, and extremely practical and doable. Go for it. You could use an ordinary PVC toilet pipe as the main pipe and a coke bottle water rocket as the tank. Then the valve can just be a pull-string operated flap in the cap of the coke bottle.

If it doesn't work in a pool, put the same device on a wheels or ice-cubes.