Question

Here's an proposal on how to get from point A to point B in zero-gravity without using any propellant and the question why it wouldn't work:

A closed tube, filled with water and a round (solid) object. If you need equations, the volume of enclosed water is the same as the round object, but the round object is 10 times lighter. (imagine a glass with water plus a ping-pong ball).

On earth the round object will float on the water inside the tube (subject to one G).

In zero-gravity the round object has no preferred position.

If we accelerate the tube in zero-gravity by one G, the situation is the same as on earth, the round object "floats". In this example we are accelerating the tube from the left side to the right side. The round object will consider floating to the right AS LONG AS THERE IS acceleration.

Now consider adding a pipe to the bottom of the tube and connecting it to the top, a loop. Inside the pipe is a small water pump.

If we give this apparatus a push, say one G in a zero-g environment, the round object will move "up", but now we start the water pump and spray the water on the round object, we try to submerge it. It will resist and impart a impulse on the water. like trying to hose down a air balloon floating on the pool. The pump will feel a resistance and hence the whole apparatus will move.

Just running the pump at constant speed, same volume of water per second, will do nothing. but if we run the pump faster and faster the whole apparatus will start moving:

the amount needs to be geometrical. The point is that we need to keep the apparatus feeling an acceleration, since only then will the round-object "float" and resist the incoming water at the top, hence we have something to "push on".

Before you blow the "foul whistle": consider the situation if there were no round floating ball in the apparatus.

(the pump runs on solar power or pre-charged battery)

(disclaimer: i know standing on a sailboat and blowing into the sail will not get me anywhere, action<->reaction)

thanks a bunch Sklivvz for the edit. sometimes the idea just needs to get out, never the mind how it looks like : P

Answer 1

Consider the following scenario:

• A gun is on the left side of a boat, which is very long.
• The gun shoots, recoil makes the boat moves left.
• The bullet hits the right side, the boat stops.

The end point of the process is that the boat has moved, but is at rest.

Why does this work? It works because there is a time between the shooting and the impact.

Consider the following scenario:

• A gun is on the left side of a boat, which is very long.
• The gun shoots repeatedly with a precise frequency so that at each recoil corresponds an impact at the same time.

The boat will start moving at the first shot (which is unbalanced) and will stop at the last impact (which is also unbalanced).

Why does this work? It works because in the beginning and at the end there is no flying bullet.

Consider the following scenario:

• Instead of a gun there is a pump shooting a continuous stream of water in an empty tube
• Initially the recoil makes the boat move left
• Once the tube is full, the impact of the water on the right side balances the recoil on the left side
• When you turn off the water, there is a final impact period on the right side that makes the boat stops

The boat will start moving as the tube fills, and it will stop moving as the tube empties

Why does this work? It works because there is no water in the tube initially and at the end.

Finally, consider the following scenario:

• Same setup as the previous, with an always full tube (e.g. with a reservoir which is simultaneously filled and emptied)

The boat will not move.