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Comment by LastMuel | Hacker News Reader

There is an almost unbelievable experiment where a fan is used to blow into a sail attached to a boat and it shows that you can propel a boat using a fan that is mounted on the boat itself. It's a bit mind bending.

When the Oxygen and Hydrogen are burned from this engine, they should recombine into water.

If we fired an engine like this in space toward a "sail", would it be possible to produce a craft long enough and with a system attached to the aft that could re-collect the water for reuse?

Possibly by having stiff wires that use the surface tension of the water to stickily grab on as the craft moves forward?

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> you can propel a boat using a fan that is mounted on the boat itself

You mean like an air boat [1]? Using a sail just makes it less efficient when self powered.

It wouldn't work like that in space because there is no working fluid (air) against which to push - which is what the fan is doing, whether it's using a sail or not. Conservation of momentum would eliminate any propulsion if the propellant is recollected.

[1] https://en.wikipedia.org/wiki/Airboat

Edit: I originally posted that I did not think it was like an "airboat at all". But, I see that they are indeed using an airboat. I am mistaken.

See: https://www.dailymotion.com/video/x8hvhih

The wind from the fan on the boat was just bouncing off the sail. It would be more efficient to just take down the sail, point the fan aft and turn it on.

For propellent-less drives, I would be looking at light-sails or mag-sails, rather than trying to collect and unburn the trillions of water molecules your engine is scattering across the vacuum of space.

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.

But what happens when the boat speed matches the propeller airspeed? Wouldn’t sucking the air from the ground and deflecting to a variable angle allow for higher top speed until you are power-limited (as opposed to gearing limited)?

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