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My spacecraft's propellant is water, so an idea I came up with is that it would melt icy bodies in the rings of gas giants into water vapour. Is it possible to collect water vapour with some sort of Ramjet? Perhaps it could charge the water vapour first?

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    $\begingroup$ This doesn't sound like a Bussard Ramjet. Those guys are powered by hydrogen and use an electromagnetic scoop to gather it. $\endgroup$
    – Daron
    Jun 13, 2022 at 14:25

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Not really

You appear to be mixing up some ideas. The Bussard Ramjet, like all ramjets, collects nothing. Unlike turbine engines, ramjets have no moving parts and use the force of incoming matter (in the case of Bussard, hydrogen) to enhance the explosive force of fuel to create thrust.

The concept of a Bussard collector or scoop is SciFi technobabble for a mechanism used to collect and store fuel. Whatever that fuel may be.

The idea Bussard pitched was the use of enormously powerful magnetic fields to draw hydrogen atoms into the ramjet.

  • Hydrogen atoms are really light and (despite being weakly magnetic due to its tendency to bind to other atoms) is prone to magnetism. In open space, hydrogen can be "easily" (*cough*) found floating around unbound.

  • Water molecules are comparatively heavy and water is almost completely non-magnetic. Curiously, oxygen is more magnetic than hydrogen, but it's heavier and really likes to bond to other atoms.

Having said that, a basic rule of thumb (and I'm making angels weep saying this) is that like bumble bees flying, everything is "magnetic" if the magnet is strong enough.

Your problem is that the energy needed to move water molecules (much less fragments of ice...) is so enormous that the energy you'd get out of the collection system isn't worthwhile.

Should that stop you?

Heck no! If you want to use Bussard's ideas to collect shattered ice for fuel, do so with gusto and ignore anyone and everyone who thinks science fiction should only reflect reality.

But if I happen to have put you off your feed for Bussard...

Consider netting the ice. It's much more energy efficient. The concept of a "monomolecular rope" has been used before in SciFi, so there's nothing stopping you from creating an unbreakable net that's thousands of kilometers on a side. And it solves the problem of having to have some kind of crusher/vaporizer at the inlet of the Bussard collector, which would be a great setup for a space opera/horror movie, but would be inelegant from a purist's point of view. :-)

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  • $\begingroup$ In a water-collecting system, you want the water for its mass, not for its energy content, so the argument that it's not worth the energy requires more explanation (e.g. how just using H2, fusion it, and eject the He as propulsion mass: is that more or less efficient than collecting water?) -- I actually don't know how such calculations pan out, i.e. under what if any circumstances collecting water is the best option. $\endgroup$
    – toolforger
    Jun 13, 2022 at 16:37
  • $\begingroup$ @toolforger I see your point, but I'm still not convinced it's worth the energy cost to use magnetism to collect it (vs using a net to collect chunks of ice). Economics wins out more often than people want to believe vs. physics. Moving something heavy and difficult to magnetize like a water molecule would be, from the perspective of realism, insane. $\endgroup$
    – JBH
    Jun 13, 2022 at 17:08
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Asteroids

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Some asteroids contain ice. It is easier to gather from asteroids than planets, since you don't need to design your spaceship to be capable of entering and then leaving the big gravity well of a planet. You just roll up to the hunk of rock and get to work.

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    $\begingroup$ Water ice likes to hang out with its friends - ammonia, salts, methane clathrates. Maybe even methanol. You might need to distill the H2O unless you are OK with that bunch crashing your propellant party. $\endgroup$
    – Willk
    Jun 13, 2022 at 14:41
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    $\begingroup$ @Willk but can you convert any of that stuff to a cheap, make-you-go-blind bootleg Vodka to offset the cost of distillation? 😜 $\endgroup$
    – JBH
    Jun 13, 2022 at 17:10
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    $\begingroup$ @JBH - that sounds like a Heinleinesque side project. Then there are pirates. Then they get drunk with the pirates. Then later the pirates show up with friends looking to party and wind up saving their bacon. Then they party after! $\endgroup$
    – Willk
    Jun 13, 2022 at 17:39
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Sounds legit, I see no reason why you can harvest from gas giants, the Kuiper Belt and Ort Clouds are other likely areas like harvesting ice from cometary coma.

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