> Surely they weren't storing it as H20 on the ship?

Why not? It's not as if water is flammable, toxic, or highly corrosive - attributes most other fuel sources do have at least one of.

You have to have water storage if you've got people on board. I'd rather have one system that stores both fuel and life-support than have to have two separate systems and then guess exactly what proportions of both I'll need...

Water storage could also be used as a way to mitigate the radiation problem in space. If you had a 'sleeve' inside the hull that contained water, you'd have both a large storage capacity for fuel as well as inbuilt radiation shielding?

(*Note, I am not an engineer)

I have a friend studying radiology with the hopes of working in the space industry. From what he tells me, I believe this would work.

Why would you want to store your propellant water separately as O2 and H rather than as water? You'd have to react it to turn it into water, water is pretty inert, there's no mass advantage (more of a problem than volume). What am I missing?

I was thinking of volume, but if the craft doesn't need to ever be in an atmosphere then that wouldn't be an issue as I had first thought.

A bigger volume might even be an advantage - easier to turn your ship if you can mount engines far from the center of mass. Plus, built in shielding.

> Where did the water come from?

Good question! His story "The Martian Way" addresses this. I don't really want to spoil it from you, but basically: where you'd expect.

> Surely they weren't storing it as H20 on the ship?

Why not? It's stable, it's useful for other reasons, and when you've got free energy and easy propulsion, there's no reason not to. (There probably would be some issues with sloshing, actually, but he didn't get into it. Asimov wrote a lot of stories that could be considered Hard Science, but I don't think he'd consider himself an engineer.)

Fair enough, it is just fiction after all. I was thinking that storing it as H20 would be a challenge as storing all the water you might need would take a large amount of space. But I suppose if it was for a craft that would never enter an atmosphere the volume wouldn't really be an issue.

Without revealing too much (and possibly this qualifies as spoilers anyway), in Seveneves (Stephenson, not Asimov), it is a comet that, in a sense, 'becomes' the ship.