Hacker News Reader

Comment by pavel_lishin | Hacker News Reader

Reminds me of some of Isaac Asimov's stories, where all ships are powered by nuclear reactors that flash water into steam for propulsion.

View on HN

Would this increase the humidity of the local biome? Should we avoid using it in deserts?

Filling the tanks with Earth-sourced water would mean Earth bacteria, tardigrades, etc. The bacteria that stuck to the side of the tank, instead of being pulled out with the water-fuel, would reproduce once the tank is refilled from extraterrestrial (sterile) ice & water sources.

Could this be a cheap and dirty way of kickstarting terraforming? By landing and taking off on a planet like Mars with this engine, the steam exhaust would contain some of the bacteria and other denizens of the tank, some of which could be anaerobic. And this mixture could easily be intentional. The engine's steam exhaust could condense into a cloud of bacteria, and even precipitate to the surface.

Or the space ship could just have a sticker on the water-fuel tank door that says: Only use (sterile) space ice. No planetary ice.

> Would this increase the humidity of the local biome? Should we avoid using it in deserts?

Space is pretty big, I don't think anyone's ever considered it a problem there.

And re: reproducing in the tank, the temperatures generated would absolutely sterilize everything that was coming out as propellant. You could shove a zoo in there, and it would come out as fine particulate mist of mostly water and maybe some small burnt carbon particulate matter.

There are probably easier ways of spreading bacteria than via clouds, but it's actually an interesting idea - the bacteria could be used as the condensation locus, and cause rain that brings them to the surface. Just do a pass through the upper atmosphere, farting the whole way...

That's an interesting approach. Where did the water come from? Surely they weren't storing it as H20 on the ship? Seems like you'd only want to keep as much as needed for life support in that state. Stored as separate oxygen and hydrogen?

> 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.

the small vehicles in the expanse are also propelled by steam.

Reply on news.ycombinator.com