Let’s say you’re outfitting your spaceship to transport water to a distant mining outpost in the asteroid belt. The outpost has requirements for a fixed mass, X kg, of water. However, volume is at a premium on your craft. What would be the most compact way to transport the water: A) a tank of water at 4 deg. C, where it’s densest, B) separate tanks of liquid oxygen and hydrogen, to be recombined at the outpost, or C) it’s the same either way. Assume the extra volume of the plumbing and structure of two tanks is negligible, just consider the volume of the components.
By my estimation I figure the LOX/LH combination would need about 98% of the volume of liquid water, but I’m not so sure I calculated correctly.
Why carry it inside your craft? As heat is radiated away, the water is going to freeze anyway. Better to put in a sphere, in frozen form, spray over a layer or sealant (to prevent sublimation) and wrap it in foil (to reflect incident sunlight). Wa-la, instant cargo hold. (If you’re not worried about losing a few percent, you don’t even need to insulate it; losses, especially once you get into the outsystem, should be minimal.)
Liquid hydrogen is not very dense (68 kg/m[sup]3[/sup]); it takes more volume to hold just the hydrogen fraction than it does to hold all of the water. (Liquid oxygen is more dense than water, but not by enough to make up for hydrogen’s lightness.) You can do better, volumewise, than LH[sub]2[/sub] by using hydrides; I haven’t found numbers for the hydride density but I suspect that these can be made good enough to beat liquid water. Of course, there are also higher-density allotropes of water ice, if you’re willing to use pressurized containers.
But if you send them separately, you can burn them to generate electricity and then drink the combustion product.
This is an important point. On the Apollo missions, the hydrogen/oxygen fuel cells which were the main source of electrical power generated so much (drinkable) water that it had to be regularly dumped overboard. However, if your spacecraft uses another source of electrical power (solar panels, nuclear) and doesn’t carry hydrogen for fuel, liquid water might be more efficient to take up with you.
Well, duh. I was assuming this is happening in the future. We’re still trying to get back to the moon. A Bussard ramjet is really for interstellar travel anyway. For intrasystem use, the best currently feasible method is probably O-skeptic’s metal hydride idea, or whatever new-fangled fuel cell someone’s thought up recently.
What Omphaloskeptic said. If you take 1kg of water (=1 liter at 4[sup]o[/sup]C) and separate it into 0.11kg of H[sub]2[/sub] and 0.89kg of O[sub]2[/sub], then liquify both, the LH[sub]2[/sub] alone would take up 1.58 liters at 20 K.
By the way, it’s very unlikely for a spaceship capacity to be limited by volume. In space (vacuum), extra volume doesn’t add any drag, and therefore doesn’t increase your fuel requirements.
You might be able to save some volume by carrying high density hydrogen-rich compounds and similar oxygen compounds, but then you really would be increasing the mass overall.
I tried a few back-of-the-envelope calcs with methanol, methane, ammonia or hydrazine burned with hydrogen peroxide, but they all came out with a higher starting volume than the water they produced. Not sure if there’s a combination that’ll work.
What I meant was that mass is the only parameter that matters. You don’t gain anything by packing the same mass into a smaller volume, which is what the OP was asking about.
Actually that’s not strictly true if you’re going to and/or from a planet with a dense atmosphere. Air resistance comes into play there. But even then, water is dense enough that its volume is tiny compared to the rest of the spacecraft needed to carry it.
I was in a savage mood (the noble kind) yesterday. You should have seen what I did to the palm tree outside the building. On the other hand, I now have a rather nice desert island shack in the parking lot.
Wow! interesting - not what I expected. What if you solidified the H[sub]2[/sub]?
Actually, wouldn’t it take only .79 liters at 10K? (assuming ideal gas?)
Assuming something is an ideal gas at 10K is a profoundly bad idea. You’re going to get results that are way, way off. Even something with very small intermolecular forces like He will still have non-ideal characteristics at 10K.
