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Isn't electrolysis of water less efficient for a spacecraft than pressurized oxygen tanks? The ISS uses electrolysis but to do this you have to send water to space which weighs more than oxygen and only contains only 1 of every 3 parts oxygen. So wouldn't it be better just to send oxygen up, and obviously mix it with other gases?

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    $\begingroup$ The atomic weight of oxygen is 15.9994 +/- 0.0004 (essentially 16). The atomic weight of (single neutron) hydrogen is 1.00794 +/- 0.00001 (essentially 1). So the ratio of mass in water is around 8 oxygen per 1 hydrogen. Also, water does not require a pressure vessel, and solar panels in space effectively have 24 x 7 sunshine. $\endgroup$ – Andrew Thompson Dec 6 '16 at 23:59
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    $\begingroup$ Solar panels in LEO are shaded by the Earth for almost 12 hours a day. $\endgroup$ – Russell Borogove Dec 7 '16 at 5:09
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    $\begingroup$ @RussellBorogove this is actually a bit more complicated due to solar precession $\endgroup$ – Antzi Dec 7 '16 at 5:35
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    $\begingroup$ Fair, depends a lot on the inclination. $\endgroup$ – Russell Borogove Dec 7 '16 at 5:40
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    $\begingroup$ AFAIK, on ISS they electrolyse water that was recovered from air moisture and urine. (and which, after filtration is drinkable, but nobody really likes the idea of drinking it.) $\endgroup$ – SF. Jun 23 '17 at 9:11
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As Andrew Thompson's comment hints, it's a matter of tankage mass vs the trifling extra mass of hydrogen. Tankage is no small thing; it's the main reason hydrolox ($H_2 / O_2$) rockets, despite their unparalleled* $I_{SP}$, don't have much better performance than other liquid fuels with considerably lower specific impulse but much better density impulse — the hydrogen requires much larger tanks. Carrying oxygen efficiently in liquid form requires a pressure vessel and cryogenic handling (or else a much larger pressure vessel for compressed gas), and has extra safety requirements, since pure oxygen is a severe fire danger.

In particular, a large tank I found for sale contains up to 250 kg of lox at 2.3 MPa, and the tank itself is 151 kg, which is a really unpleasant 60% overhead (although this tank is not, of course, optimized for aerospace work). Compared to water, which can be contained in a much lighter tank and with an overhead of 11.9% hydrogen, you can see that it's a significant penalty, even without the additional handling hassles and safety problems.

*Well, unparalleled among practical, vaguely safe chemical fuels, anyway. $H_2/F_2$, $H_2/Li/F_2$, and other weirder combinations have even higher $I_{SP}$, but require enormously complex and failure-prone tanks and engines.

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    $\begingroup$ "..and cryogenic handling.." I knew there was something I was forgetting! $\endgroup$ – Andrew Thompson Dec 7 '16 at 4:42
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    $\begingroup$ One way of looking at this is that water is much better at compressing oxygen to high density than pressurized steel tanks are. $\endgroup$ – Crashworks Dec 7 '16 at 9:34
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    $\begingroup$ @Crashworks I would prefer to say '"hydrogen bonds" compress oxygen better'. $\endgroup$ – Aron Dec 7 '16 at 10:07
  • $\begingroup$ Remember, also, that you can get water from a dehumidifier. $\endgroup$ – Loren Pechtel Dec 7 '16 at 18:36
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The ISS does receive regular shipments of oxygen as a gas in specially built, very high pressure (about 6000 psi or 400 atmospheres), scary looking tanks, as pointed out in this interesting answer. They are called NORS (Nitrogen/Oxygen Recharge System). You can read more about them here. They require careful handling and high pressure feed connections. A tank of water is a lot easier to handle, and you can easily transfer water from one tank to another using low pressure tubes and a simple pump.

Besides the very high pressure, handling oxygen and even transfer in high pressure tubes is potentially dangerous because a leak can lead to fires.

This pressurized supply of oxygen is used to re-pressurize oxygen tanks for space suits, and in an emergency can be an immediate source of oxygen to (re)pressurize the station. But for a steady, lower rate supply of oxygen, electrolysis of plain old water works quite well, considering there is plenty of electrical power.

One important caveat - it is very important to vent the hydrogen to space, since it is dangerously flammable and explosive in some cases.

enter image description here

above: NASA NORS tank, photo from here.

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  • $\begingroup$ I've asked this question about the "suit top-ups". $\endgroup$ – uhoh Dec 7 '16 at 3:01
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Some numbers about density, liquid oxygen has 1.141 kg/l, water is 1.00 kg/l and 88.8 % of its mass is oxygen and compressed oxygen at 300 bar has a density of only 0.406 kg/l. I had no numbers for oxygen at 400 bar, but the density should be lower than 0.53 kg/l due to Van der Waals forces. But compressed oxygen requires a very heavy container and liquid oxygen a very good isolated container. The mass of a container for water is much lower, the total overhead for container and the mass of the hydrogen in the water is about 15 to 20 %. Thus water is the most effective way to transport oxygen to the ISS.

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