To answer your last point first, in this world, yes, the energy costs are non-trivial, but in Traveller, with casual use of fusion reactors capable of providing enough energy to push ships around at multiple Gs for weeks, I think you can ignore it.

Of course, Traveller has never explained what happens with the waste heat from a reactor like that, unless it is magically 100% efficient at producing useful power, and the systems powered by it are as well. If you work out the energy yield from the power plant fuel for 2 weeks operation in most versions of Traveller, it is *huge*.

Of course, not all of the fuel will be fused, but the amount if fuel used suggests either the bulk of the fuel isn't fused or Traveller ships are casually handling power levels much higher than the rules indicate. The former requires that for some reason it can't simply be recycled until it is fused. Maybe a lot of coolant is needed externally and must be ejected after use? That, though, raises all kinds of questions around the running of a ship's power plant in an atmosphere.

Anyway, to return to the subject at hand, H2 is only liquid at below 33K, regardless of pressure, IIRC O2 can be liquid at reasonable temperatures, but only at insanely high pressures. In neither case would water be liquid.

Cracking cryogenically cooled water ice raises the problem of keeping the hydrogen and oxygen separate, without them just recombining, not to mention how to supply the energy required to crack the water molecules without heating it again.

In short, I think this remains something best done in either the liquid or gaseous phase, yielding gaseous H2 and O2, then cool the results, although you could handwave that there is a way if doing it not available to us today.

Of course, that brings us back to how you deal with the heat extracted from the H2 and O2 in cooling it...

Cheers

Jim

On Sun, 11 Oct 2020 04:25 , <xxxxxx@gmail.com> wrote:

When you try to separate the water into its bits, does it have to be in a particular phase or pressure for that to work?

Can you put the whole operation under high pressure and thus have the outcome be liquid H2 and O2? Or can you run the process on water ice and get O2 and H2 ice or at least liquid?

Just curious. If you have to burn a bunch of energy to compress or cool the byproducts of the separation to a liquid state, that sounds like you'd be burning a bunch of the fuel you are trying to harvest....

TomB

On Sat, Oct 10, 2020 at 10:58 PM James Catchpole - jlcatchpole at googlemail.com (via tml list) <xxxxxx@simplelists.com> wrote:
I agree with all the salient points you make, but I think you have made a mistake in your working out, as 1.56 + 9.78 <> 14...

14 / 9 = 1.56, rounded to 2dp.

14 × 8 / 9 = 12.44, rounded to 2dp.

So, the oxygen is 12.44 tonnes, taking up 0.78 DTons if cryogenically cooled.

I would add that cryogenically cooling the oxygen is trivial if you can cool H2 sufficiently to make a liquid, only needing to be kept below 90K at 1 atm, rather than the 33K maximum for H2, or 20K at 1 atm. Indeed, liquid H2 is cold enough to freeze 02...

On Sun, 11 Oct 2020 01:18 Rupert Boleyn, <xxxxxx@gmail.com> wrote:

Water masses 1 tonne per cubic metre, and thus 14 tonnes per
Displacement Ton. Of this mass 1/9th is hydrogen, 8/9ths are Oxygen.

Liquid Hydrogen masses 1 tonne per Displacement Ton (by definition).
Liquid Oxygen masses 1.14 tonnes per cubic metre, and thus 15 tonnes per
Displacement Ton.

So if you crack one DTon of water (14 tonnes), you get 1.56 tonnes of
hydrogen that takes up 1.56 DTons of volume (when cooled to a liquid),
and 9.78 tonnes of oxygen that takes up 0.65 DTons of volume (when
cooled to a liquid).

The result of all this is that water is a volume efficient way of
carrying hydrogen, compared to carrying it as a pure liquid, but it has
a huge mass penalty. To get 52 tonnes of hydrogen they need 468 tonnes
of water, which takes up 33.4 DTons of space. They'll want tanks for
this, as I doubt a normal freighter's cargo spaces are rigged for 'free'
liquid, and nor will the holds normally have facilities for pumping it
to the refinery. They should be cheaper than fuel tanks, as water isn't
cyrogenic. They'll also want a 20 DTon tank for the oxygen, and that
will need to be a 'proper' cyrogenic one.

Fortunately Traveller ships generally care very little about the mass of
their cargo in most rule sets, only the volume.

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