On 11Oct2020 0815, Timothy Collinson - timothy.collinson at port.ac.uk
(via tml list) wrote:

> So they've come up with a scheme to fill the cargo hold with water.  > Remember they already have 50 tons of collapsible fuel tanks in the
 > 205 ton hold so they can make two Jump 1 jumps. Now with a cargo of >
water as well, they can refine that on the second Jump to do a > *third*
immediate Jump. > After (some considerable) calculations by the engineer
and assistant, > they decided that 99ish tons of water would be
sufficient for 52 tons > of fuel. (I have to trust them on that as I've
no idea...) Even > better, the remaining fifty tons or so of hold can be
used to store > the oxygen that they'll crack out of the water to make
the fuel. It > wouldn't even need all that space. We think. > Now
assuming the maths is correct, which I'm happy to do, I guess my >
question is: why can none of us remember this having been suggested > or
done before? Is it reasonable, or am I (are we) missing > something?
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.

As for why it's not been done before - I remember variants of this
discussion way back when. I think it wasn't popular back then because
Fire, Fusion, & Steel was the ship-building rule set in use at the time,
and it was mass-sensitive. As people tended to build fairly optimised
ships, water didn't look too attractive. Also, the volume savings isn't
huge and you need the refinery and the time to crack the water and move
the hydrogen to the main fuel tanks. When all's said and done unless
you're really tight on space it's easier to just use hydrogen.

--
Rupert Boleyn <xxxxxx@gmail.com>