And more to the point, how can you convert it to something useful?
Well, of course it's a volume measure.
As a rough guide, one ton equals 14 cubic meters (the volume of one
ton of liquid hydrogen).
(Original Traveller book 2, p. 13)
For clarity, I'll use "dton" (displacement ton) for that, "ton" for
the short-ton mass measure of 2,000 lb. As always, the
units tool is my friend when
working with American measure.
Most things in space, particularly GURPS Spaceships, care much more
about mass than they do about volume. There will, of course be no
single answer to the question of how to convert between them, as there
are various ways of finding plausibly equivalent numbers. Essentially
what I'm asking for here is the overall density of a spacecraft.
-
GURPS Traveller (pp. 84 and 158) assesses 5 tons per dton for
typical cargo mass. Most of its ships are rather lower overall; e.g.
the Beowulf far trader is 200 dtons, loaded mass 598 tons, ratio
of 2.99. (Empty 257 tons, but let's not worry about that.)
-
A standard cabin in Traveller takes up 4 dtons. In GURPS
Spaceships you can fit two of them into a single system slot on an
SM+7 ship (and the ratio is consistent above that), which takes up
15 tons. So that's 1.875 ton/dton.
-
In the real world, a forty-foot cargo container (40 × 8 × 8.5 feet)
has an external volume of about 5.5 dtons, and fully laden it's
limited to 32.500kg, which comes out to about 6.5 ton/dton.
-
An Apollo command module has an approximate volume of 13.8m³ and
launch weight of 5.3 tons. That.s 6.0 ton/dton. If we add the
service module, we're up to a total of 68m³ (not counting the engine
bell) and 32,390lb, which brings the ratio down to 3.3.
-
If the ship is able to float on water, it can't weigh more than
15.4 ton/dton.
It's up to you; I don't think any of these has a better claim to being
a firm figure than any other, but my preference is to use 5 for ease
of calculation.