http://en.wikipedia.org/wiki/Ton
[Though its wikipedia, so it makes mention of weight instead of the more proper, mass - so 'also be used as a measure of energy' is a bit redundant and incorrect, eh!
]
Classic Traveller was
very explicit about how this worked (At least in the Second Edition). This didn't keep people from being confused anymore than they are confused in RL about seagoing vessel tonnage!
In
Classic Traveller, starship design is based on 'mass displacement' tons (dTons), which is
strictly a volume measurement. It is described in LBB1 (reprint edition) as being 'as a rough guide, one ton equals 14 cubic meters' (the volume of one ton of liquid hydrogen)'. For deck planning purposes, in
Classic Traveller, this is typically represented using 2 grid squares assumed to represent 3 meter height, as recommended in LBB5. The volume of two 1.5 meter squares, 3 meters high, being 13.5 cubic meters, or 'approximately 14 cubic meters'. 'Finally, a leeway of plus or minus 10% to 20% should be allowed'.
Re: 2300AD -
Your Edition May Vary!
In the RW, the amount of mass that volume could hold is dependent on the structural and operating characteristics of the holding object - which, in turn, would depend on acceleration experienced. I.e. weight - which is a measure of acceleration. Remember, acceleration is a vector measurement. I.e. any gravity is a directional application of force. Which, in turn, must be netted with any additional acceleration. Further, except in limited cases, acceleration is constantly changing (such as when changing distance from center of mass).
An accelerating spaceship, notably operating in gravity wells, would therefore have a very wide range of operational mass tonnage capacities with regards to actual safe load. Even in 'weightlessness' there is still (inertial) mass to consider if the ship is accelerating. In a 'hard science' setting, the useful mass for say cargo, would be explicitly spec'd with given gravity/acceleration limits. (And even more realistically, mass per unit area limits... the pressure footprint of a tank may be much different than that of a similar mass semi.)
However, its volume would generally be invariant - i.e. not change (ignoring relativistic effects!).
Since cargo mass would vary by design and performance characteristics, if the designers didn't spec one (and assuming there is no way to derive from movement rules, such as fuel equations) then pick the density of some RW object and derive mass tons from mass displacement. (Ex: mass of a design (displacement) volume of a specific specie of aluminum. Thus, heavy (steel) equipment would require more cargo 'tonnage' than say loads of wood.)
