[Barnacle Bill]

I believe that kT is actually a unit of volume, not mass.

In real life, the volume of a ship is expressed in "tons" of "displacement", where 1 ton of displacement is the volume occupied by 1 ton (mass) of seawater. "Displacement" means only the part of the hull below the water line counts - it "displaces" the water that would otherwise fill the volume of its hull below the water line.

Numerous science fiction backgrounds assume that this ancient nautical tradition will continue to be used with space ships. Examples include David Weber's "Honor Harrington" novels (which actually use the "kT" terminology) and the RPG "Traveller". Taking "k" (as in "kilo" on the units is probably a reflection of the common assumption that space ships of the distant star-faring future will be huge compared to current maritime vessels. For example, the USS Enterprise of the original 1960's Star Trek TV series was supposedly about the same length as the real-life CVN USS Enterprise, but was classified as a "heavy cruiser" and had about 1/10 the crew as the wet-navy carrier. Another example is the derelict Galactic Empire cruiser found & refurbished in Asimov's "Foundation", which was described as over 1 mile long. In both cases, "cruiser" is apparently intended to refer to a mid-sized ship. Another example of huge ships is in the "Honor Harrington" novels, where a Superdreadnought is about 3 km in length!

And no, I do not suggest that spaceships will be measured in tons of vacuum displaced. What I mean is that the volume of 1 ton (mass) of seawater is an established unit of measurement for the volume of large maritime vessels, so it is reasonable to assume that if they start making spaceships of similar or larger size (and using all sorts of other wet-navy traditions) they will likely use the same measure of volume.