One of the great debates (among many debates, both great and not-so-great…) in the Traveller RPG universe is the Small Ship vs Big Ship universe.
The original Traveller rules (and a selection of “basic rules” since then) have a methodology for ship construction that caps ship sizes at around 5000 “displacement tons” (the volume of 1000kg of liquid hydrogen, or 14 cubic meters). That’s about, either, twice the size of a WW2 “Liberty” ship, or about half the size of the Titanic, roughly the size of the USS Arizona, or about 10 Los Angeles-class submarines, or about 500 tractor-trailers.
This is also known as the “Adventure Class Ships” in some quarters. It is based on roughly fixed (but easy to extrapolate) hull sizes, and fixed size engines and power plants, that become available at various tech levels, in conjunction with computers. Smaller ships can go faster, but this is also limited by computers – which provide the real limitation on interstellar speed. The drives can move ships faster than the computers can calculate at lower technological levels.
When then Book 5: High Guard came, it gave a radically new design and construction paradigm. Most notably, ships could be up to a million tons in displacement. Plus, engines and power plants are a percentage of space depending upon the performance desired, and interstellar speed is directly linked to tech level, not computer performance per se.
As can imagine, these are largely incompatible. The canon Traveller universe clearly converted over to a “Big Ship Universe” while keeping “Adventure Class Ships” around for the PC’s. There are reasons – the BSU model is far more thorough, and covers vastly more options for construction and wargaming. It also more closely parallels “real world” naval construction in that ships became bigger and bigger as materials science and propulsion technology has allowed.
One of the quirks of the SSU is that while larger sizes of ships became available with increased technological development, and faster interstellar travel, but faster came with a decrease in hull displacement – e.g. the ships get smaller when you want them to travel faster, with significant differences. This actually suggests some differences in a SSU version of Traveller from the BSU version.
The big one, in my mind, is that ship size is limited because by some other factor of the interstellar medium. Additionally, that this limiting factor is not merely a function or characteristic of interstellar travel (“Jump Space”) because the same size limitations apply to boats (non-starships) as well. Either that there is some life-sustaining quality of terrestrial existence that is lacking, or there is some additional threat that has not yet appeared in our (real-world) explorations of space. I tend to run with the idea that the starships in Traveller also maintain some sort of magnetic or biotic field that protects the inhabitants from prolonged exposure to dark matter (along with perhaps acting as a deflector for space dust and micrometeors).
Now, many of us that prefer a SSU would also prefer ships a bit bigger than 5000 dtons (that’s “displacement tons”) in size. Luckily there are some interesting extrapolations of the SSU ship construction rules that suggest that ships could get up to a 10,000 dtons. There is also a methodology in the Mongoose Traveller High Guard supplement that creates ships with up to six “sections” (for better damage tracking). I like using these both in conjunction with an additional idea that was meant to create faster “big ships” (on a SSU scale) but that I instead tweaked to allow these larger, multisectional ships that merely operate like the original SSU ships. The end result is ships IMTU (“In My Traveller Universe” as opposed to the “OTU” or “Official Traveller Universe”) up to 60,000 dtons in size (though they are, slow, slow, slow…).
But this lets me build ships big enough to transport an entire battalion of troops at a single time (important for campaign-related, flavor text reasons), as well as warships that are both impressive in size and utterly overpowering without being utterly ridiculous when compared to whatever ship the player group is likely to be travelling around in. Player-character types ships tend to be in the 100 dton to 400 dton (perhaps a bit larger if they are lucky), and this puts Corvettes in at around 300 dtons, Frigates around 700 dtons, Destroyers around 1000 dtons, and Cruisers in the 2000 to 3000 dton range.
Yeah, ok, I know… I know… get around to catching up on game logs…
Plus, rules for Shades for 5E.
Plus, I promised my players some guidelines about multiclassing.