I have a "dumb question" and this post seems like one to ask it. "Which version of Traveller is T5? I currently have T20 and Mongoose Traveller? Doing so brieff calculations: CT, Mega-Traveller, Marc Millers Traveller, GURPS: Traveller, T20, and now Mongoose Traveller- or is there another version out which I have yet to see?
It's another version you have yet to see.
The order is; Classic Traveller (CT, or LLB (Little Black Books)), MegaTraveller (MT), Traveller the New Era (TNE), Marc Miller's Traveller (T4), Gurps:Traveller (GT), T20 (T20), Hero:Traveller (HT, I think), Mongoose Traveller (MgT), Traveller5 (T5)
T5 has been released as a set of raw Playtest Documents available on CD from the FarFuture website.
Nine different editions, well you could say ten if you count the releases under the Gurps line, as Gurps did a edition change also.
It amuses me that we have enough here to do a Cladistic Diagram for evolution of Traveller.
In Game Mechanics terms CT, MT and MgT are all pretty interoperable, as they use 2d6 as their core mechanics.
As will in general T4 and T5, though you will need to decide which dice throwing mechanic to use, as when as which technical base.
TNE, stands alone mechanically, but you have the bulk of the other games produced by GDW in that period to draw on for addition material under that rule-set. It also comes the closest to Hard SF of all the things produced under the Traveller Clade of games.
The others all are pretty much stand alone, or can draw upon their respective game system for additional material. With Gurps being the king amongst these as there are something like 300+ source books in that line covering a amazing amount of ground thematically, or as Steve put it more than 4 linear feet of shelf space.
Sorry, if this ran on, once on fact cam out more screamed to be included.