The title of this post combines two entries from my old Tough Guide to the Known Galaxy. We're in no rush, so read them and report back for further discussion.
These subjects are brought back to mind by the comment thread on Space Warfare XIII, which has now reached a preposterous 836 comments, taking it past the mere 820 of Space Warfare XII. I have no moral grounds for ragging on the commenters for being bloodthirsty, given that I served up the topics. One of the many subthreads of the discussion turned to language, particularly (of course) military language, in all senses of the phrase.
How soldiers (or even civilians) talk is an issue that science fiction, fantasy, and historical fiction all face on much the same terms. Roman legionaries presumably swore in Latin, but surely not Ciceronian Latin. Nor was it even Caesarian Latin - at any rate not the Latin of De Bello Gallico, though Gaius Julius could no doubt express himself eloquently in good centurions' Latin when he needed to.
In a few cases we have direct evidence of how soldiers spoke: English troops in France during the Hundred Years' War said "God damn" so much that goddams, or godons, became French slang for the English. This is rather reassuring.
Contrary to a popular trope, most military swearing is not actually very imaginative. Yes, there is the occasional CPO or sergeant with Shakespearean mastery of English invective, but service field language relies mostly on a few very basic concepts, generously repeated.
Scroll down this old Language Log post (it isn't long) for what, from my recollection, is a very good summary of gruntspeak.
The example of godons also reveals another basic truth - styles in swearing change, over time and from culture to culture. In particular, while the Fourth Commandment still surely comes in for very frequent violation by service personnel, at least in 'Murrican military service the pride of place - as reflected in the Language Log example - now surely goes to what, with a delicate nod to spam filters everywhere, I shall call 'the eff word.' Even simpler military evolutions than passing pliers would be impossible without it.
Of course the eff word is no recent coinage. While its use by medieval English grunts went unrecorded, it surely did not go unspoken. (Shakespeare makes a sly grammatical reference to a 'focative case.') But it probably was used only in a fairly literal sense. In particular, acronyms such as FUBAR and the now-generalized SNAFU - like most other military acronyms - seem only to have become widespread during World War II.
And this is the point at which things get tricky. "Eff you!" strikes me as fairly timeless, nearly as home in the 31st century as in the 21st - or even in the 11th century, if the speaker is a Viking, given that "Odin damn!" somehow just doesn't work in our era. On the other hand, "all effed up" has - to my ear - a bit too much contemporary flavor, as though the speaker is not just a universal grunt, but specifically a current era 'Murrican grunt.
In this case, not only the YMMV principle but intended audience comes into play. Much military SF is, let's be honest, not just war porn but specifically Ameriwank war porn. Thus, for the stereotyped Baen audience (by no means identical to the full Baen readership, but surely a non-trivial part of it), 31st century espatiers who sound like they just shipped out from Camp Lejeune are a feature, not a bug. Note that this probably applies to an audience with inverse Ameriwank politics as well.
And it applies with even more force when we move away from swearing like a soldier to the more technical aspects of military language, such as names for weapon systems. SPQR may evoke Rome, but SPQ-31B evokes recent era Western militaries, and particularly (again) the 'Murrican military.
(Somewhat off message, but it surprises me that no one else seems to use USAF style sequential type numbering. Even the Soviet era Russians used manufacturer-centric aircraft designations, e.g. TU-95.)
Alphanumeric designations strongly evoke the recent era - great if the connection is intended, awkward and even frame-jolting if the setting is meant to have a more distant flavor.
More ambiguous are generic terms for weapons or ship classes, such as battleship and cruiser for combatant spacecraft. I have tended in asides and comments to come down fairly hard on such terminology - perhaps more so than is justified. For one thing, the opposite extreme of renaming familiar items can be horribly clunky. I've been unable to track down, among hundreds of comments here, a brilliantly devastating invented example, weapons with an imaginary name, described as "like swords, but more awesome." (Whoever wrote that, step up and claim your well-earned prize.)
Looking at different eras is, alas, rather unhelpful. Older terms for ship types, such as carrack or galleon, seemed to refer not to the ship's functional role but to structural features that are often now thoroughly obscure. (On the other hand, a 17th century Florentine type was called a bastardella, 'little bastard' - an expression that may not clarify its mission, but is timelessly nautical.)
Battleships and cruisers, by comparison, at least refer to familiar missions, at least in their 'classical' usage. (And by SF convention, cruiser seems to retain the sense of a large, fast independent patrol craft, not a heavy escort type.) I have argued against the automatic lifting of this nomenclature, as introducing a bias into our thinking about space warfare. But having said that, it is hardly implausible, in operatic settings, to have some heavy spacecraft, optimized for fighting power, that operate in main-force constellations, and somewhat smaller ones, optimized for mobility, that operate independently.
Details of an imagined technology come into play here. If the 'cruisers' are individually larger than 'battleships,' as is plausible (more propellant, larger crew), the classical terminology becomes a bit misleading. But this does give you a great excuse for battlecruisers, and the Rule of Cool is hard to resist in this case.
Another consideration is particulars of a future history. My old 'Human Sphere' setting was a variant on the standard First and Second Empire theme, and centered on nascent 'Second Empire' trade federations in the 28th century.
The 'First Empire' had no heavy space force in its salad days because it needed none. (Who was it going to fight?) So that particular setting should display a great discontinuity in military affairs. People might well cast back into history books to revive terms like frigate, but on the whole their combatant spacecraft and military institutions would more likely be adapted from police, exploration, or other activities and organizations that have some quasi-military features. Thus, in this particular setting, survey ship comes to have nearly the usual connotation of cruiser.
On the other hand, if your setting has space militaries established by terrestrial Great Powers in the 22nd century, with the latter having substantial continuity with the present day, your nomenclature jumps through a very different set of historical hoops, and cruisers can just be cruisers.
The image of a space battlecruiser comes from this SF website. I nabbed it via Google Images - one of eight space battlecruiser images that come up before the first image of the seagoing kind, HMS Hood. Not until the second page do you find any image of a ship that fought at Jutland.