What about the "up to 19,999dt" missile platforms? (I'm deliberately not calling them escorts, frigates, destroyers or cruisers so as to not derail the question. ...)
Well, if the world's navies can't agree on what a frigate is, no great surprise we can't either. Pocket cruiser? Battle destroyer? Missile cruiser? Siege frigate?
...Missiles pack a huge wallop in all of the various Traveller rules and for those ships a spinal is both unnecessary and adds vastly to costs, due to Power Plant if nothing else. ...
A spinal is often a one-hit kill, therefore it pays to spread your missile "eggs" among as many "baskets" as possible so they don't all get broke in that one hit. At about a thousand dTons, most of the vitals are percentage-based and you earn your first bay. At 2000 dTons, you lose an important edge in size. Therefore, I find the ideal missile bombardment platform to be in the 1000-1999 dT range, preferably as small as possible while still mounting a full array of weapons to absorb weapons hits and keep the missile bay at strength - 1400-ish seems to be a happy medium. Building a larger ship for more missile bays is just building something that's easier to hit with more weapons to go silent when it does get hit.
You can design a very effective destroyer escort in the 1400-1800 dT range: agile, good jump range, heavily armored, with that one missile bay, inexpensive so you can have lots of them to minimize the effects of meson fire, its role to escort the capital ships and to survive heavy fire just long enough for the capital ships to make their escape in a retreat.
You could design a larger ship, a destroyer in the 3000-5000 dT range, say, or even larger, but having made it a somewhat easier and higher value target, I wouldn't take it up against capital ships as an escort. I'd give it roles where I needed the larger size but where it wasn't likely to encounter the big guns, maybe have it flying solo in reconnaissance missions or deep commerce raiding. I'd use the extra space to have it carry troops and fighters, make it an inexpensive multi-purpose platform, but I'd still prefer it small so I can have them cover more ground and don't lose as much when one is lost.
I don't really have a combat role for something in the 10,000 to 19,999 dT range, with or without a spinal mount, not that couldn't be more effectively done by a squadron of destroyer escorts or a division of destroyers. 19,000 dT makes for a serviceable if lightly armored light cruiser but, as you point out, you pay a stiff price for the spinal. However, putting all your eggs in a 19,000 kT missile platform is not an improvement, not when you can build 4 or 5 good destroyers or ten DE's who can run the same missions it could run without the same degree of vulnerability to meson fire.
A missile turret in CT has enough missiles for seven turns of fire (triple launcher has 9 missiles, the turret can store a further 12 according to SS3).
If you want to add ammo rules have 7 tally marks and cross them off as you use the missiles. ...
A dTon is, what, 135 missiles?
The classic Book-2/SS3 arrangement is one missile tri-turret with its 9 missiles and then the 12 extras and a crew position. I take that as the civilian norm. However, in High Guard there are these batteries of up to 10 turrets run from one crew position. Seems to me you could extrapolate from that to come up with more than 7 battery-rounds for your turrets. Me, I'd consider 20 battery rounds for a triple launcher if I were going to impose an ammo-tracking rule. On the other hand, one could as easily imagine the other half-dTon per turret being some sort of delivery system for getting missiles from some cargo hold to the missile turret, and then calculate on the basis of 135 missiles per dTon of available ammo space.
Only problem is that missiles, despite their supposed power, lose it when you actually have to account for them. Burning through 30 missiles at a salvo, you can easily use up a couple of dTons of missiles for every hit you manage, meaning you'd either run out quick or have to bring missile transports along, either option significantly reducing the usefulness of missiles (and incidentally elevating the importance of spinal mount capital ships). And, if you go with the Trav price for the things, it can be more than the cost of a dreadnought to take a dreadnought out of the fight with missiles; when it costs you more to neutralize a target than it costs him to field it, the war is not going your way.
) on its military. And half of that goes to bases. This means that a pocket empire on the Imperium's border can spend proportionally 22 times as much (10%) on its military than the Imperium spends on its (planetary forces of member worlds not included in this).
