So, thinking the military options through a little bit.
For my dollar, I'm not sure I can justify equipping the fleet with drop tanks as a standard operational model.
First, by doctrine, I don't think any fleet action is going in to a system with the expectation of "losing" the battle. That is, with the expectation that the fleet will have to immediately jump back out to safety.
Can it happen? Sure. But does it normally? No, I don't think so. I should say not normally enough that the contingency of preparing for that for every operation is worth the costs involved.
When a fleet jumps in, it's not like a fighter bomber making a stealth insertion attack on some specific target, slipping in to the trench to hit the exhaust port and sneak out. Those kinds of missions are certainly possible, but I think unlikely.
Rather operations are more sending overwhelming forces in to a system with the intent to make some kind of beachhead and thus interdict that system from the enemy. So, I visualize the fleet jumping in and making a direct approach on the fuel resources, and then making their attack on other resources.
Because once the fleet jumps in, barring dumb luck, it's not like the system can react. The system has what forces is has available, and there's no cavalry coming. Any possible help triggered by this action is 2 weeks away. So, the attacking fleet has 2 weeks to crack the defense surrounding (say) the gas giant. (Clearly if they have to skim water on a defended planet, it's a completely different operation, but that operation would be planned for approriately).
Odds are high that when the fleet commits to this action, they have the intelligence on what defenses are guarding that gas giant, and plan accordingly. If they pop in and there happens to be a large battle fleet waiting for them, then that's a massive intelligence failure, and you can file that under "Bridge too far".
That situation CAN happen, but it doesn't solely for the reasons that operations do their damnedest to NOT let it happen. Wars are operational in nature, the battles are secondary. The great stories are of the last ditch defenses thwarting a crushing assault. Nobody goes to battle expecting to lose, and do their best not too.
So, I don't think the doctrine of drop tanks "just in case" makes much sense.
There's the premise of a deep penetration ship, where its equipped with higher rated jump drive, but can only normally fuel a smaller jump. When using the tanks, the ship can travel farther (or enter the system with some reserve), but again as a doctrine, it seems wasteful to design that capability in to the entire fleet. You're wasting money on drives and power plants that you simply are not normally using. Better to build more ships and come to stay than a fewer ships designed to hit and run.
Next, there's the fleet train. If you're going to follow a hit and run (or runaway) doctrine, then when they arrive at their ultimate destination, they'll be out of fuel. That destination needs to have a friendly, static fuel supply, or a fleet train available to supply those ships.
Also, the "hit and run away" tactic will, overall, slow down the fleet. When an attack fleet with a "run away" doctrine lands, the fleet train is sitting, and waiting, for notification as to where to go next. The fleet will need to X-Boat (or whatever) a message back to the fleet train to let them know how the battle went. If the battle was successful, they can jump in to the target system. If it was not, they need to jump in to the "run away" system to meet the fleet.
If the fleet basically "knows they're going to win", the fleet train can jump soon after the attack fleet (like a day to keep them out of the main engagement), and not wait for the results of whether it's successful or not. Granted, this is potentially risky, but, as doctrine, with decent intelligence, I think fleets mostly go in to battle with confidence. The strategic end result is assumed (i.e. fleet will arrive and gain space superiority), the details may not be (i.e. at what cost, what ships damaged). They may not be able to take the world, per se, with heavy defenses, but they can at least maintain space superiority. When the outcome is less certain, the fleet train can wait for word.
This assumes that the fleet simply doesn't jump back where it came from. And that's the other rub.
How do you get the tanks to the ships? If the jump off point for the attack is a star system with an adequately equipped starport, then that's just peachy. Fleet arrives, gets equipped and fueled, powers up, drops them, and proceeds with the attack.
If the launch point is NOT so equipped, then the ships have to get to the launch point either with the drop tanks, or have the drop tanks carried and installed in the remote system.
Let's consider something like the AHL cruisers. 60K tons, J-5. That means 30K tons devoted to fuel for J5 (for HG, pretty sure, 50%). When you bolt tanks on to that ship, for a J5 jump, you now have a 90K tons ship, with 60K tons of fuel. Assuming that "a jump drive is a jump drive is a jump drive" and it's ultimate jump number is simply a ratio of drive tonnage to volume (that is, specifically, that a J-5 Drive for a 60K ton ship can drive a 90K ton ship AT ALL, much less at a lesser capacity), then a J5 drive for a 60K ton hull is 3600 tons, which is equivalent to a J3 drive for a 90K ton ship. With the drop tanks equipped, the AHL has fuel for 6 jumps, so it can make 2 J-3 jumps in a row. It would need to proceed at this slower pace to the jump off point for the attack. Naturally, there is no requirement that the drop tanks must supplement the entire on board jump capacity. You could simply bolt on another J1 worth of tankage to give the fleet a "get out of jail free" card, they just likely can't go back to where they came from and the fleet train will have to hunt them down and bring them home.
If the jump drive can not be used like this, then the drop tanks need to be shipped and equipped at the launch point. That implies that large, empty holes in space are flown to the launch point where maintenance crews somehow bolt them to the ships, then the ships fuel up. For the AHL class, that's 30K tons of empty space that has to be shipped to support the cruiser. I'm assuming that they can not be manufactured in the launching system. That is, I assume that the fleet train can't bring "drop tank kits" and assemble them. They may be able to install the tanks, without a starport, but not assemble them.
Now, if a starport is simply required for the mounting of drop tanks, then that means that the only systems suitable for launching such an attack are those systems with adequate starports. That can certainly limit the deployment options.
For those times when an attack is deigned risky enough to warrant a "get in/get out" strategy, those can be handled sans drop tanks via forward deployment of the fleet train. Clearly those attacks aren't particularly spontaneous, and require more planning and, more specifically, more staging, which means more time. Bringing fuel ships up to top off the attack fleet before they jump in, and then meeting them when (if) they jump out. Implicitly, if the ships are not intended to be equipped with a full external fuel load, that is, that they will be entering a system with partial tanks, and will (likely) not be able to come back to where it came from, then there is need for this fleet train and operations to potentially create deep space service points to service the fleet even if you have drop tanks. If you're going too tool up that kind of capacity, you can then use it in lieu of drop tanks at all (by staging early, and within range).
Now it could be argued that if the cost of facilitating drop tanks is simply the plumbing and fittings on the ship, then its probably fair to say that the added cost of these fittings is incidental to the overall fleet cost. This provides drop tank capability to the fleet, and offers the flexibility to deploy with them should they ever come up. However, building ships with overcapacity jump drives and infrastructure on the off chance that they'll use a drop tank seems like a waste of money for little used capability.
So, drop tanks can be practical, and useful in specific operations. But as an overall fleet doctrine, I don't think it makes sense.
Actually...
