The Cavern Dropship is the ideal logistic device for a jump circuit. It's available cargo tonnage of 74621+72(
Spacebus)*160=86141 tons exceeding a Behemoth while still requiring only a single docking collar on a jumpship.
The time to undock from one jumpship (1 minute) and then safely redock on another jumpship is a key limiting factor. The base time to redock is 30 minutes, but it requires a control roll with a -1 bonus per extra 15 minutes. In addition, there is a +2 penalty associated with any dropship larger than 20K tons. Thus the time to safely redock with a baseline control roll of 5 is 30+15*5 = 105 minutes. Adding in a small cushion for transfers and convenience, this implies 2 hours per jump in the circuit. In other words, we can move 85K tons 30 light years every 2 hours or 1.3M ton-light years/hour. Although faster jump circuits certainly exist, it's difficult to improve ton-light years/hour.
Given this number, we can also compute the maximum length of the circuit. Safely recharging a jumpship requires 175 hours with a control roll of 5 which is 7.3 days. Rounding up slightly for convenience, this is 7.5 days or 4 jumps / month for a jumpship. For a dropship riding the circuit though, it can travel 90 jumps covering 2700 light years in 7.5 days, which is sufficient distance for most purposes.
Another key question is: how much tonnage can be interchanged during a 2 hour stopover in a system? Recovery of smallcraft is typically the limiting factor since that takes 5 times longer than launching. As an example, a Spacebus can be recovered in 5 minutes, unload containerized cargo in <2 minutes using 4 teams of 5
Strongarm exoskeletons, load in <2 minutes, and then launch by the 10th minute. A single door can recover 2 craft at a time or 4 over that duration and there are 9 doors, implying 36 smallcraft bays could be kept in constant use with a full duplex cargo delivery of 2880 tons every 5 minutes after the first 5 minutes. Overall this works out to 66240 tons in 115 minutes which empties and refills 78% of the Cavern in a single 2 hour stop. The excess tonnage available provides spare capacity for cargo traveling more than one hop.
There are 72 smallcraft bays to support noncontainerized cargo as well. The Spacebus can do a full cycle with general cargo in 22 minutes. For this, all 72 smallcraft bays must work, with the capacity to do 20 full duplex cargo exchange cycles per door recovery slot for 18*20*160 = 57.6K tons offloaded and loaded on. Bulkier items like assault mechs take yet more time with a maximum of a 67 minute cycle time for a 160 ton mech. In that situation, the bottleneck becomes the number of bays, each of which can complete 2 cycles for a total of 144 superheavies offloaded and a new set of 144 loaded on.
One key factor for any jump circuit rider is cost. The cost here is substantial, although pains have been taken to minimize it. The Cavern itself is 433M, significantly cheaper than the Behemoth. The 72 Spacebus attached to the Cavern cost 336M and the 1440 Strongarm cost 219M for a total systems cost of 988M. That's pricy, but amortized over all the nodes of a jump circuit it is relatively small. On a maximum length jump circuit, the cost is just 11M/stop. Even on a 9 stop circuit, the 110M/stop amortized cost is small compared to the cost of a jumpship.
The 'who owns what' question is important to address. The Cavern keeps the crew of all Spacebus and has extra quarters for all associated bay personnel. By rotating the pilot every 8 hours and taking a Spacebus offline for a 90 minute maintenance cycle at slow stops the Spacebus can be constantly used. The exoskeletons also mostly stay on the Cavern with occasional 10 minute maintenance cycles using tech teams that visit briefly at stops. The exoskeleton crew members however rotate 1/4 every stop so each crew member serves through 4 stops doing an 8 hour shift, then spend the rest of the week getting ready for the next shift.
Successfully executing the 2 hour stopover really requires a computer-mediated schedule to efficiently handle the mixture of containerized, noncontainerized and bulk cargo which needs to offloaded and loaded in each direction quickly. Luckily, computers are pretty good at that sort of thing.
Cavern Dropship
Type: Military Spheriod
Mass: 100,000 tons
Technology Base: Inner Sphere (Standard)
Introduced: 3025
Mass: 100,000
Battle Value: 660
Tech Rating/Availability: D/X-E-D-D
Cost: 439,185,600 C-bills
Fuel: 300 tons (3,000)
Safe Thrust: 1
Maximum Thrust: 2
Heat Sinks: 210
Structural Integrity: 10
Armor
Nose: 64
Sides: 64/64
Aft: 64
Cargo
Bay 1: Small Craft (72) 9 Doors
Bay 2: Cargo (74621.0 tons) 0 Doors
Ammunition:
None
Escape Pods: 14
Life Boats: 14
Crew: 5 officers, 19 enlisted/non-rated, 360 bay personnel, 144 passengers
Notes: Mounts 36 tons of standard aerospace armor.
Weapons: Capital Attack Values (Standard)
Arc (Heat) Heat SRV MRV LRV ERV Class
None
Tradeoffs:
Civilian or Military? Military has much better strategic thrust but is a little bit more expensive. Strategic thrust is irrelevant in the primary role, but neither is cost since it's amortized over many jumps. Overall, I went with military on the theory that it substantially improves independent use.
Where are the crews? Keeping enough exoskeleton crews on staff for shifts eats up quite a bit of tonnage and greatly increases the cost. It plausibly makes more sense for them to rotate off to stations or planets where over the intervening week they can assist with other cargo movement.
Fuel capacity? The 300 tons of fuel is more than adequate for a trip in-system from a zenith/nadir jump point.
Smallcraft bay count? It's not particularly clear whether a smallcraft being recovered needs a smallcraft by to recover into. I assumed 'yes' conservatively, and then doubled the number of smallcraft to improve the flexibility to deal with noncontainerized cargo. You could go for more smallcraft bays if the goal was constantly transporting super heavies. The wasted tonnage isn't a serious concern since the Spacebus is 80% cargo, but the cost does go up due to the extra support requirements.
Alternate Use Repurposing into a Behemoth style usage pattern when/where is feasible. The fuel reserves are small enough relative to potential smallcraft that operation in this capacity should be done in systems which can resupply fuel. Since smallcraft and exoskeletons are a significant expense, having just a few associated with the Cavern in a Behemoth usage mode with the majority in use around a planet makes sense. As an Alt-Behemoth, the Cavern is somewhat cheaper, with somewhat greater overall storage capacity, and a significantly greater smallcraft capacity for faster cargo turnover.
Comments?
Edits: swapped from civilian to military design for fuel consumption reasons.
