Ok, I was doing some figuring to work out the economics of using tankers to fuel long range starships.
Here's my assumptions:
100KTon freighter, J4, TL 15
I designed ships using FF&S1, and using TNE universe (this has a dramatic affect as we'll see). This is the only ruleset that I have handy to work with.
We're assuming that the drop tanks can be ships themselves. i.e. a Drop Tank with a MDrive and crew.
You can call them tankers if you like. You can call them "fuel shuttles" that move the tanks from A to B, but, when combined, operate pretty much like a ship. So for all intents and purposes, I think, there's no reason a Drop Tank can't be an actual ship.
I used good 'ol Sol as the star system of choice for my model.
Basically, using FF&S and TNE rules, a 100Kton ship needs 25000 Dtons of Jump Fuel for a J4 drive.
So, I then decided on a tanker designed to carry a net 5000 Dtons of fuel (after burning maneuvering fuel -- the tanker is an 8000 Dton ship, FYI), so the 100Kton ship would require 5 such tankers to make the jump.
I also allocated enough fuel for "1 jump" (in this case, 1/4, 6250 Dtons) to the Freighter as a buffer for Jump fuel, assuming that 100% of the fuel can't come from the tankers.
I also gave 30G hour of fuel for the Freighter, as this seems to be a Best Practice for civilian ships (using the few in the TNE book as an example). In truth, the ship really doesn't need anywhere that much for normal operations.
Finally, since we're working Sol, I assumed that the tankers would be skimming a gas giant (Jupiter) while the freight would be delivering to Earth.
I calculated the cost of the ship, and threw in the monthly expenses including salaries and maintenance. I also charge 1/240 of the cash price as ship payments. For salaries I used a flat 1000Cr per crewmember since the rules base them on expertise and officers and what not. I think I'm overpaying at that rate, but it's a reasonable guesstimate.
I took this monthly cost and divided it by the cargo volume of the freighter to get a gross expense for each cargo Dton to break even.
Since I have the tankers gassing up at Jupiter, it turns out that it takes quite a bit of time to make the trip when you're limited by G-Turn burns for fuel. A tanker fleet can make the rountrip from Jupiter to Earth in roughly 4 weeks. So in order to maintain a 2 week turn around on a simple J4 route for the freighter, I need 2 tanker fleets per system of 5 ships each. So, 10 tankers per system, 20 tankers total.
In the end, on the freighter I had a bit over 56000 Dtons free for cargo. M-Drive fuel was the top consumer of space at ~26,800 Dtons for 30 G-Turns. Next was the J Drive itself at 5000 Dtons.
So, what do we end up with? Does the math work out? Does maintaining that large tanker fleet pay off?
It does, and it pays off well.
Using the above scenario, it works out to an overall expense of 386Cr/ton. If we boost the JDrive fuel, and dump the tankers, it works to 411Cr/ton. A 6% savings. Nothing to sneeze at.
But wait, that 6% is misleading. Why? Because when we have our own tanker fleet to fuel the ship, we don't have to pay for fuel! At 100Cr/ton for unrefined fuel, the expense shoots up to 689Cr/ton! A 78% increase over operating your own fleet!
But wait, that 386Cr/ton is for operating your own fleet of drop tank tankers. If you were to operate your own fleet of fuel ships to simply fuel the ships internal J4 jump fuel tank, the price goes up to 581Cr/ton. About 16% cheaper than fueling from the starport with unrefined fuel.
If drop tanks are not "allowed", then with only a 16% savings, you would probably see fuel fleets for only the largest carriers that would have capital to build the tanker fleeets in the first place.
But if drop tanks ARE allowed for the large ships, then the large ships would almost always have tanker support. They'd also run over well controlled space lanes, and would probably not even have the 30G hrs of maneuver fuel. There would probably be 3rd party tanker fleet operators (like the companies who operate the fuel trucks at airports).
Mind, this assumes that they can fill the cargo ship. At stock 1000Cr/ton for generic Freight, all of these scenarios are viable and profitable. Even at under 50% capacity, this is a profitable enterprise.
Now, you thruster plate folks have a completely different set of math involved, since the you lose all of that reaction mass and gain cargo, and also your insystem travel times are much better, again since you have no reaction mass. It's ~140G hours one way from Earth to Jupiter, but that's at 1G. You can speed that up with higher G drives at minimal fuel cost. At 4G it takes 1/2 as long, so with 4G drives you need 1/2 the tanker fleet. And your tankers don't have to be as big either (my tankers consume over 1400Dtons of fuel just to make the round trip to the gas giant).
With reaction mass, hours are hours, no matter how fast your drive can spit them out, so it makes no sense to have more than a 1G drive, it's cold most of the trip anyway.
Anyway, it was a pretty enlightening excercise. I must say that running a fleet of 20 ships to support a larger ship coming out cheaper than the single ship itself was quite interesting.
Feel free to break out your kibitzing hats.
