TL;DR:
You can probably save cost on freighters by splitting them into two designs: a tug ship and an unarmored cargo station.
User xenoscepter had a post a while back (as in, 364 days ago) about
using engineless container vessels as carriers.
What if we applied the same concept to freighters?
Here is a typical single-hold freighter design:
1 standard cargo hold
1 one cargo shuttle bay
1 bridge
1 engineering space
some engines
some fuel storage
crew quarters as needed
armor as needed
Civilian shipping companies use this outline for the "Small" design.
In my current game, the civvy design has three size-60 engines, which gives nearly optimal net speed per cost for a single cargo hold at my tech levels.
(You can get roughly 9% more speed per cost by adding a 4th engine, another 4.5% by adding a 5th engine, and another 2% by adding a 6th engine. A 7th engine squeezes out another 0.3% gain, and after that it's downhill; the marginal cost per engine outweighs the marginal speed gain.)
Optimizing throughput cost is good, and raw speed is good, but it is also good to keep your designs small (to minimize shipyard costs), so I mimic the civvies early and use the 3-engine design.
Design specs:
302.3 BP
35,568 tons
759 km/s
12.6 Bkm range (more precisely: 12.578...)
Care to guess what the most expensive component is?
It's armor. High Density Duranium, comprising 31.8% of the cost.
(The engines are second at 26.7%.)
I don't really need armor on these things. If they are ever attacked, they are going to die. The armor is only going to delay their destruction, and not for long.
But I can't take the armor off and keep the engines on it.
So why not split it in two? Let's make a tractor-trailer.
The trailer has the cargo hold and shuttles, and no armor.
The tractor has the engines and fuel, and a tractor beam, and must have armor.
They both have a bridge and crew quarters and a standard Engineering Spaces component.
Cost of trailer: 111.9 BP
Cost of tractor: 172.2 BP
(You could make a logical argument for leaving the bridge and the engineers off of the trailer, if the trailer is always going to be traveling with a tractor, but a) the game won't let you make a bridgeless ship over 1000t, and b) a commercial ship without engineering spaces will have a very high AFR, as if it were a military ship. You can work around these game limitations by adding some custom ship components to the database, though.
Caveat hackor.)
Anyway, design specs of the combo:
284.1 BP
35,415 tons
762 km/s
12.6 Bkm range (more precisely: 12.633...)
That's a cost savings of 6% while actually increasing speed and range by 0.4%.
There are other benefits to using the tractor/trailer combo:
Trailers can be built by industry.
--It's probably not a great idea in the long run to be using your industrial capacity to build cargo holds, but the flexibility can come in handy.
Tractors aren't married to trailers; they can be used to tug other things.
--For example, I might build small(ish) orbital miners/terraformers if I already have a small tug design to use with them.
You can leave a trailer parked somewhere by itself for storage.
--You could store an entire system's mineral output at a JP, where a long-range hauler will pick it up for intersystem distribution.
Retooling shipyards for new engine designs is cheaper and faster as well.
--This actually might be a bigger deal than the 6% cost savings on the design. It takes a long time to retool a big freighter yard with many slipways. Anything that lets me get that done faster is a boon for my empire.
Drawbacks?
The obvious one: the trailer has no armor.
--I don't think this matters much.
--I almost never send freighters anywhere near hostiles. If my freighters are surprised by hostiles somewhere, they are probably going to die regardless of armor. I keep at least a minor patrol ship at every colony, and a very-short-range fighter docked in a small carrier at every JP, but a) the space between JPs and colonies is big, and b) the fighter at the JP has <1Mkm range. If a surprise hostile appears in some random location near a freighter in one of my "safe" systems, odds are very high that my response time to that location is way, way longer than it will take a hostile ship with any weapon to blow up that freighter.
--In fact, this might be an advantage rather than a drawback. If a hostile appears near a tractor-trailer, at least I have the option to ditch the trailer and run away with the tractor, which has ~4x the speed of the integrated freighter.
A bit more micromanagement.
--For every combo, you have to give the tractor an order to tug the trailer at least once. You can forget about it after that, and treat it just like a normal freighter, but those are extra clicks that will add up over the course of the game.
--You also have to manage two shipyards building these designs at different rates. In the time it takes to build a tractor, you can build nearly 2.5 trailers (if building in a shipyard). You'll be adding slipways to the tractor yard more than twice as often as the trailer yard, which means it won't be as simple as just remember to build a trailer every time you start a new tractor. If you are like me, you'll probably just decide to build enough extra trailers to cover one year's output of tractors, and then
check once per year to see if you need to build more trailers this year. (This would be less annoying if you could queue up shipyard orders like you can industry orders.)
Extra shipyard cost up front.
--But in the long run, shipyard costs are lower. Details
below.
Any other drawbacks? Am I overlooking something?
