Yeah, there doesn't seem to be one. Though your question got me brainstorming. Here is an idea I had that I hope you like.
Probably the most straight forward way to model armor in Bk2 designs while still keeping the Bk2 flavor of simplicity would be to simply say that a hull is either armored or unarmored. I like the idea that you can armor your main and engineering sections separately (for reasons you'll see later), so I'm going with that idea.
An armored hull costs 3% more than an unarmored hull, and the armor takes up 5% of the hull's "main" section volume. The first time you get a hit on locations 6-8 (Hull/Hold - the most statistically likely locations), the armor blocks it. After blocking one hit, the armor is compromised and must be repaired before it can block any more hits.
You can buy more armor "layers" (just pay the cost and tonnage), each one of which blocks another hit. Note that each named location is individually armored - your armor will still protect your hold if your hull is hit, and the "other hull" is still protected as well. (I suggest making one of them the passenger deck and one the crew deck, or something like that.)
You can also buy armor for your drive section, provided you have room, which protects locations 2-4. The tonnage for that armor is 5% of the drive section's size and costs 2% of the hull's total cost. (Armoring the whole ship would cost 5% total, and would protect the drives, hull and hold from one hit each.)
Armor must be repaired at a shipyard of C class or better, and cost 1% of the hull's cost per hit to repair. (You don't need to repair the whole armor installation, just the "hole" that compromised it. You should never pay more than the armor's original cost to repair it, even for the drive section.)
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If you are ok with a little more complexity, you could add different armor types for different tech levels. This table gives the TL and hull cost percentage. (All take up 5% of the hull section they are armoring):
NOTE: all of these take up 5% of the hull section's tonnage they are protecting. The higher tech armors are MUCH more efficient in terms of space, though not cost. I had some verbiage to describe how each type of armor worked, but in the end I think a table is easier.
The table is the number of "hits" protected at each location, from 2-12: (2-4 are engineering, 5-12 are main.)
Titanium Steel (TL 7+): 3% / 2% hull cost
[1 1 1][0 1 1 1 0 0 0 0]
Crystaliron (TL 10+): 12% / 8% hull cost
[2 2 2][1 2 2 2 1 0 0 0]
Bonded Superdense (TL 14+): 50% hull cost
[3 3 3 2 3 3 3 2 1 1 1]
(I based armor costs off of the MGT SRD's costs, they may need to be adjusted.)
That you can't mix different types of armors when adding layers, though you could armor different sections with different types of armor if you wanted to.
The reason you'd pick a higher tech type of armor is to get better coverage for your 5% tonnage. The cost of that armor doesn't scale with the protection, so high tech armors will make a ship much more expensive, though it will be much better protected.
If you're using these rules, the starport performing the repairs must match the armor's tech level in order to perform the repair. Optionally, you could make the repair cost twice as much if the world has access to regular trade with a planet whose TL could repair the armor.
So let fancy programs and sandcasters make you harder to hit and let armor soaks up some of the damage for you. High tech SDBs can armor the whole ship with bonded superdense, traders in bad neighborhoods can armor their hulls and cargo to protect their passengers and merchandise, and quasi-military craft can use crystaliron to additionally keep their fuel and computer intact so they can jump away if necessary.
I haven't tried to playtest these rules at all, but they seem reasonable. You might notice that in 100 and 200 ton standard hulls, there's not any engineering section room to install armor. Oh well, that's what custom hulls and larger ships are for.
