Spaceships hull armor…solid?

[Varyon]

As noted, each system is 5% of the mass of the ship, so if you assign one as armor, said armor has a mass equal to 5% that of the ship, and you only have 95% to work with for the rest of it. Said armor systems are spread over that section of hull, however - that dDR protects everything (except for Exposed Systems, naturally) in the same section where it is set.

Armor is incredibly dense, however, so there are some optional rules in Pyramid #3/34 to account for this - having a lot of armor makes your vessel smaller, which increases the effectiveness of armor (less surface area to cover means thicker armor for a given mass), and can even make your vessel harder to hit (-1 or -2 to SM, due to being rather small compared to less-armored vessels of the same mass). Personally, I also don't see a serious issue with allowing armor to be added on in excess of the 20-system limit, but if you do so you'll need to adjust acceleration and delta-v. For example, let's say you've got a ship - 2G acceleration, 9 fuel tanks, total delta-v 31.5 mps - where all the systems are already taken up, but you decide to toss on another 3 armor modules (one for each section) to increase dDR. Your ship is now 115% - 23/20 - of the mass that its drives were designed for. You would divide acceleration by this ratio, giving you around 87% of your originally-calculated acceleration - around 1.7G. For delta-v, you would actually divide your number of fuel tanks by this ratio, then recalculate delta-v from there. 9 fuel tanks becomes 7.8 fuel tanks, which works out to only 23.4 mps delta-v (9 fuel tanks has a multiplier of x1.4, but 7-8 fuel tanks only has a multiplier of x1.2, hence the loss of more than 13% of your delta-v).

Making a vessel extremely heavy by piling on armor should make it harder to handle, as your Control Room (or, rather, the attitude gyros/thrusters/etc that actually make up the bulk of the Control Room's mass) is too small. -1 to Hnd if you're 150% mass (10 extra armor systems) or larger; at 300% mass (40 extra armor systems), you should just redesign the vessel to be one SM larger, but with enough armor to be targeted as a smaller vessel.

Originally Posted by The Colonel

I'm guessing there are no Washington Treaty style shenanigans then with using the ship's bulk water tanks as armour or what have you...

I don't think there are any rules for doing so, no. Spaceships doesn't directly track the weight of water (presumably, it's included in the mass of any system that provides life support), but you could probably work something out (I believe water canonically grants around DR 1 per inch).