After replying to this post where I mulled over how hard it is to make an armored ship in Spaceships it got the old think box ah' turning about some ways to make ships a little more survivable.
One criticism that is often leveled at the GURPS Spaceships is that ships made with its system are basically "eggshells with sledgehammers". That it's impossible to make a warship that's worth it. Now there are those who feel that this is just a reflection of reality, one shot one kill after all, and to a degree they are right but real life combat is seldom that simple. In 1987 the Oliver Hazard Perry class Guided Missile Frigate USS Stark was stuck by two Exocet anti-shipping missile and while heavily damaged was make sea worthy again after a couple of repairs made by a nearby destroyer tender. The Oliver Hazard Perry class is hardly what anyone would call armored and a very small ship by modern standards and yet it showed that one shot, one kill is still a hard thing to pull off. That's not to say modern weapons are brutal and that the anti-shipping missile that our space cruisers of the future will sling aren't going to be down right terrifying, but ships do go down perhaps a bit too quickly.
So then, how do we make it so that ships made with GURPS Spaceships a little more survivable?
Well, the truth is we're never going to get a system that fully works so long as we are using a Hit Point based system. Things do not take damage in a liner manner. Even if 10 punches might have the total energy of a 9mm bullet, that doesn't mean 10 punches will do the same amount of damage! Yes, if you land enough blows in a short enough amount of time you will do as much damage but it might take fifty or even a hundred blows! Even the Harpoon series of war games, the most realistic naval simulators on the civilian market, has to constantly rethink their hit point equation since it keeps finding that ships at certain break points keep either being too survivable or not survivable enough! Though this fix is a little beyond the scope of this post and would take a total reworking of how GURPS works to fix... so maybe I'll tackle this one at a later date.
Despite this, there are two fixes that we can do. In real life, just because two ships have the same tonnage it doesn't mean they'll both be equally susceptible to damage. While they will probably be in the same ball park, it is possible to design a ship to be more survivable by adding more bulkheads, reinforcing seams and joints and other factors. This is why naval warships, even if not "armored" by modern standards, can survive a few hits and still keep fighting.
The other fix that is used is the idea of armor belts. Rather then trying to cover the whole ship in the heaviest armor, ship designers would just up armor critical areas.
Also another issues is that you're not giving a lot of armor options for
anyone TL and for most ultra-tech armor what is listed is that TL's
equivalent to mild steel. A quick fix here is for parts of the ship that
needs better protection just use armor 1TL higher to represent higher
but more extensive grades of armor that would available.
This all being said, here are two options that takes these concepts and applies them to the Spaceships design system.
New Spacecraft System Option
Structural Reinforcement (TL1) [Hull]
By reinforcing the ships hull, adding more bulk heads and shock mitigation factors a vessel can be made more resilient to battle damage. A ship can have up to three Structural Reinforcement systems, one in each hull section. The first Structural Reinforcement system increases the ships dHP by 15%, the second by 30% and the third by 60%. Any hull section that has a Structural Reinforcement system is counted as remaining sealed if any of the other sections of the ship are breached. If three systems are installed, besides giving the max increase of 60% to the ships dHP, the extra reinforcement also make the vessel more resistant to long term stress giving it +1HT.
Structural Reinforcement Table
SM +4 +5 +6 +7 +8 +9 +10 +11 +12 +13 +14 +15
Cost 2K 6K 20K 60K 200K 600K 2M 6M 20M 60M 200M 600M
Repair Skill: Mechanic (Vehicle Type)
New Design Feature
Very few real world warships that were considered armored had the best or heaviest armor covering the whole ship. This was far too costly and would add too much weight. Instead they would only cover both the likely place shots would land as well as the most critical areas of the ship with "belts" of thicker and usually higher grade armor. This way ship had a good chance of surviving without being over burden or too costly but this also meant that lucky shots could still get through.
If using this design feature buy any system of armor as normal. It will still takes up a system space but instead of covering a given section of the ship entirely, you get six belts of armor. You can then choose which systems to place these belts on and can place any number of them on any one system (up a max of 6 per belt armor system you have placed) you want with each belt giving their full dDR per belt placed. However the dDR will only offer protection if the belt covered system is hit. Each belt placed also counts as an extra system for providing extra PF against radiation (see Spaceships 5: Exploration and Colony Spacecraft pg. 41).
Example: Let's say that your front hull of your unstreamlined SM+10 TL10 space cruiser has one system of Nanocomposite giving dDR50 , a Advanced Tactical Array , two Habitats [3-4], a Command Room , and decide to give location  another Nanocomposite armor system but with the belt option.
The whole front system already has dDR50 and now has 6 extra belts that can placed with each also giving an extra dDR50. You figure it's best to cover were the crew is going to be so you place two belts on each Habitat and the last two belts on the Command Room giving them each dDR150 or three times as much armor as the systems not covered by belts. However if sections , , or  are hit in combat only the base dDR50 protects the ship but if sections , , or  are hit they are protected at dDR150. For protection against radiation the Habitats and Command Room are each covered by the Nanocomposite armor system, the Advanced Tactical Array and two Armor Belts for a total of 4 non-core systems protecting them giving them each a PF of 400. If an normal Nanocomposite armor system was used instead of the belt armor then only three systems would cover them for a PF of only 300. A crewman inside the Advanced Tactical Array however would only have the Nanocomposite armor system covering them giving them only PF 100.
Thanks to Refplace from over on the GURPS forum for suggesting this!