Vehicle hit points
 

Pretend you’re in charge of choosing the hit point system for GURPS Vehicle Design. Unlike David Pulver in the real world, you have no mandate to be even slightly consistent with established 4e vehicle stats, but do have to remain consistent with the rest of GURPS mechanics and weapon damage values. You can choose any of the existing systems or build your own.

3e: multiply surface area by 1.5, with a 0.1x to 2x modifier based on frame strength. Cars have around 200 HP, an SM +10 space cruiser might have around 150,000 HP.

4e: multiply the cube root of empty weight by 4. Cars have around 60 HP, the same space cruiser has around 1,000.

Unused 4e (from Pyramid “Extreme Damage”): multiply the square root of empty weight by 1.7. Cars have around 100 HP, the space cruiser has around 7,000.

Damage never changed from 3e to 4e, but vehicles are anywhere from 2x to 150x or more easier to destroy. I know there’s lots of ways to mitigate that via house rules, but suppose you were staring over: what kind of HP formula actually makes sense for vehicles and makes them appropriately easy or hard to kill?

Re: Vehicle hit points
 

My own ideal would be to have a Frame for each vehicle, which is what determines if the vehicle is physically destroyed, and everything else is made up of separate locations that are damaged on their own (albeit with some possibility of bleed-through)... but I suspect that would end up being a bit complex in play.

From what's available, I'd probably go with the 4e default, but use something like Conditional Injury so damage doesn't accumulate so quickly (a 120 damage collision* being able to destroy that car is probably fine, a few dozen 1d arrows doing so probably isn't).

*On that topic, revamping of how HP interacts with collisions would be in order. As it stands, a catapulted cow does half as much damage as it would if you killed it first (Unliving objects have twice as much HP as living ones), or only 1/4th as much damage as if you butchered the cow and wrapped up the remains in a burlap sac (Homogenous objects have 4x as much HP as living ones).

Re: Vehicle hit points
 

For the original question, I don't think you can really make one number solve the overall problem. 200 HP and 60 HP are both wrong if you're destroying a car by shooting small arms through its passenger compartment...

Really, not sure HP are a good solution for vehicles at all, and not sure exactly what problem we want them to solve.
If you're splitting vehicles into a bunch of modules, why not do the same for the structure? You don't really need a 'the vehicle is destroyed' track, let the players decide whether they consider a plane with one wing, a flooded boat with half a usable engine, or a car with no roof and two wheels to be 'destroyed'.

Re: Vehicle hit points
 

The idea of tracking the Frame separately would be that it's a component that, if sufficiently damaged, will cause the entire vehicle to break apart completely. It's also something you can't really replace - you need to either repair it, or just cannibalize the working compartments for parts and scrap the rest - whereas doors, armor, airlocks, engines, weapons, warp drives, etc can often be swapped out, at least with a bit of work.

But, yes, often a vehicle would be rendered unusable long before the Frame gave up the ghost.

Re: Vehicle hit points
 

I would use the 4/e rule, because I use it for characters, and I don't want incompatible mathematical models in the same game.

Re: Vehicle hit points
 

I'm afraid that's what 4e already does. 20 HP is either the limit for PC humans or a 1000lb bear. That looks like two different mathematical models to me.

That's for realistic games too. A practitioner of the Sumo Wrestling Slyle from MA could have 40 hp. Thats as much as a small car.

If I thought vehicle or lare animal combat was likely to come up in an important fashion in a prospective game I was running I'd be very likely to fix it some how.

Re: Vehicle hit points
 

I don't think anything says 20 HP is the limit for a 1000 lb bear. It's just the baseline for one in the same way that 10 HP is for humans.
Some vehicles have a central portion of their structure whose loss leaves all the rest severed, but others don't particularly. A (seagoing) ship, for instance, can continue to be at least one ship if you up and chop it into two pieces somewhere along its length. Provided it's compartmentalized successfully so at least one piece doesn't sink. (Probably at most one piece will have functional propulsion in that case.)

It's probably true if your vehicle has a structural frame, you can't replace it in the sense of pulling it out and swapping another in while leaving everything else in place. Because while its out everything else can't remain in place, at a minimum. You can replace it in the sense of getting a new one and migrating all the insides over though. Or replace it piecewise in place.

Re: Vehicle hit points
 

The most basic question is how they scale all else being equal: with surface area (3e), length (4e), or something else. Then it’s just choosing a multiplier that gets you the number of hits you want it to be able to take. I notice Harpoon (allegedly very “realistic”) scales hit points by surface area with modifiers for materials, compartmentalization, etc. But that’s also dependent on how damage scales. If Spaceships used 3e HP, for example, unarmored mid-sized ships could shrug off multi-terajoule laser blasts.

Re: Vehicle hit points
 

In the end, 4e is dependent on DR to keep things balanced, so if you're not going to do something more radical like Conditional Injury, the easiest way to make sure large objects are appropriately tough is to give things a special type of DR that works against all attacks (even ones that normally reduce or bypass DR) of around HP/20.

Re: Vehicle hit points
 

For this specific issue, I have a house rule that I think helps here (not perfect, but still better than the actual rules).

First I use log ST from Knowing Your Own Strength (KYOS) from Pyramid #3/83. I am simplifying (and definitely rounding) the actual conversion, but in approximately comes out to:

OLD ST: 15 KYOS: 13
OLD ST: 20 KYOS: 17
OLD ST: 30 KYOS: 20
OLD ST: 50 KYOS: 23
OLD ST: 70 KYOS: 27
OLD ST: 100 KYOS: 30

The levels in my above list where chosen as they represent the "average ST" for each SM, so SM +1 would be ST 13 (KYOS) while SM +3 is ST 20 (KYOS)

While KYOS changed the ST score of creatures, it didn't change your HP. So, if a SM +4 Giant in the old rules had ST 50 / HP 50, in the KYOS rules it will have ST 23 / HP 50. This was because the log ST rules didn't want to revamp the entire HP, Dmg, and DR rules, so they made it to keep it matching with the current combat and damage rules.

All of the above is from KYOS. My house rule also changes the HP so that they match the new ST. So the former SM +4 ST 50/HP 50 giant is now a SM +4 ST 23/HP 23 giant. No, instead, those of SM +1 get a Damage Reduction that divides the injury incurred along a log pattern of /1.5, /2, /3, /5, /7, /10, etc.

So, if that SM +0 Dungeon Fantasy warrior with cinematic ST 20 / HP 20 faces a SM +3 troll also with ST 20 / HP 20, while they might seem on equal footing, the SM +3 troll also divides all injury by /3, giving it the advantage.

While at first glance, the injury /3 (for this example) might seem the same as multiplying HP by x3 (to make 20 HP the equivalent of 60 HP... which is quite a bit more compared to the mere 30 HP it would have in the old rules at SM +3), but the fact that you drop fractions when you divide the injury actually makes it a little tougher than merely having 60 HP.