[3e Vehicles] In need of a few clarifications...

[Varyon]

So, I've decided to start work again on my project to meld the old Vehicles and the new Spaceships into a cohesive whole. The reason for the generic title for this thread is because I intend to use it for the various questions that crop up during this project.

My first round of questions all revolve around Frame Structure (p18-19). In Vehicles, you set Frame Strength (how thick the supports are) and Materials (what they're made of). Strength adjusts the frame's weight, cost, and HP roughly linearly - you can get 1/4 the weight for 1/4 the cost, but this results in 1/4 the HP. Materials keeps HP the same, but lets you use cheaper materials for higher weight and lower cost, or better materials for lower weight and higher cost.

First off, does this influence anything other than HP? I did find that Frame Strength influences Crush Depth, but that's a niche application. Secondly, I understand that 3e and 4e scale HP differently, but don't know how 3e actually did its scaling. Is the above linear relationship between weight and HP typical of 3e, or is it an exception - and how should this be handled in 4e? With 4e's almost pure reliance on mass to determine HP, there's also the fact that cheaper materials (higher weight) would give higher HP, and better materials (lower weight) would give lower HP, but I intend to handle this using Vulnerability and IT:DR (essentially, HP stays the same regardless of weight due to materials, although there will be some nuances).

Secondly, there's the Robotic Structure. I'm somewhat unclear as to what this truly represents. It seems like it's something you'll need if the entire vessel can be controlled by a single crew station, because I don't see any real functional difference between that and a robot brain (the latter is simply automated), but Vehicles states it's only needed for a vessel with a robot brain.

[Ottriman]

From what I know hp, damage and DR numbers scaled way differently past the human scale in 3e vs 4e. I think 3e had linear hp and kinetics with square root based lasers while 4e has cube root based hp and square root based for most damage and DR.

This makes converting from vehicles to spaceships something of a bear. I honestly advise against it from a pragmatic standpoint of effort vs reward.

[RyanW]

I always took robot structure to include all the positional sensors necessary for a machine to move like an organic creature. Without it, you get the 1968 walking truck instead of Big Dog. No particular reason it should be needed for less complex mechanisms like self-driving cars or drone aircraft.

[Varyon]

Originally Posted by Ottriman

From what I know hp, damage and DR numbers scaled way differently past the human scale in 3e vs 4e. I think 3e had linear hp and kinetics with square root based lasers while 4e has cube root based hp and square root based for most damage and DR.

My understanding is that damage - at least for firearms - and DR are functionally the same. My work with the Weapon Design portion of Vehicles certainly gives this indication, at least for low TL firearms (I get very similar statistics to what is found in Low Tech).

Originally Posted by Ottriman

This makes converting from vehicles to spaceships something of a bear. I honestly advise against it from a pragmatic standpoint of effort vs reward.

I'm basically trying to take the best approaches from each. Sails in Vehicles are overly complicated affairs, so I'll probably go with the ones from Spaceships, but I favor kW over PP, so I go with Vehicles there (although the real implementation is a hybrid - I use kW per ton of vehicle mass). It's a lot of work, which is why I let it fall to the wayside for a bit, but I think the end result will be worth it (assuming VDS isn't released anytime soon, as it will likely invalidate basically everything I'm doing).

Originally Posted by RyanW

I always took robot structure to include all the positional sensors necessary for a machine to move like an organic creature. Without it, you get the 1968 walking truck instead of Big Dog. No particular reason it should be needed for less complex mechanisms like self-driving cars or drone aircraft.

That makes a lot of sense, thank you. I'll probably include it for things with advanced neural interfaces and machines that, as you note, are meant to move more like organic creatures, but will leave it out on less complicated vehicles.

[RyanW]

Originally Posted by Varyon

That makes a lot of sense, thank you. I'll probably include it for things with advanced neural interfaces and machines that, as you note, are meant to move more like organic creatures, but will leave it out on less complicated vehicles.

Do understand that this is what I think makes sense, but it's not how the official rules read.

[Varyon]

Originally Posted by RyanW

Do understand that this is what I think makes sense, but it's not how the official rules read.

The official rules boil down to "At late TL7, you can control the entire vehicle from a single computer terminal. Unless you're an AI running on that computer. Then the vehicle needs extra hardware." An AI running elsewhere and remotely piloting the vehicle? Good to go. An AI in the vehicle? Nope, can't do it, sucks to be you, better get a more expensive frame. That doesn't make sense to me, but your explanation does.

[Pomphis]

Originally Posted by Varyon

Secondly, I understand that 3e and 4e scale HP differently, but don't know how 3e actually did its scaling.

Two steps: surface area (VE2 p.20) and materials (VE2 p.19).

[hal]

Originally Posted by Pomphis

Two steps: surface area (VE2 p.20) and materials (VE2 p.19).

Not to rain on anyone's parade, but ever since the thought experiment involving age of sail warship combat, I honestly have doubts about either the realism level for hitpoints of vehicles in 4e rules. A first rate ship of the line can not withstand realistic combat using 4e damage for cannons and hitpoints based on mass. If the rules that come closest to realistic results are GURPS Vehicles 2nd edition as compared against GURPS 4e rules, it might be better to stick with 3e rules for vehicles instead. Just a thought.

Hal

[Kalminos]

Originally Posted by hal

Not to rain on anyone's parade, but ever since the thought experiment involving age of sail warship combat, I honestly have doubts about either the realism level for hitpoints of vehicles in 4e rules. A first rate ship of the line can not withstand realistic combat using 4e damage for cannons and hitpoints based on mass. If the rules that come closest to realistic results are GURPS Vehicles 2nd edition as compared against GURPS 4e rules, it might be better to stick with 3e rules for vehicles instead. Just a thought.

Hal

Current GURPS 4e vehicles don't withstand much damage, but with a few rules usages such as application of injury tolerance rules from some pyramid article I don't recall, (and another rule I don't believe I'm at liberty to discuss yet-) it can be helped. Though I think I can say you come up with some rather large HP values for the larger space ships that are best represented in scientific notation.- which while accomplishing the task, can be cumbersome.

[Varyon]

One thing that gives vehicles (and people for that matter) more realistic injury tolerances is to use a semi-cumulative wounding system, and I actually intend to do just that. The primary issue with GURPS HP is how they accumulate. A good quick-and-dirty way of handling this, while it's not as realistic as some other approaches, is to determine the immediate effect of any given attack normally, but for purposes of actually reducing HP, divide it by 5 first. So a 20 HP injury on a 10 HP character requires 3 HT rolls (one to stay alive as the attack would drop you to -10 HP, one to stay conscious as the attack would drop you below 0 HP, and one to avoid knockdown/stunning, as the attack does more than 50% HP injury), but actually only reduces HP by 4 - getting hit with three such attacks is likely to render you unconscious, and has a good chance of killing you (3 death checks), but won't instantly kill you.

Of course, I'm also considering some insanity where each component has its own HP completely separate from the vehicle's actual HP, with each component giving Cover DR to the vehicle's two Core systems and its Frame. Frame is a "free" (by mass; it's distributed equally among all components and is somewhere between 1% and 5% of total vehicle mass, I haven't decided yet) system, and only damage to it counts directly against the vehicle. My current intent is to simply give the Frame HP appropriate for an Unliving object of its mass, with Injury Tolerance: Armored Flesh (basically, IT:DR where Armor Divisors reduce your level of IT:DR) based on the materials it's made from... but some work* indicates the IT:AF divisor is likely to be quite large - ultra-high strength TL 7 steel has IT:AF of 50 or so. So, an SM+5 vehicle with a TL 7+ steel frame that accounted for 1% of its total mass would have around 30 HP and IT:AF 50, giving it functionally 1500 HP (Spaceships only gives an SM+5 vehicle 200 HP), which is... problematic.

*My methodology here is to take a ~125 lb character - HP 10, and thus Cover DR 10 - and assume they have the density of water. This body is then replaced by an equal volume of the material in question and HP is recalculated based on the new weight (we use a multiplier of 2, as Cover DR is what we're interested in). As Cover DR 10 would correspond to around 10" of flesh, we multiply the material's DR/inch by 10 to determine what Cover DR it should give, then divide this by its HP to determine its level of IT:AF. For example, the Wood found in Low Tech Armor Design (Pyramid #3/52) has DR 1.5/inch and a density of 25.2 lb/cf. Replacing our 125 lb character with one made of this wood would reduce weight to around 51 lb and HP to around 7.5, and 10" means DR 15, so IT:AF is 2. For the above TL 7 steel, we're looking at DR 90/inch and a density of 486 lb/cf, for around 980 lb and a hair under 20 HP, and DR 900. This gives us IT:AF of 45, which I opted to round up to 50 (trying to keep with SSR). Building the Frame out of ablative materials (note S on the Armor Design articles) means that any damage to it is multiplied by 1.2 although IT:AF still plays a roll. For example, if we take that SM+5 vehicle from above and give it a Fiberglass Frame instead of a Steel one, it would have HP 30 and IT:AF 15 (functionally 450 HP). Hit it with a 75 damage attack, that means the attack is treated as a 90 damage attack, but IT:AF reduces this to a mere 6 HP. If the attack was from a laser with AD (2), however, that would reduce IT:AF to 7, for 13 damage.

EDIT: After calculating values for all of the armor types found in the two Armor Design Pyramid articles, I've decided the above methodology doesn't quite work - for an equal mass of two materials, the one that gives a higher DR should have a higher IT:AF, but there are several cases with my method where this isn't the case. If I instead use WM ratios (maintaining the DR 1/inch and 62 lb/cf values from above, flesh has WM of around 5.17), dividing WM(flesh) by WM(material), things become much more workable - TL 7 steel has IT:AF of 10, for example. There are still some high divisors for really good armor materials - the various nanocomposites have IT:AF 50 - but things aren't nearly as bad as they were before.