So, for my Fellow-Travellers ($3+ Patrons), I have the first draft of that conversion document. It was written with an eye towards Psi-Wars specifically; while useful as an example, I'm sure that many people will want to see Vehicles converted to their own specific campaign needs, and with an eye towards that, I have written the Vehicle Commentaries for all Dreamers ($1+ patrons), which discusses my observations on how to update Vehicles from 3e to 4e, and how you can use the same resources I did to do the same. Some of the answers may not satisfy you, as 4e is less consistent than you might think about vehicles, but it does at least offer you a map that might help.
But I didn't want this to just be about GURPS Vehicles, nor did I want to write something exclusively for Psi-Wars Patrons, so I wanted to stop and discuss GURPS Spaceships, its shortcomings, and its brilliance.
The Problem with GURPS Spaceships
When GURPS Spaceships first came out, it garnered much praise. Where GURPS Vehicles had terrified people with its use of high school algebra and cube roots, GURPS Spaceships was plug and play: choose a size, slot in 20 modules, and you were done. It was great!
Only, as people used it more and more, they found it wasn't really all that great. The aircraft built using its rules proved far too fast, its ships too lightly armored, and it created weird things, like multi-million tons open spaces or blimps despite being crafted of balsa wood and tinfoil. On the other side, with the release of Alternate Spaceships, people could build tanks and discovered that they couldn't hold a candle to the armor and firepower of tanks of a smaller size found in GURPS Ultra-Tech or even GURPS High-Tech. What was going on? How could GURPS Spaceships be so wrong?
As I worked with GURPS Spaceships and GURPS Vehicles, I think I've been able to glimpse at and guess at some of the design decisions that went behind the creation of the book, so I think I can point to why some of these weird inconsistencies exist.
First, GURPS just isn't consistent across the board. I know that some people have the impression of a lurking design system hiding in the background, but the values for vehicles found in GURPS Campaigns conflicts with those found in Ultra-Tech and High-Tech, which conflict with GURPS Spaceships. The truth is, some where guessed at while others were built with something approaching a coherent system, but not all of those systems agreed with one another. One must make concessions when building a system, and that will create inconsistencies of those concessions don't agree with one another across systems. So some of these inconsistencies aren't explicitly the fault of GURPS Spaceships.
Second, GURPS Spaceships definitely simplifies a lot of design decisions. GURPS Vehicles demanded that you work out the volume, mass and surface area of all of your vehicle components and what they looked like when bundled together. GURPS Spaceships, by contrast, asks for you to only know the size modifier and conflates volume with mass.
This is not as crazy as it sounds. GURPS Vehicles assumes most components weigh 50 lbs per cubic foot. This makes sense: given the same basic designs, a heavier engine is also going to be bigger than a lighter engine, all other things being equal. GURPS Spaceships clearly uses volume as the core metric for its size modifier (which is why it says things like "Ships might be twice as long as noted," despite the fact that GURPS Basic says that the longest dimension indicates size, so a ship that's twice as long should have a larger Size Modifier), and it derives its relationship between SM and volume from GURPS Vehicles (or, at least, uses similar assumptions). If you take the expected mass of the vehicle in pounds, and divide it by 50 and then look up the resulting volume in the GURPS Vehicles volume-to-size-modifier chart, you'll find that each spaceship mass matches its expected size modifier (though it tends towards the smaller side of its size modifier band).
The problem arises when we realize that not every component has the same density. A cubic foot of feathers does not weigh the same as a cubic foot of lead. GURPS Vehicles acknowledges this, placing human density at 20 lbs per cubic foot while armor clocks in at as heavy as 400 lbs per cubic foot, while empty space can be vanishingly light (no more than 6 lbs per cubic foot at TL 7-8, assuming structure alone). This explains much of the weird results found in GURPS Spaceships, including the relatively unarmored tanks, and the overly heavy blimps or empty cargo-haulers.
GURPS Spaceships also simplifies surface area, assuming a fairly low, modest amount of surface area for a given size modifier. This is largely correct for boxy or spherical, simple designs, but it gets more and more wrong the more and more complex you make the ship: jet fighters and Star Trek vessels definitely have a more complex surface area than GURPS Spaceships allows, and this impacts things like armor (GURPS Spaceships are too lightly armored for simple shapes) and air speed (GURPS Spaceships are too fast for typical aerial vehicles, which tend to be slowed by their complex shapes).
Most of the problems found in GURPS Spaceships can be laid at the feet of this problem. Beyond that, Spaceships suffers the fate of all systems that simplify their topic. I often have a hard time connecting how a Spaceship subsystem relates to the real-world (how much computer is in a control system? Are the hangar values remotely accurate? etc).
The Brilliance of GURPS Spaceships
Even with these flaws, GURPS Spaceships turned into my top resources for updating GURPS Vehicles into 4e, and the more I worked with it, the more brilliant I realized it was. The "problem" with Spaceships is not that it's badly written or that the author made mistakes, but that he needed to make it user friendly and, in so doing, had to lose some elements and make some assumptions to make everything work. The problem is not that it's bad, but that its assumptions might not match yours.
GURPS Spaceships seeks to simplify a great deal of design, and it succeeds at this magnificently. For example, GURPS Vehicles treats cargo space as volume, which makes sense. Nobody asks for a 200 lb capacity trunk, instead, you get a trunk with space, and how much the cargo weighs depends on the density of the cargo: a trunk full of feathers weighs less than a trunk full of lead. This, however, is not useful to us when calculating performance, as the speed of your truck or, especially, the delta-V of your spaceship, is determined more by its mass than its volume, so we need to know the mass of cargo we can carry.
By conflating mass with volume, GURPS Spaceships sidesteps an enormous mess of issues that, for the most part, don't really matter. If you don't overly armor your ship, or fill it with nothing but empty space, for the most part, the 50 lbs per cubic foot works, given the wiggle room within the size modifier, you should be close to correct. This gives you an idea of what your mass will be, which means you can start to use ratios to good effect: if 1/20th of your ship's mass is dedicated to fusion rockets, then given the efficiency of that rocket, your speed and fuel efficiency should be about such-and-such value.
And like that, all the guess work, the cube roots, the niggling little details about volume, mass and surface area evaporate in one set of assumptions that mostly work. We go from terrifying math to sublime simplicity.
That simplicity gives us a few advantages as well. GURPS Vehicles requires you to handle your kilowatt usage, which varies from "Why am I bothering with this?" such as the handful of kilowatts necessary to run your control system, to gigantic, such as the energy requirements for most beam weapons or reactionless drives. However, just as with mass, you start to notice certain ratios: a particular system uses, pound for pound, about half of what a particular generator outputs. "Power Points" are simplifications of these ratios: some things uses so little power that you don't need to worry about them, and other things use a substantial proportion of your power, and power points tell you roughly what that proportion is. This makes it much easier to depower and repower certain systems on demand; if you want to play Scotty, instead of stopping to do math on giant kilowatt expenditures, you just know the order of magnitude each system consumes and move those around ("If I turn off the phasers, I have more than enough power to double the engine output.")
Similarly, GURPS Vehicles has an interesting combat system, but I'd never want to run it at the tabletop. It involves turn-by-turn vehicle logs, where you track the Gs the vehicle is currently pulling and compare them to the vehicle's maneuver rating and use that to determine the control roll modifiers, etc. It's so tedious that my eyes glazed over as I tried to read it. GURPS Spaceships definitely takes those rules and simplifies them with an eye towards space combat.
And its description of space combat is pretty spot on, more so than many people are willing to admit. Realistically, space combat would largely be determined by missiles (just as modern combat is) with beam weapons and guns relegated to point defense and late-phase combat ,once the missiles had been expended and destroyed. Most of this fire would occur when the other spaceship was an impercetbiel dot in the distant, inky black sky, picked up only by your sensors. This makes the system virtually useless for swooping space opera starfighter combat, but that's because that doesn't resemble actual space combat at all.
So GURPS Spaceships set out to give relatively realistic rules for space combat, and usefully simplified rules for vehicle design, and it succeeded. When I finished my update, I realized that much of it was unnecessary, and that your own spaceship designs will largely give you the same results spaceships would have, unless you're building with different constraints or very specific components in mind, or you really need that high level of detail. Even then, I often find tossing together a GURPS Spaceship as a "first draft" to greatly simplify my vehicle construction process, as it gives me a solid idea of what the ratios might be.
I get the sense that many people see Vehicles as a sort of holy grail, that if they had it, then their vehicles would make much more sense, and Vehicles does offer a great deal more detail, but having dived very deep into it and surfaced again, I can say that much of that detail is unnecessary for the average user. I do not say this to dissuade you from using Vehicles; I'll be using it and I think it works far better for ground vehicles and aircraft, but if you want to design realistic to semi-realistic spaceships, and you're not extremely worried about high detail, Spaceships is an excellent resource.
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