Tuesday, August 6, 2013

Intermediate Sizes in GURPS Spaceships

GURPS Spaceships uses a very broad scaling: every 2 increases to a system's SM increases its size and cost by a factor of 10. A single SM increase is roughly a threefold increase. As it works out, that means that official Spaceships are all 30 tons, or 100 tons, or 300 tons, or whatever. Which is simple and straightforward, but it makes converting real world equipment inconvenient: a fully loaded F-16 weighs about 21 tons, which doesn't match either 10 tons or 30 tons very well. It's also annoying when converting a more finely grained fictional setting such as Battletech: 20 ton light mechs and 40 ton medium mechs can be fudged as 30 ton GURPS Spaceship designs, but things get muddled when 50 ton medium mechs, 75 ton heavy mechs, and 90 ton assault mechs (with notably different capabilities in the original game) all get smeared into 100 ton GURPS Spaceships designs.

As solution, I think the rules have space for some intermediate multipliers, so there can be multiple levels of  oversized or undersized designs. Here's a sample table that can be scaled to any arbitrary base SM.

1.0SM +0 +0%1.0
1.25SM +0 +25%1.1
1.5SM +0 +50%1.2
2SM +1 -33%1.3
2.5SM +1 -16%1.4
3SM +1 +0%1.5
4SM +1 +33%1.6
5SM +1 +66%1.7
6SM +2 -40%1.8
8SM +2 -20%1.9
10SM +2 +0%2.0

This is pretty straightforward. A SM+11 spaceship is nominally 30,000 tons, but a Midway class carrier weighs 45,000 tons, so it would SM+11+33%. A Midway's chemical power plant would cost $200M (the normal $150M for an SM+11 chemical power plant +33%) and have 4 workspaces. It's habitats would costs $40M apiece and have 260 cabins each.

Armor systems are slightly more complicated, since armor goes up by a factor of 2 for every 2 increases in SM. I think that increases armor dDR by 10% for every step on the table works, but I haven't done the math as carefully. The sample Midway carrier would get 11 dDR for each system of streamlined steel armor incorporated in the design.

What Bought This Up?

I've been thinking about this for a while, but now I'm also thinking about writing up some Jovian Chronicles vehicles in GURPS Spaceship terms so I can use that system to run my local group through the Odyssey from the very first Mekton Jovian Chronicles book. I don't know if that's going to happen, but I thought I would get this down while I was here.

I suspect I'm the only gamer on the web who is interested both in Jovian Chronicles and GURPS (I certainly felt I was the only gamer in Austin who cared about both games) so even if I do the conversions, I won't litter this blog with them. Unless I get an overwhelming response in the comments demonstrating that I vastly underestimated the popularity of both systems.


  1. I want to go on record as a guy who is interested in both GURPS and Jovian Chronicles. I like Heavy Gear better, but Dream Pod 9 made some gorgeous RPG books, and I really enjoyed the settings they created. I think GURPS suits me better than Core, but DP9 did wonderful work in exactly the areas SJG has neglected with GURPS. If you're looking for online players, down the road...

  2. I was just thinking about the lack of granularity in Spaceships, and whether or not a fully-flexible system could be cobbled together from what we have been given. This is a nice work-around, though, and I will likely steal some of your ideas here.


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