This house rule exists to allow for more precise sizes in GURPS Spaceships. This is done by applying fractional SMs, in increments of 0.2: if SM 4 is 10 tons, then SM 4.2 is 12 tons; SM 4.4 is 15 tons; SM 4.6 is 20 tons; and SM 4.8 is 25 tons. Likewise, if SM 5 is 30 tons, then SM 5.2 is 40 tons, SM 5.4 is 50 tons, SM 5.6 is 60 tons, and SM 5.8 is 80 tons.
More generally, every table in Spaceships uses one of three standard progressions, depending on how many SMs you need to go up in order to get to ten times the quantity. The Volume progression is a two-step of either 3/10 or 1.5/5; the Area progression is always 2/5/10; and the Linear progression is 1.5/2/3/5/7/10. I have more granular replacements for this that let you interpolate systems that fall between these values based on whether they're ±0.2 or ±0.4. The tonnages given above are an example of applying this principle to a Volume progression; but similar interpolations exist for Area and Linear progressions as well. Also: while I'm technically using decimal SMs, I round to the nearest SM in gameplay; so instead of talking about adding or subtracting 0.2 or 0.4 to the SM, I introduce size categories to use within each SM. (Kudos to mlangsdorf for this revision.) The centerline, “medium”, represents the programs found in the book; start by finding the one that matches that, then shift up or down up to two ranks for more precision.
For those who want to use them, my interpolation charts follow:
Volume Progression[]
This is the default progression.
Tiny: 0.6, 2, 6, 20, 60, 200 Small: 0.8, 2.5, 8, 25, 80, 250 Medium: 1, 3, 10, 30, 100, 300 Large: 1.2, 4, 12, 40, 120, 400 Huge: 1.5, 5, 15, 50, 150, 500
Sometimes the progression centers around 2/6 or 5/15 instead of 1/3. To handle these, just shift the rows:
Tiny: 1.2, 4, 12, 40, 120, 400 Small: 1.5, 5, 15, 50, 150, 500 Medium: 2, 6, 20, 60, 200, 600 Large: 2.5, 8, 25, 80, 250, 800 Huge: 3, 10, 30, 100, 300, 1000 Tiny: 0.3, 1, 3, 10, 30, 100 Small: 0.4, 1.2, 4, 12, 40, 120 Medium: 0.5, 1.5, 5, 15, 50, 150 Large: 0.6, 2, 6, 20, 60, 200 Huge: 0.8, 2.5, 8, 25, 80, 250
Area Progression[]
Used by Open Space, Soft Landing System, and Stasis Web.
Tiny: 0.7, 1.6, 3.5, 7, 16, 35 Small: 0.8, 1.8, 4, 8, 18, 40 Medium: 1, 2, 5, 10, 20, 50 Large: 1.2, 2.5, 5.5, 12, 25, 55 Huge: 1.4, 3, 6, 14, 30, 60
Linear Progression[]
Used by Armor, Force Screens, and Weapons.
Tiny: 0.8, 1.3, 1.8, 2.6, 4, 5.8 Small: 0.9, 1.4, 1.9, 2.8, 4.5, 6 Medium: 1, 1.5, 2, 3, 5, 7 Large: 1.1, 1.6, 2.2, 3.2, 5.2, 7.5 Huge: 1.2, 1.7, 2.4, 3.6, 5.5, 7.8
Weapon Systems[]
Weapon systems introduce two complications which the above doesn't address: gun calibur/launchers, and D-damage. The former doubles every +4 SM; so it should be possible to build an interpolation chart. I simply haven't bothered to, as I don't know of any rules where this would matter.
The latter is more important: when you're talking about dice of damage, interpolation gets tricky; especially when you want to keep the existing damage dice in place.
From what I can tell, it looks like the average damage is supposed to follow the Slow Progression given above, starting around 10 D-damage. If we allow for “(xd±y)×n” for the dice formulas, we shouldn't have any problems matching any of the data points:
D-damage ratings[]
Tiny: 3d–2, 4d–1, (3d–2)×2, (4d–1)×2, (3d–2)×5, (4d–1)×5 Small: 3d–1, 4d+0, (3d–1)×2, (4d+0)×2, (3d–1)×5, (4d+0)×5 Medium: 3d+0, 4d+1, (3d+0)×2, (4d+1)×2, (3d+0)×5, (4d+1)×5 Large: 3d+1, 5d–1, (3d+1)×2, (5d–1)×2, (3d+1)×5, (5d–1)×5 Huge: 3d+2, 5d+0, (3d+2)×2, (5d+0)×2, (3d+2)×5, (5d+0)×5
Unfortunately, I don't see any way to do this using only xd+y and xd×y; that combination simply doesn't allow enough resolution. I also can't get it to align with the values given in Spaceships as written. But if you're willing to flat-out replace Spaceships' D-damage values, the above should do the trick.
Alternately, note that Medium Batteries have three weapons, Secondary Batteries have 10 weapons, and Tertiary Batteries have 30 weapons. Instead of adjusting the D-damage, you can adjust the number of weapons in Secondary and Tertiary Batteries:
Battery: Medium, Secondary, Tertiary Tiny: 2, 6, 20 Small: 8, 25 Medium: 3, 10, 30 Large: 4, 12, 40 Huge: 5, 15, 50
This doesn't work for Spinal or Major Batteries, because there's no such things as 1.2 weapons in a fixed mount or a turret. It also doesn't work for Small Medium Batteries.
Fine-tuning[]
Finally, you have the option to vary the spaceship's length by up to ±5%. Apply that directly to all Linear progression figures; apply double that to all Area progressions; and apply triple that to all Volume progressions.