[Spaceships] Sub-SM Ships

[Michael Thayne]

This is not an original idea, but I was having trouble finding a write-up of it by googling, so I thought I'd do my own quick. The basic idea: any ship SM+5 or larger can be designated a "sub-SM" design, which means while officially it has its normal SM, it only has about 60-70% of the loaded mass and 85-90% of the length of a normal ship of that SM. Here, "officially" being a given SM includes, but is not limited to, providing the modifier to use for detection and attack rolls.

The ship's loaded mass will be twice that of a ship one SM smaller. Similarly, Systems will cost twice as much as one-size-smaller systems and, in most cases, will be twice as effective. For weapons batteries, "doubled capability" means doubling the number of mounts rather than making individual mounts more powerful. For fuel tanks, double tonnage of fuel but keep delta-V the same. Sensor and comm systems get +1 to all rolls but reduce the maximum bonus for time spent by 1. For DR provided by armor and force screens, and number of station provided by a control room, take the average of a full-sized system and a system one size smaller, rounding down. Computer network complexity is unaffected (but see note below).

When using the rules for smaller systems, each normal-sized system is only equivalent to two smaller systems. Similarly, a large system is equivalent to six normal-sized systems. Small fuel tanks provide half the usual delta-V rather than a third.

Note: I've thought for awhile that control rooms with an odd-numbered SM should be able to run three times as many programs as ones with an even-numbered SM. If you use that rule, you can interpolate the following progression for maximum number of programs running at one time (TL10, for other TLs apply the usual modifiers):

• SM +4: two complexity 6 programs
• SM +5(Sub): four complexity 6 programs
• SM +5: six complexity 6 programs
• SM +6(Sub): twelve complexity 6 programs, or one complexity 7 program and two complexity 6 programs
• SM +6: two complexity 7 programs
• (etc.)

I feel pretty comfortable with this treatment on the whole. It's not quite mathematically exact (particularly in the case of armor), but it's not meant to be. Feed back is appreciated though—it's totally possible I've overlooked something.

[Anthony]

The easy way to scale for intermediate SM is:

1. Build at the SM you want (optionally, build at one lower; there are a few exploits possible if you don't, though they're generally minor).
2. Decide on a mass ratio R, which is the mass of your ship divided by the standard mass for a ship of that SM.
3. Multiple all quarters, cargo, and similar things by R.
4. Multiply cost by R.
5. Multiply weapons per mount by R or multiply weapon damage by R^1/3.
6. Multiply DR and linear size by R^1/3.

Done.

[Michael Thayne]

Originally Posted by Anthony

The easy way to scale for intermediate SM is:

1. Build at the SM you want (optionally, build at one lower; there are a few exploits possible if you don't, though they're generally minor).
2. Decide on a mass ratio R, which is the mass of your ship divided by the standard mass for a ship of that SM.
3. Multiple all quarters, cargo, and similar things by R.
4. Multiply cost by R.
5. Multiply weapons per mount by R or multiply weapon damage by R^1/3.
6. Multiply DR and linear size by R^1/3.

Done.

...fair. I'd add that you should probably be required to increase weapons per mount rather than damage for kinetic weapons, because kinetic weapon scaling is wonky. Otherwise fair.

[Pomphis]

Interesting. One Question: any opinions on what SM to use for such ships? Next higher or next lower?

[ericbsmith]

I fully incorporated intermediate SM Spaceships into my Spaceships Design Spreadsheet. I only went for +0.5 SM increments, because the granularity of the system really isn't low enough to go lower than that anyways. The basic scaling is pretty easy for most ship systems, as there are only two standard scales used throughout the books - 1-3-10 and 1-1.5-2-3-5-7-10. For each you just need to insert an appropriate intermediate value for the scale; since it uses two different logarithmic scales it's pretty to calculate the intermediate values to get the following two progressions:
1-2-3-6-10
1-1.2-1.5-1.8-2-2.5-3-4-5-6-7-8-10

There's a few other places you need to make a value judgement because it uses a non-standard scaling, and for all weapons you need to choose a damage that's between two listed values, but for the most part you can get half-SM scaling very easily.

[ericbsmith]

Originally Posted by Pomphis

Interesting. One Question: any opinions on what SM to use for such ships? Next higher or next lower?

Anything less than double the mass of the lower SM I would round down, anything between 2x the the mass and the next SM up (which is either 3x or 3.333x) round the SM up to the next higher.

[Michael Thayne]

Originally Posted by Pomphis

Interesting. One Question: any opinions on what SM to use for such ships? Next higher or next lower?

I would generally use "next higher". The SM to ship length table in Spaceships exactly matches that on p. 19 of the Basic Set. Furthermore, according to B19, "If a creature’s longest dimension falls between two entries on the table, base its SM on the higher value." GURPS does not strictly apply this rule everywhere: for example the rules suggest a 6'7" should be treated as SM 0 and you don't count as SM+1 until you reach 7' at least.

[AlexanderHowl]

One simple way to build in between ships is to just have less components than normal for the SM. For example, a 5,000 metric ton spaceship would be an SM+10 spacecraft with only 10 components. In that case, you should probably use smaller components for some of the components.

When using a stripped down spaceship, I would retain the three sections (front, central, and rear), but you could reduce the core systems to just the central core system and spread out the remaining systems as much as possible. The above example would have three front components, four central components (including the core), and three rear components.