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Old 03-20-2013, 11:43 AM   #11
Join Date: Feb 2005
Location: Berkeley, CA
Default Re: [Space] GURPS

Originally Posted by Rabek View Post
Is there a feasible cap to creature size at various levels of gravity, then?
Almost certainly. Figuring out what that cap is, however, is not trivial, because size limits are a function of both structure and metabolism, and those scale differently. It's possible that the -2/3 power is an adequate rule of thumb, it's just not immediately obvious why it would be correct.
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Old 03-20-2013, 03:54 PM   #12
Join Date: Jun 2005
Default Re: [Space] GURPS

Originally Posted by Brett View Post
Fortunately, RPGs are a great way to improve your knowledge of science, social science, and history. Designing settings is especially good. And I have been doing it for longer than you have been alive: 32 years since my first D&D "milieu".
There's a lot of that here. I started DMing in 1975 or 1976. Then I started wondering how to estimate costs for things, and I started reading medieval economic history, and, well . . .

Bill Stoddard
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Old 03-20-2013, 03:55 PM   #13
Join Date: Feb 2005
Location: Berkeley, CA
Default Re: [Space] GURPS

Doing a bit more math: let's say we have a creature that is double linear scale (and thus 8x more massive) in 0.5 gravity.

Strength is multiplied by 4. Weight is also multiplied by 4.
Walking speed varies as sqrt( gravity * stride length ), and stride length is x2, so walking speed is unchanged.
Accelerating the limb to its desired speed requires acceleration that scales as (speed^2 / limb length). We thus need half the acceleration. Since we have 4x strength and 8x mass, that works out.
Energy requirement for movement scales as Speed*Resistance. Resistance generally scales as Weight*X + Area*Y*Speed^2; X and Y will be constant for any particular creature; both weight and area are multiplied by 4, so energy requirement is multiplied by 4.
Energy requirement for maintaining surface temperature scales with area, so that's x4. Absent a circulatory system heat transfer is an issue, since it scales as (core temperature) * (area) / (temperature difference between core and skin), but with a circulatory system I believe this problem goes away.

Thus, x2 size, x4 metabolic rate, x4 weight, x4 strength, x8 weight. That actually hangs together pretty well.

The usual scaling for metabolic rate is given as the 3/4 power of mass, not the 2/3 power of mass, but that may have to do with larger creatures having different body shape from small creatures (which the above math was ignoring). As such, going with max size ~ 1/gravity seems fair.

And I first played D&D in 1977, though I sort of left the hobby and came back again around 1989.
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aliens, gurps 4e, space

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