GURPS Vehicles non-earth performance

[Kelzar]

I am trying to design Mars-capable vehicles according to the rules provided in GURPS Vehicles, but the rules seem to assume that gravity and wind resistance are constants. I'm not an engineer, or even good at math or science, so I am hoping for suggestions on how to do this. Does the following sound reasonable?

To account for gravitational differences, simply multiply the weight by the gravitational difference (so for Mars, multiply by 0.38) when figuring ground pressure.
To account for differences in drag, multiply the speed factor by the ratio of the difference in atmospheric pressure (assuming Martian atmosphere of 0.087 psi vs. Earth atmosphere of 14.69 psi the ratio is about 168.9). On p.128 the rules say "Find the square root of that quotient. Multiply by motive system speed factor to get ground speed.", so it seems that drag, weight, motive power, and speed factor depending on motive type (tracked, wheeled, etc.) are the factors that determine base performance.

If this is completely or partially wrong-headed, or if you have any helpful suggestions, please let me know.

[Fred Brackin]

Originally Posted by Kelzar

I am trying to design Mars-capable vehicles according to the rules provided in GURPS Vehicles, but the rules seem to assume that gravity and wind resistance are constants. I'm not an engineer, or even good at math or science, so I am hoping for suggestions on how to do this. Does the following sound reasonable?

To account for gravitational differences, simply multiply the weight by the gravitational difference (so for Mars, multiply by 0.38) when figuring ground pressure.
To account for differences in drag, multiply the speed factor by the ratio of the difference in atmospheric pressure (assuming Martian atmosphere of 0.087 psi vs. Earth atmosphere of 14.69 psi the ratio is about 168.9). On p.128 the rules say "Find the square root of that quotient. Multiply by motive system speed factor to get ground speed.", so it seems that drag, weight, motive power, and speed factor depending on motive type (tracked, wheeled, etc.) are the factors that determine base performance.

If this is completely or partially wrong-headed, or if you have any helpful suggestions, please let me know.

I seem to rememebr that this was addressed at least somewhat in the vehicle entries for Transhuman Space or at least brought up in the playtest. Possibly during the Gurps Mars playtest too. It was all rather a while ago.

I believe you are basically correct on the ground pressure but are greatly overestimating the air pressure thing. I would simply recalculate the vehicle's speed assuming maximum streamlining. That's what you do for aircraft above 100,000 ft.

[Kelzar]

Excellent advice! I wasn't aware of the upper atmosphere streamlining rule, but it makes sense. Btw, while browsing through the Vehicles book I noticed p.166 has a sidebar w/ rules for these exact issues. Turns out they are exactly what you suggested!