[Pyramid #3/34] Alt Spaceships: Basement PF?

[DaltonS]

I'm trying to design a largely underground Mars base using the "Building" rules in Pyramid 3/34: Alternate GURPS Alternate Spaceships p.9-10, and would like to figure out the radiation protection factor (PF) for inhabited "Basement" systems (ie. those that are underground). There are two such systems (including the core) in the upper hull section, with both middle and lower hull sections completely buried. Spaceships 5 p.41 describes calculating PF for systems of a ship in space, but this assumes all hull sections are equally exposed and nothing outside is adding its protection. I would like to be able to calculate cumulative long term exposure based on the following:

Average exposure on the surface of Mars is about 1/4 rads per day at the zero datum altitude. <snip> Mars suits cut the dose in half (PF 2), and the habitats used by the first manned missions would be PF 20. Later permanent structures sheltered by bricks and dirt have a PF of 200 or more.

GURPS Mars, page 20

Any help would be greatly appreciated.

Dalton “GURPS is about realistic roleplaying, right?” Spence

[sir_pudding]

It's effectively going to be high enough to not worry about, a yard's thickness of regolith is PF 1,000 and each additional yard of depth would multiply the the PF by a factor of 1,000. Gamma doesn't penetrate very far into the ground.

[malloyd]

Originally Posted by sir_pudding

It's effectively going to be high enough to not worry about, a yard's thickness of regolith is PF 1,000 and each additional yard of depth would multiply the the PF by a factor of 1,000. Gamma doesn't penetrate very far into the ground.

And neutrons and high energy charged particles aren't very different. A good first approximation is each foot (30 cm), divided by your shield density, cuts radiation to about a tenth. Soils like regolith are somewhere between 1 and 2 g/cm^3, so a yard is somewhere between PF 1000 and 1 million.

If a few tons per square meter of stuff between you and a radiation source fails to cut radiation to survivable levels, you probably need to start worrying about stuff like the radiation melting your shield.

[Anthony]

Originally Posted by malloyd

If a few tons per square meter of stuff between you and a radiation source fails to cut radiation to survivable levels, you probably need to start worrying about stuff like the radiation melting your shield.

You want on the order of 5 tons per square meter for GCR shielding on long term habitats, they're more penetrating than any radiation associated with fusion, fission, or radioactive decay, and you'll get a detectable (but not dangerous unless things are really strange) muon count quite far underground as muons don't have strong force interactions so they don't get stopped by collisions with nuclei, but yeah, underground structures do not generally need to worry about radiation.

[The_Ryujin]

Well, a system worth of protection weights 1/20th of the ships total weight and a cubic foot of soil weights around 92lbs/ft^3 or 0.046tons/ft^3. Figure how much a system weighs for you building then figure how many cubic feet or soil are covering the basement and multiply the cubic feet by 0.046. Divide the weight of the soil by the weight of a building system, that's how many systems of protection the basement has. Finally multiply this product by the PF that one system of protection gives.

Example: A system for a SM+14 building weighs 50,000 tons and a single system of coverage gives PF400. Now let's say that it's basement is covered by 50,000 cubic feet of soil which weighs 2,300 tons. 2,3000/50,000 is 0.046. So this means that 2,300 tons of soil gives PF 400×0.046 or 18.

[sir_pudding]

Originally Posted by The_Ryujin

Well, a system worth of protection weights 1/20th of the ships total weight and a cubic foot of soil weights around 92lbs/ft^3 or 0.046tons/ft^3. Figure how much a system weighs for you building then figure how many cubic feet or soil are covering the basement and multiply the cubic feet by 0.046. Divide the weight of the soil by the weight of a building system, that's how many systems of protection the basement has. Finally multiply this product by the PF that one system of protection gives.

Example: A system for a SM+14 building weighs 50,000 tons and a single system of coverage gives PF400. Now let's say that it's basement is covered by 50,000 cubic feet of soil which weighs 2,300 tons. 2,3000/50,000 is 0.046. So this means that 2,300 tons of soil gives PF 400×0.046 or 18.

18 is at least two orders of magnitude too low.

[Anthony]

Originally Posted by sir_pudding

18 is at least two orders of magnitude too low.

Well, that's lower than actual shielding, but that might just be because 50,000 cubic feet is a low estimate. It's not totally clear how much area a building is supposed to have, but if we set a SM+14 building to 1,500' long and 300' wide, one foot of shielding is 450,000 cf.

[The_Ryujin]

Originally Posted by sir_pudding

18 is at least two orders of magnitude too low.

I was going off of a how Spaceships would handle it. But yeah, the figures in Spaceships is a bit off. It has 50,000 tons of coverage only gives PF400 or PF 8,000 vs solar flares or PF4 against cosmic rays. I wonder if this was supposed to be dPF. Though I should of also treated the building like a core systems doubling the PF.

My example would give negligible FP vs cosmic rays and PF360 vs solar flares (or 720 if treated as a core system). If the PF is supposed to be decade scale then it would give PF 180 (or 360) normally, PF 2 (or 4) vs cosmic rays or PF 3,600 (or 7,200) vs solar flares which isn't as bad.

Anthony: Yeah, I pulled the number out of thin air. With 450,000 cf you'd get PF 166 or PF 3,320 vs Solar Flares. That does seem a bit more in line.