Rules for combat and physical feats in spin grabity

Michael Thayne · 03-01-2020, 05:57 PM

An odd gap in GURPS's rules, when it seems to have rules for everything, is doing stuff in spin gravity. So I'd like to do a thread to try to come up with some rules. My general understanding of the physics of spin gravity is this: if you try to move spinward, you will feel heavier. If you try to move anti-spinward, you will feel lighter—or even weightless if your movement exactly cancels out the spinning motion. Falling objects will drift anti-spinward due to the coriolis effect, while people and objects moving upward will drift spinward for the same reason. How do you put all this into game rules?

For starters, it seems plausible that being in spin gravity could be treated as "bad footing" for combat purposes, giving -2 to attacks and -1 to defenses. As with other penalties for specific types of bad footing these penalties could be bought off with a perk. Other stuff is trickier. I think maybe you could steal some rules for performing physical feats on an incline (with spinward being treated as "uphill" and anti-spinward being treated as "downhill"), but I'm not sure how to translate degree of coriolis effect into a "slope" (I think generally lower radii and slower spin will tend to translate into greater "slope", but I'm not sure what the details should look like).

Thoughts? An important constraint is that I want something that's easy to calculate on the fly at the gaming table, and it's okay if the rules are only approximately realistic if that's what's necessary to keep the calculations simple.

AlexanderHowl · 03-01-2020, 06:08 PM

It really depends on the spin rate. Supposedly, anything <1 rpm only takes a few minutes to get used to for the first time. From 1 rpm to 3 rpm would probably take days to get used to and >3 rpm is probably quite harsh. Bad footing would apply for 1 rpm to 3 rpm until an individual gets familiar with the motion while it would probably always apply at >3 rpm unless someone bought the perk. And you have to account for the gravity level.

Michael Thayne · 03-01-2020, 10:29 PM

Quote:

Originally Posted by AlexanderHowl

It really depends on the spin rate. Supposedly, anything <1 rpm only takes a few minutes to get used to for the first time. From 1 rpm to 3 rpm would probably take days to get used to and >3 rpm is probably quite harsh. Bad footing would apply for 1 rpm to 3 rpm until an individual gets familiar with the motion while it would probably always apply at >3 rpm unless someone bought the perk. And you have to account for the gravity level.

What's the basis for this claim? Also, is rpm the most relevant number, or might something like "linear velocity at the edge" be more relevant? In terms of feeling heavier or lighter when you walk spinward or anti-spinward, I think linear velocity will tell you more, but there might be effects where rpm is the key thing.

Verjigorm · 03-01-2020, 11:28 PM

Quote:

Originally Posted by Michael Thayne

What's the basis for this claim? Also, is rpm the most relevant number, or might something like "linear velocity at the edge" be more relevant? In terms of feeling heavier or lighter when you walk spinward or anti-spinward, I think linear velocity will tell you more, but there might be effects where rpm is the key thing.

I'm not sure where he got his data, but the ideas about RPM and what's tolerable to people seem to vibe with the information from Atomic Rockets.

Agemegos · 03-01-2020, 11:39 PM

It's the Coriolis acceleration that is of concern here, and the formula for that is

-2 Ω'×v'

where Ω' is the rotation vector of the frame of reference (i.e. a vector with magnitude proportional to the rotation rate), and v' is the instantaneous velocity vector of the object relative to that frame of reference.

Now, it is a rotating frame of reference, so the magnitude of the object's relative velocity v' will change if its trajectory takes it closer to or further from the axis of rotation. But instantaneously v' is independent of r'. Surprisingly, the magnitude of the Coriolis acceleration is the same near the axis as near the rim; it depends on object speed and the rotation rate of the frame of reference.

Note that that is a vector cross product, with magnitude sin(θ) |v'| |Ω'|. Components of v' parallel to Ω' don't contribute to the magnitude of the effect, and the direction of the Coriolis acceleration is given by the Right Hand Rule.

Michael Thayne · 03-02-2020, 07:34 AM

The discussion I'm seeing on Atomic Rockets focuses on whether people get nauseous or can function at all. But there are other problems. Consider a wheel with a 100 meter radius rotating at 3 rpm. Apparent gravity is 1G. Let's say you've gotten used to the nausea. Now try going for a jog in the wheel. Instantaneous velocity of you feet when standing still is 31.4 meters per second. Felt gravity is proportional to that speed squared. So even a relatively leisurely pace of around 3.2 meters per second spinward increase your felt gravity more than 30%. A 180 lb. man will suddenly feel like he's carrying a 40 lb. weight. Jogging at the same speed anti-spinward will reduce your felt gravity about 20%, which, if you aren't expecting it, is a recipe for falling flat on your face because the floor wasn't quite where your feet expected it to be.

Prince Charon · 03-02-2020, 03:48 PM

It may help to look at a website called SpinCalc, linked for your convenience, which calculates spin-based artificial gravity based on radius of the centrifuge, angular velocity (rpm), tangential velocity, and acceleration (g-force), as well as providing details on each.

Anthony · 03-02-2020, 04:18 PM

In general I'd just call it "-1 DX unless you have G-Experience (Spin)", plus maybe treating the direction towards the spin as slightly uphill (given the severity of slope required to affect combat in GURPS, it's probably below the resolution of the rules).

Verjigorm · 03-02-2020, 04:21 PM

Quote:

Originally Posted by Anthony

In general I'd just call it "-1 DX unless you have G-Experience (Spin)", plus maybe treating the direction towards the spin as slightly uphill (given the severity of slope required to affect combat in GURPS, it's probably below the resolution of the rules).

I think I'd do something like -2 for "bad footing", with a DX check to avoid falling for steps and move and attack. Allow a Terrain adaption(Spin Gravity) to offset. This could be an integral part of space based martial arts.

Michael Thayne · 03-02-2020, 05:54 PM

I looked at the rules for gravity gradients in Ultra-Tech and if you applied them to smaller spin-gravity stations the results are going to be pretty brutal. Basically if you move at more than about 5% of the tangential velocity of the system, you need to roll at DX to avoid falling over. The roll will take a -2 penalty if you try to move at more than about a third of the tangential velocity.

03-01-2020, 05:57 PM	#1
Michael Thayne Join Date: May 2010	Rules for combat and physical feats in spin grabity An odd gap in GURPS's rules, when it seems to have rules for everything, is doing stuff in spin gravity. So I'd like to do a thread to try to come up with some rules. My general understanding of the physics of spin gravity is this: if you try to move spinward, you will feel heavier. If you try to move anti-spinward, you will feel lighter—or even weightless if your movement exactly cancels out the spinning motion. Falling objects will drift anti-spinward due to the coriolis effect, while people and objects moving upward will drift spinward for the same reason. How do you put all this into game rules? For starters, it seems plausible that being in spin gravity could be treated as "bad footing" for combat purposes, giving -2 to attacks and -1 to defenses. As with other penalties for specific types of bad footing these penalties could be bought off with a perk. Other stuff is trickier. I think maybe you could steal some rules for performing physical feats on an incline (with spinward being treated as "uphill" and anti-spinward being treated as "downhill"), but I'm not sure how to translate degree of coriolis effect into a "slope" (I think generally lower radii and slower spin will tend to translate into greater "slope", but I'm not sure what the details should look like). Thoughts? An important constraint is that I want something that's easy to calculate on the fly at the gaming table, and it's okay if the rules are only approximately realistic if that's what's necessary to keep the calculations simple.

03-01-2020, 06:08 PM	#2
AlexanderHowl Join Date: Feb 2016	Re: Rules for combat and physical feats in spin grabity It really depends on the spin rate. Supposedly, anything <1 rpm only takes a few minutes to get used to for the first time. From 1 rpm to 3 rpm would probably take days to get used to and >3 rpm is probably quite harsh. Bad footing would apply for 1 rpm to 3 rpm until an individual gets familiar with the motion while it would probably always apply at >3 rpm unless someone bought the perk. And you have to account for the gravity level.

03-01-2020, 11:39 PM	#5
Agemegos Join Date: May 2005 Location: Oz	Re: Rules for combat and physical feats in spin grabity It's the Coriolis acceleration that is of concern here, and the formula for that is -2 Ω'×v' where Ω' is the rotation vector of the frame of reference (i.e. a vector with magnitude proportional to the rotation rate), and v' is the instantaneous velocity vector of the object relative to that frame of reference. Now, it is a rotating frame of reference, so the magnitude of the object's relative velocity v' will change if its trajectory takes it closer to or further from the axis of rotation. But instantaneously v' is independent of r'. Surprisingly, the magnitude of the Coriolis acceleration is the same near the axis as near the rim; it depends on object speed and the rotation rate of the frame of reference. Note that that is a vector cross product, with magnitude sin(θ) \|v'\| \|Ω'\|. Components of v' parallel to Ω' don't contribute to the magnitude of the effect, and the direction of the Coriolis acceleration is given by the Right Hand Rule. __________________ Decay is inherent in all composite things. Nod head. Get treat.

03-02-2020, 07:34 AM	#6
Michael Thayne Join Date: May 2010	Re: Rules for combat and physical feats in spin grabity The discussion I'm seeing on Atomic Rockets focuses on whether people get nauseous or can function at all. But there are other problems. Consider a wheel with a 100 meter radius rotating at 3 rpm. Apparent gravity is 1G. Let's say you've gotten used to the nausea. Now try going for a jog in the wheel. Instantaneous velocity of you feet when standing still is 31.4 meters per second. Felt gravity is proportional to that speed squared. So even a relatively leisurely pace of around 3.2 meters per second spinward increase your felt gravity more than 30%. A 180 lb. man will suddenly feel like he's carrying a 40 lb. weight. Jogging at the same speed anti-spinward will reduce your felt gravity about 20%, which, if you aren't expecting it, is a recipe for falling flat on your face because the floor wasn't quite where your feet expected it to be.

03-02-2020, 03:48 PM	#7
Prince Charon Join Date: Dec 2012	Re: Rules for combat and physical feats in spin grabity It may help to look at a website called SpinCalc, linked for your convenience, which calculates spin-based artificial gravity based on radius of the centrifuge, angular velocity (rpm), tangential velocity, and acceleration (g-force), as well as providing details on each. __________________ Warning, I have the Distractible and Imaginative quirks in real life. "The more corrupt a government, the more it legislates." -- Tacitus Five Earths, All in a Row. Updated 12/17/2022: Apocrypha: Bridges out of Time, Part I has been posted.

03-02-2020, 04:18 PM	#8
Anthony Join Date: Feb 2005 Location: Berkeley, CA	Re: Rules for combat and physical feats in spin grabity In general I'd just call it "-1 DX unless you have G-Experience (Spin)", plus maybe treating the direction towards the spin as slightly uphill (given the severity of slope required to affect combat in GURPS, it's probably below the resolution of the rules). __________________ My GURPS site and Blog.

03-02-2020, 05:54 PM	#10
Michael Thayne Join Date: May 2010	Re: Rules for combat and physical feats in spin grabity I looked at the rules for gravity gradients in *Ultra-Tech* and if you applied them to smaller spin-gravity stations the results are going to be pretty brutal. Basically if you move at more than about 5% of the tangential velocity of the system, you need to roll at DX to avoid falling over. The roll will take a -2 penalty if you try to move at more than about a third of the tangential velocity.