Guns in spin gravity?

[fdsa1234567890]

Is there a rule for this somewhere? I can't seem to find any.

Also, would gyrojet weapons be affected in the same way if they are accelerating? This could be a reason to use guided projectiles despite the cost.

Lasers would probably be the best option as of TL10 at least, as they would not really be affected.

[Hide]

What do you mean by “spin gravity”? Is this the "Higher-spin theory"?

The easiest way to solve this dilemma is to decide whether or not regular guns will work in such “environment”. If guns work, then pick a penalty. IMO -5 sounds like a good starting point.

[Anthony]

I forget where the general rules for spin gravity are (Space?), but guns don't need any special case rules, they're about as inconvenienced by it as other DX skills.

[Rupert]

Given the velocity of bullet compared to the velocity of a spun environment, unless you're in a really big O'Neil cylinder (or similar) and firing a long way it's not going to matter in terms of skill.

What will matter at ranges over say 50m for pistols and a couple of hundred metres for rifles if the gun was zeroed in a gravity field because the point of impact will be off. At close ranges this won't matter any more than the usual issues of sight line not matching the bore line already do (and they're presumably accounted for by skill and familiarity).

[Fred Brackin]

Originally Posted by Anthony

I forget where the general rules for spin gravity are (Space?), but guns don't need any special case rules, they're about as inconvenienced by it as other DX skills.

I don't remember any specific rules for using tools/weapons in spin gravity as opposed to real or artificial gravity.

The rules we do have for using guns in reduced or microgravity are probably wrong anyway. At least in a Newtonian sense a gun recoiling is governed by inertial mass rather than gravitational weight.

[Varyon]

Originally Posted by Rupert

What will matter at ranges over say 50m for pistols and a couple of hundred metres for rifles if the gun was zeroed in a gravity field because the point of impact will be off.

Zeroing it for spin gravity is only valid if you're shooting in the same direction as you zeroed it - a projectile's trajectory will always (appear to) curve anti-spinward. You'd need some sort of smart sight that automatically shifts the projected aimpoint depending on what direction you're facing.

And, of course, how far away the target needs to be for this curving to have an impact depends on velocity. A quick search for what I can find on the plans for Voyager Station indicates it's set to have a spin gravity of 1/6G and spin at 1.2 rpm. That indicates a tangential velocity of around 14.2 yards/second (using this calculator). That means firing perpendicular to the direction of spin puts your point of aim off by half a yard (enough to turn any hit on a standing human into a miss) every ~0.035 seconds of travel. That's significant - a "hot" 9mm traveling at 400 yards/second would miss at around 14 yards, for example. Using the values from GURPS Tactical Shooting, a Move 300 pistol bullet misses beyond 10.5 yards, a Move 600 rifle bullet misses beyond 21 yards. However, this is roughly comparable to needing to adjust for the wind, and GURPS doesn't penalize for that - indeed, Kromm has suggested wind-speed penalties are only appropriate when there's variation in the wind, which doesn't happen in the case of spin gravity. So, treating it as Familiarity is probably about right - treating spinward as north, a character with the appropriate Familiarity already knows to aim high and to the right when firing northeast, or low and to the left when firing southwest. Familiarity is arguably specific to a given tangential velocity (as this will dictate how much you need to adjust your aimpoint), but I'd be lenient and let it cover a range.

[Rupert]

Originally Posted by Varyon

Zeroing it for spin gravity is only valid if you're shooting in the same direction as you zeroed it - a projectile's trajectory will always (appear to) curve anti-spinward. You'd need some sort of smart sight that automatically shifts the projected aimpoint depending on what direction you're facing.

Given the low relative velocity of the spin compared to the projectiles, over the ranges you'll get in all but the largest habitats there is no curve.

And, of course, how far away the target needs to be for this curving to have an impact depends on velocity. A quick search for what I can find on the plans for Voyager Station indicates it's set to have a spin gravity of 1/6G and spin at 1.2 rpm. That indicates a tangential velocity of around 14.2 yards/second (using this calculator). That means firing perpendicular to the direction of spin puts your point of aim off by half a yard (enough to turn any hit on a standing human into a miss) every ~0.035 seconds of travel. That's significant - a "hot" 9mm traveling at 400 yards/second would miss at around 14 yards, for example. Using the values from GURPS Tactical Shooting, a Move 300 pistol bullet misses beyond 10.5 yards, a Move 600 rifle bullet misses beyond 21 yards. However, this is roughly comparable to needing to adjust for the wind, and GURPS doesn't penalize for that - indeed, Kromm has suggested wind-speed penalties are only appropriate when there's variation in the wind, which doesn't happen in the case of spin gravity. So, treating it as Familiarity is probably about right - treating spinward as north, a character with the appropriate Familiarity already knows to aim high and to the right when firing northeast, or low and to the left when firing southwest. Familiarity is arguably specific to a given tangential velocity (as this will dictate how much you need to adjust your aimpoint), but I'd be lenient and let it cover a range.

The bullet has that same velocity, so what you'll see is a rather smaller rise or drop and (for east/west shots) even smaller shift to the left or right.

[Varyon]

Originally Posted by Rupert

Given the low relative velocity of the spin compared to the projectiles, over the ranges you'll get in all but the largest habitats there is no curve.

The bullet has that same velocity, so what you'll see is a rather smaller rise or drop and (for east/west shots) even smaller shift to the left or right.

I felt there was something I had to be missing, given how harsh something as leisurely as 1.2 rpm and 1/6G looked to be, and it was indeed that the bullets start with the same velocity vector as the characters, they just lack the curving aspect. In fact, I think this means I got things backward - projectiles would seem to curve spinward rather than anti-spinward.

[malloyd]

Originally Posted by Varyon

I felt there was something I had to be missing, given how harsh something as leisurely as 1.2 rpm and 1/6G looked to be, and it was indeed that the bullets start with the same velocity vector as the characters, they just lack the curving aspect. In fact, I think this means I got things backward - projectiles would seem to curve spinward rather than anti-spinward.

Are you sure it's not directionally dependent? Here on the Earth's surface coriolis forces "move" you antispinward if you fire toward the rotation axis, spinward if you fire away from it.

[Anthony]

Originally Posted by Rupert

Given the low relative velocity of the spin compared to the projectiles, over the ranges you'll get in all but the largest habitats there is no curve.

That's not really different from gravity; it also doesn't matter except at long ranges. Spin gravity probably matters more, actually.

When you're aiming at a target in spin gravity that you think of as unmoving, it's actually moving on a somewhat different vector relative to you. In the extreme case of aiming at a target on the exact opposite side of the habitat, you'll miss by 2*spin velocity*bullet travel time, which for a 100 meter habitat spinning at 31m/s, and a rifle with a bullet travel time of 0.22s for 200 meters range, is a miss by 6.8 meters (compare to the .24m miss for ignoring gravity).

In general an 'unmoving' object at any given location on a spin cylinder has a velocity of sin(θ),cos(θ)*v, so if you're at 0 degrees you have a velocity of 0,31 and a target at 30 degrees away from you (about 50 meters apparent range) has a velocity of 15.5,27; at travel time 0.06 seconds it's going to move by about a meter, so you have to aim about a meter above them when firing spinward, a meter below when firing anti-spinward.