Bullet flight time conventions

[Ulzgoroth]

Originally Posted by Joseph Paul

So a homing bullet might work because it would be able to make some corrections. Maybe. I have little confidence that small caliber rounds would be able to make much of a course change given they have no thrust to work with and often little altitude to trade. The vision of them doggedly pursuing their mark through an urban apocalypse dodging buildings and the odd burnt out car is not one that I cater to. Whether they would actually help with AA fire or air-to-air combat is something to be considered.

I think you're looking at homing weapons wrong here. A homing bullet doesn't follow its target. (Neither do missiles usually if not fired from a chase position.) It can't do that. But it also doesn't need to. It's watching the target the whole way in - as soon as they start moving, it starts maneuvering to match. It's hard to look this up, but a human probably can't generate more than 1g acceleration over any appreciable period. If the bullet's control surfaces (whatever they are) can produce a mere 1g lateral acceleration, it should have no problem at all tracking somebody who tries to run or dive out of the way of the shot.

Of course, if the target runs or dives behind total cover before the projectile arrives, the smart bullet has no prospects of a hit. It can hit the cover, or it can fly past the cover, but it can't possibly stop and turn a corner. Larger payload rounds might detonate a blast/fragmentation or directional EFP warhead to try to tag someone they can't hit directly in that circumstance, but impact rounds are just out of luck.

[Anthony]

Originally Posted by Ulzgoroth

It's hard to look this up, but a human probably can't generate more than 1g acceleration over any appreciable period.

The physics isn't that hard. Horizontal acceleration, without jumping, is capped at (horizontal offset of center of gravity from feet)/(vertical offset of center of gravity), and is also capped at the coefficient of friction of your feet on the ground. Without specialized setups like a runner's block, you aren't going to achieve 1g at all except by dropping prone.

Note that dumb homers that simply keep pointing at the target rather than predicting the target's path need significantly more acceleration than the target. With the amount of sensors and smarts in a bullet, that's probably the case at least at TL 9.

[RyanW]

Originally Posted by Anthony

Note that dumb homers that simply keep pointing at the target rather than predicting the target's path need significantly more acceleration than the target. With the amount of sensors and smarts in a bullet, that's probably the case at least at TL 9.

Guided missiles generarally use constant bearing, decreasing range rather than classic pursuit. Maybe not the sensor, but 1950s era missile brains could probably be put in a bullet sized package today.

[Gigermann]

Originally Posted by Joseph Paul

I have little confidence that small caliber rounds would be able to make much of a course change given they have no thrust to work with and often little altitude to trade.

Already in development

[DouglasCole]

Originally Posted by Ulzgoroth

If the bullet's control surfaces (whatever they are) can produce a mere 1g lateral acceleration, it should have no problem at all tracking somebody who tries to run or dive out of the way of the shot.

The radius of curvature of a bullet undergoing a 1g lateral acceleration at a velocity of 800m/s is 64 kilometers. That means as the bullet moves 800m in that second, it can bend its path about (800m/(3.14 x 64,000m) = .004 radians, or a bit less than 1/4 of a degree per second.

Now, bullets can withstand much higher loads, and the AIM-9 sidewinder can pull 35g, which will lower the radius of curvature in a linear fashion and thus boost the off-axis rate from 0.25 degrees to about 50 degrees off-axis . . . more than enough to splortch a fleeing human target.

Anything more than about 1-2 degrees of change during a 1-2 second flight time is enough to steer in on a moving vehicle (Move 20 from 800 yards, frex).

[Anthony]

Originally Posted by DouglasCole

The radius of curvature of a bullet undergoing a 1g lateral acceleration at a velocity of 800m/s is 64 kilometers.

For another comparison: a typical 5.56mm bullet loses a bit over 300 fps over 100 yards range, which it takes a bit over a tenth of a second to cover. That's about 90 Gs of deceleration.

[Ulzgoroth]

Originally Posted by DouglasCole

The radius of curvature of a bullet undergoing a 1g lateral acceleration at a velocity of 800m/s is 64 kilometers. That means as the bullet moves 800m in that second, it can bend its path about (800m/(3.14 x 64,000m) = .004 radians, or a bit less than 1/4 of a degree per second.

Now, bullets can withstand much higher loads, and the AIM-9 sidewinder can pull 35g, which will lower the radius of curvature in a linear fashion and thus boost the off-axis rate from 0.25 degrees to about 50 degrees off-axis . . . more than enough to splortch a fleeing human target.

Anything more than about 1-2 degrees of change during a 1-2 second flight time is enough to steer in on a moving vehicle (Move 20 from 800 yards, frex).

...Or, of course, you could look at how much it changes its point of arrival in terms of linear distance, which is what the target actually has to dodge in. Which can safely be approximated as 1/2 A*t^2, since you've helpfully established that the direction of velocity will change negligibly. So at one second, 4.9 meters. Can our human target cover more perpendicular distance from the original aimpoint than that in that one second after the shot is fired?

(Realistically, no. GURPS-wise, possibly if you give them a turn to do it and they're speedy, not encumbered, and facing in a good direction.)

[Joseph Paul]

EDIT - Oh my! Such mathy endeavors! Response below written before seeing these.

Nope, pretty much I understand homing. Pretty sure a lot of people are influenced by popular depictions of it though and I am nixing that.

Problems I see with rifle sized homing are that they stop accelerating once they leave the barrel - they are on a glide path with a steadily shrinking area they can reach. I am dubious of any kind of wing or fin arrangement for the 10-15mm range which leaves active skin or maybe a memory metal jacket that forms a lifting body shape. How that works while spinning several hundred times a second I am not sure. I am not sure if the Copperhead spins or not once the glide wings are deployed but I doubt it. I have my doubts about stability of spinning rounds that try to change direction. Heck they might get into a tumble if CG and CP get out of whack. And I don't know what it costs the round in range to make a turn.

Playing with a triangle calculator and assuming that the 15mm AM rifle does 1000 yps making a 1 deg change at 1000 yards after firing allows the round to move left or right ~139 yards over 8000 yards of travel. Making a 1 deg shift at 8000 yards it only covers ~17.5 yards. A 10 degree shift at the same ranges allows ~1410 yards of shift and ~176 for the last 1000.

These rules for time of flight and how hard a round can turn have to deal with more than paltry humans - various vehicles, powered armor with Superjump, Supers with enhanced move, Teleporters, magic (hello Wanda!). It pays to ask the questions and nail down how it works and of course it is GURPS so it can work lots of ways. I am just skeptical of some of them. :)

[Joseph Paul]

Originally Posted by Gigermann

Already in development

Not a bullet - look at the cutaway - more akin to a gyroc. It is lit up from the moment it comes in frame

[Varyon]

Originally Posted by Joseph Paul

Not a bullet - look at the cutaway - more akin to a gyroc. It is lit up from the moment it comes in frame

Here's a longer video (also a bit more recent, and without the annoyance of apparently splicing in an image from Runaway, at least going off of the comments on the first video). I'm not certain what tracers look like on high-speed cameras, but it's possible the glow is actually from that rather than from a small booster of some flavor. At the very least, I'd absolutely expect for such a test to use tracer rounds for increased visibility.