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Military technology: Magic bullets

economist.com

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Re: Military technology: Magic bullets

#21

Post by sharpneli »

__alexs wrote: post #20
gmaslov wrote: post #13

Earlier quoted context omitted.

Hmm, would the accelerometer really detect anything? I think the bullet would have to experience significant lift, otherwise it's in free fall and the only acceleration you'll see after it leaves the barrel is air resistance, which isn't in the right direction to change with each rotation. To a first approximation, spinning bullets don't experience lift. Maybe there's still some minuscule effect that can be detected,…

I was thinking of the accelerometer being located near the outside of the shell, rather than relatively stationary near the centre of rotation. I agree it's not entirely trivial, but it seems pretty doable to me.

The accelerometer cannot measure the earths gravity which was probably what the GP suggested. However you can measure the centrifugal force (http://xkcd.com/123/) which may be more precise than trying to measure the muzzle velocity or just assuming certain known muzzle velocity.

Re: Military technology: Magic bullets

#22

Post by GraemeL »

arethuza wrote: post #15
Sharlin wrote: post #14

Earlier quoted context omitted.

Technically true; however, in practice, you must rifle the barrel of any grenade launcher, or else have tail fins on the grenades to prevent aerodynamic tumbling. Tank guns, cannons, howitzers, and the like typically have rifled barrels but we don't call them rifles. (Mortars, on the other hand, use finned ordnance.) AFAIK rifles are usually defined to have a caliber at most 20mm.

I thought that tank guns these days are usually smoothbore rather than rifled - with the notable exception of the British Challenger 2: http://en.wikipedia.org/wiki/Challenger_2

True, but the smooth-bore guns used in most other tanks all fire fin-stabilised rounds.

Re: Military technology: Magic bullets

#23

Post by icegreentea »

dimitar wrote: post #3

$25 bullets and $25000 rifles are insanely expensive. And I don't see how better rifles will bring an advantage - most coalition casualties are from mortar fire and improvised explosive traps - rebels have already lost preference for gun standoffs. But I'm sure H&K and Alliant will love to sell lots of these to whatever army is corrupt enough to buy them. Also... aren't exploding bullets illegal under the Hague conve…

The Hague forbids bullets that expand on penetration, on the basis that it creates larger and more traumatic wounds. This round acts more like a grenade. It explodes (and wounds/kill by shrapnel) near the target. It shouldn't actually hit the person. Completely different class of weapon. It's more accurate to call this a grenade launcher than a rifle.

Re: Military technology: Magic bullets

#24

Post by gvb »

__alexs wrote: post #4

Surely it's just a 1 axis accelerometer and perhaps a lookup table or some basic maths? The military sure do like to make themselves seem important. Super secret alien assault rifles being attacked by unspecified hackers from unknown locations. Yep, better keep funding new and exciting ways to kill people.

No. An accelerometer measures change in acceleration.

When the round is fired, it will experience a huge acceleration from the propellant along its axis plus a very large rotational acceleration as the rifling spins the round from 0 revolutions per second to a large number of revolutions per second.

Once the round leaves the barrel, it is falling at a constant acceleration (9.8m/s, freefall, 0G, "weightlessness"), so it will not be able to measure the earth's gravitational force. It also will be spinning at a constant rate because there is no outside force increasing or decreasing the spin rate, thus no rotational acceleration. While there will be acceleration due to the centripetal effect of the rotation, that acceleration is constant and thus not usable with an accelerometer for counting revolutions.

Re: Military technology: Magic bullets

#25

Post by the_tubes »

humbledrone wrote: post #10
__alexs wrote: post #4

Surely it's just a 1 axis accelerometer and perhaps a lookup table or some basic maths? The military sure do like to make themselves seem important. Super secret alien assault rifles being attacked by unspecified hackers from unknown locations. Yep, better keep funding new and exciting ways to kill people.

> Surely it's just a 1 axis accelerometer and perhaps a lookup table or some basic maths? I'm more interested by the mechanism that programs each projectile's detonation distance. That seems like the tricky bit...

I am thinking something like a headphone jack right before it's loaded in the barrel. Press some buttons, cock, shoot. During this time of programming its probably then charged too with a capacitor in the grenade/bullet thing.

EDIT: I just thought of an even better way of doing it! There could be little contacts on the back of it surrounding the part that the hammer hits. During charging, it also programs with spikes of voltage. While it is still loaded the gun, the gun's computer monitors that voltage of the cap to keep it charged so there isn't a lag during when you want to fire.

Re: Military technology: Magic bullets

#26

Post by nhangen »

sbierwagen wrote: post #16

Ah, the XM25. Last publicity blitz we saw on this was a year ago, and Gary Breacher called them out on it: http://exiledonline.com/xm25-gee-whiz-how-can-we-be-losing-w... (Even the file photo is the same!) The XM25 autocannon is neat. Absolutely nobody denies this. It's also pretty useless, because any insurgent who engages security forces in a conventional gun battle is going to die. Nine times out of ten, they're d…

That's not entirely true. There are still plenty of gunfights and ambushes being staged in Afghanistan, especially outside of the major cities.

Re: Military technology: Magic bullets

#28

Post by kabdib »

__alexs wrote: post #12
humbledrone wrote: post #10

Earlier quoted context omitted.

> Surely it's just a 1 axis accelerometer and perhaps a lookup table or some basic maths? I'm more interested by the mechanism that programs each projectile's detonation distance. That seems like the tricky bit...

Something along the lines of RFID? I've heard them described as "radio controlled" before too. Since it's in the chamber protecting it from external sources of interference / attack (e.g. programmed to explode at the end of the muzzle) should be easy to protect against physically. I guess you could do it with some mechanically retractable pins or something, but electrical contacts and the inside of a bullet chamber d…

Presumably they got their security right; no recommit after firing, for instance.

If not, I can easily imagine a "bullet jammer" gizmo that broadcasts a retargeting command. "Sorry, little grenade, you have another thousand meters to go...."

Re: Military technology: Magic bullets

#29

Post by ilaksh »

In this era of global mass instantaneous communications, every citizen of every country is our neighbor. There is no excuse for the continued existence of this disgusting military industrial complex.

The only things keeping this going are racism, ignorance or misperception fueled by propaganda, and a sick, inhuman and outdated Social Darwinism.

We just cannot permit criminals and thugs to continue to have control.

Re: Military technology: Magic bullets

#30

Post by jgrahamc »

My stab at how the bullet measures its own rotation:

Suppose the bullet leaves the muzzle rotating at 300,000 RPM (quite reasonable for a rifle). That is 5kHz. It's easily within the capabilities of a Hall Effect sensor to measure a 5kHz rotation and so I'd make the very centre of the bullet contain a free rotating magnet. The rifle itself sets the magnet spinning at some known rate opposite to the rifling. When the bullet leaves the rifle it is able to measure the time between detections of the magnet and given the known rifling, the known magnet rotation speed it would be able to calculate rotations.

Alternatively, the inner magnet could be spinning in the same direction as the rifling but faster and detect complete rotations of the bullet. Either way I think Hall Effect + spinning magnet at bullet core would work.

Also, 5kHz is well within the operating range of a micro controller so this wouldn't need much computer power. Lastly the article says that the weapon is accurate at 500m and that the shrapnel kills everyone within a few meters. Also says that the soldier estimates the target distance.

That sounds like there's plenty of room for the measurement to be not too accurate. Suppose 'several meters' is 5 then you can get away with an error of 1% or so.

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