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Military technology: Magic bullets

economist.com

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Re: Military technology: Magic bullets

#81

Post by robotresearcher »

gvb wrote: post #39
gvb wrote: post #24

Earlier quoted context omitted.

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…

I got it wrong in my first sentence - in order for an accelerometer to be used, there has to be changes in acceleration proportional to distance travelled. Per the rest of the discussion, there isn't any.

You still have it wrong. An accelerometer measures acceleration.

Users usually calibrate the device at rest to get a good estimate of G so that it can be subtracted from accelerations measured later. Is that what you mean?

Re: Military technology: Magic bullets

#82

Post by robotresearcher »

__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.

All the positive acceleration happens in a very short time. It might be hard to measure it frequently and accurately enough to use the integrated results as a velocity estimate.

Perhaps measuring the negative acceleration caused by air friction could give you a velocity estimate (the velocity-to-friction function is probably non-linear). But this may vary a bit with air density, temperature, etc.

I'd be interested to see how it's done. My bet is that it's not directly measuring a near-impulse acceleration.

Re: Military technology: Magic bullets

#84

Post by gvb »

robotresearcher wrote: post #81
gvb wrote: post #39

Earlier quoted context omitted.

I got it wrong in my first sentence - in order for an accelerometer to be used, there has to be changes in acceleration proportional to distance travelled. Per the rest of the discussion, there isn't any.

You still have it wrong. An accelerometer measures acceleration. Users usually calibrate the device at rest to get a good estimate of G so that it can be subtracted from accelerations measured later. Is that what you mean?

No. A falling object is at "0G", it is "weightless" (in a "drop seat" type carnival ride, when the ride triggers, you have no force on your butt). A bullet is a falling object after it leaves the gun. You cannot measure earth's gravitational attraction from the bullet's frame of reference because it is 0. Therefore, you cannot use earth's gravitational attraction to measure the bullet's spin rate because the value is zero, and zero has no direction information.

Re: Military technology: Magic bullets

#85

Post by joezydeco »

mhb wrote: post #37
jgrahamc wrote: post #30

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…

Ten years ago, I had a $2 top with LED lights which cleverly displayed words in a fixed position by using a sensor to use the earth's magnetic field as a reference.

If the top cost $2, it probably was not a magnetometer clocking the LEDs in that toy.

Re: Military technology: Magic bullets

#86

Post by olh »

lusr wrote: post #42
jgrahamc wrote: post #30

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…

Any take on why a simple timer isn't sufficient?

For 500 meters against the wind it would not guarantee a stable velocity, thus not a stable path length over certain time (it would probably be non-linear to calculate).

So, it seems to exist a correlation between the RPM of the projectile and its velocity over wind's influence that guarantee a linear calculation over the RPM of the projectile.

Re: Military technology: Magic bullets

#90

Post by Sukotto »

kkartik wrote: post #88

Am I the only one horrified by both the technology, and the general apathy to the moral implications of military tech in the comment thread.

The morality is in deciding when it is worth ending a human life. What tool you use is a distant second, at best. In war, once you have decided to kill a person, the moral course is to kill them as quickly and effectively as possible. Preferably with as little pain as possible.

The tools are horrible, because war is horrible. If you're opposed to their use, you need to act at the political level (to stop unnecessary war from happening) or the strategic level (to win the conflict in a way that does not require direct, physical confrontation)

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