Ballistics thought experiment: (Warning: I am not an engineer and barely remember my high school physics)
You make a hollow round steel shield and make it a vacuum inside. You put a slightly smaller steel disc inside the vacuum region. You make that second disc rotate very fast. Very fast indeed.
An incoming projectile hits your shield. It is several times thicker than all three layers of your shield combined. It easily penetrates the first layer, passing through to the vacuum region. It contacts the speedily spinning inner layer, and then things get crazy… the tip of the projectile gets eroded by the spinning inner layer (Yes, it’s spinning faster than the projectile is travelling, I told you it was fast).
I’m not sure what happens next, but I can think of two plausible possibilities:
1. The projectile bounces off the inner layer, because it’s effectively been blocked not just by the material in front of it but by a whole ring of material, which collectively has more mass than the projectile. The ring slows down a bit and maybe has a dent in it now, a perfectly circular groove.
2. The inner layer disintegrates, but so does the projectile. The momentum of the ring of material that impacts the projectile is transferred to the projectile, knocking it sideways and probably breaking it into little melty pieces. The part of the inner layer outside the ring of material is now a free-spinning hula-hoop, probably a very unstable one which splits and shoots fragments in many directions.
Either way, the projectile doesn’t make it through your shield.
So, does this work?
If it works, would it be useful for anything? It seems to be a relatively light form of armor; most armor has to be of similar (or greater) thickness to the projectile in order to block it, but this armor can be an order of magnitude less thick, possibly two or three. (Well, you also have to remember to account for the weight of the outer layer that holds back the vacuum… But I expect that won’t be much, at least when we are armoring against tank shells and the like.)
So maybe it would be useful on tanks or planes, or ships. Something where weight is a big concern.
The center of the shield is a weak point I think. You can just put conventional armor there.
Problem: Aren’t spinning discs harder to move about than still discs? They act like they have lots of mass, without having lots of weight? I feel like I vaguely recall this being the case, and my system 1 physics simulator agrees. This wouldn’t reduce the armor’s effectiveness, but it would make it really difficult to maneuver the tank/plane/ship equipped with it.
Problem: Probably this shield would become explosively useless after blocking the first shot. It’s ablative armor in the most extreme sense. This makes it much less useful.
Solution: Maybe just use them in bunkers? Or ships? The ideal use case would be to defend some target that doesn’t need to be nimble and which becomes much less vulnerable after surviving the first hit. Not sure if there is any such use case, realistically...
This is well-beyond today’s technology to build. By the time we have the technology to build one of these shields we will also have prolific railguns. The muzzle velocity of a railgun today exceeds 3 km/s[1] for a 3 kg slug. As a Fermi estimate, I will treat the impact velocity of a railgun as 1 km/s. The shield must have a radial velocity much larger than the incoming projectile. Suppose the radial velocity of the shield is 100 km/s, its mass is 1000×3kg=3000kg and you cover a target with 100 such shields.
The kinetic energy of each shield is E=12Iω2. The moment of inertia is I=12mr2. We can calculate the total kinetic energy of n shields.
ETOT=14nmr2ω2=14nmv2=10043000kg(100,000ms)2=75TJ
This is an unstable system. If anything goes wrong like an earthquake or a power interruption, the collective shield is likely to explode in a cascading failure. Within Earth’s atmosphere, a cascading explosion is guaranteed the first time it is hit. Such a failure would release 75 terajoules of energy.
For comparison, the Trinity nuclear test released 92 terajoules of energy. This proposed shield amounts to detonating a fission bomb on yourself to block to a bullet.
So, does this work?
Yes. The bullet is destroyed.
Aren’t spinning discs harder to move about than still discs? They act like they have lots of mass, without having lots of weight?
Spinning a disk keeps its mass and weight the same. The disk becomes a gyroscope. Gyroscopes are hard to rotate but no harder to move than non-rotating objects. This is annoying for Earth-based buildings because the Earth rotates under them while the disks stay still.
In reality this is significantly limited by air resistance but air resistance can be partially mitigated by firing on trajectories that mostly go over the atmosphere.
I had been hoping that the shield, while disintegrating and spraying bits of metal, would do so in a way that mostly doesn’t harm the thing it is protecting, since all its energy is directed outwards from the rotating center, and thus orthogonal to the direction the target lies in. Do you think nevertheless the target would probably be hit by shrapnel or something?
Huh. If gyroscopes are no harder to move than non-rotating objects… Does this mean that these spinning vacuum disks can be sorta like a “poor man’s nuke?” Have a missile with a spinning disc as the payload? Maybe you have to spend a few months gradually accelerating the disc beforehand, no problem. Just dump the excess energy your grid creates into accelerating the disc… I guess to match a nuke in energy you’d have to charge them up for a long time with today’s grids… More of a rich man’s nuke, ironically, in that for all that effort it’s probably cheaper to just steal or buy some uranium and do the normal thing.
A cookie-cutter was shaped like an aspirin tablet, except that the top and bottom were domed more to withstand ambient pressure; for like most other nanotechnological devices a cookie-cutter was filled with vacuum. Inside were two centrifuges, rotating on the same axis but in opposite directions, preventing the unit from acting like a gyroscope. The device could be triggered in various ways; the most primitive were simple seven-minute time bombs.
Detonation dissolved the bonds holding the centrifuges together so that each of a thousand or so ballisticules suddenly flew outward. The enclosing shell shattered easily, and each ballisticule kicked up a shock wave, doing surprisingly little damage at first, tracing narrow linear disturbances and occasionally taking a chip out of a bone. But soon they slowed to near the speed of sound, where shock wave piled on top of shock wave to produce a sonic boom. Then all the damage happened at once. Depending on the initial speed of the centrifuge, this could happen at varying distances from the detonation point; most everything inside the radius was undamaged but everything near it was pulped; hence “cookie-cutter.” The victim then made a loud noise like the crack of a whip, as a few fragments exited his or her flesh and dropped through the sound barrier in air. Startled witnesses would turn just in time to see the victim flushing bright pink. Bloodred crescents would suddenly appear all over the body; these marked the geometric intersection of detonation surfaces with skin and were a boon to forensic types, who cloud thereby identify the type of cookie-cutter by comparing the marks against a handy pocket reference card. The victim was just a big leaky sack of undifferentiated gore at this point and, of course, never survived.
Somewhat similar to https://en.wikipedia.org/wiki/Ablative_armor, but I don’t think it actually works. You’d have to put actual masses and speeds into a calculation to be sure, but “spinning much faster than the bullet/shrapnel moves” seems problematic. At the very least, you have to figure out how to keep the inner sphere suspended so it doesn’t contact the outer sphere. You might be able to ignore that bit by just calculating this as a space-borne defense mechanism: drop the outer shield, spin a sphere around your ship/habitat. I think you’ll still find that you have to spin it so fast that it deforms or disintegrates even without attack, for conventional materials.
It’s the easy solution to many problems in mechanics—put it in space, where you don’t have to worry about gravity, air friction, etc. You already specified that your elephant is uniform and spherical, so those complexities are already taken care of.
Ballistics thought experiment: (Warning: I am not an engineer and barely remember my high school physics)
You make a hollow round steel shield and make it a vacuum inside. You put a slightly smaller steel disc inside the vacuum region. You make that second disc rotate very fast. Very fast indeed.
An incoming projectile hits your shield. It is several times thicker than all three layers of your shield combined. It easily penetrates the first layer, passing through to the vacuum region. It contacts the speedily spinning inner layer, and then things get crazy… the tip of the projectile gets eroded by the spinning inner layer (Yes, it’s spinning faster than the projectile is travelling, I told you it was fast).
I’m not sure what happens next, but I can think of two plausible possibilities:
1. The projectile bounces off the inner layer, because it’s effectively been blocked not just by the material in front of it but by a whole ring of material, which collectively has more mass than the projectile. The ring slows down a bit and maybe has a dent in it now, a perfectly circular groove.
2. The inner layer disintegrates, but so does the projectile. The momentum of the ring of material that impacts the projectile is transferred to the projectile, knocking it sideways and probably breaking it into little melty pieces. The part of the inner layer outside the ring of material is now a free-spinning hula-hoop, probably a very unstable one which splits and shoots fragments in many directions.
Either way, the projectile doesn’t make it through your shield.
So, does this work?
If it works, would it be useful for anything? It seems to be a relatively light form of armor; most armor has to be of similar (or greater) thickness to the projectile in order to block it, but this armor can be an order of magnitude less thick, possibly two or three. (Well, you also have to remember to account for the weight of the outer layer that holds back the vacuum… But I expect that won’t be much, at least when we are armoring against tank shells and the like.)
So maybe it would be useful on tanks or planes, or ships. Something where weight is a big concern.
The center of the shield is a weak point I think. You can just put conventional armor there.
Problem: Aren’t spinning discs harder to move about than still discs? They act like they have lots of mass, without having lots of weight? I feel like I vaguely recall this being the case, and my system 1 physics simulator agrees. This wouldn’t reduce the armor’s effectiveness, but it would make it really difficult to maneuver the tank/plane/ship equipped with it.
Problem: Probably this shield would become explosively useless after blocking the first shot. It’s ablative armor in the most extreme sense. This makes it much less useful.
Solution: Maybe just use them in bunkers? Or ships? The ideal use case would be to defend some target that doesn’t need to be nimble and which becomes much less vulnerable after surviving the first hit. Not sure if there is any such use case, realistically...
This is well-beyond today’s technology to build. By the time we have the technology to build one of these shields we will also have prolific railguns. The muzzle velocity of a railgun today exceeds 3 km/s[1] for a 3 kg slug. As a Fermi estimate, I will treat the impact velocity of a railgun as 1 km/s. The shield must have a radial velocity much larger than the incoming projectile. Suppose the radial velocity of the shield is 100 km/s, its mass is 1000×3kg=3000kg and you cover a target with 100 such shields.
The kinetic energy of each shield is E=12Iω2. The moment of inertia is I=12mr2. We can calculate the total kinetic energy of n shields.
ETOT=14nmr2ω2=14nmv2=10043000kg(100,000ms)2=75TJ
This is an unstable system. If anything goes wrong like an earthquake or a power interruption, the collective shield is likely to explode in a cascading failure. Within Earth’s atmosphere, a cascading explosion is guaranteed the first time it is hit. Such a failure would release 75 terajoules of energy.
For comparison, the Trinity nuclear test released 92 terajoules of energy. This proposed shield amounts to detonating a fission bomb on yourself to block to a bullet.
Yes. The bullet is destroyed.
Spinning a disk keeps its mass and weight the same. The disk becomes a gyroscope. Gyroscopes are hard to rotate but no harder to move than non-rotating objects. This is annoying for Earth-based buildings because the Earth rotates under them while the disks stay still.
In reality this is significantly limited by air resistance but air resistance can be partially mitigated by firing on trajectories that mostly go over the atmosphere.
Lovely, thanks! This was fun to think about.
I had been hoping that the shield, while disintegrating and spraying bits of metal, would do so in a way that mostly doesn’t harm the thing it is protecting, since all its energy is directed outwards from the rotating center, and thus orthogonal to the direction the target lies in. Do you think nevertheless the target would probably be hit by shrapnel or something?
Huh. If gyroscopes are no harder to move than non-rotating objects… Does this mean that these spinning vacuum disks can be sorta like a “poor man’s nuke?” Have a missile with a spinning disc as the payload? Maybe you have to spend a few months gradually accelerating the disc beforehand, no problem. Just dump the excess energy your grid creates into accelerating the disc… I guess to match a nuke in energy you’d have to charge them up for a long time with today’s grids… More of a rich man’s nuke, ironically, in that for all that effort it’s probably cheaper to just steal or buy some uranium and do the normal thing.
Reminds me of cookie cutters from The Diamond Age
Somewhat similar to https://en.wikipedia.org/wiki/Ablative_armor, but I don’t think it actually works. You’d have to put actual masses and speeds into a calculation to be sure, but “spinning much faster than the bullet/shrapnel moves” seems problematic. At the very least, you have to figure out how to keep the inner sphere suspended so it doesn’t contact the outer sphere. You might be able to ignore that bit by just calculating this as a space-borne defense mechanism: drop the outer shield, spin a sphere around your ship/habitat. I think you’ll still find that you have to spin it so fast that it deforms or disintegrates even without attack, for conventional materials.
Mmm, good point about space-based system, that’s probably a much better use case!
It’s the easy solution to many problems in mechanics—put it in space, where you don’t have to worry about gravity, air friction, etc. You already specified that your elephant is uniform and spherical, so those complexities are already taken care of.