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.
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