4 ms·

aerogel? plus something in front of it to fragment the larger bits? https://ntrs.nasa.gov/citations/20130012682 https://ntrs.nasa.gov/citations/20130012682 fou

aerogel? plus something in front of it to fragment the larger bits? https://ntrs.nasa.gov/citations/20130012682 https://ntrs.nasa.gov/citations/20130012682 found this idea searching for that actually..

wlesieutre 2y ago
If you fragment the larger bits now you have a bunch of small bits that cross your orbit, are harder to track, and are still going very fast
- capitainenemo 2y ago
  no. like the proposal in the article above. a sandwich of thin light layers that fragment, plus much thicker layers of aerogel behind it to catch the tiny bits. All in one piece, so no bits flying off anywhere. Similar strategy to bullet proof vests actually (deform, then catch), but with a lot lighter of materials.
  - Kye 2y ago
    How do you keep it from tumbling out of control or into the station every time it takes a hit? All that energy has to go somewhere if you catch it.
    - capitainenemo 2y ago
      Hm.. my impression was that most orbital debris was very very small, and the issue was more about making small holes with a lot of energy, than about knocking larger masses around. In any case, if it's transferred enough energy to alter a trajectory, presumably maneuvering thrusters could fix it.
      - JumpCrisscross 2y ago
        > the issue was more about making small holes with a lot of energy, than about knocking larger masses around You’re not letting it pass through with an aerogel-backed whipple shield. I wonder if you angled the whipple members you could preferentially deflect it at a high angle. That way it can carry most of its momentum away from the shield and the shielded station.
        lazide 2y ago
        Unless it is a really large (KG plus) piece of debris - which such a shield wouldn’t be able to stop anyway - the actual net momentum transferred isn’t significant. It’s the point loading over such a short area and time due to the delta in velocities that causes the damage. It’s also why being shot in a bullet proof vest with anything man portable doesn’t knock a person backwards (in real life), but gives them a heck of a bruise. Without the vest, the bullet would go all the way through them, or create a huge wound cavity. [https://whitemountainforensic.com/wound-ballistics-motion-effects-projectiles-human-body/ https://whitemountainforensic.com/wound-ballistics-motion-ef...]. Which is what causes the desired fatal effect. Even when shot with something bigger (such as a tank round), there is no time for the momentum to transfer to their body without destroying it - for the same reason it is impossible to ‘throw’ an egg 20 yards by hitting it with a hammer. These orbital debris are much much much faster than any known traditional bullets (17 km/s is 55,000 fps or 17x faster than the fastest common cartridge), so the effect is even more pronounced. Interestingly, there was work by Newton that models these types of penetrations pretty well. They only start to apply to high speed bullet impacts around 3k+ FPS. The type of impact damage becomes noticably different then, with say 5.56/223 Remington on mild steel having an impact crater that looks remarkably like a asteroid impact crater at close range, but a normal ‘flat’ crater/dent at long range once the velocity has dropped significantly. Once you past the speed of sound in the materials involved, it’s quite predictable based on the density of the material and the length of the projectile. [https://en.m.wikipedia.org/wiki/Impact_depth https://en.m.wikipedia.org/wiki/Impact_depth] It’s why modern tank penetrators are made of long, thin rods of high density material (such as depleted uranium or tungsten), and move very fast. They’ll penetrate essentially anything, with only depth varying based on density, because no material is able to transfer the force within itself effectively to ‘reject’ the penetration in time. Spacing armor also helps, as it gives room for projectile fragments to break apart and lose energy over more area instead of efficiently transferring momentum directly into the point loaded part of the armor, which can dramatically reduce the impact depth in practice. [https://en.m.wikipedia.org/wiki/Spaced_armour https://en.m.wikipedia.org/wiki/Spaced_armour] Angling the armor on the timescales involved in these impacts isn’t going to meaningfully deflect any penetrators though. It does, however, improve the effective depth of the armor. The forces involved in trying to redirect the later parts of the projectile based on the forces subjected to the first parts of the projectile don’t really work once you pass the speed of sound in the material. [https://en.m.wikipedia.org/wiki/Sloped_armour https://en.m.wikipedia.org/wiki/Sloped_armour] Counter intuitive based on our normal life experience, but true. So having many widely spaced layers of hard (to fragment the penetrator) then soft (to ‘catch’ fragments without transferring shock very well into later layers, hence spreading out the force over a wider area and often time) material in as ‘fluffy’ or an armor as possible is the most weight efficient way to stop very high speed projectiles - which is why it is used in space, because weight efficiency is really important. Notably, heavier/denser versions of this (due to space constraints and the need to stop slower moving and heavier projectiles than in space) are widely used in newer generation main battle tanks in the form of chobham style composite armor. [https://en.m.wikipedia.org/wiki/Chobham_armour https://en.m.wikipedia.org/wiki/Chobham_armour] In those cases, sometimes adding explosives on top can help, as the penetration detonates it, and the resulting shockwave helps disrupt the projectile. [https://youtu.be/zK77-8kJ69c?si=PxMue9goABlfuiXY https://youtu.be/zK77-8kJ69c?si=PxMue9goABlfuiXY] There are some neat videos of simulated tank armor penetrations on Youtube (look for “apfsds penetration test” for one example) that shows what is going on in pretty good fidelity. So, to summarize - the aggregate force is not a real issue, it’s the damage done by the nature of the very high speed penetration. Think more ‘gamma radiation’, less ‘brick’.
bell-cot 2y ago
It certainly would not be a point-defense system, but...I'd suggest placing a bunch of maneuverable little satellites in LEO, each one capable of making & high-precision dropping smallish cubes of a fast-evaporating aerogel. Game plan: Have those satellites drop cubes in front dangerous little bits of LEO space junk, to drop each bit of junk's perigee to the Kármán line. No need to match orbits, and you don't need to care (much) about debris from the high-velocity collisions, since all of that will reenter or evaporate within an hour.
- aeternum 2y ago
  The problem is you need a lot of fuel and delta-v for that. IMO lasers is a much better solution. High-power thermal laser ablates a key portion of the space-junk to slightly change its trajectory and preferably help deorbit it. All you need is energy since the reaction-mass comes from the space-junk itself. Maybe you could combine the ideas and instead of ablating to deorbit, you modify the space-junk orbits to collide with a large aerogel satellite. You could potentially even boost it in the process.
  - bell-cot 2y ago
    > The problem is you need a lot of fuel and delta-v for that. Why? If the Bit O'Junk is within ~90% of its apogee, and it hits the aerogel cube at more than ~90 degrees, the impact will yield a fatal drop in Mr. O'Junk's perigee. Ion thrusters on the cube-laying satellites, high-precision orbital tracking, and a few racks of computers on the ground (to calculate orbits and impacts) should be enough. Low-powered lasers would probably be useful in the precision-tracking systems. Blasting space-junk with high-powered lasers is cool...but those immediately get into issues with dual-use, space weapons treaties, and arms race scenarios.
    - bell-cot 2y ago
      (Corrections: "within ~90% of its apogee" should be "within ~45% of its apogee", and "hits the aerogel cube at more than ~90 degrees" should be "hits the aerogel cube at more than ~135 degrees". In both cases, the useful arc is ~90 degrees.)

wlesieutre 2y ago

If you fragment the larger bits now you have a bunch of small bits that cross your orbit, are harder to track, and are still going very fast

capitainenemo 2y ago
no. like the proposal in the article above. a sandwich of thin light layers that fragment, plus much thicker layers of aerogel behind it to catch the tiny bits. All in one piece, so no bits flying off anywhere. Similar strategy to bullet proof vests actually (deform, then catch), but with a lot lighter of materials.
- Kye 2y ago
  How do you keep it from tumbling out of control or into the station every time it takes a hit? All that energy has to go somewhere if you catch it.
  - capitainenemo 2y ago
    Hm.. my impression was that most orbital debris was very very small, and the issue was more about making small holes with a lot of energy, than about knocking larger masses around. In any case, if it's transferred enough energy to alter a trajectory, presumably maneuvering thrusters could fix it.
    - JumpCrisscross 2y ago
      > the issue was more about making small holes with a lot of energy, than about knocking larger masses around You’re not letting it pass through with an aerogel-backed whipple shield. I wonder if you angled the whipple members you could preferentially deflect it at a high angle. That way it can carry most of its momentum away from the shield and the shielded station.
      - lazide 2y ago
        Unless it is a really large (KG plus) piece of debris - which such a shield wouldn’t be able to stop anyway - the actual net momentum transferred isn’t significant. It’s the point loading over such a short area and time due to the delta in velocities that causes the damage. It’s also why being shot in a bullet proof vest with anything man portable doesn’t knock a person backwards (in real life), but gives them a heck of a bruise. Without the vest, the bullet would go all the way through them, or create a huge wound cavity. [https://whitemountainforensic.com/wound-ballistics-motion-effects-projectiles-human-body/ https://whitemountainforensic.com/wound-ballistics-motion-ef...]. Which is what causes the desired fatal effect. Even when shot with something bigger (such as a tank round), there is no time for the momentum to transfer to their body without destroying it - for the same reason it is impossible to ‘throw’ an egg 20 yards by hitting it with a hammer. These orbital debris are much much much faster than any known traditional bullets (17 km/s is 55,000 fps or 17x faster than the fastest common cartridge), so the effect is even more pronounced. Interestingly, there was work by Newton that models these types of penetrations pretty well. They only start to apply to high speed bullet impacts around 3k+ FPS. The type of impact damage becomes noticably different then, with say 5.56/223 Remington on mild steel having an impact crater that looks remarkably like a asteroid impact crater at close range, but a normal ‘flat’ crater/dent at long range once the velocity has dropped significantly. Once you past the speed of sound in the materials involved, it’s quite predictable based on the density of the material and the length of the projectile. [https://en.m.wikipedia.org/wiki/Impact_depth https://en.m.wikipedia.org/wiki/Impact_depth] It’s why modern tank penetrators are made of long, thin rods of high density material (such as depleted uranium or tungsten), and move very fast. They’ll penetrate essentially anything, with only depth varying based on density, because no material is able to transfer the force within itself effectively to ‘reject’ the penetration in time. Spacing armor also helps, as it gives room for projectile fragments to break apart and lose energy over more area instead of efficiently transferring momentum directly into the point loaded part of the armor, which can dramatically reduce the impact depth in practice. [https://en.m.wikipedia.org/wiki/Spaced_armour https://en.m.wikipedia.org/wiki/Spaced_armour] Angling the armor on the timescales involved in these impacts isn’t going to meaningfully deflect any penetrators though. It does, however, improve the effective depth of the armor. The forces involved in trying to redirect the later parts of the projectile based on the forces subjected to the first parts of the projectile don’t really work once you pass the speed of sound in the material. [https://en.m.wikipedia.org/wiki/Sloped_armour https://en.m.wikipedia.org/wiki/Sloped_armour] Counter intuitive based on our normal life experience, but true. So having many widely spaced layers of hard (to fragment the penetrator) then soft (to ‘catch’ fragments without transferring shock very well into later layers, hence spreading out the force over a wider area and often time) material in as ‘fluffy’ or an armor as possible is the most weight efficient way to stop very high speed projectiles - which is why it is used in space, because weight efficiency is really important. Notably, heavier/denser versions of this (due to space constraints and the need to stop slower moving and heavier projectiles than in space) are widely used in newer generation main battle tanks in the form of chobham style composite armor. [https://en.m.wikipedia.org/wiki/Chobham_armour https://en.m.wikipedia.org/wiki/Chobham_armour] In those cases, sometimes adding explosives on top can help, as the penetration detonates it, and the resulting shockwave helps disrupt the projectile. [https://youtu.be/zK77-8kJ69c?si=PxMue9goABlfuiXY https://youtu.be/zK77-8kJ69c?si=PxMue9goABlfuiXY] There are some neat videos of simulated tank armor penetrations on Youtube (look for “apfsds penetration test” for one example) that shows what is going on in pretty good fidelity. So, to summarize - the aggregate force is not a real issue, it’s the damage done by the nature of the very high speed penetration. Think more ‘gamma radiation’, less ‘brick’.

bell-cot 2y ago

It certainly would not be a point-defense system, but...I'd suggest placing a bunch of maneuverable little satellites in LEO, each one capable of making & high-precision dropping smallish cubes of a fast-evaporating aerogel. Game plan: Have those satellites drop cubes in front dangerous little bits of LEO space junk, to drop each bit of junk's perigee to the Kármán line. No need to match orbits, and you don't need to care (much) about debris from the high-velocity collisions, since all of that will reenter or evaporate within an hour.

aeternum 2y ago
The problem is you need a lot of fuel and delta-v for that. IMO lasers is a much better solution. High-power thermal laser ablates a key portion of the space-junk to slightly change its trajectory and preferably help deorbit it. All you need is energy since the reaction-mass comes from the space-junk itself. Maybe you could combine the ideas and instead of ablating to deorbit, you modify the space-junk orbits to collide with a large aerogel satellite. You could potentially even boost it in the process.
- bell-cot 2y ago
  > The problem is you need a lot of fuel and delta-v for that. Why? If the Bit O'Junk is within ~90% of its apogee, and it hits the aerogel cube at more than ~90 degrees, the impact will yield a fatal drop in Mr. O'Junk's perigee. Ion thrusters on the cube-laying satellites, high-precision orbital tracking, and a few racks of computers on the ground (to calculate orbits and impacts) should be enough. Low-powered lasers would probably be useful in the precision-tracking systems. Blasting space-junk with high-powered lasers is cool...but those immediately get into issues with dual-use, space weapons treaties, and arms race scenarios.
  - bell-cot 2y ago
    (Corrections: "within ~90% of its apogee" should be "within ~45% of its apogee", and "hits the aerogel cube at more than ~90 degrees" should be "hits the aerogel cube at more than ~135 degrees". In both cases, the useful arc is ~90 degrees.)