10 ms·

No, it doesn't work like that. The military is of course tight-lipped about exactly how powerful SHiELD is, but at least one site claims that the goal is 50kw [

No, it doesn't work like that. The military is of course tight-lipped about exactly how powerful SHiELD is, but at least one site claims that the goal is 50kw [1]. That's a lot of power. A few years ago, a 30kw laser test made the rounds in news outlets for destroying a few targets during testing. [2] Mirrors don't perfectly reflect light in all frequencies. Mirrors are designed to reflect mostly visible light, lasers at this power level are infrared. Any imperfections in the reflective surface would immediately heat up, damaging the reflective surface, which would then heat up even more. And even if it were perfect, and were designed to reflect whatever frequency of radiation the laser's putting out, it's still not going to reflect 100% of the laser's output, and you've probably got yourself a material that's pretty impractical for a rocket. [1]: https://phys.org/news/2018-03-air-fighter-mounted-laser-summer.html https://phys.org/news/2018-03-air-fighter-mounted-laser-summ... [2]: https://newatlas.com/lockheed-martin-laser-truck/36377/ https://newatlas.com/lockheed-martin-laser-truck/36377/

wnkrshm 7y ago
Since atmopsheric transmission windows dictate which kind of wavelengths you'd use for a laser weapon and which kinds of wavelengths can efficiently be created, multilayer coatings could conceivably reflect a lot of incoming power. In the 90s, the US DoD asked for 'laser armor' and got photonic crystal structures (1-d multilayer) that omnidirectionally reflect [1]. Further details about this are also available in the book "Photonic Crystals" (preprint for free here [2]). What is probably way easier though (and actually survives launch and particles hitting it and is already proven in practice) is to use ablative heat shields on missiles. The materials available are already optimized for mass vs. effectiveness due to the engineering constraints in space flight. Missile optics would probably need to be coated though or distributed. [1] https://www.osapublishing.org/ol/abstract.cfm?uri=ol-23-20-1573 https://www.osapublishing.org/ol/abstract.cfm?uri=ol-23-20-1... [2] http://ab-initio.mit.edu/book/ http://ab-initio.mit.edu/book/
- AlanSE 7y ago
  > What is probably way easier though (and actually survives launch and particles hitting it and is already proven in practice) is to use ablative heat shields on missiles. The word "ablative" is typically used in reference to reentry. As the vehicle encounters incredible heat as it comes back into the atmosphere (due to compression heating or whatever), the shield intentionally has some part of its surface gradually boil off. Let me get what you're saying straight. Superpower A would launch an ICBM, and then Superpower B would then laser it as it flies through the air or space. As the missile absorbs the laser heat, its surface starts to boil off, but if the laser doesn't boil off enough, then the laser counter-attack on the missile could still be survivable.
  - 0xDEFC0DE 7y ago
    Plus delaying the time to destroy a single missile is still useful, even if it doesn't eventually make it, since an actual engagement is likely to involve more than one missile being launched. You could throw enough missiles to overwhelm a laser defense system and you might still come out cheaper than the laser defense system. </armchair-general>
  - wnkrshm 7y ago
    Ablative is also used in terms of the mechanism of carrying away the heat with the evaporating material. A non-ablative heat shield like the space shuttle's just takes a lot of energy to heat up but will still carry all that energy with it and it'll slowly diffuse into connected components (that then heat up a lot due to their lower specific heat capacity). Edit: I didn't think of space but in space this could even be more viable since the ejecta from the ablative heat shield will just stay with a body in free fall and further scatter incoming radiation at least a little. I rather thought about shorter engagements where a missile only takes tens of seconds and needs to be destroyed quickly. Each second an ablative shield buys improves the chances of a hit.
  - JackFr 7y ago
    > The word "ablative" is typically used in reference to reentry. Also used when nominative, dative, accusative, genitive and vocative heat shields just won’t do the trick.
- Symmetry 7y ago
  An ablative shield will would help protect a missile from a laser burning through it but it won't protect form the shock wave caused by the rapid heating of the missile surface. I'm not actually sure which of those is the important mechanism in this case. But more importantly I'm not sure how you can have one and still have a missile guidance system on board, and also they tend to be pretty heavy.
  - wnkrshm 7y ago
    I don't know anything about mechanical shock waves due to rapid heating in the material but in terms of the weight of an additional heat shields and their effectiveness I've found this: AVCOAT 5026-39 which apparently is an ablative heat-shield material for NASA's Orion (and has been developed during the Apollo years) has a density of 0.51g/cm³ so about half the density of water and one fifth that of aluminum (of which a guided missile is made mostly, I believe). From reading some reports of the performance of Apollo era heat shields it's clear though that reentry is a way more gentle affair in terms of thermal load, than a 50kW laser in a 1cm2 spot. The endured peak heat flux per area is about 0,48 kW/cm2 for the heat shield, magnitudes off, if the 50kW laser can achieve a 1cm2 spot. The total amount of heat flux the ablator was required to endure results in a total amount of heat energy/area of 0,0141 kWh/cm2 - which 50kW in a 1cm2 spot would saturate in about 1 second. Reentry lasted 800-1000 seconds.
  - quakeguy 7y ago
    Just rotate the missile in its trajectory? Even ablation and guidance is still possible... -another arm chair general-
Retric 7y ago
50kw is actually not that powerful in terms of lasers. Petawatt systems are being built. The real question is how how much power you can deliver to a target. In that context rotating reflective missiles make this much harder as does slightly thicker casings. Anything you deploy optimized for today’s missiles is easy to design around, so you need to design for countermeasures.
- coredog64 7y ago
  Building a useful air launched missile that spins at a rate capable of being a countermeasure is difficult.
  - Retric 7y ago
    On it’s own I agree, but even fairly slow rotation such as 1 RPM becomes useful on edge cases. That’s not hard to achieve. My point is if you find the minimum power output to be X kw, you want to deploy something at ~4x power output to deal with fairly easy modifications. PS: Stealth capabilities is probably a bigger long term issue, but that’s a large modification.
  - nradov 7y ago
    The existing RIM-116 does spin in flight. However it's designed for use on surface ships, not aircraft.
- chongli 7y ago
  50kw is actually not that powerful in terms of lasers. Petawatt systems are being built. The 50kW laser that the air force wants is a sustained laser. This is not at all comparable to the petawatt lasers used by scientists, which produce pulses on the order of a femtosecond. Such a brief pulse would not be able to destroy a missile since the total amount of energy delivered is very low.
  - nthompson 7y ago
    Pulsed femtosecond lasers have repeat rates in the megahertz. The total energy is very high. When I took laser physics in grad school, they were able to drop a razor blade through the beam and slice it in half. That was in 2013. So yeah, pulsed lasers can do it, and I'm certain the Air Force would rather be using them. They are finnicky though.
D_Alex 7y ago
If you only reflect a portion of the power, say 80%, then only 10 kW is absorbed as heat. I suspect the flow of air around the missile provides effective cooling.
fit2rule 7y ago
Coat everything in VANTABLACK? https://en.wikipedia.org/wiki/Vantablack https://en.wikipedia.org/wiki/Vantablack
- Sir_Vival 7y ago
  That'd do the exact opposite of what you want by absorbing most of the light.
  - fit2rule 7y ago
    But wouldn't it be distributing it across the total surface area of VANTABLACK - like, isn't the point that the energy is dissipated across a much wider surface? I mean, I get it .. I'm no physicist and have no idea how any of this works ..
    - kosievdmerwe 7y ago
      Lasers destroy things by pumping energy into them and melting them or melting through them. Painting the missile black makes it so that it absorbs energy more efficiently. Making the laser more effective. This is why people are suggesting mirrors as mirrors reflect energy away without absorbing it, but other people have comments about why that won’t work perfectly. Think of wearing a white vs black T-shirt on a sunny day. Which shirt is hotter? From my experience it’s the black one since it absorbs all the sunlight and heats up; whereas, the white shirt reflects a large amount of it and heats up less.
bufferoverflow 7y ago
Dielectric mirrors can reflect 99.9% of light in a wide spectrum, and 99.999% for narrow spectrum and limited angles. So even if we take the 99.9% one, your 50kW laser will only heat it with 50W of power, which is nothing compared to the heating an ICMB (or its warhead) has to withstand.

wnkrshm 7y ago

Since atmopsheric transmission windows dictate which kind of wavelengths you'd use for a laser weapon and which kinds of wavelengths can efficiently be created, multilayer coatings could conceivably reflect a lot of incoming power. In the 90s, the US DoD asked for 'laser armor' and got photonic crystal structures (1-d multilayer) that omnidirectionally reflect [1]. Further details about this are also available in the book "Photonic Crystals" (preprint for free here [2]). What is probably way easier though (and actually survives launch and particles hitting it and is already proven in practice) is to use ablative heat shields on missiles. The materials available are already optimized for mass vs. effectiveness due to the engineering constraints in space flight. Missile optics would probably need to be coated though or distributed. [1] https://www.osapublishing.org/ol/abstract.cfm?uri=ol-23-20-1573 https://www.osapublishing.org/ol/abstract.cfm?uri=ol-23-20-1... [2] http://ab-initio.mit.edu/book/ http://ab-initio.mit.edu/book/

AlanSE 7y ago
> What is probably way easier though (and actually survives launch and particles hitting it and is already proven in practice) is to use ablative heat shields on missiles. The word "ablative" is typically used in reference to reentry. As the vehicle encounters incredible heat as it comes back into the atmosphere (due to compression heating or whatever), the shield intentionally has some part of its surface gradually boil off. Let me get what you're saying straight. Superpower A would launch an ICBM, and then Superpower B would then laser it as it flies through the air or space. As the missile absorbs the laser heat, its surface starts to boil off, but if the laser doesn't boil off enough, then the laser counter-attack on the missile could still be survivable.
- 0xDEFC0DE 7y ago
  Plus delaying the time to destroy a single missile is still useful, even if it doesn't eventually make it, since an actual engagement is likely to involve more than one missile being launched. You could throw enough missiles to overwhelm a laser defense system and you might still come out cheaper than the laser defense system. </armchair-general>
- wnkrshm 7y ago
  Ablative is also used in terms of the mechanism of carrying away the heat with the evaporating material. A non-ablative heat shield like the space shuttle's just takes a lot of energy to heat up but will still carry all that energy with it and it'll slowly diffuse into connected components (that then heat up a lot due to their lower specific heat capacity). Edit: I didn't think of space but in space this could even be more viable since the ejecta from the ablative heat shield will just stay with a body in free fall and further scatter incoming radiation at least a little. I rather thought about shorter engagements where a missile only takes tens of seconds and needs to be destroyed quickly. Each second an ablative shield buys improves the chances of a hit.
- JackFr 7y ago
  > The word "ablative" is typically used in reference to reentry. Also used when nominative, dative, accusative, genitive and vocative heat shields just won’t do the trick.
Symmetry 7y ago
An ablative shield will would help protect a missile from a laser burning through it but it won't protect form the shock wave caused by the rapid heating of the missile surface. I'm not actually sure which of those is the important mechanism in this case. But more importantly I'm not sure how you can have one and still have a missile guidance system on board, and also they tend to be pretty heavy.
- wnkrshm 7y ago
  I don't know anything about mechanical shock waves due to rapid heating in the material but in terms of the weight of an additional heat shields and their effectiveness I've found this: AVCOAT 5026-39 which apparently is an ablative heat-shield material for NASA's Orion (and has been developed during the Apollo years) has a density of 0.51g/cm³ so about half the density of water and one fifth that of aluminum (of which a guided missile is made mostly, I believe). From reading some reports of the performance of Apollo era heat shields it's clear though that reentry is a way more gentle affair in terms of thermal load, than a 50kW laser in a 1cm2 spot. The endured peak heat flux per area is about 0,48 kW/cm2 for the heat shield, magnitudes off, if the 50kW laser can achieve a 1cm2 spot. The total amount of heat flux the ablator was required to endure results in a total amount of heat energy/area of 0,0141 kWh/cm2 - which 50kW in a 1cm2 spot would saturate in about 1 second. Reentry lasted 800-1000 seconds.
- quakeguy 7y ago
  Just rotate the missile in its trajectory? Even ablation and guidance is still possible... -another arm chair general-

Retric 7y ago

50kw is actually not that powerful in terms of lasers. Petawatt systems are being built. The real question is how how much power you can deliver to a target. In that context rotating reflective missiles make this much harder as does slightly thicker casings. Anything you deploy optimized for today’s missiles is easy to design around, so you need to design for countermeasures.

coredog64 7y ago
Building a useful air launched missile that spins at a rate capable of being a countermeasure is difficult.
- Retric 7y ago
  On it’s own I agree, but even fairly slow rotation such as 1 RPM becomes useful on edge cases. That’s not hard to achieve. My point is if you find the minimum power output to be X kw, you want to deploy something at ~4x power output to deal with fairly easy modifications. PS: Stealth capabilities is probably a bigger long term issue, but that’s a large modification.
- nradov 7y ago
  The existing RIM-116 does spin in flight. However it's designed for use on surface ships, not aircraft.
chongli 7y ago
50kw is actually not that powerful in terms of lasers. Petawatt systems are being built. The 50kW laser that the air force wants is a sustained laser. This is not at all comparable to the petawatt lasers used by scientists, which produce pulses on the order of a femtosecond. Such a brief pulse would not be able to destroy a missile since the total amount of energy delivered is very low.
- nthompson 7y ago
  Pulsed femtosecond lasers have repeat rates in the megahertz. The total energy is very high. When I took laser physics in grad school, they were able to drop a razor blade through the beam and slice it in half. That was in 2013. So yeah, pulsed lasers can do it, and I'm certain the Air Force would rather be using them. They are finnicky though.

D_Alex 7y ago

If you only reflect a portion of the power, say 80%, then only 10 kW is absorbed as heat. I suspect the flow of air around the missile provides effective cooling.

fit2rule 7y ago

Coat everything in VANTABLACK? https://en.wikipedia.org/wiki/Vantablack https://en.wikipedia.org/wiki/Vantablack

Sir_Vival 7y ago
That'd do the exact opposite of what you want by absorbing most of the light.
- fit2rule 7y ago
  But wouldn't it be distributing it across the total surface area of VANTABLACK - like, isn't the point that the energy is dissipated across a much wider surface? I mean, I get it .. I'm no physicist and have no idea how any of this works ..
  - kosievdmerwe 7y ago
    Lasers destroy things by pumping energy into them and melting them or melting through them. Painting the missile black makes it so that it absorbs energy more efficiently. Making the laser more effective. This is why people are suggesting mirrors as mirrors reflect energy away without absorbing it, but other people have comments about why that won’t work perfectly. Think of wearing a white vs black T-shirt on a sunny day. Which shirt is hotter? From my experience it’s the black one since it absorbs all the sunlight and heats up; whereas, the white shirt reflects a large amount of it and heats up less.

bufferoverflow 7y ago

Dielectric mirrors can reflect 99.9% of light in a wide spectrum, and 99.999% for narrow spectrum and limited angles. So even if we take the 99.9% one, your 50kW laser will only heat it with 50W of power, which is nothing compared to the heating an ICMB (or its warhead) has to withstand.