4 ms·

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,

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.
- deleted 7y ago
  [deleted]
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-

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.
deleted 7y ago
[deleted]

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-