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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-23-20-1... [2] http://ab-initio.mit.edu/book/

> 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.

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.

> 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.

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>

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.

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.

Just rotate the missile in its trajectory? Even ablation and guidance is still possible... -another arm chair general-

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.

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.

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.

Pulsed femtosecond lasers have repeat rates in the megahertz.

Not for petawatt lasers, those are around 1Hz or lower [1].

[1] http://www.eli-np.ro/research-activities-ra1.php

Both systems are cutting edge and a the limit of what we can build. They are not using a continuous beam because it’s inherently better, they are using it because it’s the best compromise.

Was the time-averaged power higher than 50kW? I think that's what we're talking about here. Peak power is a measurement of something else entirely, and I wouldn't be surprised if this military laser were also pulsed. But it's quite uncommon to find a laser with 50 kW of time-averaged power output, let alone on an airplane, which is what makes this impressive.

Yeah, a Petawatt is huge, there is nothing on earth (well, see next sentence) that can provide that sort of average power continuously. For comparison, 4PJ is basically the output of a kiloton nuclear weapon, so one second of a PW laser needs a nuclear weapon powering the thing.

Averaging 50kw is impressive. However, unless you can keep targeting the same area for X unit of time the average power output over X period is not important.

That said, I would be shocked if they can dissipate enough heat to fire a continuous beam for an hour. It’s very likely designed to be pulsed over longer time periods.

> So yeah, pulsed lasers can do it, and I'm certain the Air Force would rather be using them.

Do you really think they are so incompetent so as not to have thought of this?

I’m sure the labs developing these are aware of what you’re suggesting, have probably even tried them, and have their reasons for the design they’ve chosen.

Want to and can are different things.

They likely want to used pulsed lasers assuming average power output stayed the same, but physics is a bitch. So, they don’t get to arbitrarily chose.

Ok, but if we're including things which are currently impossible then I'd assume they'd rather just use Professor X and have him mind blast the missiles out of the atmosphere.

Every new design is building something that does not exist.

We already have pulsed lasers with average power output in the kw range over several hours. Even better, this thing does not need to fire for hours at a time so you can skimp on heat dissipation. But, you can buy 2kw laser cutters off the shelf.

Really, scaling it up in a lab is one thing, but getting everything else to work like dealing with vibration, portability, and targeting is hard. So, while this design had slightly better tradeoffs, that does not say much about the design space they where working with or what they consider useful vs required. Ex: What frequency is this?

This is CW, not pulsed. The only reason short-pulse people get away with calling it "Petawatt" is because the pulse duration and focus are both incredibly small. The total energy content on the other hand is often not that much compared to say the energy content in missile. Also, the Petawatt scale is state of the art, in a lab that is finicky to being hit on the side of the laser table with ball driver. It's not fair to use the state of the art as a figure of merit for a weapon to be deployed in combat situations.

Building a useful air launched missile that spins at a rate capable of being a countermeasure is difficult.

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.

The existing RIM-116 does spin in flight. However it's designed for use on surface ships, not aircraft.

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.

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.

Coat everything in VANTABLACK?

https://en.wikipedia.org/wiki/Vantablack

That'd do the exact opposite of what you want by absorbing most of the light.

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 ..

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.