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-summ...

[2]: https://newatlas.com/lockheed-martin-laser-truck/36377/

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/

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.

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.

Also used when nominative, dative, accusative, genitive and vocative heat shields just won’t do the trick.

</armchair-general>

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.

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.

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.

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.

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

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

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

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.

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.

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.

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?

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.

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.

I suspect the flow of air around the missile provides effective cooling.

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

I mean, I get it .. I'm no physicist and have no idea how any of this works ..

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.

The question itself was fine and got interesting answers. Phrased differently, I don't think anyone would have downvoted you.

High quality mirror material with > 99.8% reflectivity near infrared region would do the job. Multi layer metal substrate, usually copper based or fused silica to withstand thermal expansion and dielectric coating. High macroscopic surface quality is not needed. It's enough that it reflects.

Even the mirrors used in laser erode. Laser weapon uses larger mirror area to focus tighter beam to the target. But unlike laser mirror missile surface don't have to withstand erosion long time and you can rotate the missile to divide the thermal power to larger area.

There are practical reasons why the best practical option is not mirror but material with high thermal resistance. Hypersonic missiles already use those materials and they are protected against current generation of military lasers (50 to few 100s of kilowatts). You need megawatt lasers if you try shooting down HGV.

From the descriptions that gramps used to give me, I'm not sure how what's described here is an advance over what he did back in the late 70s. I guess one thing is that back then, they were nowhere near being able to put the whole assembly into a pod that could be mounted on a fighter jet's wing.

(gramps also worked on the Hubble Telescope, and spy satellites)

[1] Am I getting old or is it weird realizing that 1970 was almost 50 years ago?

Aircraft these days have much better electrical power systems (since all control surfaces and a lot more engine operation is electrically powered), and new aircraft carriers and surface combatants are being designed with about 50-100% margin on electrical power to make them compatible with these systems in the future.

You’re doing that ‘I am very smart and have found an obvious flaw sitting here that none of the professional missile engineers thought of’ thing. Makes a lot of people cringe.

It is a very interesting idea, the reflectors likely will burn but you could cover the missile with them. You only need to reflect enough to disable/weaken the emitter.

Once the missiles are launched, you need to navigate them actively towards the target at roughly the same flight profile as a real missle, otherwise why would the enemy bother engaging them? So they need guidance, which adds more cost and weight.

In the end, you'll end up with a "decoy" that has everything the real missile has except for a warhead, so why not just put a warhead on it?

At that point we're talking about just building more missiles, period.