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Comment by gingabriska | Hacker News Reader

Don't we simply need a missile covered with mirrors to defy these lasers?

It's just a light bean albeit concentrated one, how hard can it be to deflect it?

Edit: don't understand the downvoting, please enlighten. Your wisdom is not obvious to me. Thanks

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-summ...

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

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.

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>

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.

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.

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.

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.

> 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?

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.

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.

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.

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.

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.

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.

The way you asked the question implied that everyone working on the project must be kind of stupid. "simply" "how hard can it be" while showing a lack of expertise on the subject made you sound conceited.

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

Despite what other commentators say, mirrors would work in theory. In fact lasers themselves have mirrors.

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.

Could a plasma channel surge counter this and destroy the plane?

My grandfather worked on high-energy laser weapons back in the 70s. The laser would, indeed, melt their own mirrors. They had to develop a whole technology of self-deforming mirrors in order to avoid hot spots on the mirrors.

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)

One would hope that they have made fundamental improvements / innovations to the tech in those past 50 years[1], and they’ll just be keeping that advantage secret for a while.

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

It is weird! :)

One of the big changes has been the shift to (mostly) solid-state lasers for this kind of work - it's taken longer to scale those up to weaponized power levels, but they're much easier to work with and miniaturize. The telecoms industry did a lot of the fundamental work in this area for fiber optics purposes. Back in the 70s they were using mostly chemical-powered lasers, which could get high powers without high-performance electrical power sources, but were a pain to work with (hence the use of large airliners).

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.

"normal" mirrors don't really work for high-power lasers. If you try to reflect a high-power laser with your bathroom mirror, in the (very short) first moment it will reflect most of the beam, the part that's not reflected will burn the mirror and suddenly it's not a mirror anymore in the spot that the laser hits.

> don't understand the downvoting

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.

Or they don't understand why their idea is flawed and would like someone to explain so that they may learn why. Also known as, asking questions is the pursuit of knowledge. That this is somehow lost on you makes me cringe, since it reminds me of reddit trolls.

"Don't we simply" "How hard can it be"

Mirrors would work for a low power laser, but not for what they were testing. There are other options though. The simplest one is a spiky elongated nose cone, which would dramatically reduce delivered power density for a missile on head-on trajectory. Add an ablative cone and you can defeat laser power levels realistically available in the near future, compact enough to be placed in a pod under a wing. Bear in mind that chemically powered laser will have limited number of shots available. Electrically charged one (with super-capacitor) will need significant delays between shots. In both cases they can be overwhelmed by a large enough number of missiles.

Would not the laser burning off the ablative armour disturb the trajectory ?

Yep, but that could be countered by putting the missile in a spin (like a rifled bullet) or by active course correction.

Mirrors aren’t 100% efficient which means they can be overwhelmed thermally and if you want your missile to be stealthy, you don’t want it reflective.

Reflectivity and stealth are not mutually exclusive. As long as you don't reflect back to the radar antenna(s), you are invisible. In this case a cylindrical missile with 100% reflective conical front and ceramic fins would be very stealthy head-on.

Reflective mirror surface is good for stealth if the angle is right.

If you could make a flying retroreflector that looked like a missile you could get the plane to shoot itself down.

Insightful. But aiming back would be all kinds of hard. There's very little time before your contraption stops reflecting due to being burned by the unreflected portion of the initial beam.

A retroreflector does not require aiming nor moving parts, it simply uses geometry to ensure that a lightbeam is always reflected back to its source. Retroreflectors are cheap to build and well-understood.

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.

Besides the other problems noted, mirrors and ablative shielding aren't really practical for guided missiles which target aircraft or ships. The missile needs an infrared/optical/radar seeker. So it would be difficult or impossible to find a material which would be electromagnetically transparent for the missile's own seeker but also protect it against incoming laser wavelengths.

That's actually a very good point. Standard dielectric mirror coatings can easily handle tens of kW hitting a ~1cm spot.

Good question - being in a somewhat cloudy part of the world I wonder what effect weather would have on this kind of system. Can you only defend yourself on clear days?

Even if the missile is not destroyed, optical sensor systems must necessarily absorb some light, and will probably be destroyed. This is a possible tactic employed by less powerful lasers, too.

I upvoted you, its a simple question, dont understand the downvoting. Its not everyone's field of expertise.

One defense is to make a missile that spirals around it's trajectory so it is hard to stay locked on.

Missile defense systems have always had a very basic and fundamentally insurmountable weakness that people just willfully ignore. It costs almost nothing to make more decoys, and it costs an astronomical amount to upgrade your system to track and handle more targets. Building any missile defense system is a nearly guaranteed way to destroy yourself. The enemy just builds more decoys for almost nothing, and then you go broke.

Well, you do have to launch those decoys,which takes fuel. If they're aircraft-carried, now the airplane has to carry decoys instead of/in addition to live missiles, so it's now less effective.

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.

So you are on to something. Why build big one missile that the laser will zap? Do it like your body does with sperm. Send a whole swarm of tiny missiles, overwhelm this huge engineering project, and one might get through.

Usually, at least in the case of nuclear weapons, more missiles aren't used but more warheads, with the vast majority being decoys. The way the missiles work is that they break apart once they reach a certain point in the trajectory, making tracking far more difficult. It's the fundamental difference between adding more target tracking and handling capability to an anti-missile system (expensive) to just throwing out more cheap decoys. This has been an opposition to missile defense systems since the early 1980s when they were first introduced and I haven't read about any system that overcomes this weakness.

I understand what MIRV is. However, as another poster pointed out, the problems are apples & oranges and have different solutions. We aren't talking about shooting down re-entering nuclear warheads here.

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