August 22, 2021

Lasers at Sea Part 3

In our previous two installments, I've taken a look at the technology behind lasers and their potential effectiveness. But now it's time to look instead at their drawbacks, and possible countermeasures against them.

A plane modified to defend against lasers

The simplest theoretical defense is to coat the target in mirrors, but this doesn't work as well as might be supposed. The highest reflectivity comes from dielectric mirrors, but these are specialized against specific wavelengths, and probably even specific geometry, so a more conventional mirror will be needed. This will definitely cut the range at which the laser starts doing damage, although the mirror is unlikely to continue to be particularly reflective after it starts to take damage. This, combined with the closer range at which damage starts, means that a target is likely to go from being fine to being dead quite quickly. Mirrors will still be helpful, although at the cost of raising visual signature, which might open up new guidance options for defensive weapons.

Defenses are more likely to come in one of two forms: armor and obscurants. Armor is exactly what it sounds like, material that is designed to slow down the laser's damage, and reduce the number of targets the weapon can deal with in a given amount of time. As we saw last time, there are materials such as graphite1 where even a thin layer will take quite a while to burn through at range. Expect these to become more common on sophisticated weapons over the next few years, as designers try to work out the lowest-cost way of defeating a potentially hostile force. Every second it takes to kill a missile at range is one second the laser can't be going after another missile, and graphite is a relatively cheap material by aerospace standards. Graphite also has the advantage of being transparent to radio waves, which allows a radar seeker to be placed behind the armor. IR and optical guidance are impossible to armor and quite easy to blind with lasers, leaving radar as the obvious winner.

The missile of the future?

But while armor is good for missiles, it's less useful in other contexts. Drones are likely to benefit some, but if they're actually drones and not just missiles being called drones because it sounds cool, long-term survivability and thus surveillance potential is still not great. Boats are even more of a problem. In theory, an entire one could be covered with graphite, but this is likely to get expensive, and I'm not sure how well graphite would hold up in the marine environment.2 But these raise a different proposition, based on the fact that lasers are made of light, and thus can be blocked by things like rain, smoke and fog which obscure normal vision, known unsurprisingly as obscurants.

Obscurants can be divided into those that occur naturally, such as rain and fog, and those produced artificially, primarily various forms of smoke. The high energy of lasers means that they will interact rather differently with these than normal light. Even in cases where there may be a partial line of sight through the obscurant for normal vision, the laser is likely to be affected far more. As discussed in part one, one of the main things acting to spread a laser beam out on the target is thermal blooming, when energy absorbed from the beam turns the air into a lens. Obviously, any energy lost to the obscurant will accelerate this process. In theory, it should be possible to burn through the obscurant with enough time and rely on the adaptive optics to counter the blooming, but in practice, this is likely to be able to sharply reduce the effectiveness of the laser, particularly as the target is likely to keep moving, bringing fresh obscurant into the line of sight.3 A bigger problem is that lasers are precision weapons, reliant for their effect primarily on their ability to hit pinpoint targets where they can do the most damage. A smokescreen makes this far more difficult, although in theory a sufficiently precise radar linked to the laser should be able to see through the smoke and give the data needed.

Obscurants are used to defend ships against laser attack

So what can we conclude from this? Obviously, lasers are far less use in rain and fog, which most other modern weapons are not particularly fazed by. Exactly how this will dictate tactics is not quite clear, and probably depends on the efficacy of other anti-laser systems. Anti-laser armor fitted to missiles will also be quite helpful, particularly for closing targets, but smokescreens are difficult to impossible for high-speed weapons. There has been some talk of providing UAVs with smokescreens, particularly out of China, but I'm skeptical of the practicality of such plans, given the area that would need to be covered and the great hindrance it would be to the UAV's mission. For surface ships, things are rather different. Existing countermeasures systems like SRBOC could easily be adapted to deploy anti-laser smokescreens, which will likely provide significant protection, at least if the winds aren't high. Of course, ships engaging each other at close range hasn't happened for decades and is unlikely to resume happening in the near future. There may be some use for this kind of system in low-intensity conflict, however, depending on norms for the use of lasers. Improvised obscurants may also be useful for protecting the small boats of non-state actors like terrorists and pirates, although in that case, the ship being targeted should be able to take it as a sign of overt hostility and use more conventional weapons.

One last countermeasure system that I ran across while doing research for all of this was an active jammer system which fires a laser back to disrupt the signal responsible for beam tracking. In practice, I suspect this is going to turn into another ECM vs ECCM battle, just like the dozens fought in the radio band over the last century. Lasers seem posed to be the biggest technological shift in naval warfare since the development of computerized combat systems and networking. Exactly how they will impact the balance of power on the world's oceans is not yet certain, but enough information exists to suggest that, much like every other weapon ever developed, they will be a useful addition to arsenals without rendering all previous weapons obsolete. For the next decade or so, these systems are most likely to be useful in situations short of an all-out war, where they give new and unprecedented capabilities. Beyond that, they'll form a useful inner ring to defenses, but only in good weather, and the lack of all-weather capability will keep missiles and guns viable for a long time to come.

1 It's worth noting that graphite may not actually be the ideal material for this, but it should be at least broadly representative of laser-resistant materials, and I don't want to go down a materials-science rabbit hole on this.

2 Note that this is different from using some graphite as internal armor over vital areas, which I could very much see being incorporated into warships in the next few decades.

3 There's also the possibility that the obscurant could be specially engineered to turn into a plasma that still absorbs the laser. There's more discussion of this in the comments.


  1. August 22, 2021ike said...

    Bean, it makes me smile to know that you use knife-sharpened pencils.

  2. August 22, 2021incurian said...

    Delivering smoke via shell airburst may be a way to deliver precision obscuration for spread out friendlies from a centralized battery. They've developed some smoke shells that are fast like WP but thicker and longer lasting.

  3. August 22, 2021bean said...


    Sorry to burst your bubble, but I grabbed the photo off wikimedia. I can't stand dull pencils, and thus use .5 mm mechanicals exclusively.

  4. August 22, 2021Lambert said...

    Would shooting the at the sea between the laser and its target with a 4.5" or so gun kick up enough water to obscure it?

  5. August 22, 2021Chris Silvia said...

    Extremely useful and thanks for this series. I enjoy this theory/future focus, and I'm curious if you have any plans to look into other future technologies in development, such as railguns (or other advanced guns).

    It's also very interesting to see the conclusion that laser weapon systems don't render all other weapons systems obsolete. That was what I thought going in, but you convinced me otherwise. Especially since vertical launch cells are relatively self-contained, I see lasers and missiles co-existing for decades

    The lower range on lasers also convinces me that lasers don't effect the "are aircraft carriers still viable" debate, as naval aircraft are unlikely to get within laser range of their targets if firing standoff weapons.

    The one application of land-based lasers which I didn't see explored was opposing an amphibious assault. Landing craft are similar to the USS Cole reference scenario for surface combat. Ease of re-targeting would mean a laser weapons system could engage a large number of landing craft in quick succession. Laser weapons could also be mounted on natural hills to have additional horizon range.

  6. August 22, 2021Chris Silvia said...

    On the topic of armor for surface targets, I think combined-arms with Lasers and artillery is potent. Heat-seaking artillery or rocket artillery can hit the hot spots of armored targets, causing damage as well as potentially cracking or detaching the armor.

  7. August 22, 2021bean said...


    There's definitely enough water in a shell splash to interrupt a beam. But I don't think one splash would be enough to obscure an entire target, and they don't last very long. Also, the time it takes to get a splash is pretty long relative to how quickly the laser can react. You'd do better if you used something like the Russian anti-submarine mortars than a normal gun, but I don't think it's a good strategy on the whole.


    I’m curious if you have any plans to look into other future technologies in development, such as railguns (or other advanced guns).

    Not immediately. Lasers are the only really new thing on the horizon, which leaves a lot more room to look at implications. It also helps that I'm a lot more familiar with them.

    The one application of land-based lasers which I didn’t see explored was opposing an amphibious assault.

    Interesting thought. I think your analysis makes sense, although it's worth pointing out that ships are much better at transporting stuff than land vehicles are, so I'm not sure you can just assume a shipboard laser is plopped on a hill.

    Heat-seaking artillery or rocket artillery can hit the hot spots of armored targets, causing damage as well as potentially cracking or detaching the armor.

    I know I mentioned signature enhancement in Part 1, but I'm not sure we're likely to see a case where a laser heats something enough to actually weaken it but can't kill it, so you have to use artillery. In which case, just use a laser designator.

  8. August 22, 2021DampOctopus said...

    Armoring against lasers seems to be a matter of how quickly you can transfer heat away from the illumination point.

    If I were designing a shipboard system for armoring against lasers, I'd have a transparent outer layer, backed by a layer of seawater doped with just enough dye to absorb the beam. Unless the laser delivers enough power to pop the imperfectly-transparent outer layer, it has to boil an arbitrary quantity of seawater before it burns through to your internals.

    With a bit of design work, an inflatable boat could incorporate a layer of armor like this, and vital components (engine, etc.) could be hidden behind it.

  9. August 23, 2021Anonymous said...


    Would shooting the at the sea between the laser and its target with a 4.5" or so gun kick up enough water to obscure it?

    Probably better to just boil some water and shoot the steam in whichever direction you're worried about taking a laser from, or even just send a spray of water over the side without bothering with boiling it.

    OTOH it'll make you very visible.

  10. August 23, 2021redRover said...

    Re laser defense, what about glorified fire hoses, either to disrupt the beam at some distance from the ship, or with a bit more work creating a deluge system where the ship is covered in an inch (or whatever) of water, such that the laser has to boil the water before it can actually get to the underlying metal.

  11. August 23, 2021bean said...

    The firehose/washdown system is something I didn't think of, but it seems like it would work quite well. Not sure exactly how the economics work out relative to smoke, but there's really no way for a plausible laser to burn through it, and you should be able to keep it up for quite a long time.

  12. August 23, 2021FXBDM said...

    I want to commend you on your pictures for this article. I've always liked your more whimsical side.

  13. August 23, 2021redRover said...


    I am sure there are more complications to it than just this, but my (rough) math suggests that for a 500kW laser hitting 30C (fresh) water, you would only need a flow of about 0.2L/s (just vapor) or ~1.5L/s (no vapor) through the swept area, also assuming that the vessel can withstand 100C temps for a while in the swept area (as well as what's on the other side of it).

    While 2L/sec is not a huge amount, sustaining it over the whole side of the ship seems like it would take a decent amount of horsepower and piping.

  14. August 23, 2021bean said...


    Thanks. It was mostly "I've used a bunch of laser photos and...Oh, hey, the B-58 was silver", and went from there.


    I don't think it's that bad. Note that for the 500 kW laser, you're losing 30% of your power at 5 km to aerosol scattering. And I suspect the "just vapor" number is probably closer to reality, as the vapor is going to tend to disrupt/absorb/disperse the beam. I'm not sure that washdown is a slam-dunk, but it's at least likely to push the effective distance in some, and make life generally harder for the laser.

  15. August 23, 2021Doctorpat said...

    Moving away from the sea, I note that land armour these days is moving towards being coated with a layer of active armour, ie. explosive plates (imagine claymore mines, but not really) aimed outwards. The idea being that the explosive goes off when an armour penetrating shell or missile is about to hit, and smashes the explosive head and/or kinetic penetrator so it no longer will penetrate the main armour. Of course you need a thick enough underlying armour that having an explosive mine go off on the outer surface is something you can just shrug off. So tanks and things, not HMMWVs.

    My point being: Being able to laser up an enemy tank means you can set off, or at least burn, their active armour, leaving them vulnerable to missiles. And of course having the active armour explode is disturbing to any unarmoured vehicles or humans that are next to them.

  16. August 24, 2021incurian said...

    I think modern tanks already automatically deploy smoke on detection of rangefinder lasers, they could probably do the same against mean lasers.

  17. August 25, 2021Emilio said...

    This post reminded me of the Traveller RPG Universe, where starship can mount sandcaster to cast sand on the trajectories of LASERs...

  18. August 25, 2021bean said...

    Those wouldn't actually work. When you run the math on lasers in space (keeping in mind the very different environments and power levels) it comes out that the mass is more efficiently used as conventional armor. This actually threw me when I first started looking into obscurants, but the power levels are different enough to make it work.

  19. August 25, 2021AlexT said...

    What's the logic of using sandcasters as space laser defenses? You're launching your armor in an ever-expanding, ever-thinning cloud, instead of keeping it welded on to your ship where it has a far greater chance to eat incoming damage. The idea makes sense vs kinetics, but lasers..?

  20. August 25, 2021bean said...

    It sounds obviously ridiculous if you frame it that way (which is correct, to be clear) but much less so if you frame it as "lasers are light, and we can deal with them by blocking the light". It's not intuitively obvious that you're dealing with light which can cut through the block, and that's not even true in all cases, as smokescreens and the like seem to work pretty well against plausible laser powers in-atmosphere.

  21. August 25, 2021DampOctopus said...

    What’s the logic of using sandcasters as space laser defenses?

    Same logic as kinetics, I think. A kinetic round hits a sand grain, vaporizing both of them, so your hull gets hit by a dispersed cloud of vapor rather than a concentrated solid round. A laser hits a sand grain, vaporizing it and being slightly refracted through the resulting turbulent vapor, so your hull gets hit by a broad, dispersed beam rather than a concentrated one.

    Without having run the numbers, I seriously doubt that this is practical, but it's not quite as silly as you've framed it.

    The real-world version of this, used for defense against micrometeoroids, is called a Whipple shield: you leave the armor attached to your spacecraft, but with some spacing, so there's room for the impactor to disperse before it hits your main hull.

  22. August 25, 2021Alexander said...

    A big difference between smoke in an atmosphere, and a vacuum, is how quickly it disperses. Even ignoring different power levels, smoke should be more effective in an atmosphere.

  23. August 25, 2021Lambert said...

    Presumably the problem with pocket sand vs space lasers is that the particles vaporise and disperse pretty quickly, compared to particles in air which a) are able conduct heat to the air and b) they don't diffuse away even once they are superheated or vaporised. Ideally, you'd want to find something that promotes ionisation of the air, so the laser just heats up a cloud of opaque plasma.

  24. August 25, 2021bean said...

    It goes further than just dispersion. In an atmosphere, the smoke can use the air around it as a heat sink, while in space, it has to deal with all of the energy itself. I'm not sure exactly how much laser energy you need to pump into a smoke cloud to clear it out, but I'd guess it involves raising all of the smoke/air mixture in the path of the beam by several hundred degrees. That said, it looks like less time than I expected. If I did my math right, I'm getting about 7000 deg C/second for a 150 kW HELIOS beam at 5 km going through a 10 m thick cloud. (I'm sure this isn't the exact number, but it's almost certainly within the right order of magnitude, and it's a lot more than I intuitively expected.) Maybe obscurants won't work as well as I thought.

  25. August 25, 2021Alexander said...

    Perhaps not, but they'd be even less useful in a vacuum. Could they make it difficult to keep the beam hitting the same spot or targeting a particularly vulnerable point, at least?

  26. August 26, 2021bean said...

    They'd definitely do that, although Lambert's suggestion of making sure that any resulting plasma is doped to be opaque would work pretty well. That's going to spread the heat out a lot, and you might end up having to turn half the smokescreen into plasma to burn through it.

  27. August 26, 2021ike said...

    Just to be clear, are we talking about soot smokescreens or mineral smoke screens (TiCl4 etc)?

  28. August 26, 2021Lambert said...

    I'd use a metal carbonate if possible, so it decomposes endothermically into an oxide. Yttrium carbonate looks like a decent option if you can find a way to disperse it. The oxide will boil at over 4,500K, compared to TiO2's 3250.

  29. August 26, 2021ike said...

    Keep in mind, even if the enemy LASER boils away some of your titania particles, those atoms are still there more or less in the path of the beam. They can still absorb and scatter light to some extent, and as cool air mixes in from other parts of the cloud they will enthusiastically recondense.

    I don't doubt there are better anti-LASER smokes than titania (though don't under estimate it. TiCl4 is very cheap and easy to handle). Keep in mind your smoke generator will want a liquid fuel that burns to your desired product - probably a metal hailide, hydride, or alkide.

  30. August 27, 2021bean said...

    @Doctorpat (slightly belated)

    Reactive armor isn't new. It dates back to the late 60s/early 70s, and has some fairly obvious limitations as well as advantages. There are some newer systems which get around a few of those limitations, but in either case, I don't really see lasers changing the balance much. Or being nearly as useful on land as they are at sea.

  31. August 27, 2021redRover said...


    Are you planning to talk about "what is going to power the lasers" at some point?

    500kW is fairly small in the scheme of things, so (at least with IEP) it seems like you could probably run it continuously off the main engines until the laser unit overheats, but I think the more power hungry options have things like flywheels and compulsators that build on rail gun theory.

  32. August 27, 2021Blackshoe said...

    This thread rules

  33. August 27, 2021bean said...

    Doesn't seem much point right now. None of the plausible lasers are going to exceed ship's power any time soon, so they'll just plug in.

  34. October 18, 2021ec429 said...

    @FXBDM: Whimsy is always welcome, but the absence of a proper caption left me curious as to the identity of the flying-boat in the Obscurants picture. According to it's a Felixstowe F.5L — a licence-built American variant of the British F.5 (itself partially an evolution of the American Curtiss H-12).

  35. October 27, 2022EngineOfCreation said...

    As for protecting against a laser, would simply rotating a missile be efficient? It would expose much more material to be ablated before actual damage is done. It would work for both crossing and closing missiles, except when the laser is aimed at the head of a closing missile, i.e. exactly down the rotational axis. Maybe you could also start rotating it only when it has line of sight to the target or its escorts.

    How much survival time could the missile gain through rotation, and would it be worth it compared to lost capability like range, payload, other defenses?

  36. October 27, 2022DampOctopus said...

    When the missile is illuminated, the laser is ablating material across one side of it. If the missile is spinning, the laser has to ablate material from both sides. So, as a rough guess, spinning the missile means it takes twice as long to destroy it with a laser.

    It could be a bit better than this, because the surface on the far side of the missile can cool off a bit between exposures, and the ablation is - because of missile-body geometry, not laser focus - a bit more concentrated than just on half of the missile. But it could also be substantially worse, as you say, when the laser is aimed at the head of a closing missile.

    The impression I get from Bean's posts is that laser range is a strong limitation, so most laser missile defence is likely to be a ship defending itself, rather than another ship it's escorting. That suggests most engagements will be of closing missiles, in which case spinning is more-or-less useless.

  37. October 27, 2022EngineOfCreation said...

    @DampOctopus Twice the life time would be a conservative lower bound. If the hot spot is exactly large enough to cover the whole height of the missile, meaning the hot spot evenly distributes its energy over that area without wasting any, then only the very center line, where the laser hits at the largest angle (perpendicular in the best case), receives the max amount of energy per armor thickness resulting in the smallest time until destruction. The areas above and below curve away from the beam and so are increasingly protected by hull/armor. So I assume the laser user wants the smallest possible hot spot in any situation when the goal is to burn through the target. Therefore, rotating a missile should afford a much higher lifetime than only twice - if the hotspot covers an average of e.g. 10% of the missile's circumference and the missile rotates quickly enough to not die while going through those 36°, then the rocket's lifetime would increase to 10x because the laser has to ablate ten of those 36° arcs.

  38. October 28, 2022DampOctopus said...

    Let me refine that lower bound a bit. If the ablation rate is proportional to the laser power, and the laser illuminates across the entire diameter of the missile, then spinning the missile means you have to ablate around the entire circumference, increasing the lifetime by a factor of pi.

    This is still a lower bound, because ablation rate increases faster than linearly with laser power: at low power, the equilibrium temperature of a patch of missile skin may be low enough that it doesn't ablate at all. So, particularly at the outermost effective range, ablation may only occur on patches of missile skin which are most nearly normal to the incoming light. And, as you point out, I'm implicitly assuming that the period of the missile's spin is substantially shorter than the time required to destroy it, and that the laser spot size is larger than the missile diameter.

    How reasonable is that last assumption? Picking some vaguely-appropriate figures from Bean's posts, let's say we have a primary mirror of diameter D=0.4m, an infrared laser wavelength of lambda=1.5um, and a range of r=10km. The diffraction-limited spot size is roughly r*lambda/D = 4cm, versus a missile diameter for e.g. Harpoon of 34cm.

    The diffraction limit sets a lower limit for the laser spot size: it will be broadened by atmospheric refraction, imperfect focusing, etc. Whether that will increase the spot size by a factor ~10 ... it's plausible, but I don't know. But certainly, as the range closes, there will come a point at which the spot size is substantially smaller than the missile diameter, making my assumption wrong.

    It's also possible that ablation is entirely the wrong model here. Bean mentioned in this post that ABM laser tests usually kill the missile by weakening the missile body through heating, and letting aerodynamic forces do the rest. If that happened slowly enough, with the heat conducted throughout the entire missile body, then it wouldn't matter at all whether it were spinning or not.

  39. October 28, 2022Anonymous said...

    You've also got any negative effects spinning may have on guidance, no good saving the missile if it just ends up crashing harmlessly into the ocean.

  40. October 29, 2022AlexT said...

    negative effects spinning may have on guidance

    What would those be exactly, other than requiring more complex flight control logic? Hardly seems fatal as long as there's time to work out the math and design it properly.

    Actually, doesn't the RAM do exactly that?

    ABM laser tests usually kill the missile by weakening the missile body through heating

    Took that to mean weakening a small portion of the airframe, not half-melting the whole thing, am I wrong?

  41. October 29, 2022bean said...

    In theory, yes, spinning would help, although I believe that the laser's tracking may well be precise enough to track its spot even as the target spins. In which case, you're looking at something very slightly above twice as long to penetrate. Of course, it's also going to make the missile designer's life considerably harder. Yes, it can be done, as with RAM, but it's not trivial and RAM is a pretty simple missile compared to a full ASM.

  42. October 29, 2022Doctorpat said...

    And all the images of a spinning missile are assuming some sort of ballistic missile, with small tailfins at most. Our current live demonstration of modern war features winged cruise missiles at least as commonly as ballistic ones. You aren't getting much of a spin on them.

    Remembering of course Bean's previous comments about how you shouldn't overweight current battles, featuring weapons and vehicles that can be decades old, and designed even earlier. They might teach lessons that do not apply well to stuff being designed now, that will have to deal with weapons designed decades hence.

    Though once again, this is illustrated so clearly in the current war, where tanks designed in the early 1960s are being shelled by artillery from the 1940s being directed by satellite guided drones from the late 20teens.

  43. October 31, 2022DampOctopus said...

    In which case, you’re looking at something very slightly above twice as long to penetrate.

    Compared to the case in which the laser is focused on the midpoint of the missile body, most nearly orthogonal to the beam, the beam spends half the time ablating the skin with efficiencies ranging from that maximum value down to zero, and half the time on the wrong side of the missile. The time to penetrate is increased, I think, by a factor of at least pi.

    Possibly more importantly, it also decreases the effective range. The increase in time to penetrate is only a factor of pi at short range, but it's substantially larger near maximum range, where the equilibrium temperature is only high enough to cause ablation when the laser focus is close to the midpoint of the missile body.

    Took that to mean weakening a small portion of the airframe, not half-melting the whole thing, am I wrong?

    For ABM lasers the ranges should be substantially longer, so I guess they're in the regime in which the laser focus covers a significant fraction of the missile. For shooting down ASMs, I'd expect it's more likely to get localised heating or burn-through.

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