Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

How to defend against lasers?

  1. May 19, 2015 #1


    User Avatar
    Science Advisor

    Suppose lasers become a common weapon for medium range anti-drone use. Maybe they've gotten too good at dodging gunfire. What kind of laser countermeasures exist?
    I was thinking, maybe some kind of mist generator to block out vision of the drone. But that would be limited to drones moving at low speeds, as the mist cloud can't follow the drone. Would it be worthwhile to make the drone out of transparent materials? Maybe the chassis is shaped to refract light away from most sensitive components. But the drone's sensors cannot be transparent.
  2. jcsd
  3. May 19, 2015 #2
    In my setting, i thought, that lasers are effective countermeasures on fighters against missiles, however it shouldnt be too easy to blind permanently something or someone.
    A strong flash of light only temporary blinds a human, it needs a longer exposure to permanently blind. I think something like that should be employed, have filter on the sensors, that allow only a narrow bandwidth to pass through, polarization could also count. Probably the sensor could be made of liquid crystals, that can be rearranged if ruined by the laser. Have sensor arrays sensitive to different wavelengths, use frequency agile radars.
  4. May 19, 2015 #3
    Most cameras (used to?) have physical irises that could shut to block out excess light like the sun. One could expect a response time of 1/60 of a second or faster, since that is the current most common speed of such cameras. Sensors seem to have gotten more rugged, since I don't see warnings about shooting the sun any more.
  5. May 19, 2015 #4
    Are you talking about defending against a directed energy weapon? Like the Star Wars program designed to shoot down ballistic missiles?

    I could imagine one using superconductors. If you could coat the surface with a superconductor, I believe that the instead of burning a localized hole, the energy from the laser would be instantly distributed over the entire superconductor. You could then have prongs sticking out of the surface to leak the extra energy into the atmosphere similar to how a plane leaks static electricity.
  6. May 19, 2015 #5
    For lasers strong enough to heat things, wouldn't a simple reflective coating like chrome reflect most of the energy of the laser away?
  7. May 19, 2015 #6
    Yes, the problem is that they are designed for a very specific type of weapon. A weapon with a slightly different frequency would burn right through it. For example: China already has this technology, but it's reflective coatings are specifically designed to counter American laser weapons. If they get in a fight with Russia though, they'd have to reengineer their coatings. http://www.scmp.com/news/china/article/1444732/us-lasers-pla-preparing-raise-its-deflector-shields
  8. May 19, 2015 #7
    Arent broadband mirrors effective enough?
  9. May 19, 2015 #8


    User Avatar
    Science Advisor

    Well, reflection certainly provides protection, but I don't know if it will be enough. Thin coatings would overheat and lose their reflective power if any power got absorbed.
  10. May 19, 2015 #9
  11. May 31, 2015 #10
    I believe you are referring to a "superconductor of heat" which was invented by Larry Niven for his novel Ringworld.

    In the real world, the only similar thing known to science is superconductors of electricity, not heat.
  12. May 31, 2015 #11
    A heat pipe might distribute the energy over a larger area. The problem with drones as used in a near future scenario would be that they are tiny, and the lasers might be truck-sized, for a weapon with no latency it'd probably mean detection => death is almost instantaneous.

    For a spaceship in a laser battle, I'd pump in coolant to reduce the effect of the beam, the vaporized cloud of coolant could also help by obscuring the target area, dispersing some of the energy. Probably some kind of graphite-oil slurry, it takes a lot of energy to vaporize carbon, and if less extreme heat causes the coolant to harden, an ablative layer could be formed in-situ.
  13. May 31, 2015 #12
    Yes, it would reflect most of the energy away. And the energy that got through will vaporize the chrome plating like a sandblaster.

    In the US military, the minimum threshold for a tactical weapons-grade laser is 100 kilowatts. A chrome mirror is about 99.9% effective, which means 10 watts of laser energy gets through. This is about the same energy as an industrial carbon-dioxide metal cutting laser.

    The chrome mirror will be flayed off, exposing the target to the full strength laser. A standard ICBM will be punctured with a laser energy concentration of about 10 KJ/cm2.

    As newjerseyrunner stated, dielectric mirrors can be up to 99.999% effective, but only on specific laser frequencies. Switch frequencies and they are zero percent effective. And in any event, in the US military the minimum threshold for a strategic weapon-s grade laser is one megawatt.
  14. May 31, 2015 #13
    Two problems with that:

    [1] making a cloud means wasting coolant on areas of space that do not contain your spaceship. And you do not have coolant to spare because on any rocket-propelled vehicle every gram counts.

    [2] if you are hiding inside a cloud, you are prevented from maneuvering around ("jinking") in order to complicate your enemy's targeting solution on you. The cloud cannot maneuver, it travels at the speed and vector it had upon creation. If your ship maneuvers, it will emerge from the cloud and loose its protection.
  15. May 31, 2015 #14
    (1)I misphrased it, any hot material boiling off into space will not form a "cloud" in the common sense of the word, even if the droplets/the soot particles are tiny they will still indeed have a significant velocity. I was thinking of ways to increase the protective effects of ablation, as well as cooling off the areas surrounding the areas undergoing "hostile vaporization". It would of course have a significantly better protective effect if someone tried to drill a hole with a high depth-to-width ratio into my ship rater than heating a surface completely open to space.

    Perhaps the term "jet" or "temporary gas layer" makes more sense.

    (2) If "every gram counts" is true depends on the type of scenario the ship operates in. FTL or no FTL? If STL, high-STL or low? If FTL, at what distance(s) are ships likely to encounter their opponents? What kind of propulsion systems are used, what fuels/reaction mass, how much waste heat do they produce, et cetera. High-STL, for example, must inherently be so efficent that the ships propulsion produces almost no, if any waste heat. Another factor is if a ship uses, say, carbon or iron for reaction mass it'll be easier to armor it than if it uses liquid hydrogen, the lower density means a significantly worse mass/unit area ratio, ergo armor or active cooling systems to prevent damage would be less useful.

    My point is, there are lots of scenarios as per above where "we must have a slightly better delta-vee" simply isn't true. Take something simple, like efficent (input energy to beam energy) and accurate UV-laser beams in a low-thrust scenario. The ship will not be able to maneuver away from such an attack to any greater degree, nor is it likely to be capable of moving fast enough to minimize the time it is exposed as it is being fired at from several light-seconds away. As long at it can get where it is going, acceleration and delta-v are not high enough on the priority list that armor becomes impossible.

    Off topic to a degree, but I really like your site. It got me thinking all those years ago :)
  16. May 31, 2015 #15
    Yes, efforts to increase the protective effects of ablation is an old trick. The way to defeat that is to pulse the beam at about five microsecond intervals.

    Well, as I stated above:on any rocket-propelled vehicle every gram counts.

    Rocket-propelled means no FTL, is STL, and no reactionless drives allowed. This means the vehicle is firmly in the clutches of the Tyranny of the Rocket Equation.
    And as you stated, if any of those are not true, all bets are off.

    Thank you for your kind words!
  17. May 31, 2015 #16
    There is still the time factor. Drones can move fast and unpredictably, while such a heavy laser might be slower. The laser could be it's own worst enemy, pushing a light drone out of it's path.
  18. Jun 1, 2015 #17

    About time factor, it took a significant amount of time to disable that speedboat, and it didnt have reflective armor, didnt spin around its axis (the last one IMHO is a good protection in space)

    Spot size is also a question.
    Otherwise i also like your web site, although i consider TV tropes analyses even better in some scenarios, they consist more viewpoints.

    As far as i know, fuel isnt free for aircraft neither, yet its viable to build bombers, not just ballistic missiles.
    In a (probably quite far) but not FTL future, i dont think its so decisive that due to extra mass, you can achieve only 100km/s closing speed instead of 105 or 110.
    Last edited: Jun 1, 2015
  19. Jun 1, 2015 #18
    And the Lockheed Martin's Area Defense Anti-Munitions laser in your link had a power of 10 kilowatts. Not 100 kilowatts as I was discussing.

    Spinning an ICBM to reduce laser dwell time on the hull is an old trick. But once the power level of the laser is high enough the target cannot spin fast enough. Not without flying into pieces. A good standard laser bolt will be composed of 1000 pulses at 5 microsecond intervals, or a total bolt interval of 0.005 seconds. If the target is a spacecraft big enough to fit a human being inside, the spin velocity to smear the bolt over even a centimeter will be high enough to do damage to the spacecraft.

    Did you read the link I supplied?

    The cost of fuel is not the problem. The upper limit on delta-V is the problem. Delta-V is

    dV = Ve * ln(R)
    dV is the rocket propelled vehicle's total change in velocity capability
    Ve is the exhaust velocity of the rocket engine
    ln(x) is the natural logarithm of x
    R is the mass ratio of the rocket vehicle, mass of rocket with full propellant tanks divided by mass of rocket with empty propellant tanks.

    So if your mass ratio is 20, this means for every gram of coolant you add, you have to add 20 grams of propellant. Every gram counts. Rocket propelled vehicles are very different from bombers.

    I go into this in more detail here.

    And as I stated this only applies to rocket-propelled vehicles. If your spacecraft are not rocket propelled then the limitations are conditional on the details of the fictional engine.
  20. Jun 1, 2015 #19
    A well reflective armored spacecraft is also different from a speedboat. The question is, whether the laser can deliver enough energy (calculate with scattering also) not just to damage the armor, but cause a fatal injury to the spacecraft, before the hit spot moves away.
    If it is so strong, that it can, the question is cooldown time.

    "One important aspect of any ICF research project is ensuring that experiments can actually be carried out on a timely basis. Previous devices generally had to cool down for many hours to allow the flashlamps and laser glass to regain their shapes after firing (due to thermal expansion), limiting use to one or fewer firings a day. One of the goals for NIF is to reduce this time to less than four hours, in order to allow 700 firings a year.[24]"

    While i appreciate your help, I have already read the rocket equation on wiki. Until mass ratio isnt that high ( required delta-V isnt that far from the exhaust veolcity), the equation is close to linear. (For my fictional spacecraft, around 100km/s exhaust velocity for fusion powered interplanetary craft, and a mass ratio around 7, for short-range striker craft, exhaust veolcity 10km/s, delta V around 10km/s, mass ratio around e.)
  21. Jun 1, 2015 #20
    Well, yes, obviously.
    If your laser weapon is the functional equivalent of a blast furnace that produce coherent light as a byproduct, the efficiency will be abysmal. The question then becomes how large your laser heat radiators are. Or if there are hostiles shooting back, the question becomes how large your heat sinks are. Because heat radiators are big targets and generally fragile.

    No, it was my mistake. Since it was not clear to me that you were talking about such ferociously high exhaust velocities, it seemed like the parameters were the standard inconveniently high mass ratios. I had no way of telling what parameter set your statement about insensitivity to propellant fractions came from. In such cases I have a tendency to take an instructive role. Sorry if I appeared condescending.

    But you should see some of the email I get. Psychoceramic.
  22. Jun 1, 2015 #21
    Someone had the idea to build large radiator wings, so they are harder to hit (unless the enemy can attack from multiple directions), but the focusing mirrors themselves are fragile targets also. A dielectric mirror has a very high reflectivity over a narrow bandwidth, but very small over other bandwidths, when it is heated by the first burst, IMHO it will be quite vulnerable, and even low powered/lesser focused lasers can damage a large mirror area, before other protected targets are taken out. Of course the attackers lasers are vulnerable too...
    But if the lasers damage each other, that buys time to get close.
    Well, we, or our descendants will see, which method is efficient, viable.

    Sorry if i were too harsh, well the setting i think about definitally wont be tomorrow, i dont intend to be diamond hard, just dont violate ten laws of physics.
    Otherwise a pretty fine job to collect so much information and graphic on your site. :)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook