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Directed energy weapons

  1. Jan 30, 2011 #1
    hello. ive come to physics forum because when it comes to physics, ill admit im clueless. So im looking for some learned assistance. Im writing a book, its not about physics, but a portion of the book has to do with the viability of directed energy weapons i.e. lasers etc. Its a political book, not a physics book, so please answer in terms of someone who has an iq probably quite a bit lower than the average physics forum poster and knows little about actual physics. So here goes.

    Part of my book as i stated has to do with the viability of directed energy weapons. So my questions are as follows. The military has developed a 100kw directed energy weapon. You can go to youtube and see it. Im thinking bigger however and i am aware that there exists a 1.3 petawatt "laser" or whatever its called. So one of the premises of my book is developing a system to defeat the intercontinental ballistic missile (ICBM), and doing so with directed energy defense. So my question to the physics forum experts is what exactly would happen if an ICBM would be hit by the beam from a 1.3 petawatt laser? From what ive read this beam splits atoms so as a physics moron if someone could please put in laymens terms exactly what would happen if said icbm came in contact with such a defense? Also what would be the detrimental factors? Are there any environmental hinderances? Are there any hindrances to the beam hitting the target? Is this possible in either theory or in reality? Are we talking vaporizing this target or are we talking this beam not getting to the target at all for some reason or when it gets there being completely ineffective? At what wattage would the target be vaporized if at all? Ive seen 40 watt lasers cut through steel. So If i was to target an icbm with 1 quadrillion watts even for a femtosecond, what exactly would be the resulting damage to said target? And what would be the time frame/ wattage in general i may need to destroy said target?

    I appreciate any help anyone on here could give me. thanks.
  2. jcsd
  3. Jan 31, 2011 #2
    Have you read http://en.wikipedia.org/wiki/Directed-energy_weapon ?

    The problem with using Lawrence Livermore's petawatt laser http://en.wikipedia.org/wiki/National_Ignition_Facility (NIF) as a weapon is 1) it is huge. It wouldn't even fit on an aircraft carrier. 2) while the peak power is a petawatt, the average power is tiny. Kind of like a camera strobe; bright for an instant but dark the rest of the time. It only fires a few times a day, if that. 3) the beam would "bloom" or defocus in the atmosphere and little would be focused on the target (unless the laser and target are in space). 4) There is nothing to aim the beam. It is only focused on a single point in a test chamber.

    There is talk of using a laser of a kilowatt of power for defense against swarms of miniature drones each of which weigh an ounce or so. Given the low power level, resultant light weight, and likelihood of the threat- might become a common battlefield weapon. http://en.wikipedia.org/wiki/Micro_air_vehicle
  4. Jan 31, 2011 #3
    The idea behind defensive lasers targeted at ICBM's are very specific.
    The laser does not vaporize the entire missle.
    Rather, the laser vaporizes a small portion of the exterior shell surrounding the launch fuel, causing it to leak and/or ignite; resulting in catastrophic dynamics for that missle.
  5. Jan 31, 2011 #4
    Thanks for the assistance. What im looking for though moreso is not the viability of the current technology, but the potential of such technology. If i could build 50 1.3 petawatt lasers, could I destroy icbms? and what effect does a weapon of that type of power have on a target should it strike such target. basically, if i was to make a direct hit on an incoming icbm with this weapon, what would be the results as far as damage? none? complete destruction of the target?

    Thanks for the assistance.
  6. Jan 31, 2011 #5


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    Well as said, yes it is possible for a laser to destroy an ICBM - or more correctly, damage it enough that aerodynamic forces destroy it.

    You don't need, nor do you want a laser in the petawatt class for this task, but if you used one, you'd probably be able to vaporize the ICBM. A megawatt class laser has been used to melt enough to destabilize them and cause them to disintegrate.
  7. Jan 31, 2011 #6
    What would be the hindrance of such a weapon? not meaning size and cost etc., but the hindrance of the actual beam connecting with the target? What would be the timeframe of contact i would need to complete the destruction? would it be immediate? or would i need extended contact with the target? would the destruction have to occur during the boost phase or is it conceivable that i could target/destroy multiple independent reentry vehicles? would it take a beam the size of a laser pointer to do it, or is it feasible scientifically to create a beam with a wider target area if necessary?
  8. Feb 1, 2011 #7
    Formally, the destruction should occur during the boost phase; as the laser can effect fuel explosion, thus destroying the missile.
    However, the laser system must be relatively close to the launch site or initial boost trajectory.

    That aspect can be a huge problem as it might be difficult to have such a system close enough, especially with highly mobile launch platforms and/or launch facilities many hundreds of miles within the protected interior of the offensive country.

    Because of this, much focus has and is being drawn towards dealing with the reentry phase.

    The now defunked SDI program(satellite-based) seeked to address this, and now airborne laser systems are on the same track, yet MUST be in the air at the time, and very much near the reentry trajector(ies)

    Yeah, it's a tough one...
  9. Feb 1, 2011 #8
    So then is it within the laws of physics to have a surfaced based missle defense system? Can this be done? I need to hit them in any phase. I need to be able to create a defense strong enough to stop an icbm, i need a laser(s) powerful enough to immediately destroy an icbm regardless of whether its in the boost phase, middle phase or reentry. the imporatnce of it is simple. Throughout history, the advantage in war has swung between the extremes of technologies offense and defense. At this point, because of the icbm, the advantage lies with the offense, and the world is catching up. The next swing of the pendulum is to the advantage of defense. The ability to stop the icbm delivery system is the next step, the next advantage in war.

    So i guess the main jist of my question, is a surfaced based laser based missile defense system feasible within the laws of science, using lasers of the strength that currently exist? if so why not? This may be getting into the more theoretical than realistic, but theoretical is fine, as long as it can be conceivablly done. Im not concerned about cost or positioning or where they have to be. I can put them where they need to be and pay the cost of them. I just need to know from a scientific standpoint, can it be done, even in theory.
  10. Feb 2, 2011 #9
    Correction on my previous post: It appears that airborne laser missle defense systems are designed for boost-phase interdiction of such missle, not reetry interdiction.
    My apologies.

    Source: http://www.boeing.com/defense-space/military/abl/ [Broken]
    Last edited by a moderator: May 5, 2017
  11. Feb 2, 2011 #10


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    The answer, for at least the third time, is yes. Your constraints need some work and there are technical issues that may make it impractical (but not impossible), but otherwise, again, yes, it certainly can be done.

    Note: In general, the only ICBM you can shoot down in the boost-phase from a ground-based laser is your own. The Earth will tend to get in the way of shooting at an enemy's missile in the boost phase.
  12. Feb 2, 2011 #11


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    Staff: Mentor

    It's my understanding that some of the inherrent limitations of this are why it was cancelled:

    Range limits plus boost-phase use mean it has to be located near the threat. That's fine for an AEGIS destroyer in international waters off the coast of Japan, but it's more complicated, expensive and less safe to operate an Airborne Laser in the same location, for the same purpose. It's probably OK if you're protecting against Iraqi SCUDs for a couple of months, but it gets expensive if you want a handful always hanging around North Korea just in case Lil Kim is a belligerent drunk.
  13. Feb 2, 2011 #12
    This is great. This is what im looking for. What technical issues make it impractical? And this may make no sense, but if i could park these wherever i wanted, why would the earth get in the way? I can put it anywhere. If i can put it on a 747 i can put it anywhere no? yes? I can put it on a submarine? i can put it on an island somehwere, i can put it on a boat? i can take the earth out of the way? yes? no? and what are my constarints? this is great. this is exactly what im looking for. Thanks.
  14. Feb 2, 2011 #13
    Uh, I think opium is his problem, but that's another story... :wink:
  15. Feb 2, 2011 #14
    Well, can you shine a laser from the grouind at NYC and hit a plane flying over the Soviet Union?

    The earth's curvature comes into serious play at even marginal distances.
    I would seriously doubt that a ground laser at NYC could even hit a plane 600 miles away at 30,000 feet altitude.
  16. Feb 2, 2011 #15
    what distances?
  17. Feb 2, 2011 #16
    at what distance does it come into play? please include in your explanation if you would fire a ground based laser and hit an icbm in the middle phase and reentry phase whatever those altitudes are.
  18. Feb 3, 2011 #17
    Here's a general primer:


    For observers on the ground with eye-level at h = 5 ft 7 in (5.583 ft), the horizon is at a distance of 2.89 miles (4.65 km).
    For observers standing on a hill or tower 100 feet (30 m) in height, the horizon is at a distance of 12.25 miles (19.71 km).

    Source: http://en.wikipedia.org/wiki/Horizon

    This is "line of sight" A requirement for laser interdiction.

    Obviously this changes with high altitude reentry missiles. Don't know the math with respect to your question.
  19. Feb 3, 2011 #18
    Can I ask, what would be the effect of a Zettawatt laser in atmosphere? If you had enough power, would you see ElectroWeak effects, and if so, what would they be?
  20. Feb 3, 2011 #19
    also in follow up to nicodemus, what are electroweak effects and at what power would you need your weapon to be to still be effective considering taking such effects into account?
  21. Feb 5, 2011 #20
    I just found this, and I remembered doing some math once to find the distance to cirrus clouds just at the horizon just for fun. (Yes, not only was I interested with clouds but I also did math about clouds for fun. Probably pretty normal here, but as a highschooler that's pretty funny where I come from...)

    Certainly you know high school math, algebra?

    So I'm no scientist, but I'm sure this is right and will give your range. You can put this ground-based laser system anywhere not in a hostile nation, since naturally anybody launching ICBM's at you is probably hostile, and countries like NK probably wouldn't take kindly to us planting ICBM-killing laser sites on their soil...

    So range is very, very important.

    I drew a right triangle, it assumes that anything at 90* is in Line-of-sight, and that the laser is on the ground, with an airborne target. Given these facts, this is the range of the system:

    The three points are "The Laser", on the surface. This point has the 90* angle to the other two. "The Target", at some altitude in the air, and the center of the Earth, which has some obscenely small angle to the other two. (This angle isn't important.)
    The line connecting "The Laser" to the Center of the Earth is parallel to our Y-Axis, and is equal to the radius of the Earth, "r".
    The line connecting "The Target" and "The Laser" is parallel to the X-axis, leaving the distance in-between the target and the center of the Earth as the Hypotenuse.
    The hypotenuse line is equal to the radius of the Earth, "r", plus the altitude of the target, "Alt".

    H = r + alt

    r^2 + D^2 = H^2

    D^2 = H^2 - r^2

    D = Sqrt( (r+alt)^2 - r^2 )
    r = 6,371 km
    D = Sqrt ( (6,371 + alt)^2 - 40,589,641 )

    Give an altitude in km, and you'll get a range in km.

    However, anything just on the horizon will either probably be blocked by clutter (trees, mountains, etc.), and the atmosphere will block too much of your laser light to be effective. Assuming you can intercept at a 60* angle, that means your range is only 86.6% the given value.

    Here's some values up front:
    Altitude (10km is ~32,000 feet). Range, and range adjusted to 86.6% original value.
    Large bombers fly ~50,000 feet tops, which is ~15km.

    Altitude: 10 km, range: 357 km, range, adjusted: 309 km
    Altitude: 15 km, range: 437 km, range, adjusted: 378 km
    Altitude: 20 km, range: 505 km, range, adjusted: 437 km
    Altitude: 25 km, range: 565 km, range, adjusted: 489 km
    Altitude: 30 km, range: 619 km, range, adjusted: 536 km
    Altitude: 35 km, range: 669 km, range, adjusted: 579 km
    Altitude: 60 km, range: 876 km, range, adjusted: 759 km

    I don't know much about ICBM's, but I do know a lot about Space lift vehicles. ICBM's are solid fuel, so they're somewhat a different animal from liquid-fuelled/space rockets.
    Liquid fuel rockets can burn longer on less fuel, and the Space Shuttle will stop burning (MECO for Main Engine CutOff) around 109 km altitude. It's Solid Rocket Boosters (which use the same fuel as ICBM's) burn out around 30 km.

    I'd take an educated guess that an ICBM will burnout somewhere in-between 25km-60km. Anything below 25 km there's too much air friction, anything above 60 km would mean either a very long burn time for a solid fuel rocket or an unrealistically high acceleration. It would be nice if someone knew the facts on this.

    Everybody's said it a few times and I'll say it again, destroying the entire missile is incredibly difficult. However, you probably understand from Challanger, if not a few other things, that rockets have a wonderful tendency to blow up on launch. If you can target a missile during launch, you don't have to destroy it, instead, you just damage the wall of the fuel tank, and the thing destroys itself. Even if it doesn't, though, an ICBM needs an extremely specific amount of speed and a very specific heading, if you just damage it during launch you'll probably have set it off course or under-speed so it won't hit it's target. They're relatively easy to destroy during launch.

    For firing time, IIRC, it takes 12 seconds for the airborne laser to get a lock and down a missile.

    Any other time, they don't have a big explosive fuel tank attached, they just glide to their target after launch. So then you actually have to damage them enough that they can't re-enter correctly and will break up on reentry. Think: Columbia. Any time after re-entry would be incredibly difficult. You'd probably be better off using kinetic kill missiles for point-defense, like the THAAD system, at least until laser technology improves significantly.
    Last edited: Feb 5, 2011
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