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Looking for a ballpark estimate of the wattage of a pulse laser that would do a similar amount of damage as a .22 LR round. Something farmers would use for pest control. This is for flavor in a far-future story, so doesn’t have to be super accurate.

(Edited) E.g. The raccoon was back again. The farmer grabbed his 400-Joule pulse laser carbine. “This is the last time you go after my chickens,” he sneered.

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    $\begingroup$ Comparison point to get some idea of orders of magnitude: a million watt pulse laser puts some small craters in black tungsten, but only hurts like hell when shot at a human hand, even after adjusting to get the colour to something that will absorb into his skin colour reasonably well. If you want to be killing raccoons, then, you probably need significantly more than that (albeit only very briefly). $\endgroup$ – user3482749 Feb 20 at 19:49
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    $\begingroup$ Depends on what you want to actually accomplish. Blinding a racoon could be almost as good as killing it. Here's what you get at 200W youtube.com/watch?v=IzUoe-9bKa0 $\endgroup$ – DWKraus Feb 20 at 20:10
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    $\begingroup$ u basically answered u question - replace " 300-watt pulse laser rifle" to " 300-Joule pulse laser rifle" and it is done. a pulse 1e-6 second with energy 300J will land damage, if the dot is similar to .22 $\endgroup$ – MolbOrg Feb 21 at 1:59
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    $\begingroup$ Quick writing note: Who cares what the calibre is? If there's a specific name for a rodent-killing gun, you call it that, and then maybe add a scifi qualificative in front of it, like "laser-ratgun". Or you just call it a pest control rifle and then describe the laser bolt (or lack thereof) when it's fired. Because however many joules or watts it ends up being, it'll just be an abstract number that won't speak to readers. You might know 12-gauge is a lot for a shotgun, but can you visualise 400J? $\endgroup$ – AmiralPatate Feb 22 at 7:41
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    $\begingroup$ Unless your readers are scientists or engineers, I would just have the character pull out his "varmit laser rifle". $\endgroup$ – NomadMaker Feb 22 at 14:04
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I am going to go on a limb here, and say that if there is a farmer that uses a laser rifle as varmint repellent, then the tech level of that world is off the charts (why then there still is varmints to be repelled is another question, maybe the technology has peaked and has become apocryphal, like in StarWars or Dune?).

So let's get with it. As the other answers have already shown in quite some detail, it is not advisable to use a laser where a .22 would suffice. IF, that is, you want to go the path of Boom-Splat.

But what if you where to utilize the laser in a different way?

Five Punch Laser Exploding Heart Technique

(maybe go for the acronymically more pleasing Six Punch Laser Aorta Tearing Technique)

Aim, press trigger. A plethora of different-wavelength lasers scans the target - UV to millimeter-wave. A microsecond later, the target is identified, probability-vectors of movement for the next millisecond (1cm at varmint-speed) are locked in, reflectivity and absorption of different wavelengths by different tissues in the target mapped. The gun still feels as if you are in control, but in reality the phased array emitters will keep a lock on the target irrespective of your jitter. The tactile feedback from the triggerpress has not even reached back to your brain, and the rifle now knows exactly how to end the critter. A barrage of high-wattage ultrashort laser pulses goes down on the target, vaporizing miniscule amounts of matter in a preplanned rythm, scattering impacts all over the body, while building to a destructive, well actually constructive, crescendo when the soundwaves associated with the microimpacts interfere to burst the aorta of the victim.

A sound like a dull pop filters back, right as you experience fully depressing the trigger, and the critter rolls, twitching and slightly smoking. Dead.

You congratulate yourself on the ...paralegal upgrade for your rocksplitter tool, set the dial back to the crystal-plane cutter and continue the carving you had underway.

See this paper for a guesstimate of Lasers to push Space debris by ablation at several 100kms

  • Rough estimate on the wattage / energy of the above laser rifle at .22-range : 100kW, 10 Joule per Varmint
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It's not so much the wattage, it's how you focus it

Square cube laws answer is ballpark correct; the figure I got was 2g impacting at 200m/s has 0.5 * 0.002 * 200^2 joules of kinetic energy. That's works out to about 400 joules. And Mike's comment is correct, a kw laser for 100ms it's only going to slightly warm the skin on impact.

A bullets impact doesn't go into heating the skin, it goes into ripping it. A laser is a poor choice of weapon for this, but if we have to use it:

Focus the energy into a smaller region than a typical bullet impact. Those laser guns have adaptive optics that auto focus all that energy into a pinprick a few square microns across.

A 10 square micron surface exposed to 2kw of power is going to be damaged much more severely than a normal laser impact. What happens depends exactly on the surface its hitting, but fires, nasty skin burns and surfaces liquifying and wrupting are all likely to happen. Sweat turns to steam and flash boils, as your only heating a miniscle mass of skin, but your heating that miniscle amount of skin to extreme levels. Possibly plasma.

A direct hit on a rat by a farmer with a 2kw laser rifle will likely take a chunk out of the rat from moisture flash boiling alone. Itll be a tiny hole and it might die a painful slow death, but if you wanted to kill it faster, use projectiles.

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    $\begingroup$ It seems like lasers are less efficient than bullets at converting energy to death. Could you just up the energy in a shot to get similar damage? E.g. a 20 kW, 100 ms pulse wasting a lot of energy heating the rat, but still splattering it? $\endgroup$ – BillThePlatypus Feb 21 at 4:16
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    $\begingroup$ Do note that such an extreme focus seems highly unlikely in atmosphere. $\endgroup$ – Jan Dorniak Feb 21 at 4:29
  • $\begingroup$ @BillThePlatypus 100ms on-time isn't really a pulse. Pulses need to be fast and very high power... 100ms on high power will waste energy heating gas. Multiple short pulses are the way to go. But yes; lasers do have inefficiency issues. Range issues, too. There's a tradeoff to be made against their accuracy, flight time, and armour penetration. $\endgroup$ – Starfish Prime Feb 21 at 9:26
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    $\begingroup$ @JanDorniak it is theoretically doable... with 1000nm light it is below atmospheric breakdown in normal air, for example. Problem is, to produce a spot that small at, say, 100m, you need a lens that's almost that wide, and you have to consider that pushing over a 70m slab of glass onto your target might just be a more effective way of zapping them... $\endgroup$ – Starfish Prime Feb 21 at 10:26
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that would do a similar amount of damage as a .22 LR round

This criteria makes the question a non-starter. It is impossible for an pulsed energy beam weapon to do a similar amount of damage to a projectile weapon.

The damage done by a projectile weapon is almost totally due to impact energy. It is a kinetic energy weapon. F=ma. The force applied is from the acceleration and deceleration of mass. It smashes bones, gashes flesh, renders skin to create huge gaping holes, penetrates deep into the body tearing blood vessels asunder. The destruction goes far deeper than just surface damage, and it is widespread.

The damage done by a pulsed energy beam is due almost exclusively to localized heating. The conversion of one form of energy to another, particularly heat. A pulsed energy beam has almost no impact (kinetic) energy. It can not break bones, or rupture blood vessels. It can burn a hole in a bone or blood vessel, but it can not smash it and tear it apart. And the burn is entirely localized. Unless the beam is continuous, it can not 'cut'.

I am afraid that, sci-fi writing aside, a pulsed beam weapon makes a horrible device for killing rodents. Kinetic energy weapons will always win over pulsed beam weapons. The energy ratio between the two in order to achieve a 'kill' shot are so extreme as to be absurd. For a pulsed energy beam to do the damage of a kinetic energy weapon, the localized heating would have to be so extreme and so sudden as to cause the immediate vaporization of liquids, and the sudden expansion of the steam explosion would have to be so extreme as to allow the shock wave to propagate through the flexible and elastic flesh and muscle layer sufficiently to cause physical structural damage. That is a very big demand.

Now, if you were to propose a kinetic energy weapon that was projected by a light beam, say the photons could somehow be converted to REAL mass, or real mass could be driven by light, you might have something. But then you would have the perfect reaction mass - light propulsion spaceship drive.

I mean, really, I can stand directly in front of the world's most powerful spotlight and not be propelled back at all. Zero impact damage. Maybe blinded, and perhaps badly sunburned, but zero impact damage.

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  • $\begingroup$ Another option would be for the light frequency to match the the bond-dissociation energy of whatever the major component of human tissue is. Then you could ablate the flesh away. en.m.wikipedia.org/wiki/Laser_ablation $\endgroup$ – DrMcCleod Feb 21 at 22:30
  • $\begingroup$ @ DrMcCleod But how much damage could be done in a single short pulse? My cat is really, really good at ablating my flesh, no lasers needed. $\endgroup$ – Justin Thyme the Second Feb 22 at 2:15
  • $\begingroup$ Isn't "real mass that's driven by light" just a different word for a solar sail powered spaceship? $\endgroup$ – Nzall Feb 22 at 10:37
  • $\begingroup$ Basically, once you got enough compact power to have fun with lasers you might have even more fun with Gauss rifles :] $\endgroup$ – PTwr Feb 22 at 12:48
  • $\begingroup$ @Nzall Nope photons don't even come close to 'real mass'. I am talking about something than can form into molecules and then solids. I have never seen any theory that says photons can 'mass' together into larger particles or clumps of photons. The discontinuity between quantum physics and real life is that quantum physics particles never hang together and act as one. They are always discreet, lone actors. $\endgroup$ – Justin Thyme the Second Feb 23 at 1:22
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"It depends"

I'm of the opinion that until someone can make a decent pulse laser, then lasers just aren't very useful weapons (and a quick glance at EDL and Ash's answers will confirm this). A pulse laser needs to deliver enough energy to vapourise a chunk of the target, and deliver enough of those pulses in a short enough time to carve a hole through the target. Your question presupposes the existence of consumer-grade, effective laser weapons, so that's what I'll run with.

A low powered laser might deliver, say, twenty pulses of 20 joules each in under a millisecond. Focussed down to a spot less than half a centimetre across, a short enough pulse duration will in fact drill a hole through meat. Not a big hole, or a wide one, but it should do the job. A longer pulse train would be preferable for drilling bigger holes, as would better focus.

Peak power will be enormous... 10-20MW, probably. Pulse lasers need that kind of massive power because they need to turn room-temperature matter into hot gas or plasma. You might not want to refer to the weapons as a "20MW pulse whatever" though... 400J would do a better job of informing people about the weapon's capabilities. Maybe "20x20" would be better, including both pulse energy and the number of pulses in a shot.

If you wanted to be more pedantic, it probably wouldn't be a "rifle" either. Rifling on lenses isn't useful! It'd probably look more like a little video camera than 22LR rifle. Here's a vintage super-8 that seems like a reasonable proxy:

Canon 518 Super 8 Camera

(a laser weapon would probably have a trigger guard, mind you)

Not shown: cooling vents. There might be a little fan to provide forced-air cooling to the heatsink that stops the laser frazzling itself as well as whatever it is pointed at.

The eyepiece for the reflex gunsign is at the back; you can't see it from here. The front element of the lens is ~40mm across. A civilian weapon might sensibly produce a wavelength of 1000nm... near IR, invisible to human eyes. It blinds by burning the cornea rather than the retina, making eye damage potentially repairable by surgery. At this wavelength, the diffraction limited range for a 5mm diameter spot (about the largest you'd want for zapping pest species) would be ~160m, but real-world range would be probably more like 100m and maybe less. This seems roughly comparable to a 22LR, though the (probably) shorter effective killing range of the laser is more than made up for by the fact that it has negligible flight time, no bullet drop or windage, and as it uses a reflex gunsight (eg, you look down the barrel at the target, using the same set of optics) you'll hit exactly where you're pointing.

At very close range, say 20m or less, you'd have no problem blasting a hole clean through an unarmoured human, or fatally wounding big game. At maximum range, the wound could be fatal for humans, but I wouldn't rely on it. bad news for raccoons, though.

(note that at longer ranges, the laser is still dangerous, it can blind and burn and seriously wound, though it is unlikely to deliver an immediately fatal wound even to small game)

You could probably have a folding stock on the back if you wanted, but for this sort of weapon and purpose you probably wouldn't need one. If battery technology hasn't marched on enough (and current advances suggest it probably will) a stock might be a good place to put the battery.

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    $\begingroup$ Yeah, “rifle” is sort of an overloaded term in Science Fiction. $\endgroup$ – Ryan Williamson Feb 20 at 22:56
  • $\begingroup$ "look more like a little video camera than 22LR rifle." - not necessarily u may need an optical resonator cavity $\endgroup$ – MolbOrg Feb 21 at 1:51
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    $\begingroup$ You might have a little video camera mounted at the end of simething shaped like a rifle just for the intimidation effect. $\endgroup$ – The Square-Cube Law Feb 21 at 3:07
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    $\begingroup$ @TheSquare-CubeLaw the dark counterpart to the selfie-stick... the youfie stick. I can see that striking fear into the heart of my scifi enemies, and no mistake ;-) $\endgroup$ – Starfish Prime Feb 21 at 9:55
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    $\begingroup$ @MolbOrg the laser itself is likely to be fairly compact; again, if you can't make it compact, you can't make personal laser weapons. A phase-locked diode array needn't be that big, but the optical elements can't really be shrunk down much. Same with power source, and probably cooling source too. $\endgroup$ – Starfish Prime Feb 21 at 9:57
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A .22LR bullet has 41 Joules of kinetic energy at leaving the muzzle.

$J_{vaporize\ water} = 2260 kJ/kg$

A Watt is $Joules \over seconds$, so how dangerous your Laser Rifle is depends on the pulse width.

1 ms pulse width = 0.3 Joules
1 second pulse width = 300 Joules if and only if you can keep the rifle trained on the critter. And as was mentioned before collimation is important. If your beam is tiny enough and burns through the critters fur, and hits the skin, you'll start vaporizing its flesh -- assuming its effectively water -- at rate of $$ Vaporize_{critter} {kg \over sec} = {{W_{laser} \times Pulse\ Width} \over J_{vaporize\ water}} $$

Your 300 W laser with a pulse width of 1 second will vaporization 1 milligram of Racoon per second.

So not very lethal. Might hurt. Only dangerous maybe to the eyes -- as has been observed in previous answers.

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    $\begingroup$ @RyanWilliamson, about the same since you kind of multiplied by 10 on one side and divided by 10 on the other. $\endgroup$ – EDL Feb 21 at 1:16
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    $\begingroup$ "1 ms pulse width = 0.3 Joules" - u look at it from 180 degree wrong angle and circular polarisation - for destruction in a short pulse u need energy, not the wattage, and from the energy, u may or may not deduce the wattage of a pulse which honestly is meaningless, especially for short pulse, as it tells u nothing about practical things like how big a battery pack it has to be, how big a discharge for battery pack there should be. $\endgroup$ – MolbOrg Feb 21 at 1:47
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    $\begingroup$ @RyanWilliamson was looking for 10kJ capacitor-discharge, it does bang out of a sheet of foil, but yeah, was expecting more. Delivering 76kJ in a short pulse, a 0.01ms or less is equivalent to a blast of 1 cubic centimeter(1.6gamm) of TNT on the surface of a target. Not sure how much damage it will do being on the surface, looking at different boomsticks - 6gramm 300kJ probably may be enough, bit overload your trusty 300kJ to 1MJ - let it be him or the rifle, make a suspense. $\endgroup$ – MolbOrg Feb 21 at 7:55
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    $\begingroup$ @RyanWilliamson i chose my 400J energy rating with care ;-) $\endgroup$ – Starfish Prime Feb 21 at 10:08
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    $\begingroup$ You miss the point, The joules of a kinetic energy projectile are delivered almost instantaneously at the point of impact. It is all delivered as sudden impact energy. F=ma. Rating a pulsed energy beam would have to be time-dependent. 300 joules of energy applied in a microsecond is distinctly different than delivering 300 joules over a second. So not only would it have to be rated in joules, but in joules per microsecond. $\endgroup$ – Justin Thyme the Second Feb 21 at 17:42
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The ammo type you mention delivers in the range of 178 to 259 joules per shot. Just divide this value by the pulse duration to find the wattage. I.e.: if the pulse lasts 100 ms, then the wattage would be in the range of 1,780 to 2,590 W.

If one shot is composed of multiple pulses, use the total pulse duration.

This is for short to medium range. For really long shots atmospheric attenuation might have a larger role, so you'd have to increase the output to get the same energy delivered to the target.

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    $\begingroup$ The energy is the same, yes ... but the .22 bullets have 2.6 g x 370 m/s or 1.9 g x 500 m/s = 0.95 to 0.96 kg m/s. But even 300 J / 3E8 m/s = 1E-6 kg m/s. This is like being hit by a milligram at a couple of miles an hour - not quite the level of "stopping power" the gun fans like to talk about. And the laser pulse, being deposited as superficial heat rather than pushing flesh apart, may be limited in the damage it can do even to muscle, let alone internal organs. $\endgroup$ – Mike Serfas Feb 20 at 19:55
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    $\begingroup$ The sort of tissue damage that lasers cause is entirely different to the damage that bullets cause, though. The amount of energy you need to inflict a similar degree of injury might be entirely different. $\endgroup$ – nick012000 Feb 21 at 8:51
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As long as the pulse is short enough, the width of the pulse hardly matters to the racoon at all, so quoting wattage doesn't tell you anything about the killing power of the pulse rifle.

Better to quote the rifle's pulse energy in Joules.

A 300 Joule rifle puts out pulses with energy similar to bullets, but you have to worry about how well it's absorbed.

Obviously you'd want a Kill-o-Joule rifle instead.

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As the majority of the answers indicate, radiation weapons are a joke versus projectiles for damage. But maybe we can exploit their properties to use them smarter.

You can easily realign your EM beam with mirrors (auto-aim). With (very) high tech, it is possible that a weapon with an enemy anatomy database could direct a high-power penetrating (xray/gamma) beam to fry/massively mutate a point in the brain that will cause almost instant death due to cessation of some important function, like heart beating.

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