Energy weapons and Hard Science

Copyright 2000© Luke Campbell

Others have already noted the limitations on the batteries or power cells you need to carry around to feed to your energy weapons.  To put some hard numbers on this, the maximum specific energies you will be getting from any material relying on chemical bonds to either store its energy or hold it together is somewhere around 40 megajoules per kilogram or so.  This is about what you will be getting if you are burning gasoline in air (and neglecting the mass of the air) or if you are storing energy using the elastic properties of perfect carbon graphene sheets or nanotubes to hold your power cell together (with no extra safety factors to make sure the slightest jolt or imperfection doesn't cause your power cell to explode).  In the near future, expect specific energies of closer to 1 to 3 megajoules per kilogram if you want to extract the energy quickly enough to power a beam weapon.

In order to have the above make any sense, we need to also talk about the power outputs.  For comparison, the 5.56 mm NATO bullet fired from an M16 has about 2 kilojoules of energy.  Pistols generally have a little less, high powered rifles a little more, but that is a typical scale for the energies of bullets, and the sort of energies you need to kill or at least reliably incapacitate a person with a fast projectile.  As long as lasers require more energy to kill a person than this, you might as well use the energy to power a coil gun or rail gun or electrothermal gun or some other projectile launcher.

Now how much energy do you need to deliver to someone using a laser to kill or incapacitate that person?  100 kilojoules will do so.  I can say this with confidence because we know what radiant intensities (integrated over time and delivered in a small time frame, say, 1/10 of a second or less) are required to produce third degree burns on exposed skin.  The same integrated radiant intensity will cause second degree burns under light clothing (such as soldier's uniforms), and will ignite most (but not all) clothing.  100 kilojoules focused into a 1 meter diameter spot will deliver this integrated radiant intensity.  A one meter spot will cover roughly one quarter of the body, and second and third degree burns over that much of the body is almost always lethal.  It is, unfortunately, not instantly lethal, burn victims take a long time to die (the mechanism is usually fluid loss or infection).  It should be instantly incapacitating, however, as people who've had their faces and the skin on their hands burned off are not likely to try shooting back.

100 kilojoules is a lot of energy, 50 times more than the energy of 5.56 mm NATO bullet.  Can we make lasers more efficient?  Maybe.  The details of the interaction of intense light beams with condensed matter and their subsequent physiological effects are not well known.  Concentrating the beam into a smaller spot on the target will produce worse burns over a smaller area, but they may not affect vital organs.  If the laser does not affect vital organs, it is not going to be quickly killing or reliably incapcitating soldiers if it only produces a localized effect.  If you held someone down and applied a 100 kilojoules to a square centimeter of his body over a few minutes, you would burn a square centimeter hole right through him (and a couple of his friends as well, if they were behind him).  The problem is that people generally don't sit still while you are burning holes through them.  If you try to apply this 100 kilojoules quickly enough that the person will not be able to squirm and spread out the effect of the beam, you will find that the beam will not so much drill a hole as cause a surface explosion.  The plasma and debris from the initial explosion will help to block much of the rest of the beam, so not all of the beam energy will be delivered to the target.  Your target may just have a nasty looking surface wound that may kill him eventually, but it will not reliably kill or incapacitate - people under life threatening stress will perform remarkable acts even when they have sustained horrific wounds.

Now, one option is to make the beam into a very short pulse.  At nanosecond timescales or less, almost all of the energy of the beam is converted into the energy of a shockwave in the condensed matter the beam hits.  This shockwave can vaporize additional material while it is intense enough, and even when it is no longer significantly heating the target, it will be stretching and tearing tissue and breaking bone.  25 grams of TNT lets off about 100 kilojoules of energy when it detonates, so a 100 kilojoule nanosecond pulse would cause at least as much structural damage as strapping 25 grams of TNT to the target and setting it off, probably more due to the efficient coupling of the beam energy to the shockwave energy in the target.  Is this enough to kill?  Probably, if it hits the head.  I am less sure about torso hits. Remember, you want to affect the vital organs, not just blow out a surface chunk.  If the shockwave shatters bone it might send secondary bone projectiles flying through the body and perforating vital organs.  A grazing hit to the chest might therefore be much more immediately lethal than a direct hit to the abdomen.

A refinement of the above idea is to deliver a train of rapid pulses faster that the target has time to reposnd.  The first pulse blows out a small amount of tissue and causes a significant  temporary cavity due to its shockwave in the target.  The second pulse hits the back of the temporary cavity from the first pulse and blows out some more tissue, and causes an additional temporary cavity reaching further in.  Repeat this process until one pulse blasts through the heart or a major blood vessel or some such, or until it hits bone and sends secondary bone projectiles flying around in the body and shredding organs.

This last mechanism might end up being about as efficient as bullets for causing deep life threatening wounds.  No one really knows, though.  It might require more energy for the same effect as a bullet.

Other issues come up as well, besides power consumption and storage.

There is the mass of the weapon itself.  The lasers we have today that can kill a person on a time scale short enough to be useful in combat are all huge structures, taking up at least a few semi-trailers worth of space.  There is no fundamental reason why some types of lasers couldn't get much smaller, but no obvious way of making them so.

There is also cooling.  Some of the big pulsed lasers used in research take an hour to cool between pulses.  This need not always be a concern, some free electron lasers operate at efficiencies of more than 99%.  More conventional lasers, however, are doing good if they get more than 20% efficient.

The sorts of things people will probably be using handheld lasers for will be destroying other people or lightly armored vehicles at relatively close range.  For these applications, they might as well use projectile weapons. The places where lasers will likley be used is where they can perform much better than projectiles.  The two most likley such used are explained in the following two paragraphs, along with reasons why hand-held lasers are not suitable.

Mounted laser weapons can be aimed with pinpoint accuracy from stable firing platforms and guided by big beam pointers using radar guidance, auto-stabilization and image tracking techniques to reliably destroy quickly moving, distant, lightly armored targets (missiles, aircraft, artillery shells) with pinpoint accuracy compared to unguided projectiles and a very low cost per shot compared to expending a guided missile.  Human hands and arms make for an unstable firing platform (full of jitters from respiration, pulse, muscle tremors, and so on), and human optical guidance coupled with human arms and hands for beam pointing are not really sufficient for tracking and destroying the sorts of things lasers will most likely be used against (the aforementioned missiles, aircraft, and artillery rounds).  Electronic image stabilization coupled with adaptive optics could get rid of the jitters problem, but not the tracking and pointing problem.

For space combat, lasers can be accurately directed at enemy spacecraft tens of thousands of kilometers away, even a light minute away if you are allowing x-ray lasers.  Unguided projectiles cannot do this effectively, and guided projectiles are very expensive.  People cannot point handheld weapons with sufficient accuracy to target spacecraft at those distances.

Also, the effective range of a laser beam is determined by both the wavelength of light being used (shorter is better) and the aperture through which it is focused (bigger is better).  A tank or aircraft or spacecraft could easily carry around a 1 meter diameter aperture mirror for its beam pointer, a human would probably not use a gun with an aperture much greater than 10 cm or so. Thus, if both lasers fired the same wavelength of beam at the same power, the mounted laser would be able to focus its light into a lethal spot at ten times farther away.  Because they can carry larger laser generators, the tank or aircraft or spacecraft could also have a more powerful beam.  If using free electron laser technology, bigger also means shorter wavelength laser light, so you also get longer ranges (up to a point, once you get past the UV-B at about 0.1 microns, the atmosphere absorbs your light, so you will not see anyone using light shorter than UV-B wavelengths in air).

Further, lasers produce a very bright flash where they hit and may produce multiple specular reflections.  The former may dazzle anyone looking at it, the latter may cause permanent blindness if the light is reflected into an eye.  Laser equipped vehicles will have sufficient protection on their sensors that their users need not worry about blindness (temporary or permanent).  If ground troops are issued lasers, everyone will also need eye protection as well, even those not using lasers.