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.