Laser beam weapons have some attractive features, but also several
probably insuperable problems.  However, the need for a large
electrical power supply is not necessarily one of these.

There are any number of chemical fuels which when burned produce hot
gases in which many or even all of the molecules are formed initially
in upper quantum levels, the requisite condition for laser action.  If
this very hot, high-pressure, highly excited gas is then expanded
through a supersonic nozzle or nozzles, one ends up with a cold,
low-pressure, rapidly moving, but still highly excited gas laser
medium -- a kind of "jet engine" laser medium pumped purely by its own
combustion.

The supersonically expanded gas coming out of the nozzle can in fact
be considerably colder than room temperature so far as its kinetic
motion is concerned, as a consequence of the supersonic expansion.
This leads to comparatively long upper-state lifetimes and
substantially reduced doppler broadening of the molecular transitions.
At the same time the population inversion into upper laser levels can
be large, leading to substantial laser gain on the molecular
transitions.  And, following the laser action the spent molecules
automatically flow on out of the active laser region and are removed
from the system, so they don't hang around and spoil the laser action
by reabsorption.

Useful examples of such fuel combinations include atomic hydrogen or
deuterium plus fluorine or some other halide, leading to lasers such
as the HF, DF or HCl lasers which lase over a wide range of
wavelengths in the middle infrared; or cyanogen or other fuels which
contain carbon, oxygen and nitrogen, and which when burnt lead to
powerful gasdynamic CO2 laser action at 10.6 microns in the mid IR.

[The clever Israelis even made a small single-shot carbon dioxide
laser which used nothing except ordinary gasoline ignited by an
automobile spark plug in a thick-walled chamber with a heavily
spring-loaded door.  This door blew open after ignition, letting the
hot gases exhaust through an expansion nozzle.  There was enough hot
nitrogen and CO2 in the exhaust gases to give a short burst of CO2
laser oscillation at 10.6 microns.  What this device could be good
for, however, I can't imagine.]

In a crude picture of say an HF chemical laser, when an H atom and a
fluorine atom come close together in the hot gaseous fuel mixture they
attract each other, pull each other toward one another, and the two
atoms then in essence "hook together" with a large amount of residual
vibrational motion.  In quantum terms the two atoms bond together
producing an HF molecule in a very highly excited vibrational state.

A rough rule of thumb for chemical-gasdynamic lasers is that the
combustion of one pound of such fuels can produce enough excited
molecules to extract, with proper design, several hundred kilojoules
of laser energy output energy -- in a good case, perhaps 500 kJ of
output laser energy per pound of fuel burned.  Thus, 10 lb of fuel can
give you a 1 megawatt laser beam for 5 seconds of total firing time.
Lasers in this general range of performance have been built.  A
one-kilowatt laser beam focused at close range will produce very
impressive results, burning through a firebrick or piece of armor
plate in a few seconds.  A one-megawatt diffraction-limited beam
transmitted and focused by, say, a steerable 1-meter-diameter focusing
mirror can do the same thing over distances of many kilometers.

Attractive features of such beam weapons include:

1) The "photon bullets" travel outward at the speed of light -- about
Mach 1 million.  A lot better than trying to hit an incoming missile
with an outgoing one having about the same velocity and launched from
a cold start.

2) You only have to point and aim the final pointing and focusing
mirror, not the entire device, making possible rapid retargeting on
multiple targets (assuming sufficient tracking and computer power).

3) It really is a "directed-energy" weapon. It can direct nearly all
of the energy (but not the mass) of a combustion/explosion process
taking place *here* onto a target located way out *there*.

Practical problems include:

1) The good chemical laser fuels are almost always horrendously
corrosive, flammable, explosive and toxic.  The idea of trying to
store large quantities of fluorine and hydrogen, for example, and pipe
them around a ship, especially one that might come under attack, fills
the Navy with well-deserved horror.

2) The optical power levels inside the laser devices themselves are so
horrendously high that the high-reflectivity laser mirrors operate
just on the verge of self-destruction.  Any flaw or blemish or dust
particle on the mirror surface causes the mirror reflectivity to
decrease or its absorption to increase.  As the absorbing spot gets
warmer, its absorption goes up, and the situation goes to pot in a
runaway fashion.  The result is near-instantaneous catastrophic
runaway thermal damage which blows the surface off the mirror faster
than you can possibly shut things down.  The supersonic nozzles are
extremely fragile and touchy also.

3) You have to really hit directly on the target to do any good; no
"near miss" benefits.

4) The full power output from good chemical lasers is inherently
distributed over a very large number of separate infrared wavelengths,
corresponding to transitions between many different rotational and
vibrational quantum levels of the laser molecules.  All the optics
therefore has to be very broadband, high-quality, and achromatic over
a very wide wavelength range.

5) Constructing a laser of the size needed for a weapon, which will
also operate in a single transverse resonator mode so as to produce
the necessary "diffraction-limited" output beam, as well as
maintaining the necessary mirror alignments (and the delicate nozzles)
in the presence of a roaring multi-megawatt combustion process, are
very difficult, probably unsolvable technical problems, for a variety
of fundamental technical and practical reasons.

My personal opinion is that chemical lasers at the multi-megawatt
level can surely be built (probably have been built) in the
laboratory.  _Diffraction-limited_ lasers at the same power level
might just barely be accomplished, for short periods of time, in the
laboratory, with painstaking adjustment and extraordinary expense.
But to think of making hundreds or thousands of such lasers; launching
them into space; having them not just work but maintain their
performance once they reach orbit; having them stay in operable
condition, available for use at short notice, for years or decades;
aiming and pointing them remotely; and, not so obviously, being able
to have any confidence at all that they will actually work when and if
needed -- these are absurd fantasies, not worth taking seriously.

From: schillin@spock.usc.edu (John Schilling)
Newsgroups: sci.military.moderated
Subject: Re: Nukes in Space
Date: 16 Oct 2000 17:22:26 -0700

rmoderator@rime.org (terry vernon) writes:

>DD> So what would be an effective weapon in space, railgun, laser, other?

>Definitely 'other'.

>Lasers are largely neutralised by reflective coatings.

Other way around, at least if we are talking about laser weapons.

Reflective coatings are a: imperfect and b: *fragile*.  The reflectivity
is entirely a function of a micron-thick layer at the reflecting surface;
it doesn't take much energy to turn it into something else.  And what
you get when you burn, melt, ablate, or shatter a reflective surface,
isn't likely to be very reflective on its own.

The difficulty in ensuring that the laser's own necessary internal mirrors
can stand up to the beam, is one major limiting factor in laser weapon
development.  And the laser designer has all the advantages here; he only
needs to mirror a few specific surfaces, those surfaces can be in the
protected interior of the vehicle, and he can devote massive cooling
systems to those specific surfaces.  The relective-armor designer has
none of these working in his favor, and so his armor will not survive
as intense a beam as the laser designer's mirror can deliver.


>Pellets in space orbit tend to return eventually to the area
>they were launched from - still at high velocity.

Only in the simplistic two-body case, only if the projectiles are launched
at less than escape velocity, and only if their trajectory doesn't hit the
planet you are presumably orbiting.  And even then, you've got a full
orbital period to get out of the way.

All told, not a crippling operational restriction on railgun or other
projectile weaponry.  Yeah, I know Clarke(?) wrote a neat short story
using the premise once upon a time, but he turns out to have been wrong.


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