Do mirrors REALLY deflect lasers?
João Luis
One of the most comonly used defences againts laser (or other energy
beams) in "serious" SF are mirrors.
How can this be? Unless the mirror is perfect (100% reflection) even a
10th of a second of a heavy laser (X-ray ?) will burn it up...
---- /// ----
I'm here to chew bubblegum and kick ass... and I'm fresh out of bubblegum.
Nyrath the nearly wise
Thus spoke João Luis (nop1...@mail.telepac.pt):
>
> One of the most comonly used defences againts laser (or other energy
> beams) in "serious" SF are mirrors.
> How can this be? Unless the mirror is perfect (100% reflection) even a
> 10th of a second of a heavy laser (X-ray ?) will burn it up...
Actually, IIRC, there ain't no such thing as
an x-ray mirror.
In most cases where a mirror is possible, the
defensive value, as you stated, hinges on the
reflectivity of the mirror, and the energy
in the beam.
As long as the amount of power that leaks
through the mirror is small enough, you have
a defense. Otherwise you are just making
yourself a conspicuous target.
So the answer to your question is: It Depends....
Another popular defense in SF novels is
"anti-laser aerosols". Sometimes in spraycan
form for streetfighting, othertimes in forms
like the Traveller starship mounted "sandcaster",
that ejects a cannister of prismatic sand.
* A B S I T * I N V I D I A * V E R B O ** I D E M * S O N A N S *
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Thomas Womack
João Luis wrote in message <3468d043...@news.telepac.pt>...
>
>One of the most comonly used defences againts laser (or other energy
>beams) in "serious" SF are mirrors.
>
>How can this be? Unless the mirror is perfect (100% reflection) even a
>10th of a second of a heavy laser (X-ray ?) will burn it up...
But if you have a 99%-reflective mirror which is 0.5% transmissive and 0.5%
absorptive, the megawatt laser that's shining on it can be dealt with using
a pair of 5-kilowatt cooling devices. I think the lasers in the ABL planes
will have actively-cooled optics.
I agree that you've a problem if someone can shine microsecond pulses from
petawatt lasers at you, unless you've got everything in intimate thermal
contact with an amazingly good heatsink.
Tom
Das Miller
from schi...@spock.usc.edu (John Schilling) in <64b4qt$gep$1...@spock.usc.edu>
>"Thomas Womack" <mert...@sable.ox.ac.uk> writes:
>>Joo Luis wrote in message <3468d043...@news.telepac.pt>...
>>>One of the most comonly used defences againts laser (or other energy
>>>beams) in "serious" SF are mirrors.
>>>How can this be? Unless the mirror is perfect (100% reflection) even a
>>>10th of a second of a heavy laser (X-ray ?) will burn it up...
(snip of stuff about actively-cooled mirrors)
>Problem #1 - How do you *keep* a mirror 99% reflective when it is
>exposed to the environment? Every little one-micron dust particle
>your mirror picks up becomes a hot spot or an ablative firecracker,
>and thus a chink in your armor as it degrades the underlying surface.
If someone is using a laser so powerful that it will make dust specks explode,
the laser is *way* overpowered. Vaporize, yes. Explode, no.
>Problem #2 - Multiply your five-kilowatt active cooling devices by
>the ratio of target surface area to laser spot size. If you're
>building a laser, you get to chose the beam path and design mirrors,
>cooling systems, etc, to match. If you're designing armor, it's
>the other guy and/or lady luck who decides where the beam will
>hit, and you have to cover all the bases.
My feeling is that both of these rationales are like saying "Body armor won't
protect a soldier for a direct hit by a 155mm artillery shell, so what's the
point?"
Look at this problem from the other side.
Let's say I'm desiging a laser. It's supposed to shoot at rapidly maneuvering
targets, and there are a bunch of targets, so I'll have a short dwell time.
At typical combat range, let's suppose I decide I'll need 10 MW for 0.1 sec to
kill a target with an albedo of 0.1 (nice white paint). All well and good.
Now, to make my life miserable, they've come up with this 99% reflective
mirror surface. For as long as the surface lasts (which may be quite a
while), my 10MW laser is now only 10% as powerful as it needs to be. I'll
need to go to 1 second dwell (and kill only 10% of my targets), wait until my
targets are only 1/3 the normal distance (and probably die in the counterfire)
or find 9 more lasers.
Or, I could just make the laser 10x as big in the first place, but it'll be
that much more expensive and heavy and I'll be able to buy fewer of them.
Moral - mirrors would not be an absolute defense against lasers, but they'd
make them a lot less effective.
-----------------------------------------------------------------------
Mark S. Miller dasm...@aol.com
-----------------------------------------------------------------------
"The opinions expressed are mine alone and do not reflect
those of my employer, family, the U.S. Government, or any
national television program. They all evidently know some-
thing I don't."
Fyrehold
>
> João Luis wrote in message <3468d043...@news.telepac.pt>...
> >
> >One of the most comonly used defences againts laser (or other energy
> >beams) in "serious" SF are mirrors.
> >
> >How can this be? Unless the mirror is perfect (100% reflection) even a
> >10th of a second of a heavy laser (X-ray ?) will burn it up...
>
> absorptive, the megawatt laser that's shining on it can be dealt with using
> a pair of 5-kilowatt cooling devices. I think the lasers in the ABL planes
> will have actively-cooled optics.
the least. Also, if I remember correctly, even a 99% mirror will have
other problems not related to heating from being hit by a high yeild
laser.
> I agree that you've a problem if someone can shine microsecond pulses from
> petawatt lasers at you, unless you've got everything in intimate thermal
> contact with an amazingly good heatsink.
>
> Tom
---Jon Demers
---fy...@DNACo.net
Das Miller
from dasm...@aol.com (Das Miller) in
<19971112154...@ladder02.news.aol.com>
(yes, I'm quoting myself again)
(snip of a bunch of stuff)
>At typical combat range, let's suppose I decide I'll need 10 MW for 0.1 sec
>to
> kill a target with an albedo of 0.1 (nice white paint). All well and good.
This was a typo. It should have been an albedo of 0.9, and I used 0.9 in the
subsequent calculations, as I meant to.
An albedo of 0.9 would be an *absorptance* of 0.1, assuming a transmittance of
0.
John Schilling
dasm...@aol.com (Das Miller) writes:
>from schi...@spock.usc.edu (John Schilling) in <64b4qt$gep$1...@spock.usc.edu>
>>Problem #1 - How do you *keep* a mirror 99% reflective when it is
>>exposed to the environment? Every little one-micron dust particle
>>your mirror picks up becomes a hot spot or an ablative firecracker,
>>and thus a chink in your armor as it degrades the underlying surface.
>If someone is using a laser so powerful that it will make dust specks
>explode, the laser is *way* overpowered. Vaporize, yes. Explode, no.
You might want to do a bit more research on the ongoing development of
real laser weapons before you make that sort of absolute statement.
Effective laser weapons will most likely be pulsed in nature, rather
than CW. Maybe pulsed at such high frequency as to appear CW to the
human eye, but you *want* the laser to make dust specks (and the top
few microns of the target's surface in general) to explode. Mechanical
damage by vapor explosion is a *much* more efficient kill mechanisim
than simple heating - cuts the required laser size and energy by about
an order of magnitude.
Not that it matters; even if you were to use a pure-CW laser, out of
stupidity or necessity, dusts specks would still negate mirrored armor.
If the heat is applied slowly enough that the dust speck does not
explode, it will have time to conduct to the mirror surface and
degrade it before the speck has fully vaporized. Reflectivity
doesn't stop conduction.
>>Problem #2 - Multiply your five-kilowatt active cooling devices by
>>the ratio of target surface area to laser spot size. If you're
>>building a laser, you get to chose the beam path and design mirrors,
>>cooling systems, etc, to match. If you're designing armor, it's
>>the other guy and/or lady luck who decides where the beam will
>>hit, and you have to cover all the bases.
>My feeling is that both of these rationales are like saying "Body armor won't
> protect a soldier for a direct hit by a 155mm artillery shell, so what's the
> point?"
The problem is that not only will body armor not stop an artillery shell,
it will not stop an ordinary rifle bullet either. Armor, body or otherwise,
is almost never very useful against point-target weapons from a comparable
opponent.
The point of body armor in modern warfare is to protect against area-effect
weapons. A 155mm shell can penetrate any concievable body armor with a
direct hit, but disperse its energy across a few hundred square meters
and a light kevlar jacket will do.
Since I have yet to see anyone propose lasers as area-effect weapons, we
are back to the body armor vs. rifle bullet problem, and rifle bullets
win that one almost every time.
>Look at this problem from the other side.
>Let's say I'm desiging a laser. It's supposed to shoot at rapidly maneuvering
> targets, and there are a bunch of targets, so I'll have a short dwell time.
> At typical combat range, let's suppose I decide I'll need 10 MW for 0.1 sec to
> kill a target with an albedo of 0.1 (nice white paint). All well and good.
> Now, to make my life miserable, they've come up with this 99% reflective
> mirror surface. For as long as the surface lasts (which may be quite a
> while), my 10MW laser is now only 10% as powerful as it needs to be.
If you can design a mirror that will survive even 0.1 seconds under ten
megawatts of concentrated laser energy, yet be light and rugged enough
to cover a vehicle with, I know quite a few optical designers who would
like to talk to you. White paint won't do it by a long shot; it will
be charred to flat black in millisenconds.
You've conveniently omitted spot size from your example, which precludes
detailed mathematical analysis. But the bottom line is, the ammount of
laser energy per unit area needed to degrade the best mirror you can
reasonably field as armor, is small compared to the ammount of laser
energy needed to destroy even soft, unarmored targets. The mirror may
reflect 90% of the beam *at first*, but it won't last long enough to
make any real difference.
--
*John Schilling * "You can have Peace, *
*Member:AIAA,NRA,ACLU,SAS,LP * or you can have Freedom. *
*University of Southern California * Don't ever count on having both *
*Aerospace Engineering Department * at the same time." *
*schi...@spock.usc.edu * - Robert A. Heinlein *
*(213)-740-5311 or 747-2527 * Finger for PGP public key *
Bill Woods
Nyrath the nearly wise wrote:
> Thus spoke João Luis (nop1...@mail.telepac.pt):
> >
> > One of the most comonly used defences againts laser (or other energy
> > beams) in "serious" SF are mirrors.
> > How can this be? Unless the mirror is perfect (100% reflection) even a
> > 10th of a second of a heavy laser (X-ray ?) will burn it up...
>
> Actually, IIRC, there ain't no such thing as
> an x-ray mirror.
For astronomical purposes there is. It has to be almost edge-on to the x-rays,
so it's not practical in a military context.
--
Bill Woods
"Space is big. Really big. You just won't believe how vastly hugely
mind-bogglingly big it is. I mean, you may think it's a long way
down the road to the chemist, but that's just peanuts to space."
-- Douglas Adams
Das Miller
from schi...@spock.usc.edu (John Schilling) in <64fd6v$29t$1...@spock.usc.edu>
>dasm...@aol.com (Das Miller) writes:
>>from schi...@spock.usc.edu (John Schilling) in <64b4qt$gep$1...@spock.usc.edu>
>>>Problem #1 - How do you *keep* a mirror 99% reflective when it is
>>>exposed to the environment? Every little one-micron dust particle
>>>your mirror picks up becomes a hot spot or an ablative firecracker,
>>>and thus a chink in your armor as it degrades the underlying surface.
>>If someone is using a laser so powerful that it will make dust specks
>>explode, the laser is *way* overpowered. Vaporize, yes. Explode, no.
>You might want to do a bit more research on the ongoing development of
>real laser weapons before you make that sort of absolute statement.
>Effective laser weapons will most likely be pulsed in nature, rather
>than CW. Maybe pulsed at such high frequency as to appear CW to the
>human eye, but you *want* the laser to make dust specks (and the top
>few microns of the target's surface in general) to explode. Mechanical
>damage by vapor explosion is a *much* more efficient kill mechanisim
>than simple heating - cuts the required laser size and energy by about
>an order of magnitude.
It took me a while, but I finally realized that you and I are talking about
fundamentally different types of laser attacks. I'm considering space combat,
at ranges of tens of thousands of kilometers, where the spot size will
probably not be significantly smaller than the entire target.
In the very few real-world cases I've worked, if the attacker had enough power
to quickly vaporize the top few microns of the s/c's surface, we didn't even
try to defend it. We worried about them 'cooking' the spacecraft, or parts of
it. Obviously, highly reflective surfaces would make the spacecraft more
survivable. No, it wouldn't make the spacecraft immune to lasers, but it
*would* make it much harder (much longer dwell time, or much larger laser
needed) to kill.
>Not that it matters; even if you were to use a pure-CW laser, out of
>stupidity or necessity, dusts specks would still negate mirrored armor.
"negate" meaning "might as well not have the mirror, because it's not making
the spacecraft more difficult to kill?" Or "negate" meaning "keeping the
spacecraft from being invulnerable?"
>If the heat is applied slowly enough that the dust speck does not
>explode, it will have time to conduct to the mirror surface and
>degrade it before the speck has fully vaporized. Reflectivity
>doesn't stop conduction.
Also, a few tiny hotspots are *still* a lot better than having the whole
surface be a hotspot.
>>>Problem #2 - Multiply your five-kilowatt active cooling devices by
>>>the ratio of target surface area to laser spot size. If you're
>>>building a laser, you get to chose the beam path and design mirrors,
>>>cooling systems, etc, to match. If you're designing armor, it's
>>>the other guy and/or lady luck who decides where the beam will
>>>hit, and you have to cover all the bases.
>
>>My feeling is that both of these rationales are like saying "Body armor
>won't
>> protect a soldier for a direct hit by a 155mm artillery shell, so what's
>the
>> point?"
>The problem is that not only will body armor not stop an artillery shell,
>it will not stop an ordinary rifle bullet either.
Depends on the range and the circumstances. If it will stop or deflect *some*
of them, then it may be a useful defense. ("May" means that you have to
consider the downsides of the defense, too).
>Armor, body or otherwise,
>is almost never very useful against point-target weapons from a comparable
>opponent.
But most soldiers aren't killed by point-target weapons, at least from the data
I've seen. In WWII, artillery caused nearly 60% of casualties (Dunnigan, "How
to Make War", 1988 version, p. 97)
>The point of body armor in modern warfare is to protect against area-effect
>weapons. A 155mm shell can penetrate any concievable body armor with a
>direct hit, but disperse its energy across a few hundred square meters
>and a light kevlar jacket will do.
I was considering relatively soft kevlar body armor, not the Star-Wars stuff.
>Since I have yet to see anyone propose lasers as area-effect weapons, we
>are back to the body armor vs. rifle bullet problem, and rifle bullets
>win that one almost every time.
<sigh> again, I don't consider a defense useless simply because it doesn't
confer absolute immunity.
>>Look at this problem from the other side.
>>Let's say I'm desiging a laser. It's supposed to shoot at rapidly
>maneuvering
>> targets, and there are a bunch of targets, so I'll have a short dwell time.
>> At typical combat range, let's suppose I decide I'll need 10 MW for 0.1 sec
>to
>> kill a target with an albedo of 0.1 (nice white paint). All well and good.
>> Now, to make my life miserable, they've come up with this 99% reflective
>> mirror surface. For as long as the surface lasts (which may be quite a
>> while), my 10MW laser is now only 10% as powerful as it needs to be.
>If you can design a mirror that will survive even 0.1 seconds under ten
>megawatts of concentrated laser energy
"concentrated" is a relative term. With a spot size that's many meters across,
this does not sound undoable.
>, yet be light and rugged enough
>to cover a vehicle with, I know quite a few optical designers who would
>like to talk to you. White paint won't do it by a long shot; it will
>be charred to flat black in millisenconds.
Even if the laser is well-focused (small spot size), holding the spot on the
same patch of surface over very long ranges against a maneuvering, possibly
rotating target will be problematic at best. And the mirror will increase the
amount of time you have to hold the laser on the target.
For large spots (like I'm expecting), this isn't an issue.
>You've conveniently omitted spot size from your example, which precludes
>detailed mathematical analysis. But the bottom line is, the ammount of
>laser energy per unit area needed to degrade the best mirror you can
>reasonably field as armor, is small compared to the ammount of laser
>energy needed to destroy even soft, unarmored targets.
It depends on how you're trying to destroy the target, and whether you're using
a small spot or a large spot. If you're talking about using a large spot and
slagging the thing, then I agree. If you're trying to raise the bulk
temperature until you cause critical failures, then the mirror will be useful.
If degrading the surface is, itself, enough to cause a target kill, then the
mirror will increase the time and energy needed to degrade the surface and
will still be of some value.
>The mirror may
>reflect 90% of the beam *at first*, but it won't last long enough to
>make any real difference.
As I noted above, you and I are talking about fundamentally different types of
laser attacks. If the attacker has enough power to near-instantly vaporize
the top few microns, then the laser is *way* more powerful than I'd be trying
to defend against. And if it *can't* do that, then I'm sticking to my
position that highly reflective surfaces will enhance target survivability.
John D. Gwinner
Folks:
(Side Note)
Das Miller wrote:
> from schi...@spock.usc.edu (John Schilling) in
<64fd6v$29t$1...@spock.usc.edu>
>
>
> >dasm...@aol.com (Das Miller) writes:
>
> >>from schi...@spock.usc.edu (John Schilling) in
<64b4qt$gep$1...@spock.usc.edu>
....
> >>My feeling is that both of these rationales are like saying "Body armor
> >won't
> >> protect a soldier for a direct hit by a 155mm artillery shell, so
what's
> >the
> >> point?"
>
>
> >The problem is that not only will body armor not stop an artillery
shell,
> >it will not stop an ordinary rifle bullet either.
>
> Depends on the range and the circumstances. If it will stop or deflect
*some*
> of them, then it may be a useful defense. ("May" means that you have to
> consider the downsides of the defense, too).
Actually, body armor is not really designed to stop rifle bullets; the
Kevlar vests used today will generally not stop a rifle bullet, depending
on a lot of factors (angle of incidence, etc).
Machine gun fire is right out; the superstructure of an Amtrack (Amphibious
Tractor, the 'APC' the Marines use, capable of open water operation) can be
penetrated by an M-60, and anything except a tank can be penetrated by a
50 cal.
However, what kevlar body armor does stop is shrapnel -- which came from
the 155mm shells everyone said body armor wouldn't stop <G>.
With kinetic weapons, lot of "Stuff" flies around; that's the purpose of
body armor, not direct fire.
I'm not sure what effect on unit tactics directed energy weapons would
have, but it might do away with armor entirely. The WORST thing you could
wear on the battlefield would be a silver body suit as you're an immediate
target. The whole idea with small unit tactics, depending on the terrain,
is NOT to be seen in the first place. I think this would override any
considerations of 'armor', but of course it's hard to say.
I'd guess that with technology that's this high, active camouflage would be
the outer wear of choice. Better to not be seen at all, than to be even
partially armored and be an instant target.
== John ==
Isaac Kuo
In article <346C1ABE...@ix.netcom.com>,
>> Thus spoke João Luis (nop1...@mail.telepac.pt):
>> > One of the most comonly used defences againts laser (or other energy
>> > beams) in "serious" SF are mirrors.
>> > How can this be? Unless the mirror is perfect (100% reflection) even a
>> > 10th of a second of a heavy laser (X-ray ?) will burn it up...
It just might be that practically "perfect" mirrors can be
constructed. In just the past 5 years breakthroughs in high
reflectivity coatings turned the cancelled Zenith Star which
would have required active cooling of its mirrors into Star
Lite, which won't. I wouldn't rule out the possibility of
even better mirrors.
Also, even if a practical mirrored surface is still vulnerable
to a pulse laser, it may be possible to layer them. Even if
each pulse is capable of destroying a dozen layers, it would
take thousands of pulses applied to the same spot to break
through a hundred thousand layer defense.
>> Actually, IIRC, there ain't no such thing as
>> an x-ray mirror.
OTOH, it's hard to make a practical X-ray laser. Maybe it
will be suddenly become easy with the development of lasers
capable of lasing without population inversion, but maybe not.
>For astronomical purposes there is. It has to be almost edge-on to the x-rays,
>so it's not practical in a military context.
--
_____ Isaac Kuo k...@bit.csc.lsu.edu http://www.csc.lsu.edu/~kuo
__|_)o(_|__
/___________\ "Mari-san... Yokatta...
\=\)-----(/=/ ...Yokatta go-buji de..." - Karigari Hiroshi
Brian Trosko
John D. Gwinner <gwi...@northnet.org> wrote:
: Machine gun fire is right out; the superstructure of an Amtrack (Amphibious
: Tractor, the 'APC' the Marines use, capable of open water operation) can be
: penetrated by an M-60, and anything except a tank can be penetrated by a
: 50 cal.
Are you certain about that? I'm pretty sure that modern IFVs like the
Bradley or Warrior are armored against .50, at least in the frontal arc.
João Luis
On 12 Nov 1997 00:23:47 GMT, nyr...@clark.net (Nyrath the nearly wise)
wrote:
>Thus spoke João Luis (nop1...@mail.telepac.pt):
>>
>> One of the most comonly used defences againts laser (or other energy
>> beams) in "serious" SF are mirrors.
>> How can this be? Unless the mirror is perfect (100% reflection) even a
>> 10th of a second of a heavy laser (X-ray ?) will burn it up...
>
>
> Actually, IIRC, there ain't no such thing as
> an x-ray mirror.
>
Not an x-ray MIRROR, an x-ray LASER, suposedly very powerfull...
Nelson Cunnington
> Actually, IIRC, there ain't no such thing as
> an x-ray mirror.
>
> In most cases where a mirror is possible, the
> defensive value, as you stated, hinges on the
> reflectivity of the mirror, and the energy
> in the beam.
>
> As long as the amount of power that leaks
> through the mirror is small enough, you have
> a defense. Otherwise you are just making
> yourself a conspicuous target.
>
> So the answer to your question is: It Depends....
>
> Another popular defense in SF novels is
> "anti-laser aerosols". Sometimes in spraycan
> form for streetfighting [snip]
>
As opposed to "anti-mirror" aerosols, which spray nice and
absorbtive (is that a word?) black paint all over your shiny
mirror armour, making it nearly useless.
- Nelson
My address has been anti-spammed
"It's true that he takes only 4 hours sleep a night.
He also takes two 3-hour naps during the day." -Tesla, about Edison
Tony Gardner
João Luis wrote:
>
> On 12 Nov 1997 00:23:47 GMT, nyr...@clark.net (Nyrath the nearly wise)
> wrote:
>
> >Thus spoke João Luis (nop1...@mail.telepac.pt):
> >>
> >> One of the most comonly used defences againts laser (or other energy
> >> beams) in "serious" SF are mirrors.
> >> How can this be? Unless the mirror is perfect (100% reflection) even a
> >> 10th of a second of a heavy laser (X-ray ?) will burn it up...
> >
> >
> > an x-ray mirror.
> >
>
perfectly transparent to x-rays. As humans do not absorb very much
x-radiation, this would significantly increase the power requirements
for any laser hitting the craft, as it would have to act on such a small
area.
Of course, you could equally start making spaceships out of glass to
reduce the effects of visible lasers. Of course this would require that
the crew were not agoraphobic.
Tony
-did I get the right phobia there? I mean fear of open spaces
bla...@freenet.edmonton.ab.ca
Isaac Kuo (k...@nil.csc.lsu.edu) wrote:
: Basically, if you've got lots of high energy lasers lasing around
: the battlefield, anyone without very good eye protection will be
: blind and useless in short order (even if only one side is using
: them). OTOH, wearing a blindfold or really really dark goggles
: is almost as bad as being blind already.
: Therefore, soldiers will need some sort of indirect vision devices,
: like adjustable "night vision" goggles, TV cameras, or whatever.
You know, I remember a _Doctor Who_ episode which had troopers working for
the Daleks, wearing helmets with heavy visors and a Dalek-like eyepiece.
I thought that was a stupid set-up at the time, but it makes sense.
The visors keep out light, and the eyepiece is a camera projecting an
image on the visor's inner surface. The soldier has eye protection,
from optical lasers and nuclear fireballs, but can see anyway. Of
course, the scriptwriter probably never thought of that, but what the
heck?
: Assuming these things remain relatively expensive, there's incentive
: to equip each soldier with other relatively expensive things which
: let him do his job better, even on a cold economic level.
: This might mean not bothering with traditional infantry, and using
: entirely AFVs and maybe "powered armor suits". You no longer have
: to worry about the guy with the RPG from behind a bush since he's
: blind anyway.
Or _Battletech_ style mechs?
: However, TV goggles seem like the sort of thing which will get very
: inexpensive in a few decades...
As soon as an army can afford portable lasers for its troops, it can
afford indirect vision goggles. Besides, such goggles could also be
sensitive to IR, UV, radar, x-rays, ultrasound, maybe one day even
gravitational waves.
Also, even with good eye protection, there will be many soldiers going
blind from laser fire. Perhaps that will also stimulate research into
advanced eye surgery, transplants, and working artificial eyes.
Perhaps one day, it will be mandatory for all military personnel to have
their own eyes replaced by cyborg ones immune to laser fire, with superior
perception, and able to function on the whole spectrum. (Like Geordi in
_First Contact_.)
===================== ====================================
BLAINE GORDON MANYLUK email: bla...@freenet.edmonton.ab.ca
EDMONTON, AB
Jeffrey B. Siegal
jchase wrote:
> In addition, the reflection of 80-90%
> of the energy in a reverse or random direction could mess up someone's
> day- the ultimate ricochet.
Which brings up a good point: How effectively would corner reflectors defend against lasers?
John Schilling
>jchase wrote:
Worse than useless. As mentioned earlier, the best mirrors you can reasonably
expect to maintain on the exterior of a military vehicle will be degraded to
the point of uselessness by far less energy than is required to damage tthe
underlying structure, which means they won't survive long enough to reflect
a damaging ammount of energy back towards the shooter. Add to this the fact
that the divergence of the reflected beam will be quite large, due both to
the divergence of the incident beam and the diffraction limit of the smallest
mirrors in the corner reflector, and the energy density faced by the shooter
will be no threat at all.
It will, however, make a very nice beacon to tell the shooter that yes,
he *is* on target and should just keep on doing what he is doing. It
is quite likely that laser weapons intended for use against air or space
targets will use feedback from the reflected target signal for fine-tuning
the fire control solution, and corner reflectors will just make the gunner's
job that much easier.
Bill Woods
Jeffrey B. Siegal wrote:
> jchase wrote:
>
> > In addition, the reflection of 80-90%
> > of the energy in a reverse or random direction could mess up someone's
> > day- the ultimate ricochet.
>
> Which brings up a good point: How effectively would corner reflectors defend against lasers?
Less effectively.
Photons get two or three chances to degrade the reflector; failing that they go back to tell the
attacker he's on target.
--
Bill Woods
"The dinosaurs became extinct because they didn't have a space program"
-Larry Niven
Isaac Kuo
In article <64t9d7$mc7$1...@spock.usc.edu>,
>>Which brings up a good point: How effectively would corner reflectors defend
>>against lasers?
It depends.
In terms of avoiding damage, they aren't as good as a plain flat mirror.
They weigh more, and if the laser is hitting a large area, they will be
forced to absorb 3 times as much power.
However, they have potential to blind enemy sensors.
>Worse than useless. As mentioned earlier, the best mirrors you can reasonably
>expect to maintain on the exterior of a military vehicle will be degraded to
>the point of uselessness by far less energy than is required to damage tthe
>underlying structure, which means they won't survive long enough to reflect
>a damaging ammount of energy back towards the shooter.
You can already forget about damaging the shooter because of diffraction
limitations, of course.
However, I wouldn't count on not being able to maintain a highly
reflective surface. In the vacuum of outer space, a "bubble"
shield spaced away from the mirror surface could prevent most
degredation. It could naturally be blasted by a powerful pulse
laser, but the mirror surface beneath would remain (filling the
space with a gas could prevent small fragments from the bubble
shield from contaminating the mirror).
Also, the mirrors could be layered, so that only the outer layers
get degraded.
>Add to this the fact
>that the divergence of the reflected beam will be quite large, due both to
>the divergence of the incident beam and the diffraction limit of the smallest
>mirrors in the corner reflector, and the energy density faced by the shooter
>will be no threat at all.
The energy density may be a serious threat to the shooter's sensors,
which must be calibrated at extreme sensitivity (in case a normal
flat mirror is hit by the illuminating beams).
More isn't always better.
>It will, however, make a very nice beacon to tell the shooter that yes,
>he *is* on target and should just keep on doing what he is doing.
Alan Braggins
Tony Gardner <gar...@physics.uq.edu.au> writes:
> What about this for a laser defense. At the front of your spaceship,
> put a container full of mercury. Have the artificial gravity generators
> on your ship make "down" for the mercury be towards the back of the
> ship.
[...]
> the enemy, though the invention of a reflecting metal which works better
> than mercury would be an advantage.
I think the invention of artificial gravity generators would be an
even bigger advantage.
Tony Gardner
What about this for a laser defense. At the front of your spaceship,
put a container full of mercury. Have the artificial gravity generators
on your ship make "down" for the mercury be towards the back of the
vapour pressure, and anyway you can replicate some more when you
need it.
When the dish is spun along the axis of the ship, it forms a parabloic
reflector. The focal length depends on the rate of spinning. Now when
the enemy is firing his laser at you, point his laser over at one of
his friends, focus it to a point and laugh maniacally.
Because you are not returning the beam, your enemy will not be able
to range in on you. Because the mercury will evaporate faster when hot,
you have a self cooled mirror. Because you are using your enemy's
weapon back upon him, it means that you can argue in UN meetings that
this does not meet the criterion of a weapon under the "big guns
non-proliferation treaty". Also you are spared the expense of updating
your laser weapons, since this is automatically as updated as those of
Kylua100
In article <3472FD0B...@physics.uq.edu.au>, Tony Gardner
<gar...@physics.uq.edu.au> writes:
Pretty good.
Colin
Nil illigitimo carborundum
Thomas R Nelson
On 13 Nov 1997, John Schilling wrote:
>
> If you can design a mirror that will survive even 0.1 seconds under ten
> megawatts of concentrated laser energy, yet be light and rugged enough
> to cover a vehicle with, I know quite a few optical designers who would
> like to talk to you. White paint won't do it by a long shot; it will
> be charred to flat black in millisenconds.
Define "concentrated" laser energy. There are laser mirrors that are
commercially available which can take much more power than 10 megawatts,
at fairly high power densities, like on the order of gigawatts per square
centimeter. Most laser mirrors that are higher than 95% reflective are not
metal mirrors, but rather use layers of dielectric coatings which in the
right order can give very high (99.9%) reflectivities, and can be much
more durable than metal mirrors.
And dust can cause a problem, but usually the way dust would damage a
mirror is through the dust heating up by absorbing the laser energy, which
can burn the mirror coating.
Also, you are correct that high power lasers would be pulsed, rather than
CW. But the light pulses last much less time than 0.1 seconds. For
lasers that put out gigawatts or even terawatts, typical pulse durations
are on the order of 100fs or less. (1fs = 10^-15 seconds).
> You've conveniently omitted spot size from your example, which precludes
> detailed mathematical analysis. But the bottom line is, the ammount of
> laser energy per unit area needed to degrade the best mirror you can
> reasonably field as armor, is small compared to the ammount of laser
> energy needed to destroy even soft, unarmored targets. The mirror may
> reflect 90% of the beam *at first*, but it won't last long enough to
> make any real difference.
>
Although I didn't see the original analysis, it seems there are many more
things that are omitted. I would be curious to see the original post on
this subject, if someone could repost it.
Tom
************************************************************************
Thomas Nelson
Laboratory for Atomic Molecular and Radiation Physics
University of Illinois at Chicago
Office phone: (312) 413-2109 Lab phone: (312) 996-5444
Fax: (312) 996-8824 Email: tne...@uic.edu
************************************************************************
Thomas L. Billings
In article <mvpEL9...@netcom.com>, m...@netcom.com (Mike Van Pelt) wrote:
> In article <Pine.A41.3.96.971215...@tigger.cc.uic.edu>,
> Thomas R Nelson <tne...@uic.edu> wrote:
> >Define "concentrated" laser energy. There are laser mirrors that are
> >commercially available which can take much more power than 10 megawatts,
> >at fairly high power densities, like on the order of gigawatts per square
> >centimeter.
> Hmm... I heard a talk by Dr. Arthur Kantrowitz back around '82 or so,
> and he said that at a power density of more than "x" megawatts/cm^2,
> you get a laser propagated detonation, no matter what the surface is.
> (At least, if the mirror is in an atmosphere.)
> I don't recall what "x" was, but it was MW/cm^2, not GW.
Yes. Dr. Liek Myrabo is doing work at Phillips Lab with a 15cm diameter
laser reflector/rocket which uses atmospheric laser detonation. Larger
models will go to orbit using a hydrogen tank full of propellant after
they leave the atmosphere. The laser is pulsed to produce a detonation
wave, which propagates at right angles to the surface.
The shape of the vehicle allows localized focusing of the laser light, so
that the energy density needed to attain the detonation wave is not
approached in the beam until it is focused right next to the rear surface
of the vehicle, which has an extended aerospike shape. The latest SSI
Space Manufacturing Conf. Proceedings (#22) has a picture of the small
vehicle now undergoing test. "The Future of Flight" by Liek Myrabo and
Dean Ing (ISBN 0-671-55941-9) has a number of such concepts using laser
detonation waves in it. Dr. Myrabo hopes to be allowed to fly it to
several hundred feet altitude by sometime next spring. Laser range safety
issues were a major restriction at the time of the SSI Conference.
Regards,
Tom Billings
--
Institute for Teleoperated Space Development
it...@teleport.com(Tom Billings)
ITSD's web site is at, http://www.teleport.com/~itsd1/index.html
Mike Van Pelt
In article <Pine.A41.3.96.971215...@tigger.cc.uic.edu>,
Thomas R Nelson <tne...@uic.edu> wrote:
>Define "concentrated" laser energy. There are laser mirrors that are
>commercially available which can take much more power than 10 megawatts,
>at fairly high power densities, like on the order of gigawatts per square
>centimeter.
Hmm... I heard a talk by Dr. Arthur Kantrowitz back around '82 or so,
and he said that at a power density of more than "x" megawatts/cm^2,
you get a laser propagated detonation, no matter what the surface is.
(At least, if the mirror is in an atmosphere.)
I don't recall what "x" was, but it was MW/cm^2, not GW.
--
"A clown is a warrior who fights gloom." | Mike Van Pelt
- Red Skelton | m...@netcom.com
| KE6BVH
****WARNING: I complain to your ISP if you send me spam! ******
Thomas R Nelson
On Tue, 16 Dec 1997, Mike Van Pelt wrote:
> In article <Pine.A41.3.96.971215...@tigger.cc.uic.edu>,
> Thomas R Nelson <tne...@uic.edu> wrote:
> >Define "concentrated" laser energy. There are laser mirrors that are
> >commercially available which can take much more power than 10 megawatts,
> >at fairly high power densities, like on the order of gigawatts per square
> >centimeter.
>
> Hmm... I heard a talk by Dr. Arthur Kantrowitz back around '82 or so,
> and he said that at a power density of more than "x" megawatts/cm^2,
> you get a laser propagated detonation, no matter what the surface is.
> (At least, if the mirror is in an atmosphere.)
>
> I don't recall what "x" was, but it was MW/cm^2, not GW.
ok, a couple of points here. Firstly, the laser technology, as well as
our knowledge of laser interactions with matter has increased dramatically
over the last 15 years.
Secondly, and more importantly, the physics which govern the laser
interaction with matter (i.e. the damage mechanism) vary markedly
depending on the timescale of the interaction. If you're talking about
using a CW laser source, or even a relatively long pulse (i.e.
nanoseconds which are 10^-9 seconds, or longer) then MW/cm^2 may very well
be correct. Although rigorously, for long pulses like that, the important
quantity becomes energy density, and the damage threshold becomes less
dependant of the temporal pulselength. (CW lasers are the extreme limit,
and there it only makes sense to speak in terms of power densities
though.) For shorter pulses however, the power density can be quite high,
while the energy density is still fairly low.
But the type of interactions that take place on shorter timescales are
very much different as well. On shorter timescales, (i.e. femtoseconds,
or 10^-15 seconds) certain physical processes don't occur because they
don't have time to do so before the laser pulse has been and gone, and
other processes which can occur much quicker dominate the interaction. On
longer timescales, the slower processes can be the dominant process.
I can tell you with certainty, that on the femtosecond timescale, damage
thresholds regularly approach or exceed 1GW/cm^2. I work with these types
of lasers on a daily basis, and at am painfully familiar with the numbers
for damage thresholds for that type of laser pulse. If you doubt what I'm
saying, I would encourage you to look up some references on the subject.
One reference that may be a good starting point is a book called "Solid
State Laser Engineering" by Koechner. Also you could look at some of the
more recent literature dealing with laser induced damage.
Don't get me wrong, I'm not trying to be smarmy or tell you you're
absolutely wrong or anything. I'm merely trying to point out that the
problem is not as one-dimensional as your posts implied. There are
circumstances where your numbers may be right, but there are also
circumstances where your numbers are wrong.
Mike Van Pelt
In article <Pine.A41.3.96.971216...@tigger.cc.uic.edu>,
Thomas R Nelson <tne...@uic.edu> wrote:
>On Tue, 16 Dec 1997, Mike Van Pelt wrote:
>
>> In article <Pine.A41.3.96.971215...@tigger.cc.uic.edu>,
>> Thomas R Nelson <tne...@uic.edu> wrote:
>> >Define "concentrated" laser energy. There are laser mirrors that are
>> >commercially available which can take much more power than 10 megawatts,
>> >at fairly high power densities, like on the order of gigawatts per square
>> >centimeter.
>>
>> Hmm... I heard a talk by Dr. Arthur Kantrowitz back around '82 or so,
>> and he said that at a power density of more than "x" megawatts/cm^2,
>> you get a laser propagated detonation, no matter what the surface is.
>> (At least, if the mirror is in an atmosphere.)
>>
>> I don't recall what "x" was, but it was MW/cm^2, not GW.
>
>interaction with matter (i.e. the damage mechanism) vary markedly
>depending on the timescale of the interaction. If you're talking about
>using a CW laser source, or even a relatively long pulse (i.e.
>nanoseconds which are 10^-9 seconds, or longer) then MW/cm^2 may very well
...
>But the type of interactions that take place on shorter timescales are
>very much different as well. On shorter timescales, (i.e. femtoseconds,
>or 10^-15 seconds) certain physical processes don't occur because they
...
>Don't get me wrong, I'm not trying to be smarmy or tell you you're
>absolutely wrong or anything. I'm merely trying to point out that the
>problem is not as one-dimensional as your posts implied. There are
>circumstances where your numbers may be right, but there are also
>circumstances where your numbers are wrong.
Hazards of the net... The perceived tone can be radically different
from the intended tone. My *intent* was not to say you were wrong
(I knew from your .signature file that you know more about this
than I do) but to say "Hmmm, I heard something conflicting from
another expert way back when, what's going on here?"
The answer is.... Dr. Kantrowitz was talking about a gigawatt CO2
laser for launching a spacecraft, which would be either CW or
much-more-than-femtosecond pulses.
(Actually, it was a low-power pulse to excite the water and CO2 in
the atmosphere so it would be transparent to the power pulse
following right along behind it -- in effect "bleaching" a column
of atmosphere between the laser and the spacecraft.)
Thomas R Nelson
On Wed, 17 Dec 1997, Mike Van Pelt wrote:
>
> Hazards of the net... The perceived tone can be radically different
> from the intended tone. My *intent* was not to say you were wrong
> (I knew from your .signature file that you know more about this
> than I do) but to say "Hmmm, I heard something conflicting from
> another expert way back when, what's going on here?"
Sorry, I didn't mean to be overly defensive. But I've had a couple of
conversations with people on the net who not only had no clue what they
were talking about, but when I honestly tried to explain something to
them, they became extremely hostile about it, insisting that they were
right, and I didn't know what I was talking about. It was one of those
cases where a little knowledge is dangerous.
But anyway, I digress...
John Schilling
Thomas R Nelson <tne...@uic.edu> writes:
>On 13 Nov 1997, John Schilling wrote:
>>
>> If you can design a mirror that will survive even 0.1 seconds under ten
>> megawatts of concentrated laser energy, yet be light and rugged enough
>> to cover a vehicle with, I know quite a few optical designers who would
>> like to talk to you. White paint won't do it by a long shot; it will
>> be charred to flat black in millisenconds.
>Define "concentrated" laser energy. There are laser mirrors that are
>commercially available which can take much more power than 10 megawatts,
>at fairly high power densities, like on the order of gigawatts per square
>centimeter.
Not for a full tenth of a second they can't. You can wave lots of large
numbers about when dealing with the peak power of large pulsed lasers,
but what usually matters is pulse energy and average power.
>Most laser mirrors that are higher than 95% reflective are not
>metal mirrors, but rather use layers of dielectric coatings which in the
>right order can give very high (99.9%) reflectivities, and can be much
>more durable than metal mirrors.
But not durable enough to serve as practical armor.
>And dust can cause a problem, but usually the way dust would damage a
>mirror is through the dust heating up by absorbing the laser energy, which
>can burn the mirror coating.
Or by exploding under the influence of a high-energy laser pulse and
causing mechanical damage, depending on just how you set up the laser.
Whatever the mechanism, the point is that dusty mirrors don't work
under high-energy laser illumination.
>Also, you are correct that high power lasers would be pulsed, rather than
>CW. But the light pulses last much less time than 0.1 seconds. For
>lasers that put out gigawatts or even terawatts, typical pulse durations
>are on the order of 100fs or less. (1fs = 10^-15 seconds).
Again, at that point talking about laser power in anything other than the
time-averaged sense is usually irrelevant. One terawatt for one picosecond
is pretty much the same as one gigawatt for one nanosecond as far as any
macroscopic target is concerned. The important question is how rapidly you
can repeat the one-joule pulses in question. If you can fire your
terawatt/picosecond or gigawatt/nanosecond laser at one megahertz, then
in macroscopic terms what you've got is a megawatt laser.
Thomas R Nelson
On 17 Dec 1997, John Schilling wrote:
> >Define "concentrated" laser energy. There are laser mirrors that are
> >commercially available which can take much more power than 10 megawatts,
> >at fairly high power densities, like on the order of gigawatts per square
> >centimeter.
>
> Not for a full tenth of a second they can't. You can wave lots of large
> numbers about when dealing with the peak power of large pulsed lasers,
> but what usually matters is pulse energy and average power.
Apparently you missed my second post on the subject? If it lasted for 0.1
seconds, it would have to have a really high energy density to reach
intensities on the order of 1GW/cm^2, and ON THAT TIME SCALE, the damage
threshold becomes fairly low. So yes, if you had a laser pulse that
lasted 0.1 seconds, and still reached 1GW/cm^2 (a non-trivial thing to
do), you would fry the shit out of something based on the energy density.
But in most pulsed lasers, the pulse duration is MUCH shorter than 0.1
seconds. Actually, one of the most common pulsed lasers is a Nd:YAG
laser. Nd:YAG is a solid crystal, and can be made fairly powerful while
still being pretty compact. Actually it would be fairly well suited for
some sort of light weaponry. But the pulse duration there is still
nanoseconds, fairly long by today's standards, but still much shorter than
0.1 seconds.
>
> >Most laser mirrors that are higher than 95% reflective are not
> >metal mirrors, but rather use layers of dielectric coatings which in the
> >right order can give very high (99.9%) reflectivities, and can be much
> >more durable than metal mirrors.
>
> But not durable enough to serve as practical armor.
>
Depends on what you're armoring yourself against. But you can still place
a dielectric coating on top of a metal coating to provide optimum
coverage.
>
> >And dust can cause a problem, but usually the way dust would damage a
> >mirror is through the dust heating up by absorbing the laser energy, which
> >can burn the mirror coating.
>
> Or by exploding under the influence of a high-energy laser pulse and
> causing mechanical damage, depending on just how you set up the laser.
> Whatever the mechanism, the point is that dusty mirrors don't work
> under high-energy laser illumination.
>
Depends on how you define "work". Dusty mirrors exhibit a lower
reflectivity under any intensity illumination, because the dust scatters
the light diffusely, rather than allowing it to be reflected from the
mirror surface. But it doesn't increase the absorption of the mirrored
surface.
If you're interested in shielding, yes you are correct in that sense, the
dust can lower the damage threshold. But if you have enough energy
density to blow up the dust particles (which are actually very hard,
typically), then damage from the dust is the least of your worries.
>
> >Also, you are correct that high power lasers would be pulsed, rather than
> >CW. But the light pulses last much less time than 0.1 seconds. For
> >lasers that put out gigawatts or even terawatts, typical pulse durations
> >are on the order of 100fs or less. (1fs = 10^-15 seconds).
>
>
> Again, at that point talking about laser power in anything other than the
> time-averaged sense is usually irrelevant. One terawatt for one picosecond
> is pretty much the same as one gigawatt for one nanosecond as far as any
> macroscopic target is concerned.
I know several physicists who disagree with you.
>The important question is how rapidly you
> can repeat the one-joule pulses in question. If you can fire your
> terawatt/picosecond or gigawatt/nanosecond laser at one megahertz, then
> in macroscopic terms what you've got is a megawatt laser.
>
Actually, the higher peak power can be a huge advantage. In terms of how
laser radiation interacts with matter, there are several effects that are
intensity dependant (intensity meaning peak intensity, due to the strength
of the electric field). These effects increase nonlinearly with
intensity. The two lasers you describe above, while would have the same
AVERAGE power, would not be equally as effective. The interaction of the
matter and the radiation would largely be governed by the electric field
strength, which is given by the peak intensity, not the average intensity.
The laser system I use here in the lab produces pulses on the order of
1TW, but only fires at 0.4 Hz, which only has an average power of 120mW
(300mJ once every 2.5 seconds).
But trust me, you don't want to be in front of it when it fires.
Incidentally, since I came in to the conversation a little late, what
wavelength would this laser be?
Tommy the Terrorist
In article <Pine.A41.3.96.971217...@tigger.cc.uic.edu>
Thomas R Nelson, tne...@uic.edu writes:
>The laser system I use here in the lab produces pulses on the order of
>1TW, but only fires at 0.4 Hz, which only has an average power of 120mW
>(300mJ once every 2.5 seconds).
>But trust me, you don't want to be in front of it when it fires.
This seems to vindicate my idea in the "Horn of Jericho" thread that 4
Watts *is* enough power in a musical instrument, to do some pretty
serious damage. That and the "sonic laser" thread...
By the way, how large is the laser system you describe and associated
power supply? Is it a basis for a conceivable man-portable ray gun,
right out of sci-fi?
Thomas R Nelson
On 19 Dec 1997, Tommy the Terrorist wrote:
>
> By the way, how large is the laser system you describe and associated
> power supply? Is it a basis for a conceivable man-portable ray gun,
> right out of sci-fi?
>
Not a laser that powerful. A high power laser system is typically
comprised of many components. A front-end laser usually generates the
pulse to be amplified, and then it propagates through several amplifiers
before reaching the final energy. It's a bit more complicated than just
that, but suffice it to say, by today's technology, a man-portable
terawatt laser is not possible.
The laser system we have in the lab here at UIC fills a very large room.
Part of it is fairly compact, consisting of solid state materials, but our
final amplifiers are gas amplifiers, and are very large in order to
provide a large gain.
However, all this is not to say that it's impossible to have a powerful
laser be small enough to be carried around by a man, but there are other
issues involved that would make it difficult to use a laser as an
effective portable weapon.