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Russian military leaders have expressed concern about US plans
for a national missile defense system. Will defense technology be
limited by possibilities for a strategic imbalance? Is this just SDI
all over again?
(1719 previous messages)
- 06:41pm Mar 29, 2001 EST (#1720
Don't be so sure he's "probably right."
I haven't seen the reports either, and I asked Dirac for them
The energy requirements to shoot down a jet with a lasar are
large, and if they've got that working, I'm most impressed -- I'm
not saying it isn't done, quite -- but it is surely a most
impressive feat -- quite a bargain for 60 billion dollars and twenty
Even so, the feat, even if it is true (and usually rumors about
classified equipment are disinformation) 99 that's a long way from
shooting down a missile ( for example, you have to know where the
missile is -- and they don't have the resolution to do that
comfortably for even ONE incoming -- much less a few, with a number
of decoys )
And if that lasar is big enough to shoot down a plane --
how big is it?
- and how fast can it be aimed to the ENORMOUS
accuracy in space and time that MD needs ?
(Assuming that the data for aiming is there, and I have no reason
to believe that it is.)
The absorbtion is surely going to be very large through the
and the techniques used to "correct for atmospheric turbulence"
are entirely unavailable -- they take feedback that isn't there, and
they depend on compensating to optimize for resolution of a
fixed object over a substantial period of time (tens of
seconds and more).
Anyone who really had something in the Star Wars business would
arrange a demonstration of what was there, to respectable engineers
with security clearance -- who would then give their word, subject
to reasonable crossexamination, that the stuff works.
If that has happened, with real engineers, with real reputations
--- I'm interested. Also surprised.
Every engineer I've ever talked to about the schemes has felt as
I do -- that they are, in H.L. Menken's wonderful phrase
"as devoid of merit as a herringfish is of fur."
- 06:46pm Mar 29, 2001 EST (#1721
>with (potentially deliberate) variations in
>>acceleration preventing accuracy beyond more >>than
a 20-30cm, (due to microsecond relay time >>between
observation and beam aim correction),
I don't know how you got that. It is true that
>at intercontinental distances the distance the >missle will
move in the time it takes light to >make the round trip is on
the order of 20 cm. >But the trajectory is ballistic. We have
known >since Kepler how to predict where the missle >will be
next. And the motion is not at right >angles, so it would be
much less than 20 cm. And >it is not technically possible at
this time to >make a change in velocity that would make any
>difference. The dern thing is running 20,000 >miles/hr. It
would take a heck of a rocket >engine to make much difference
in the time it >takes light to make the trip.
The 20cm is an estimate based on light relay times, as you said.
On purely ballistic missiles it might be technically possible to
target, but the reference to deliberate variation would be if a
designer included (relatively cheap) auxilary rockets designed
purely to fire randomly in half-second intervals. Although the angle
of deflection of the missile's path will be minuscle, we can resolve
the sideways and forward components of the rockets motion
independently. If a 7000kg rocket had a 1400N rocket thrust applied
for 0.1 seconds, the sideways speed of movement would only be 1cm a
second, but as the laser would not be able to track faster than say
100ms, a 10mm beam would have only one second to heat the intended
material area to failure point. If the tracking were faster, a 0.5
correllation in laser and rocket movements would allow the
equivalent of 2 seconds time, if 0.9 correllation were possible, 10
seconds etc. To get the correllation to 0.9, however, with minimum
delays of ~20ms (at 500km) due to relativistic effects, would seem
quite impressive, if actually possible.
At shorter distances (1km, etc.), it would be easily possible, as
the delay would be more like 20ns - almost neglible, and the
deviation is far lower from error.
>and atmospheric absorption taking over 95% of
>>the beams energy
How'd you get that? For all the electromagnetic
>spectrum? Regardless of modulation or effect on >the
atmosphere of such energy density?
Visible, 95% = worst case scenario - clouds.
(Headline: "It would have saved us, had it been sunny!")
UV or greater will be heavily absorbed by upper atmosphere (laser
beam won't compare to stellar UV). IR will be absorbed by all gases
in atmosphere (water vapour especially).
Microwave, radio, etc. All too low power. (Microwave will be
reflected mainly, radiowave will just pass through).
Again, though, I agree - a good clear day, 95% is too high for
- 06:58pm Mar 29, 2001 EST (#1722
even if the trajectory is ballistic, the resolution for
reasonable confidence of hits isn't there -- for ANY KIND of half
way noisy environment -- they haven't hit single incomings yet-- .
But you're exactly right. Just a little "jiving" (and compressed
air would do-- you wouldn't need anything so fancy as rockets) and
you'd bugger the ballistic calculations, even if they were "perfect"
-- or just good enough- -- and the input data to go into the
calculation from radar IS NOT good enough, with any reasonable
All the position-time data points have error bands, and so
fitting your "simple hyperbolic section" you end up with a lot of
scatter, relative to what the system is being asked to do.
That's true even if the earth was a gravitationally perfect
point. And gravitational anamolies and nonsphericity of the earth
matter at these scales too.
Star Wars has been nonsense from the beginning.
Now if you are 1-2 kms from lauch - you can hit a missle with
artillery -- and if you're a few hundred yards from emergence (of a
submerged missile, for instance) you can take it out with a 50
caliber. But for the systems anybody's discussing, we're dealing
with fantasy and fraud.
And clouds cover a substantial amount of sky - a
substantial period of the time --and stars are "hard" to resolve in
daytime -- all good reasons to be "wary" of atmospheric turbulence
compensation schemes, at night and especially in the daytime.
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