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    Missile Defense

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?

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rshowalter - 06:41pm Mar 29, 2001 EST (#1720 of 1732) Delete Message
Robert Showalter

Don't be so sure he's "probably right."

I haven't seen the reports either, and I asked Dirac for them previously.

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 years.

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 atmosphere --

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."

eurocore - 06:46pm Mar 29, 2001 EST (#1721 of 1732)

>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 visible.

rshowalter - 06:58pm Mar 29, 2001 EST (#1722 of 1732) Delete Message
Robert Showalter

eurocore 3/29/01 6:46pm

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 engineering margin.

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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