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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?

rshowalter - 05:23pm Jul 6, 2001 EST (#6709 of 6715)
Robert Showalter showalte@macc.wisc.edu

High energy fluxes and ultra-precision geometry, as a general thing, don't mix.

Things expand when heated, and do it unevenly, and things distort.

And for Space Telescope levels of presision (not good enough for a death ray) just a little distortion messes up the optics beyond all hope and caring.

rshowalter - 05:29pm Jul 6, 2001 EST (#6710 of 6715)
Robert Showalter showalte@macc.wisc.edu

There's a problem -- and I'd call it a big one.

That is that, for controls, you need optical quality (small spreading angles) maybe 10-20 times more precise than Space Telescope has.

Even if your information input for targeting was perfect (and it isn't).

rshowalter - 05:50pm Jul 6, 2001 EST (#6711 of 6715)
Robert Showalter showalte@macc.wisc.edu

I'm having a problem finding a reference -- on angle, but have the analogous one on time -- there's Draper's Dictum (after Charles Stark Draper)

"To control a given system of any kind, the control elements of the system, whether automatic or human, must be able to effect changes in the system in times that are less than one-fourth, and preferably less than one tenth, of the characteristic time of change of the parts that are to be controlled."

Really, for the temporal precision to be meaningful -- dimensional precision must be, too -- and in about the same proportion. To control something within N cm -- you need to be able to measure where it is, real time -- to within .25 and (much better) .1 N cm.

The same goes for angles for an angular control -- and a death-ray aiming device is an angular control.

If you want targeting precision of 10e-7 radians (really not enough) you need something like 10e-8 radian feedback precision --- and that's WAY beyond Space Telescope - (which for ordinary light is about 5 x 10e-7 radians precision.)

rshowalter - 06:01pm Jul 6, 2001 EST (#6712 of 6715)
Robert Showalter showalte@macc.wisc.edu

Then there's the issue of "unwanted motion" -- especially vibration.

Suppose there is an angular vibration angle of theta --- -- the physical motion of the beam (perpendicular to the beam) at a distance d due to this angular vibration is

sine theta d

or, for theta in radians, for the small angles here,

theta d

Now, for a d of a few hundred or thousand kilometers, plug in your sweep angle due to vibration.

That vibration caused beam sweep will disipate energy that needs to be concentrated if the lasar is to do any damage.

Even a few cms of sweep causes a serious weakening of the destructive power of the beam.

And vibration is essentially impossible to get rid of -- in practical cases -- especially so on a rocket or an airplane, or anything , even in orbit, that you have to be able to move fast -- which takes hefty forces on structures which are not infinitely stiff.

rshowalter - 06:03pm Jul 6, 2001 EST (#6713 of 6715)
Robert Showalter showalte@macc.wisc.edu

The engineers, twenty and more years ago, who concentrated on smart rocks, rather than lasar beams, had some good reasons for their choice.

As terribly unattractive as "smart rock" approaches can be, for complicated or long distance targeting, "death ray" approaches are worse.

rshowalter - 06:06pm Jul 6, 2001 EST (#6714 of 6715)
Robert Showalter showalte@macc.wisc.edu

Questions so far ?

I can get farther into these things -- and there is a LOT of stuff in the open literature that should make judgement of the technical possibilities ( and required miracles ) more and more clear.

rshowalter - 06:07pm Jul 6, 2001 EST (#6715 of 6715)
Robert Showalter showalte@macc.wisc.edu

I'm taking a half hour break.

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