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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?

(6721 previous messages)

gisterme - 08:13pm Jul 6, 2001 EST (#6722 of 6732)

rshowalter wrote ( rshowalter 7/6/01 5:01pm ): "...It isn't so easy to blow something up with a lasar. Lasar welding works nicely, but some very stout lasars make some very small, concentrated (and pretty) spot welds..."

Industrial lasers used for cutting 1/8" metal use power levels on the order of 200 W. So if the beam area is about 0.2 cm^^2 then it delivers the same 1kW/cm^^2 needed to destroy an ICBM. And those lasers are GREAT cutters and welders.

"...To destroy a booster or a warhead --even without countermeasurs -- people are talking about more damage than that -- even if problems with distance could be ignored -- and they can't be..."

Even a 1MW laser focused to a beamwidth about as big around as a coffee cup at some distance could deliver that same energy, 1kW/cm^^2, at whatever that focal distance is (through vacuum).

You are right about one thing though, Robert, maintaining the "hotspot" in one place on the rocket body would be a difficult control problem. If the "hotspot" were 1/10m in diameter, about the diameter of a coffee cup you could only tolerate jitter slightly less than the diameter of the spot, perhaps +/- 8cm, such that the 1kW/cm^^2 was maintained in at least one area. If the 1kW/cm^^2 "hot spot" was 1m in diameter (10 MW laser) then you'd be able to tolerate perhaps +/- 98 cm of jitter...more than 10 times as much with much greater total energy delivered to the target.

So at 1000km, for the 0.1m^^2 "hot spot" a +/- 8cm jitter would amount to about 0.16 microradians of peak-to-peak beam deflection. That's about 33 milliarc seconds of deflection. For the 1.0m^^2 "hot spot" at the same distance the allowable beam jitter of +/- 98cm would be about 1.95 microradians. That amounts to about 402 milliarc seconds of deflection. In each case a 4mm diameter area would be maintained at 1kW/cm^^2.

Let's see how those requirements compare to HST pointing performance...

http://www.stsci.edu/hst/CP7overview.html#0.2.440A6O.QKNXHC.QFSSSD.B1

"...Fine Guidance Sensors (FGS)

In normal operation, two of the FGSs are used for spacecraft attitude control. The third FGS thus has the potential of carrying out astrometric and photometric observations, including (1) measuring the relative positions of sources to a precision of a few milliarcseconds; (2) measuring the separations and magnitude differences of binary stars; and (3) measuring stellar angular diameters..."

Precision is "a few milliarcseconds". If a "few" means 3, then 3 milliarcsecons is about 10 times better than needed for the 33 milliarcseconds requirement of the 0.1m diameter beam and 100 times better than needed for the 1.0m diameter beam. Looks to me like the precision of the HST controls would be plenty good enough to hold the required level of beam-jitter at 10,000km or more. Of course the controls would need to be optimized for the particular application.

gisterme - 08:17pm Jul 6, 2001 EST (#6723 of 6732)

Out for today.

smartalix - 08:28pm Jul 6, 2001 EST (#6724 of 6732)
Anyone who denies you information considers themselves your master

Well, you can answer me later then on the other points of my post.

Plus this: why would Saddam (or any tinpot dictator) buy a very complex missile system from Russia or anyone, when all one needs is a warhead and those very same people you suggest would sacrifice themselves, but in a terrorist-style attack? Why? A second-hand missile launched from a second-party site may hit its target, but a boat with a nuke in its hold (or a truck) will 100% hit its target with the same, if not more, deniability.

Your science is well-based, but your attack scenario holds no water.

lunarchick - 08:43pm Jul 6, 2001 EST (#6725 of 6732)
lunarchick@www.com

Watching through binoculars:

rshowalter - 08:55pm Jul 6, 2001 EST (#6726 of 6732)
Robert Showalter showalte@macc.wisc.edu

I'm wondering about units -- and have only just a little time - I'll look at much more in the morning. How much TIME on target do these lasars have to have?

To melt a piece of ice, you have to heat it to its melting temperature, and then add the heat of fusion -- everybody knows that. Same with a welder - melting metal. To vaporize a metal, you have to get it to a temperature where it has a large vapor pressue, and add the heat of vaporization. That takes energy -- total energy -- not just a high energy per unit time for an unspecified time.

Gisterme , you're quoting units of watts -- that's energy per unit time.

How much time?

1 gigawatt for a nanosecond
1 megawatt for a microsecond
and 1 watt for a second
are the same amount of energy -- and in no case, enough to do any significant damage, no matter how well you focus it.

The question -- can these things actually burn holes in things? is a key question.

Also -- if they can burn a big hole in a target (and that's what military function takes) how hot does the lasar itself get -- and what does that do to optical precision of the beam?

Other questions, too. I'll deal with them tomorrow.

I'll repeat my position as of now. I think lasar weapons are wildly far fetched.