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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?

gisterme - 04:58pm Jul 5, 2001 EST (#6644 of 6649)

smartalix wrote ( smartalix 7/2/01 7:37pm ): "...The fact remains that you can train a laser on a target, but that pesky inverse-square law that Robert alludes to gets in the way..."

Didn't see this post before, smartalix or I would have answered right away...

Yes, the inverse square law does apply but you need to consider that it applies within the solid radial volume of the beam. For example a point source like a distant star radiates all its energy into a spherical volume, 2pi radians on each axis. With a laser beam, like the ones we've been discussing, the energy is all contained within the radial volume of the beam. So for a beam with a given divergence, travelling in vacuum, you can easily calculate what the energy/unit-area would be at any given distance. An example I recall from one of the laser development web sites previously posted ( gisterme 7/3/01 7:52pm ) was that a 1m diameter beam (at the source) had a 12m diameter at 500km distance. So if the laser has a 1 Megawatt output that means that at 500km it will deliver about 8.8kW/square meter to whaterver gets into its beam. At double the distance it would deliver about 1/4 as much. So at 1000 km it would deliver about 2.2 kW/square meter. That's what the square law means. To get a real-world idea of how much heating that is, the broiler element in an electric oven delivers on the order of 3 kW/square meter. That's not enough to destroy an ICBM but is enough to gather test data or broil steaks from space. :-)

The beam divergence in the example is about 11m/500000m => 22.0 microradians. A 0.22 microradian beam divergence (100x better than the example and similar to HST optical performace) would give a beam width of about 1.11m at 500km with an area of about .97 square meters. At the same source power that would be about 1.03MW/square meter, or about 104W/square centimeter delivered to the target. At 1000km the beam width would be about 1.22m, an area of 1.16 square meters with about 67.2W/square centimeter delivered.

There's no reason to believe that 1MW is any sort of a physical limit for laser power or that only a single satellite based laser would engage a target at any time. Likewise, there's no reason to believe that the 15-year-old optical technology used on the HST hasn't been improved upon since that time (IF it was really the best that could be done back then). Still, a single order of magnitude improvement in laser output power alone (over what's published) using optics as good as the HST would give the estimated 1 kw/square cm energy required to destroy an ICBM booster at 500km. There would be no need to keep that beam focused on a 1cm square point on the rocket booster as Robert has suggested would be necessary. That's for a nominally parallel beam. If the optics were designed to focus the beam at a certain distance (negative divergence), at say a few thousand kilometers, even larger energies could be delivered to the target at that distance.

continued...

gisterme - 04:59pm Jul 5, 2001 EST (#6645 of 6649)

gisterme 7/5/01 4:58pm continued:

Your cogitation about spinning ICBM boosters is nuts, smartalix and verifies that you, not dirac is lacking in knowledge. During the boost phase the rockets don't travel in a straight line. ICBMs need to use a navigation system, usually inertial, based on gyro stabilized platforms, to guide their payload onto a very specific trajectory for release; they use vectored thrust to accomplish this. In addition, the launch platform for MIRVs would be severly challenged if it were spinning since it must launch each of the MIRVs onto a very specific DIFFERENT trajectory after separation from the booster. Spinning gyro-stabilized platforms just won't cut it, smartalix. You need to go to your local library and check out a high-school physics book. Read about gyroscopes...

Forget armor plate if you want the ICBM to have any payload to speak of. Adding something like the tiles used on the space shuttle to an existing ICBM would greatly reduce its range and payload, meaning less bombs and less decoys. Tiles should be far more effective and lighter than the titanium armor you suggested. That's a non-starter unless much more powerful ICBM boosters were designed and built. Not a project that N. Korea, Iraq or Iran could likely afford or hide.

Mirrored surfaces on the booster might have some effect on a laser attack but making the surface of an ICBM THAT reflective at the wavelenghth of the laser would probably be more challenging than building all the rest of the ICBM. The entire surface of the rocket would have to be a telescope-quality reflector that could remain so while its substrate containted the huge pressures that are built up within the rocket. That means that the rocket tube (assuming a solid fuel rocket) could not flex AT ALL. Not likely.

WRT decoys...expense was never a consideration, smartalix, although I'd love to see a 500 lb. gold nugget fall into my back yard, even if it did have laser burns on it or had been clobbered by a collision with an interceptor. :-) Still, you see my point that THREE or even TEN targets each coming from only a few ICBMs could be engaged during re-entry far more easily than the thousands of decoys that could be deployed while the payload is in space. Remember, the proposed BMD system is NOT intended to stop an attack by more than a very few ICBMs launched simultaneously. That's why Russia or even China shouldn't feel that their strategic strike force is rendered usless by such a limited system. Existing ICBMs already can't be used by any rational nation as an attack representitive of sane national leadership. Russia and China have both got better things to do with their limited investment capital than build bigger but equally useless ICBMs.

Ablative material on the fat end of a re-entry vehicle would make it much harder to destroy from below by a laser. Once the vehicle got deep into the atmoshere, I'd guess below 200,000 feet, it would also become hard to engage from above with a laser because it would be engulfed in a cacoon of ionized material from the heat shield. Above those altitudes, it would seem vulnerable to attack from above since it's heat shield would be facing AWAY from a satellite based laser. An overall ablative shield or mirroring would seem more practical to protect relatively small re-entry vehicles from a laser attack than they would be for protecting boosters. That's why there's a need for terminal interception...just in case all the ICBMs aren't destroyed during the boost phase. The ablative shield or mirroring on the re-entry vehicle won't make a bit of difference in a high-energy collision with an interceptor.

WRT aircraft based lasers. I think the aircraft would need to be within a few hund

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