Lasers vs. Mirrors

i recently saw a program on the history channel about the development of an airborne laser defense system. a powerful chemical driven laser is to be installed in a 767. the laser fires out of the nose thru an aiming system. this is a variation on the Strategic Defense Initiative, alias Star Wars, where satellites fire and reflect the laser.

everyone has seen cars with chrome plated bumpers. what would happen if you fire a laser at such a bumper? are we to assume that people who can design missles won’t think of making the beam reflect off the missle? even if it is just reflective/ablative paint. if the missle is also rotating the hot spot would rotate from under the beam. this plane will only have seconds to acquire the target, aim and fire. if it only has time to burn a helical track in the paint around the missle it is useless. these television presentations assume the system will magically work, they don’t say anything about countermeasures.

is this a government boondoggle?

Russian Rocketeer: Hey Uri, go polish the missle we nuke amerikanski day after tomorrow. Use that simonize from Dupot. The capitalist swine make really goot wax.

Dal Timgar

Well, even with my limited knowledge of thermaldynamics, I would assume the aiming optics for Boeings Airborne Laser (ABL) would have to be of the highest quality in order to limit the amount of energy absorbed by the beam. Optics of this quality are very delicate and are not likely to survive as the coating for a missle as it rockets from the launch pad.

Its like saying “an M1A1 is coated by steel. A trashcan is also coated by steel. They should both be able to withstand a 20mm round.”

From what I have heard and seen in news footage, the laser doesn’t cut through missles like a welding torch. It acts more like an exploding grenade as all that thermal energy hits the target in a millisecond.

Read here before that ICBM’s are actually pretty fragile in the overall scheme of things. A relatively small amount of damage could cause them to veer WAY off course or shake themselves apart from turbulance/vibration at high speeds due to being “unbalanced” by the damage. Throwing an object several thousand miles with any accuracy is not childs play. Any problem or damage that makes it more dificult makes it damn near impossible to hit what you are aiming for.

A laser’s primary destructive force comes from its heat energy, not its light energy. For a mirror to protect anything from heat it would be acting as an insulator rather than a reflector, and the glass layer of the mirror would be the only real bebefit. A chrome bumper is thermally conductive, and would absorb the destructive energy rather than reflect it.

This is something I’ve wondered about for a long time (isn’t this more of a GQ than a GD?).

So is the conclusion then that in environment where lasers are threat, it would be useless to coat a missle or aircraft with a mirrored or chrome-like finish?

Even heat is still radiation though and can be reflected, no?

Yes, but a substance that reflects certain frequencies may not reflect other frequencies. There’s glass, for instance, that passes visible light while blocking UV light. Similiarly, something that reflects visible light may not reflect IR radiation. Just a semi-informed WAG, but I think the longer wavelengths of IR make it harder to reflect.

Mmm, what materials are there that effectively reflect IR? (some metallised coatings must do, to a certain extent as they are used in thermos flasks and ski wear).

Moderator’s Note: I think these questions have at least some chance of having actual answers, so I’ll send it over to GQ.

Of course, now it will turn into a raging debate on the merits of SDI, and it will just have to get moved back over to here…

Well, I think most thermos bottles have a layer of vacuum between two layers of metal and/or glass. It’s the vacuum which blocks the heat transfer—no medium for heat to flow through. And ski wear is similar, if you mean stuff like polar fleece (or wool, for that matter)—it’s the tiny air spaces between the fibers. But it doesn’t so much reflect it as simply not allow it to pass very well (insulation).

I’ve heard some convincing arguments on why reflective coatings wouldn’t work.
But what altitude does a missile have to reach for a laser system to be effective? I assume that a transpolar trajectory would bring the missile in range. But could a laser based system work against missiles launched from submarines off shore?

The vacuum in a thermos does not block heat radiation; it blocks heat loss by conduction; the silvering reflects the heat radiation (or most of it) back into the hot liquid.

The kind of ski wear I was thinking of is that which is stuffed or interwoven with shiny metallised plastic strands.

Here’s the Federation of American Scientists’ analysis of the Airborne Laser.

Emphasis added.
This weapon is meant for theatre defense (think Scud missiles), not for shooting down ICBMs. The missile will be engaged in its boost phase, over its own territory. I don’t know what altitude the missile would be, but probably in the range of 20,000 to 200,000 feet or so. How high do theatre ballistic missiles (say a range of 100 to 500 miles?) go, anyway?

The missile is not expected to be destroyed by a millisecond-long laser burst. The beam will be held on the target for up to three to five seconds, burning through the missile’s fragile shell and rupturing its fuel tanks or igniting its solid fuel. (Or wrecking its electronics - that’d do, too.)

Spinning a boosting missile to defeat a laser attack would be impractical. All large missiles are fin-stabilised: spinning them at any appreciable rate would make their control fins ineffective and they would quickly tumble out of control.
For pretty pictures, here’s the contractor companies’ (Boeing, TRW, Lockheed Martin) Airborne Laser page.

i started watching the space program in the 60’s and i distinctly remember seeing footage of missle launches where the missle was spinning. i don’t see why this should cause significant instability, rifeling is used in guns and cannons to increase the stability of the shell which increases range and accuracy. the microprocessor in the missle could be programmed to compensate for any problems with the missle. rotating the missle once per second would greatly increase the surface area to dissipate the energy of the laser.

since the chemicals used to produce the laser beam are known, the frequency of the laser is probably known. all of the potential enemies can be experimenting with materials to best reflect that frequency. unless of course they are spreading disinformation about the laser.

All warfare is based on deception.
Sun Tzu

Dal Timgar

Either the laser can be reflected, or it can’t be. If it can be, then you can coat the target with reflective material. If it can’t be, then you can’t aim it.

Where’s CalMeacham when you need him?

I should think that spiral-shaped fins would allow both stabilization and rotation. As dal-timgar said, think rifeling.


A spinning missile is great, unless in-flight course corrections seem like something you might need. Then your control surfaces are useless.

Damn, but it’s good to feel wanted.

I’ve been away from the computer most of this Memorial Day weekend.
A lot of this stuff was hashed out back in the 1980s in the original SDI debate. It’s slippery, because in this particular debate both sides feel free to change the rules whenever they feel like. (“That coating will work against visible lasers, but not infrared lasers – HaHaHaHa!” “Yeah, well I’ll just spin my missiles – HaHaHaHa!”) It’s just like Real Life. It eventualy points out all the flaws in the scenario.

1.) There’s always a possible cuntermeasure, no matter wat you choose – just like any other form of combat. No magic solution trumps all osibilities. Choose wrong and your opponent will get his missiles through/destroy all your missiles.

2.) One example of this is the argument that “you don’t have to ‘lead’ the target – lasers are virtually instantaneous”, which was met my the ablative coating.reflective coating/rotating the missile argument, which lead to the “we’ll just let the laser dwell on the target” argument. So you trade the need to lead the target with the need to dwell on it. (“But a;; you need o do is knock the missile off course,” goes another argument, and so on.)

3.) Arguments that mirror coatings aren’t effective have to contend with the fact that mirrors have to be used to steer the beam, too – and these mirrors will see higher power densities, and more often. This one hts home to me – I’ve blown the coatings off quite a few mirrors due to high power densities – it only takes a small speck of dust or imperfection, and it’s much more likely to affect you steering mirror or (worse) your laser mirrors than it is you kilometers-away reflective target.

Perhaps it is overly obvious, but a reflective surface on a missile would be an absolute invitation to a microwave steering (read: RADAR) inbound missile attack. IOW, a monkey with a patriot missile on hand could take it out without a manual.

A shiny missile would do no good.

A chemical laser is steered by first spreading out the beam (perhaps a meter across) and directing it with a concave mirror. The beam then gradually focuses down as it traverses the atmosphere to a micrscopic point where it contacts the missile hull. The heat developed instantly sears a hole and damages the delicate innards.

lather, rinse, repeat.