Lasers against clear plexiglass?

[mmartin798]

Originally Posted by tsofian

I had another nasty thought for the corridor into Prime Base. The security post has lasers. It might be vulnerable to attack by shape charge warheads.

What if there are a number of plexiglass panels that can be placed to block the passageway? They are almost entirely transparent to the laser's wavelength. They could be fairly thick, enough to stop most bullets and to prevent the movement of people, and to trigger the detonation of shaped charge rockets and such. The Type 3 laser has an E Factor of 700 per second, so it can basically vaporize a human target every turn.

If we assume that the Project lasers are tuned to be in the IR part of the spectrum, we still have problems with plexiglass. Being almost transparent is a problem with a laser wattage of 800KW at full power. Also, if the plexiglass is thick enough to stop a bullet, then the IR transparency is significantly less. A 3rd edition Mark 3/4th edition Mark 1 laser will shred the plexiglass in short order if fired through it.

[tsofian]

How about some other material?

[mmartin798]

There are many materials that have better thermal properties than Plexiglass with good IR transmission. There is one material that seems to be the best fit for your application, aluminium oxynitride. Commercially available under the name ALON, when in thicknesses of 1.6" it is more effective at stopping .50 BMG than 3.7" of glass laminate and is lighter. It comes in 18x35 inch sheets, so your array of panels is possible. Probably not cheap though.

Found an actual report on the .50 BMG test. The ALON was a 1.6" laminate. The laminate material could degrade or fail from repeated laser blasts. So the panes will be thicker than 1.6", but you should still be able to get the desired results.

[cosmicfish]

ALON won't work. Optical extinction is down around 0.075/cm in the 1-2 um range (the best IR or visible case for this material), which means that for a 2" thickness you would be dumping about 30% of the laser energy into the material. That would be catastrophic for any kind of laser weapon even though you could see through it.

There is no real material that is both a good armor material AND optically clear at a useful wavelength. I would suggest just using turrets or firing ports, but you could always just handwave a new miracle material if you really want to do this - just don't try to pretend it is a real material.

[mmartin798]

Originally Posted by cosmicfish

ALON won't work. Optical extinction is down around 0.075/cm in the 1-2 um range (the best IR or visible case for this material), which means that for a 2" thickness you would be dumping about 30% of the laser energy into the material. That would be catastrophic for any kind of laser weapon even though you could see through it.

Yeah, I didn't think that through far enough. Even with the higher melting point, the thermal conductance is too low to allow the heat from attenuating the beam to be siphoned away.