Realistic Defense Laser Performance

[Junghalli]

Something I've been wondering about, in regards to lasers vs. missiles for space weaponry. Exactly what sort of performance would a realistic future weaponized laser have anyway? What would its effective range be? And what sort of limitations will it have on refire rate, i.e. what sort of cool-down time would it probably require after taking out a missile?

I got a figure of (at least) 7355 km for a 35 MW laser based on the distance to the horizon with a missile sat in a 3367 km high orbit from "Space Weapons: Earth Wars" (it's on Google Books but the link doesn't seem to work when I try to put it in the post). The Airborne Laser is supposed to be able to get ICBMs out to 600 km, 300 km for solid-fuel designs (which is probably more realistic for a missile designed with laser-based PD in mind). So I'm guessing maybe 5-10,000 km for a conservative figure?

I know the ABL requires several minutes of cool-down time after seconds of firing time, but presumably a future system would be better? Would a few seconds of cool-down time be reasonable? What about continuous beam, and if so what would its endurance limits be?

[SpacedTeddyBear]

It would depend on what kind of gain medium they can discover that have ideal performance characteristics. Obviously you want the laser to lase in the IR region to limit diffraction from particulates. The downside to the COIL laser's gain medium is that it can be depleted, limiting the number it can fire.

One of the limits to CW operation, is that you need constant cooling to keep the gain medium from overheating once its lasing. Overheating will reduce the efficiency of the laser since high temperatures will increase thermal collisions with the pumped electrons. A way around this for solid state lasers, is to pump it with stacked diodes, rather than pumping it with a flashlamp.

[Darth Wong]

Something I've been wondering about, in regards to lasers vs. missiles for space weaponry. Exactly what sort of performance would a realistic future weaponized laser have anyway? What would its effective range be? And what sort of limitations will it have on refire rate, i.e. what sort of cool-down time would it probably require after taking out a missile?

I got a figure of (at least) 7355 km for a 35 MW laser based on the distance to the horizon with a missile sat in a 3367 km high orbit from "Space Weapons: Earth Wars" (it's on Google Books but the link doesn't seem to work when I try to put it in the post). The Airborne Laser is supposed to be able to get ICBMs out to 600 km, 300 km for solid-fuel designs (which is probably more realistic for a missile designed with laser-based PD in mind). So I'm guessing maybe 5-10,000 km for a conservative figure?

I know the ABL requires several minutes of cool-down time after seconds of firing time, but presumably a future system would be better? Would a few seconds of cool-down time be reasonable? What about continuous beam, and if so what would its endurance limits be?

You can't just peg a number on effective range for a laser. It depends on the target it's tracking: a more fast-moving, erratic target would be more difficult to hit. Leading the target is not necessary of course, but the more powerful your laser is, the more massive its mechanism will be. This means it will be more difficult to position it precisely and quickly (the simplest rule of electro-mechanical control systems is that you can position it really quickly, or you can position it really precisely, but you can't do both at the same time).

[Junghalli]

Obviously you want the laser to lase in the IR region to limit diffraction from particulates.

Is this a concern in space, or only in atmosphere? I know the proposed SBLs are mid-infrared, but plugging in the numbers from the table on Atomic Rockets you seem to get much lower diffusion (and hence longer range) with high-energy lasers like ultraviolets.

[SpacedTeddyBear]

Obviously you want the laser to lase in the IR region to limit diffraction from particulates.

Is this a concern in space, or only in atmosphere? I know the proposed SBLs are mid-infrared, but plugging in the numbers from the table on Atomic Rockets you seem to get much lower diffusion (and hence longer range) with high-energy lasers like ultraviolets.

This is a concern only in the lower atmosphere where larger particulates and water vapor is present. Though there are bands in the IR range that can pass through the atmosphere without being blocked/diffracted. . At higher elevation, it's not much of a problem. With UV lasers, if they are powerful enough, they will ionize the atomsphere around the beam. When oxygen ionizes, one of the end products is ozone which blocks UV.