The coolest engineering project around

[russ_watters]

Main Question or Discussion Point

www.airbornelaser.com

 

[wimms]

I wonder, what is a result of missing a target and hitting ground, say city, downtown of NY?
What is consequence of people on the ground looking at the target at instant it is hit by laser? ie - How many blind people does it take to find this weapon makes unjustified collateral damage?

 

[russ_watters]

Originally posted by wimms
I wonder, what is a result of missing a target and hitting ground, say city, downtown of NY?
What is consequence of people on the ground looking at the target at instant it is hit by laser? ie - How many blind people does it take to find this weapon makes unjustified collateral damage?

People looking at the target wouldn't be affected unless the target is a perfect mirror focused on them. And the laser wouldn't ever be aimed down so it wouldn't hit anyone on the ground. Even if it were, it would move too fast to damage anything on the ground.

Remember, this laser is designed to shoot down ballistic missiles. A ballistic missile gets up to the operating altitude of this plane in something like 30-45 sec.

There are also political implications: since this is strictly defensive, collateral damage is the fault/responsibility of the country that launched the missile, not the country that shoots it down.

Collateral damage really would not be an issue with this. In any case, I said its cool because of its engineering value. Its an extrordinary project.

 

[Mr. Robin Parsons]

Originally posted by russ_watters
(SNIP) There are also political implications: since this is strictly defensive, collateral damage is the fault/responsibility of the country that launched the missile, not the country that shoots it down. (SNoP)

Hummm, how well can we rely upon that? (just curious)

 

[russ_watters]

Originally posted by Mr. Robin Parsons
Hummm, how well can we rely upon that? (just curious)

By its very nature it is not well suited to attacking ground targets.

 

[wimms]

Yes, I can agree its cool from engineernig perspective. And even more, perhaps its a way towards being able to deal with asteroids in the long run.

Originally posted by russ_watters
People looking at the target wouldn't be affected unless the target is a perfect mirror focused on them. And the laser wouldn't ever be aimed down so it wouldn't hit anyone on the ground. Even if it were, it would move too fast to damage anything on the ground.

Here I'm afraid you are quite wrong. Its a megawatt class laser onboard, and any, even faintly reflecting surface (which laser resistent missiles will obviously be), scatters laser in random directions. Even if you receive 1 millionth of the laser power, it would be 1Watt laser into your eye. Thats fatal to your eyes. And it takes less than a blink to become blind. And your eyes might not even detect it to blink because its out of visible frequency range.
Focusing of laser is done mainly to overcome normal scattering of beam, which is also why its so large diameter beam.

As to hitting NY city example, quickly sweeping over your skin megawatt laser beam would leave you unforgettable memories of the event, but more dangerous is zillions reflections of laser from all the shiny buildings around. Again, massive amount of blind people is the collateral damage.

There are also political implications: since this is strictly defensive, collateral damage is the fault/responsibility of the country that launched the missile, not the country that shoots it down.

Well, planting h-bomb to offensive country can also be called strictly defensive, but collateral damage of this is unjustified, and now agreed to be against conventions. You have to combat weapons with similar weapons. I can't imgine how do you imagine justifying ten blind kids who have nothing to do with what their government does, to shoot down a (eg. conventional) missile.

 

[russ_watters]

Wimms, I think you overestimate the risks from the laser. There is more scattering than you realize (from both the atmosphere and the object being hit) and less of a chance that someone would be randomly looking in the right direction to see the hit. Also, the beam is pretty wide - something like 1-2m in diameter (about a million times the size of your iris). So you actually would get only about a millionth of the power into your eye if they aimed the beam at you. And with random scattering, the amount of time the energy hits you would be pretty low. Then there's the atmosphere - it absorbs more than you might think. At the operating altitude of the plane, it'll be above something like 75% (maybe more?) of the atmosphere. Atmospheric absorption and scattering is one of the biggest engineering problems in this project.

Maybe I'll come back and do some more math on it this afternoon.

 

[kerimek]

Originally posted by russ_watters
www.airbornelaser.com

Interesting money dump Take an efficiency of electricity production in engines about 30% (maximally!), efficiency of electricity to beam energy conversion 10% (maximally) and finally plane to target energy transfer efficiency 20%. Then you have total efficiency about 0.6%. So for 1MW of continual thermal damage you require minimally 130MW energy source (without laser cooling consumption and normal engines consuption). Have you any dilithium, captain Kirk?

 

[Tyro]

Kerimek, your calculation is correct if you assume the process is a continuous one. Most lasers, despite having large wattages, only operate for a split second. As such, even a low-yield power source which charges capacitors, etc. over time would suffice. This does not affect the efficiencies, only the feasibility of various power sources.

 

[wimms]

Originally posted by russ_watters
Wimms, I think you overestimate the risks from the laser.

Maybe, but when I dealt with 5W argon lasers in the lab years ago, was strictly instructed to never fool with beam path optics without protective glasses, as single random reflection from any surface not limited to mirrors is sufficient to make permanent damage. That was 1mm beam.
20KW solidstate microsec pulses were tested strictly in separate room through cameras.

There is more scattering than you realize (from both the atmosphere and the object being hit) and less of a chance that someone would be randomly looking in the right direction to see the hit.

Interesting. How do you know what I realise?
Laser beam isn't flashlight, that makes it useful, and that makes it dangerous. It doesn't follow R-squared rules. Reflections from surfaces continue to be coherent beams. You can view surfaces reflecting laser as large amount of discrete beam splitters. That means that if there is 1mm flat spot on target, it deflects 1mm laser beam off it. Collimation is reduced, but its still dangerous stuff.
Penetration density is in tens of MW/mm, and 1mm flat spot deflects beam with huge density. Even if it dissipates fast after reflection, it can easily exceed hazardous levels for your eyes.

As to chance of looking, if it destroyed missile in subsecond time, noone would be even looking up. But it takes its time, and all sort of spectacular visual effects happen before it blows, so alot of curious people siteseeing are likely.

Also, the beam is pretty wide - something like 1-2m in diameter (about a million times the size of your iris). So you actually would get only about a millionth of the power into your eye if they aimed the beam at you.

Not quite sure what you mean here. The very beam is meant to vaporise metal spot of missile in few seconds. The energy density needed is immense. The output beam diam is about 1.5m it seems, and is focused to be much tighter at target, and all the way towards it beam diameter reduces.
And 1 millionth is 1W entering your iris, focuses on retina and blows it with concentration of few hundred watts per square mm as its concentrated to way smaller spot.

And with random scattering, the amount of time the energy hits you would be pretty low. Then there's the atmosphere - it absorbs more than you might think. At the operating altitude of the plane, it'll be above something like 75% (maybe more?) of the atmosphere. Atmospheric absorption and scattering is one of the biggest engineering problems in this project.

This absorption is dependant on energy density of laser, iirc. Its a main problem to deliver 100% of energy to target as energy density is high. Reflections are subject to way lower absorption and scattering from air.
Of course energy hitting me would be relatively very low. Out of 1MW only 0.1W perhaps. It doesn't kill, it blinds. 100mW laser cuts paper, btw.

And afterall, I don't quite get how they are expecting missiles to be absorbing surfaces at all. Cover missiles with perfectly reflecting coating, and all the energy goes into deflections. Imo, its a race where laser can't win, its a weapon against clueless.

 

[kerimek]

Originally posted by Tyro
Kerimek, your calculation is correct if you assume the process is a continuous one. Most lasers, despite having large wattages, only operate for a split second. As such, even a low-yield power source which charges capacitors, etc. over time would suffice. This does not affect the efficiencies, only the feasibility of various power sources.

Yes, I suppose that the 1MW is realised in pulses on example 10GW/0.1ms every second. However you require average (it doesn't matter if pulse or continual) damage power in megawats if you want to destroy rotating ballistic missile with >1 inch thick steel jacket in a few seconds.

 

[wimms]

Re: Re: The coolest engineering project around

Originally posted by kerimek
Interesting money dump Take an efficiency of electricity production in engines about 30% (maximally!), efficiency of electricity to beam energy conversion 10% (maximally) and finally plane to target energy transfer efficiency 20%. Then you have total efficiency about 0.6%. So for 1MW of continual thermal damage you require minimally 130MW energy source (without laser cooling consumption and normal engines consuption). Have you any dilithium, captain Kirk?

I'm not sure it makes sense to think about efficiency of energy conversion here. Although I don't quite get how it works, laser mentioned seems to be not so dependant on electricity:
- Chemical oxygen iodine laser (COIL) technology
- World record for chemical efficiency set by Northrop Grumman
Seems its sort of direct conversion of chemical energy into beam.
It really comes down to cost, not efficiency so much. But it sure is pretty expensive.

Btw, it seems to be continuous laser, not pulsed. Else they'd be talking about terawatt ranges. To huge density to penetrate air efficiently.

 

[russ_watters]

The laser itself is one of the coolest parts of the project. I've heard it described as being similar to a rocket motor - it burns, but instead of producing thrust a lot of its energy goes to producing a specific wavelength of light. I'm not sure of the specifics, its been a while since I've read through the site.

The fact that the technology is largely proven and just needs to be integrated into the platform is amazing to a lot of people. Its basically a product of Reagan's Star Wars that finally works.