Zenith Star

[Forest Green]

Cause a 500 kilowatt beam of a few centermeters can do the same damage as a 3 meter 5 megawattter due to putting more energy in less space.

That's why shorter wavelengths are better (assuming they're at an atmospheric window) because the optics don't have to be as large to focus the beam.

 

[Scott Kenny]

But how much precision would be needed to ensure the beam hits the right spot? RVs are moving fast and could have last-second maneuvering. Wouldn’t that make consistently hitting a vulnerable section tricky, even with modern tracking?

The idea is to hit the missiles before they release RVs to begin with, but RVs don't do a lot of jinking until they hit atmosphere. Just some relatively small adjustments of the entire bus to make sure they're released on the proper course to land where they're wanted.

That's why shorter wavelengths are better (assuming they're at an atmospheric window) because the optics don't have to be as large to focus the beam.

Larger optics are better for handling the thermal load, though, and also give you a lot more range.

The two major determinants of laser effective range are laser wavelength (the smaller the better, near-UV or even UV-C is far better than mid-IR) and targeting mirror diameter (the larger the better).

 

[Forest Green]

You also have to bear in mind the atmospheric window though. Near UV suffers higher absorbtion than 1,000nm.

 

[Scott Kenny]

You also have to bear in mind the atmospheric window though. Near UV suffers higher absorbtion than 1,000nm.

True, but 1200nm results in ~100km effective range of a laser before spot size growth plus pointing errors exceeds target size (for a 1m diameter target and 1m diameter pointing mirror!)

200nm on the other hand results in a ~1800km effective range under same conditions.

 

[Forest Green]

200nm suffers 100% absorbtion though. All depends on the maximum size of your mirror and what power levels you can achieve at said laser frequencies. Also what size your mirror has to be for thermal load.

 

[Scott Kenny]

200nm suffers 100% absorbtion though. All depends on the maximum size of your mirror and what power levels you can achieve at said laser frequencies. Also what size your mirror has to be for thermal load.
View attachment 760443

Not sure it does (various UV does make it to the surface, hence why I burn in the sun), and the targets are climbing into thinner/no air. For ICBM targets, I think UV is acceptable since you're only losing ~3min of shooting over a ~30min flight time. And you gain a hell of a lot of range in exchange.

 

[blackstar]

This thread doesn't really have anything to do with Zenith Star anymore.

 

[Forest Green]

Not sure it does (various UV does make it to the surface, hence why I burn in the sun), and the targets are climbing into thinner/no air. For ICBM targets, I think UV is acceptable since you're only losing ~3min of shooting over a ~30min flight time. And you gain a hell of a lot of range in exchange.

Perhaps not all but 95+% as per the graphic. 3 minutes is a significant portion of the boost phase though, which is often less than 3min for solid propellant rockets.

 

[Scott Kenny]

Perhaps not all but 95+% as per the graphic. 3 minutes is a significant portion of the boost phase though, which is often less than 3min for solid propellant rockets.

So UV lasers end up as midcourse engagement or the tail end of boost phase.

Again, we're talking about over an order of magnitude of improved range.

 

[publiusr]

A recent article had it that light, not heat--helped evaporate water--the band where light was most effective--green--was the band water was least affected according to old textbooks.

Go figure

 

[Forest Green]

So UV lasers end up as midcourse engagement or the tail end of boost phase.

Again, we're talking about over an order of magnitude of improved range.

Where are you getting your order of magintude from? Surely it's roughly equivalent to the change in wavelength? There an order of magnitude difference in speed by that point too making it harder to focus the beam in one spot. Hitting the missile as soon as it leaves the hole is very appealing. Blue around 450nm may be appealing for many reasons however, it penetrates almost as well as 1000nm and it has less reflectance at metallic surfaces and smaller optics. The only problem is finding efficient and flexible high energy laser types at that wavelength. With UV the absorbtion is too high though.

Once outside the atmosphere NPBs probably work best but targeting is difficult.

 

[Byeman]

Once outside the atmosphere NPBs probably work best but targeting is difficult.

NPB was for target discrimination and not destruction

 

[Firefinder]

NPB was for target discrimination and not destruction

originally yes but it was later discovered that it can do both by a slight power increase.

This can pretty late in the program so didn't get much news on it but you can find imagines of people holding up holed plates with lables of NPB target XX on them.

Either last year or the year before they release a paper saying they fix the major issues with the system, able to do search discriminate and kill, just now need to do the devil details.

 

[Forest Green]

NPB was for target discrimination and not destruction

MJ for MJ I'd bank on a beam with real mass and KE that suffers zero surface reflectance taking out a target far quicker.

 

[Scott Kenny]

Where are you getting your order of magintude from? Surely it's roughly equivalent to the change in wavelength? There an order of magnitude difference in speed by that point too making it harder to focus the beam in one spot. Hitting the missile as soon as it leaves the hole is very appealing. Blue around 450nm may be appealing for many reasons however, it penetrates almost as well as 1000nm and it has less reflectance at metallic surfaces and smaller optics. The only problem is finding efficient and flexible high energy laser types at that wavelength. With UV the absorbtion is too high though.

Came from a combination of many formulas on Project Rho/Atomic Rockets.

Beam spread and pointing accuracy being the biggest, IIRC. I don't think I included beam quality in that, and I really should have.

I'll need to re-derive it, though. That spreadsheet was on an old computer that got drowned and I never bothered to dump a backup disk image onto an external HDD to recover the data.

Like I said earlier, it was beam spread and pointing accuracy to keep the total beam within a 1m diameter(? IIRC) circle. Need to find the pointing accuracy formula so I can recreate the formula I used, think it was diffraction-limited optics (since even Hubble is diffraction-limited).

Spoiler: Some of the formulas

Beam Spread:

RT = 0.305 * D * L / RL

where:

• RT = beam radius at target (m)
• D = distance from laser emitter to target (m)
• L = wavelength of laser beam (m, see table below)
• RL = radius of laser lens or reflector (m)

(Important formula)


Beam Divergence angle

θ = 0.61 L/RL

where:

• θ = beam divergence angle (radians)
• L = wavelength of laser beam (m, see table above)
• RL = radius of laser lens or reflector (m)

Note that this is the theoretical minimum size of the divergence angle, it will be [MUCH!] larger with inferior lasers.

Bold+Italics added by me.


Beam Intensity at target

BPT = BP/(π * (D * tan(θ/2))2)

where:

• BPT = Beam intensity at target (megawatts per square meter)
• BP = Beam Power at laser aperture (megawatts)
• D = range to target (meters)
• θ = Theta = Beam divergence angle (radians or degrees depending on your Tan() function)
• π = Pi = 3.14159...

Kerr notes that if you already know the beam radius at target RT, the above equation simplifies to:

BPT = BP/(π * RT2)

 

[Forest Green]

Came from a combination of many formulas on Project Rho/Atomic Rockets.

Beam spread and pointing accuracy being the biggest, IIRC. I don't think I included beam quality in that, and I really should have.

I'll need to re-derive it, though. That spreadsheet was on an old computer that got drowned and I never bothered to dump a backup disk image onto an external HDD to recover the data.

Like I said earlier, it was beam spread and pointing accuracy to keep the total beam within a 1m diameter(? IIRC) circle. Need to find the pointing accuracy formula so I can recreate the formula I used, think it was diffraction-limited optics (since even Hubble is diffraction-limited).

Spoiler: Some of the formulas

Beam Spread:

(Important formula)


Beam Divergence angle

Bold+Italics added by me.


Beam Intensity at target

Ah okay, it's radius that is proportional to wavelength but spot size is proportional to radius squared (for a given lens size), okay. The other two formula are really just different ways of expressing the first. So 1000nm gives you just under 9x the spot area of 350nm.

RT = 0.305 * D * L / RL

• RT = beam radius at target (m)
• D = distance from laser emitter to target (m)
• L = wavelength of laser beam (m, see table below)
• RL = radius of laser lens or reflector (m)

It could work at ~350nm. Less diffraction too. Maybe an Xe laser of some kind.

 

[Scott Kenny]

(Sorry, it's been more than 10 years since I thought about that set of formulas! And I am definitely out of practice mathematically speaking...)

And what matters is energy at the spot-on-target. So a 350nm laser will be able to put the same energy downrange at a target 9x farther away than a 1050nm laser could.

 

[Forest Green]

(Sorry, it's been more than 10 years since I thought about that set of formulas! And I am definitely out of practice mathematically speaking...)

And what matters is energy at the spot-on-target. So a 350nm laser will be able to put the same energy downrange at a target 9x farther away than a 1050nm laser could.

Which does actually make the 3x atmospheric attenuation worth it after all. Also, less atmospheric diffraction with shorter wavelengths, which probably helps with targeting.