[Spaceships] Statting AU-range Lasers?

[DataPacRat]

One classic trope of space-hard-SF is setting up a pile of solar collectors near the Sun, such as on Mercury or in zillions of individual satellites in a Dyson Swarm; and use them to power lasers zapping across the Solar system, such as to push lightsails, get focused into rocket exhaust to improve efficiency and thrust, for industrial processes, or as outright weapons.

GURPS has lots of stats for lasers - but I can't find much to help with numbers for an artificially-generated network of rivers of light that are multiple AUs long. Anyone know where I can get some?


I know that focusing beams over AU scale distances is a non-trivial problem, but also that it's not an insoluble one. I've read of a proposal for a 300-meter diameter, 25-ton lens-type collimator that was projected to focus a 100-MW beam at 1 AU with 94% efficiency. My back-of-the-envelope estimate is that using similar tech, a lens could do a similar job at 3 AU for around 27 times the mass.

(I'm particularly fond of the idea of an 'aerosol lens', as proposed by NASA's "Orbiting Rainbows" study; take a large pile of 0.3mm beads, made of a glass with a highly nonlinear index of refraction, and you can use a pair of lasers to act as optical tweezers to push them all into shapes that act as holographic-grating lens structures.)

[Polkageist]

Well, like you said, what are you using it for?

Stats are useful for things that the players use, or can carry, or can affect. A network of solar collectors that feed into a laser-forming megastructure isn't exactly something that a PC would carry around in their backpack. It's kinda like statting a skyscraper, or a railway network. Aside from knowing the functional capabilities and how to use the ticket kiosk, does it really matter what the aggregate HP of the east-cost railway corridor is?

So again, what do you want it to do?

[DataPacRat]

Originally Posted by Polkageist

Well, like you said, what are you using it for?

Stats are useful for things that the players use, or can carry, or can affect. A network of solar collectors that feed into a laser-forming megastructure isn't exactly something that a PC would carry around in their backpack. It's kinda like statting a skyscraper, or a railway network. Aside from knowing the functional capabilities and how to use the ticket kiosk, does it really matter what the aggregate HP of the east-cost railway corridor is?

So again, what do you want it to do?

In background terms, form the basis of plot points involving melting asteroids. (And to provide new and interesting sorts of hardware that can go wrong and be available to be repaired or salvaged by some PC Vacuum Cleaners.)

In PC terms, having the option for their ship to have a small-scale lens-cloud emitter-gadget built in, with which they can redirect friendly fire they call in for their own purposes. They'd need to have an account with a provider, ala present-day utilities. Plus, with light-speed delays, they'd probably only be able to order a beam of N watts to be directed to coordinates XYZ at time T; so unless they have their own collimator to leave at those coordinates to redirect the beam, they'd have to keep their ship on a very particular course after transmitting their request to receive their light, which will hopefully provide some interesting conundra as Complicating Events Happen while the beam is on its way.

[Anthony]

I mean, any laser has unlimited range, the issue is how fast it spreads. For a visible light laser to not spread to larger than the size of its emitter requires a a diffraction-limited lens that's around 150 meters across; focusing to useful weapon intensity will require a couple orders magnitude larger lenses, or much shorter wavelengths. None of this will be terribly useful to stat up in GURPS terms.

[DataPacRat]

Originally Posted by Anthony

I mean, any laser has unlimited range, the issue is how fast it spreads. For a visible light laser to not spread to larger than the size of its emitter requires a a diffraction-limited lens that's around 150 meters across; focusing to useful weapon intensity will require a couple orders magnitude larger lenses, or much shorter wavelengths. None of this will be terribly useful to stat up in GURPS terms.

One set of stats that would be handy: For a given megawattage of capacity, how heavy and expensive a shipboard collimator/lens/receiver/thingy would be. So that if the ship's generator gets shot up, a call to the Light Company and turning the thing on would let them get power, or if they come across something whose position they can predict that they really want to melt, or whatever other unexpected plan a group of PCs might come up with. (Other useful stats might be how long it takes to deploy or retract, how it responds to getting shot at with various kinds of weaponry, and what a likely set of prices for a given amount of megawattage would be...)

I've been poking around at various GURPS stats for laser-boosted lightsails, or solar mirrors, but the closest I can find to this particular technodoodad is 3e Vehicles' "Beamed Power Receiver". For 100 MW, it would weigh 500,000 lbs and cost $101k; but that doesn't include any capacity to use the beam for anything but electricity, and is also ten times heavier than the estimate for a laser-maneouvered 300-meter aerosol lens I found.

[Anthony]

Originally Posted by DataPacRat

One set of stats that would be handy: For a given megawattage of capacity, how heavy and expensive a shipboard collimator/lens/receiver/thingy would be.

Basically unrelated. There's a bit of scaling based on the thermal load on the mirrors, but that's mostly covered by the normal weight of the weapon system, extreme range lasers are mostly just really really big mirrorrs. Basically, a 150 yard (SM+13 -- 11, +2 because circular) lens can focus on a similar size target at 1 AU, and each +1 SM would reduce the SM it can focus on by one.

[DataPacRat]

Originally Posted by Anthony

Basically unrelated. There's a bit of scaling based on the thermal load on the mirrors,

Any thoughts on how much thermal load it would take to start breaking down a lens made of a cloud zillions of 0.3mm glass beads (each bead around, oh, 30-70 micrograms, depending on ellipticity)?

but that's mostly covered by the normal weight of the weapon system, extreme range lasers are mostly just really really big mirrorrs. Basically, a 150 yard (SM+13 -- 11, +2 because circular) lens can focus on a similar size target at 1 AU, and each +1 SM would reduce the SM it can focus on by one.

Do you have any numbers on how far smaller lenses/mirrors could focus light onto similar-sized ones? Or how much larger a lens would have to be to have a longer range, such as 3 AU instead of 1?


For example, I'm imagining that light is 'stored', somewhat lossily, by having it circle around and around Earth's orbit 1 AU from the sun, redirected by a rosette of appropriately-sized lenses; with whatever's drawn out, or lost to inefficiencies, pumped back in from Mercury-orbit lasers to keep everything as smooth as possible. The big-and-expensive collimators would be spread far apart, with increasing numbers of gradually-smaller lenses near higher-demand areas for better response times to unexpectable local requests. Presumably that particular setup would be tilted to Earth's orbit so that none of the Big Beams are ever actually aimed at the planet. And similar setups would extend the network to other solar orbits with reasonably high demands.

The longer the range of the Biggest Available Collimators, the fewer of them there'd have to be to keep the large-wattage backbones of the network ticking over. And if I know the lens-size scaling factor for range, I can work out response times for PC requests for beamage, depending on whether they're in a densely-settled region with lots of small relays or are out in the middle of nowhere depending on a bit of a giant lens's cloud of granules being tweaked to aim their direction from light-minutes away.


If I take my source's figure of 25 tons for a 300-meter lens as given, I should be able to use the cube-law for scale changes; a one-half-diameter lens weighing an eighth of that, 3.125 tons for 150 meters; or 0.2 tons for 60 meters. (Not counting, of course, the hardware for the optical-tweezer lasers to manipulate the clouds of granules. Though I suspect such hardware would scale at least as much on the dimension of how much acceleration could be imparted to a given mass of lens-cloud as how big a lens-cloud could be manipulated... time to start estimating some numbers on forces, likely based on laser-boosted lightsails. :) )

[Anthony]

Originally Posted by DataPacRat

Any thoughts on how much thermal load it would take to start breaking down a lens made of a cloud zillions of 0.3mm glass beads (each bead around, oh, 30-70 micrograms, depending on ellipticity)?

Similar to any other mirror, but also irrelevant because such a cloud is fundamentally incapable of accurately focusing an AU-range laser.

Originally Posted by DataPacRat

Do you have any numbers on how far smaller lenses/mirrors could focus light onto similar-sized ones? Or how much larger a lens would have to be to have a longer range, such as 3 AU instead of 1?

Multiply range by both lens size and target size. Hence the 0.3mm beads reduce range by a factor of 1 million relative to the 300m lens.

Originally Posted by DataPacRat

For example, I'm imagining that light is 'stored', somewhat lossily, by having it circle around and around Earth's orbit 1 AU from the sun, redirected by a rosette of appropriately-sized lenses

Unfortunately, each reflection results in additional loss of focus so there's no appropriate lens size.

[DataPacRat]

Originally Posted by Anthony

Similar to any other mirror, but also irrelevant because such a cloud is fundamentally incapable of accurately focusing an AU-range laser.

The technical description I'm drawing from (the "orbiting rainbows" study) is that, optically, the individual beads aren't treated as individually doing the refractions; the cloud they're optically-tweezed into is formed into a three-dimensional, holographic-grating Fresnel-lens type structure. And that the overall structure can be treated as a single lens.

Some sources:
- https://www.centauri-dreams.org/2014...iting-rainbow/
- https://www.americaspace.com/2015/04...ge-exoplanets/
- https://www.photonics.com/Articles/L...ainbows/a57154
- https://www.nasa.gov/sites/default/f...s.v2tagged.pdf
- J. Palmer, “Plasmon-resonant aerosols for space optics”, J. Optical Society of America, Vol. 73, p. 1568 ff, 1983. , doi 10.1364/JOSA.73.001568

Multiply range by both lens size and target size.

That, at least, I can start applying to my incomplete design notes. :)

[Polkageist]

With respect to a shipboard system that accesses the system-wide laser power delivery network, the basic shipbuilding component is the ship system slot. The size of the ship helps define the capabilities of the system, but if you have the Spaceships book you'll have lots of examples of systems that can give you notions of scale.

Very basically, for a power system, one system slot containing a laser receiver would provide 1 power point to the ship. Adding in limitations like having a particular orientation or course is fine, there's ship systems that have quirks about their usage in the description so that's easy enough to toss in. The ship system mechanics abstract hard numbers so you don't have to worry about how many mm thick the mirror is or anything like that.

0.3mm glass bead cloud superscience? You bet!

3 AU vs 1 AU mirror size limitations? Bigger ship systems means a bigger mirror which means a longer access range from a source! SM+4 is 1 AU, SM+5 is 2, etc. etc.

Light's 'stored' in a mirror racetrack? Absolutely, why not, now laser access and strength is a solar system geography. Or access privileges (low-end subscribers get 'old' laser power)

For your asteroid melting purpose, which I'm guessing could also be craftily used to melt an dastardly pirate, you can either use the receiver component to provide the power points necessary to power a weapon array. Mechanically you use all the existing rules of a power-generating system and a weapon system, and descriptively it's the redirector/focusing thingy that you described below. Ditch the power-generating part if it's purely a rediretor that can only go through the one weapon system.

Don't sweat the mirror thickness, focus on how to access it, and the results of a PC using this system.