On Lasers

So, I gather more’n a few laser fans are coming to visit these days, so just to save time, here’s the canonical reason that lasers are the ‘verse’s secondary weapons system, not its primary one:

(It turns out that this is really a recapitulation of points raised in Non-Standard Starship Scuffles, so if you’re already nodding along to that, you can more or less skip the rest. I’ll just hit a few high points.)

Lasers, for the most part, are useful weapons systems under many circumstances. (Obviously they have to be, given their use as point-defense; if you couldn’t get effective results from lasing a k-rod, they wouldn’t be used.) As mentioned elsewhere, you can get an effective result out of a laser weapon, due to collimation, up to around a light-second, which is the entirety of the inner engagement envelope, and as such every military starship mounts a passel of phased-array plasma lasers for point-defense, and larger classes cram in some broadside offensive lasers too.

You can actually collimate reasonably effective beams at rather longer distances than that, as the existence of starwisp tenders demonstrates – although they themselves are of little use for military purposes despite the incidents mentioned in that article, seeing as they shift angular vector and alter their focus with all the grace and speed of apatosauruses mating. One would, however, make a dandy generator for a laser web.

(Yes, they exist in the ‘verse, and have done ever since the Admiralty paid the Spaceflight Initiative to launch Sky-Shield, the homeworld’s first orbital defense grid, back in the day. Orbital defense grids remain their main military use, along with civilian beamed power.)

It’s just that the IN sees no particular point in paying in either cashy money or mass/volume budget for collimation to make them effective beyond the inner engagement envelope, because you aren’t going to hit any actively evading targets at that range anyway, golden BBs and spies having gotten you a copy of their drunkwalk algorithms aside, and kinetics/AKVs work better for the geometry games played in the outer envelope.

Here, though, is the spoiler in the deck where military lasers are concerned:

Thermal Superconductors.

(The laws of physics do permit them, I am assured, and local materials science is more than up to producing them.)

In up-to-date designs, starship armor is woven through with a dense mesh of the stuff, with wicking into big heat-sink tanks of thermal goo. This causes something of a problem for weaponized lasers, because it makes it ridiculously hard to create a hot spot that’ll vaporize – instead, you just add heat to the whole starship. Which is not useless by any means, if you can manage lots of repeated hits or keep a beam on target, because if you can pump enough heat into a starship, either it, the crew, or both, will go into thermal shutdown; but this is what lasers are for in ‘verse starship combat. If you want to blast things apart, you go for kinetics, because you can’t tank (sic) big lumps of baryons.

Of course, this defense has its limitations: a laser grid at short range can hit its target with enough power to overcome the armor and, indeed, to chop its target neatly into a pile of small cubes. But that’s for definitions of short range meaning “inside knife-fight range”, and any Flight Commander who let the range close that much without having his entire propulsion bus shot off first would be summarily cashiered for incompetence.

And that’s why lasers aren’t the primary or only weapons system around these parts.

 

16 thoughts on “On Lasers

  1. Of course, now I’m wondering if anyone in the Eldraeverse has managed to weaponise the (poorly-named) “atom lasers” (a.k.a. “bosers” in some circles).

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  2. Sorry if this has been answered before (I haven’t yet finished slurping the entire archive), but what about using laser-ablation or laser-supported-detonation-wave propulsion for kinetic KVs? How does that compare to the other available methods of throwing rocks at the enemy? Might it have had a period of use before some of those other methods were invented?
    Jordin Kare’s laser launch designs are interesting drives. Therefore, they are also interesting weapons 😉

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    • On top of the points Matterbeam has mentioned below, I’d imagine that one big problem with weaponizing laser-launch payload delivery is that anything that can be reliably targeted for boosting by your own lasers for propulsion can also be reliably targeted by the enemy’s for point defense.

      On the other hand, it might be useful to have a laser-launched first stage to serve as a delivery system for an otherwise (relatively) short-ranged payload like an AKV that has its own internal propulsion for combat maneuvering — a sort of new take on the old idea of the dragoon (mounted infantry that rides on horseback to the deployment site but dismounts to fight on foot).

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      • I don’t know, I’d’ve thought that the KV would be carrying a retroreflector for beam guidance (fun phase conjugation tricks etc. to steer the beam) which is only available to you (the enemy is on the wrong side of it). Then no matter what combat manœuvres it does (and note that varying the beam’s pulse characteristics can also be used to steer it, and it can be under acceleration right up to impact and has a waaay better propellant mass fraction than any regular rocket ­— you don’t need a nozzle or a tank, just a block of ‘ice’, so terminal / burnout acceleration is gonna be pretty high), as long as it stays within the beam’s ‘steering cone’ the beam will ‘automatically’ stay locked on to it. Meanwhile, the enemy has no such advantage, to them it’s just a blunt shield shedding hard rads as it ploughs through debris with an expanding thrust plume behind it, no less stealthy than any other actively-boosting missile.

        And if you’ve already developed big lasers for point defence and knife fights, why not carry a few Multiple Independent Reentry Vegetables to throw with them when the enemy’s not at knife-fighting range?

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          • The concept was invented by Jordin Kare and his laser propulsion team at LLNL (back in the days when SDIO wanted (a) lasers and (b) cheap spacelaunch and was willing to supply some funding for both). The idea was that cucumber, say, is a solid, mostly made of water, and doesn’t go and melt while you’re setting up the experiment in the way the most obvious “solid made of water” does. I’d love to know why it didn’t pan out, but sadly Jordin’s posting does not give details.

            (Incidentally, anyone who is interested in space and, like me, wasn’t sufficiently internet-enabled and/or born to read sci.space.* in the 80s and 90s, should definitely read their way through the yarchive space section.)

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  3. Be wary of the unintended side-effects of having thermal superconductors that can absorb the heat of weaponized laser intensities. They would comfortably sit on top of a fusion reaction (making fusion containment trivial) or allow heat to be transported over very long distances.

    @ec429
    Laser ablation and pulse detonation is an effective means of propulsion, but it is limited by the electricity-to-laser-to-engine efficiency, the beam’s effective range and the acceleration time leading to lower rates of fire.

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    • I didn’t think the efficiencies were that bad, apparently FELs are something like 20% and the thruster is maybe 40% giving something like an 8% wallplug efficiency for the whole system. But the advantage is that the energy source stays on your warship the whole time (and in one big lump, too!) which I reckon gives you a better whole-system performance than carrying around a tank or two of chemical potential energy (or a stick of boom, if you prefer solids) and the nozzles, turbopumps etc. to use it, on every KV.

      If the acceleration time is too long, you can always combine this with something like a railgun (or other catapult of choice) for the initial throw; use the laser just to keep the thing accelerating (in various directions) as it passes through the enemy’s defenses. To borrow your terms, the laser may be better suited to ‘attack’ than ‘boost’ propulsion.

      I concede that the question of effective range may be a problem; but since the flux needed for useful acceleration is much lower than the flux needed for useful weaponry (the KV’s propellant is trying to be vaporised, the target is trying not to be 😉 the range should be longer than using the same laser directly as a weapon.

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  4. It does rather depend on what your thermal superconductors are made of.

    Presumably there’s a certain amount of space magic involved, because anything made of conventional matter will start having all sorts of problems with its chemical bonds and crystal structure and various bound states if it starts getting ionized. Alterations to those things are associated with a breakdown of fancy emergent properties like superconductivity. Things like single photon ionization aren’t thermal effects, and so a “simple” thermal superconductor would not be safe from it.

    The muonic matter used in your torch-ship shields would make an excellent mirror for short wavelength light, so practical and portable x-ray laser weapons would be straightfoward things for space elves to make. The fact that you haven’t mentioned them (or muonic armor to defend against them) implies that your thermal shielding is highly resistant to ionization all by itself, right?

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    • Whilst I think about it, even if the material could reduce ionization to mere heating by keeping a death grip on its electrons (or whatever else it might use in place of electrons), thermal superconductivity is still ultimately thermal conductivity and as such involves phonons carrying energy from A to B through the conductor. Phonons move at the speed of sound in the material.

      In regular matter, x-ray lasers can potentially have a drilling speed that exceeds the speed of sound. If this were to happen to your thermal superconductors, the damage front would simply race ahead of the thermal shock and the target would go “foom” in short order.

      This would probably only be the case for reasonably powerful xray lasers at moderate-to-short range, but for someone sufficiently mathematically inclined there’s an equation that should drop out of that which relates the speed of sound in the thermal superconductors to the killing potential of ionizing lasers that could be fielded by space elves.

      I wonder if a thermal supercondutor would make for a good phononic computer substrate…

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