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In my eternal quest to model a decently-realistic sci-fi space combat system, I've run through the limitations of particle weapons, magnetic accelerator cannons, and fusion rockets. Now it's time to tackle the humble laser.

It's been my assumption that a laser would be an effective weapon at short enough ranges that a pulse can vaporize hull plating. Beyond that, it was my assumption that a laser would be fairly ineffective, since it would only be able to heat the target. Disabling an enemy through overheating is a viable strategy. However, I remember hearing somewhere that something like 20% of the energy put into a laser is lost as waste heat, which heats the ship that fires it. If the target is using a reflective hull material, something with an albedo greater than 0.8, combined with an effective coolant system to keep heat from building up in the outer hull, then the target is heating up less than the ship that's firing the laser - and that's assuming that 100% of the energy fired from the laser actually hits the target.

It's also my understanding that lasers can focus energy more effectively at a longer range if they have a larger lens.

So, my question is:

Can I use reflective hulls as a defense against lasers at long range? Lasers, by definition, operate at a single wavelength, so as long as the material reflects that specific wavelength, I would assume that they would. However, it has been indicated to me that this may not be the case. In other words, will wallpapering my ship with aluminum foil help?

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    $\begingroup$ Besides reflective hulls, the other common defense I've seen is some form of ablative hull (usually ice, water being relatively plentiful). The idea being that the laser burns of some of your hull, which then forms a cloud around the area, scattering the incoming laser. The ice also acts as a heatsink for firing your own weapons. $\endgroup$ – Clockwork-Muse Sep 11 at 0:02
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    $\begingroup$ Please edit your post down to a single question with a way of identifying a single best answer. At the moment you seem to be asking at least six questions, that's not how we work here. $\endgroup$ – We are Monica. Sep 11 at 1:35
  • $\begingroup$ If you haven't seen it, check out the Children of a Dead Earth blog - it's the best space combat resource I've seen. $\endgroup$ – Brizzy Sep 11 at 3:05
  • $\begingroup$ Check out the role play game Attack vector: Tactical it's go a fairly realistic system that you can take some inspiration from. $\endgroup$ – Efialtes Sep 11 at 11:29
  • $\begingroup$ @Brizzy its a good source for kinect energy weapon stuff, but the author negatively handwaves laser tech quite a lot (and has a few outright broken assumptions, too). Still all interesting though. $\endgroup$ – Starfish Prime Sep 11 at 13:46
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It depends on the power that is being fired at you.

Conventional mirrors are not 100% reflective. They normally reflect a bit more than 90% of the impinging light, meaning that around 10% of that power is absorbed or transmitted.

If you are targeted with a mW laser, 10% of that are peanuts, and you don't have to worry.

If you are targeted with a petawatt laser (10 to the 12 Watts), you are getting something like gigawatts of power, and you better worry about that.

You can improve the reflectivity of the mirrors up to 99.999...% using metamaterials like DBR, but those come with a strong directionality. This means that along some precise direction you reflect back 99.999...% of the light, but if you are off that direction you might be as good as naked against the laser.

And don't forget that you have to take of the reflected beam, too. You don't want to be hit by a reflection, don't you?

To your favor you have the distance from which the laser is fired: despite being highly collimated, laser beams are not perfectly collimated. They still have some milliradian divergence, which ensures that over cosmic distances the power is spread over a large surface. For your reference, in the Lunar Ranging Experiment, where a laser is fired at the mirrors left on the Moon surface by the Apollo missions (400k km distance, a bit more than 1 light-second), the laser reaches the surface of the Moon spread on a disc several km wide and its power per unit surface is proportionally decreased.

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It's been my assumption that a laser would be an effective weapon at short enough ranges that a pulse can vaporize hull plating.

"Short range" is a tricky thing to quantify. It very much depends on your own tech level assumptions and requirements, and as you haven't communicated them to us then I can't really speculate. The shorter the wavelength, the fancier your optics, the longer range your lasers. You more or less get to handwave this to suit your own purposes, and pick something between hundreds of km and lightseconds.

A bomb-pumped laser, for example, might have a range as little as 100km (which is more like practical casaba-howitzer range) or as long as 10000km, depending on the quality and efficiency of your lasing rods, and the effectiveness of your targetting system.

Beyond that, it was my assumption that a laser would be fairly ineffective, since it would only be able to heat the target.

Being gently warmed by x-rays may not be quite the same as being gently warmed by a heat lamp. Just sayin' ;-)

I remember hearing somewhere that something like 20% of the energy put into a laser is lost as waste heat,

An 80% efficient laser isn't implausible, but might well be considered extremely efficient. There are some diode laser designs that reach that level of efficiency, but many other kinds of laser, especially very short wavelength or very short pulse designs will have lower efficiency. (The bomb pumped lasers I mentioned above might only be 2% efficient, but when you're pumping them with a megaton bomb and you're using a thousand of them, the inefficiency isn't so much of an issue...)

Can I use reflective hulls as a defense against lasers at long range?

There will be some range where the incoming beam is juuuuuust intense enough to melt your armour, but if you buffed it a bit first then you'll be fine. This depends on the beam actually being reflectable, of course... at the far-UV and shorter wavelength range (eg. bomb-pumped x-ray laser) it becomes a bit difficult to do this, because your electrons will start falling off, but for visible light you might be able to get away with it.

Lasers, by definition, operate at a single wavelength

Generally, yes. It is possible to adjust the wavelength of a free electron laser by a small amount, limited by the available optics. A visible-light FEL might be quite adjustable, for example. It is also possible to carry more than one laser, or a laser which can be frequency-doubled (in the way that current-day commercial near-IR Nd:YAG lasers can use a KDP crystal to produce visible green light, at some efficiency cost). Ti-Sapphire lasers can have their output tuned to some extent, between 650 and 1100nm.

so as long as the material reflects that specific wavelength, I would assume that they would.

You can make highly efficient dielectric mirrors, but they tend to be efficient in a very specific wavelength band. They're expensive (much more so thanb dumb armour) and delicate (abrasion caused by micrometeorite damage will render them useless) and will be largely ineffective against any laser light of an inappropriate wavelength.

Moreover, at effective killing range you'll start to get very high laser intensities and interesting non-linear optical effects, which means that reflecting 99.99% of the incident light just isn't enough and your mirror surface will blow up in a nanosecond and be useless.

Oh, and buttering you up with a few bomb-pumped lasers at longer range might be enough to render your armour ineffective against more conventional laser fire.

For these reasons, they're probably not a good choice for warship armour.

In other words, will wallpapering my ship with aluminum foil help?

It'll help, inasmuch as putting stuff between you and their laser will keep you alive for a tiny bit longer when things start going south. Mostly what you want is something highly refractory, because you'll want your armour to still be useful at closer ranges when you simply can't reflect enough of the heat to be safe and need something that will absorb as much energy as possible before evaporating.

Some form of carbon is probably best. Boron will also work, but it is much rarer and you may find there are other, better uses for it. A weave of carbon nanotubes is probably the best thing to use here, as it'll resist being torn open by explosive effects. In any case, these materials will also be useful against other kinds of energy weapon. Maybe some harder inclusions (some form of hyperdiamond, perhaps) might help against kinetic energy weapons, inasmuch as anything helps.

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Reflective surfaces will always help deflect some of the power, however no material is capable of reflecting 100% and some of that energy will always be absorbed. So even if you have a 99.99999% reflective surface, you still absorb a tiny bit of energy which is the main point.

A laser is powerful because it can focus a decent amount of energy into a very very small area. For arguments sake, lets say I have 1 watt (not sure if this is the correct unit) spread over 1m^2. If I can focus that into 0.5m^2, then suddenly I have the equivalent of 4 watts of power applied to that surface. As I focus the laser onto a small spot, I exponentially increase the amount of power I apply to a single spot. The aim is to cause a single spot in your defense to fail, or burn up the reflective surface, which will then lead to a catastrophic failure in your defense system. So you can cover your entire ship in foil, but if my laser will only need to burn a hole smaller than a pin into that foil to compromise your system.

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    $\begingroup$ You can't focus you laser to a pin hole at orbital distances (hundreds and thousands of kilometers) en.wikipedia.org/wiki/Diffraction-limited_system $\endgroup$ – ksbes Sep 11 at 8:49
  • $\begingroup$ @ksbes sure you can. You'll need a big emitter though, or a large refocussing device closer to the target. $\endgroup$ – Starfish Prime Sep 11 at 13:43
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Wallpapering spaceship with 2-5 mm aluminum (steel would be much better) and active cooling will save you from any reasonable laser if you keep your distance.

80% dissipating energy is only for laboratoy lasers. For powerfull "battle lasers" only 0.1% - 5% of energy goes to the beam. And the beam itself greatly looses energy density with range due to difraction. It means that morden lasers in space has more chances to melt itself then opponent, even if opponent do not uses mirros.

I would say mirros are overrted as anti-laser defence. Better defence would be black material with high melting point (like titan-tungsten alloys or ceramics): heat dissapation due to radiation in "cold" space would not alow to melt such a material with laser at all (hard science is requerd here to make some firm statments). And active cooling and creative use of vacuum will keep you internal at normal temperature. You can even utilize that energy of laser beam!

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  • $\begingroup$ Given that "battle lasers" aren't even a thing that exists, I'd say the author is fairly free to handwave efficiencies. 80% is rather high, though. $\endgroup$ – Starfish Prime Sep 11 at 12:24
  • $\begingroup$ Battle laser prototypes does exist. en.wikipedia.org/wiki/Boeing_YAL-1 or en.wikipedia.org/wiki/AN/SEQ-3_Laser_Weapon_System and they have exactly this efficiencies (percents) $\endgroup$ – ksbes Sep 11 at 12:54
  • $\begingroup$ there are prototypes of all sorts of things. Doesn't mean that they work, or are fit for purpose. $\endgroup$ – Starfish Prime Sep 11 at 12:57
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The thing that kills lasers at range is beam spread.

A laser spreads a lot less than, say a flashlight, but it does spread. That makes the energy density go down. It is the energy density that burns through a ship. If the energy density isn't enough to damage the hull, it just heats the target and, as you pointed out, you lose the heat war.

So, things that decreases the energy density or increase the heat needed to damage is good.

So, spinning the ship (or just moving a lot) will increase the size of the "patch" the laser hits. A hull of curves will also increase the size of the patch. Non functional, reflective, projections could take the hit. Some energy would be transmitted down the shaft of the projection to the ship but a lot of the energy will dissipate back to space. I'm picturing a ship covered in bubbles or umbrellas. :-)

Also, substances with a high melting point and a high thermal conductivity would work well. I'm thinking of something like a diamond coating but won't shatter when hit by a high velocity projectile (though it would look very pretty as the target is surrounded by sparkles).

Traveller (an old RPG) used "sand cannons" defensively against lasers. They would throw a cloud of glittering particles between themselves and the attacker. As long as they didn't maneuver too much, and kept the cloud between them and the attacker, it would attenuate the laser hits.

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Which laser?

There are many kinds of lasers available with wavelengths from radio "masers" all the way to ultraviolet "excimer" lasers, potentially more available with technological progress. And there is no known material that fully reflects all these wavelengths. For example, aluminium does have a dip in its reflectivity in the infrared 700nm – 900nm spectrum.

On the radio end of spectrum: if enemy ship or its construction elements are made from electrically conductive material, sized similar to used maser wavelength, they will inevitably act as antenna producing electricity, resulting in EM interference, heating and arcing.

So, if you have multi-wavelength array for all conceivable types of enemy shielding, or widely tuneable laser, it would be very effective.

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I think if you build the ship wall out of retroreflectors, with a layer of high melting point material below, then you've got the perfect defence against attack lasers. Any laser directed at your hull will be, to a large percentage of the energy, directed right back to the attacking ship. And the layer below makes sure that the heat from what gets absorbed will not do any structural damage to your ship.

On the other hand, the retroreflector hull is a disadvantage if the laser is not used for attacking, but for detection and ranging (Lidar). Because you'll make your ship perfectly visible for any such technology. In that case, a perfectly black hull would make more sense.

Maybe the ideal defence would be an outer black layer that absorbs any Lidar lasers, but will be quickly burned away by attacking lasers, and then below a retroreflector layer for an automatic, passive counterattack, and then as third layer a highly heat-resistant material to protect the ship.

The outer layer will make you hard to find (but will be quickly burned away when you are found and attacked), the second layer will make you dangerous to attack (anyone attacking you effectively attacks himself), and the third layer protects your ship.

Also note that rthe laser being just one wavelength doesn't help you, since the attacker can freely choose which wavelength he uses. If you may make your ship perfectly safe against 400nm lasers, that won't help you if the attacker chooses to attack with a 600nm laser instead.

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Instead of a reflective layer of aluminium, you can technically use the principle behind gradient optical fibre and use multiple films of transparent materials to guide the laser along the surface of the hull and redirect it. The gradient has to be such that no matter what angle the laser hits, it would work.

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The easiest defense against a laser would be spin induced artificial gravity. You move the affected part of the hull under the beam fast enough to prevent lasting damage or at least keep pulse lasers from drilling too deep. Think of a carbon foam as outer layer. Either it escapes the beam glowing and radiating the energy away but otherwise fine. Or the porous structure helps keeping the beam inside the armor layer.

Of course you shouldn't feel too confident when your defenses successfully defend you against a surprisingly low powered and badly focused laser. That would be a telltale sign of a Laser coupled cold particle beam, carving your ship into a corkscrew shortly after.

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