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In my world I'm thinking of using megawatt class free electron lasers as main armament, hopefully somewhat minaturized with future tech.

Free electron lasers are lasers that use electrons as the lasing medium, they can scale up or down to any frequency of light, a megawatt class free electron laser can theoretically according to equations vaporize 20 ft of steel a second. Here is a link: https://www.nextbigfuture.com/2016/03/us-navy-plans-for-scaling-free-electron.html

Would this be somehow viable assuming I have a liquid helium cooling system and a hydrogen fusion reactor.

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    $\begingroup$ Sure. It's all reasonable-enough technobabble. Just remember that it'll need to be focused for long range shooting, and that -- just like you've developed FEL countermeasures, so will your enemy have done so. (That's why kinetic energy weapons are best: use the fusion generator to power a helium-cooled rail-gun which shoots KE penetrators at relativistic speeds.) $\endgroup$ – RonJohn Mar 15 '18 at 21:57
  • $\begingroup$ Yeah i was thinking a kinetic railgun as a secondary, a small 20mm one with high velocity.... $\endgroup$ – Efialtes Mar 15 '18 at 21:58
  • $\begingroup$ What range are you planning on using this weapon at? Space is big enough that you actually do have to worry about dispersion for lasers. And now you have me wondering about retroreflectors that could work on electrons. You could get a nasty surprise if those work =) $\endgroup$ – Cort Ammon Mar 15 '18 at 22:04
  • $\begingroup$ true thank you for the info, i was thinking in the thousand km range, maybe slightly longer. $\endgroup$ – Efialtes Mar 15 '18 at 22:12
  • $\begingroup$ 1000km is super short range in space. $\endgroup$ – A. C. A. C. Mar 15 '18 at 23:09
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Lasers aren't very effective at these distances. There are two major issues, which sort of interact. The primary one is the dispersion of the beam. It is not possible to create a perfect column of light from a laser. There's always a focusing element, and it's aperture is important for the formation of an Airy disk. The equation for this effect is:

$\theta \approx 1.22\frac{\lambda}{d}$

Where $\lambda$ is the wavelength and $d$ is the diameter of the aperture. You want a small $\theta$ to focus as much energy as possible on the other vehicle, which means large diameters and small wavelengths. Let's just say your laser has a gigantic aperture of $1m$, and you are using visible light, so $\lambda=10^{-6}$ (note: you could use UV or X-rays to improve this, but they are much harder to wrangle). This results in an Airy disk of $\theta\approx 0.00000122 radians$. At a distance of $1000000m$, as you mentioned in the comments, that's $1.2m$. That's the smallest diameter spot you can focus on.

Now I don't know where you got your 20ft/s number for melting steel. Steel has a specific heat of 480 J/K-kg. Now you'll need to get this up to at least 1500C to melt it (realistically, more because melted steel still stays in place in 0 gravity). If we are kind and assume we start at 300K (somewhere roughly room temperature), we need 576 kJ/kg to melt steel. That means our 1MW laser is good to melt roughly 2kg of steel a second. At a density of 7850 kg/m3 and an area of roughly 1 square meter (the airy disk was 1.13m^2), the steel armor in that slug we are focusing on has a mass of 7850kg/m, or 7.85kg/mm. That means that, we can melt approximately a millimeter of steel every 4 seconds. We'd love to do better, but the airy disk won't let us focus any smaller than that.

The second issue is what Renan mentioned: tracking. Consider that you need to keep that laser on target for all 4 seconds to burn a millimeter of steel. Relative velocities of vehicles are fast in space. You might see relative velocities in the thousands of m/s. The object might easily travel 10000m in those 4 seconds, relative to you, and while that's happening, you've got to keep a track to within a fraction of a meter. That kind of precision is the kind of thing that's worthy of an XKCD article!

Finally, consider the obvious solution: roll. If your opponent has to keep a laser focused on a section for 4 seconds to burn a millimeter, and you roll, they're going to have a lot of trouble because the target area will eventually roll behind you and they'll have to pick a new target point.

In all, the distances just make this hard. Kinetic weapons are more efficient because they can make adjustments along the way. There's a decent body of literature on weapons, present and future which have gone down this path, or are going down this path.

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    $\begingroup$ I'm just waiting until I can vote again so I can upvote this. $\endgroup$ – Renan Mar 15 '18 at 23:06
  • $\begingroup$ Two problems. One, it is not necessary to melt metal. You merely need to weaken the metal enough to cause structural failure. This is how lasers shoot down missiles and aircraft in real life. Two: a 1-m aperture is small. Hubble is 2.4 m, JWST is 6.5 m, SOFIA is 2.5 m. $\endgroup$ – user71659 Mar 16 '18 at 3:10
  • $\begingroup$ @user71659 If you wish, you may scale these by a factor of 10 or 20 to account for the aperture being too small and not needing to reach 1500C. Regardless, you still have to deal with the reality that that target is much further away than the missiles and aircraft shot down by modern missiles. And the finer you do succeed at focusing, the more sensitive you are to tracking issues and roll. $\endgroup$ – Cort Ammon Mar 16 '18 at 3:47
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    $\begingroup$ @Mark A pulsed laser is harder to build and keep aiming at a given point, but it is much more destructive for a given energy: the micro- or nano-pulses will explosively vaporise steel, creating a shockwave that will damage the armour. Waiting time between pulses is just enough so the vapourised or even plasma steel has time to disperse, so it doesn't block the new pulse. Also, obligatory Atomic Rocket link: projectrho.com/public_html/rocket/… $\endgroup$ – Eth Mar 19 '18 at 13:35
  • $\begingroup$ I would note that while you're correct that a visible-light laser would not be very effective at such extreme range, projectiles or guided munitions would likely be entirely useless or nearly useless, respectively. The large relative velocities and dramatic effect of minor course corrections you mentioned would make it impossible for unguided munitions to hit, and guided (and therefore propelled) weapons could be detected and intercepted by point-defense long before impact. And both would have to also account for the effects of orbital mechanics and (potentially) relativity. $\endgroup$ – MikeB Mar 31 '18 at 5:07
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Not very much.

At first it might sound like a good idea. Nothing is faster than light in a vacuum. And without an atmosphere to absorb its energy, the range is practically infinite.

In practice, though, you have to focus the laser on the target for it to be as effective as it can be. In order to focus a laser on a target, you need to know where it is. But tracking stuff in space is very hard:

See How accurately (maximum possible accuracy) can future satellite positions be predicted?, in space.stackexchange.com.

The answer for that question might sound like rocket science. That's because it is rocket science.

So, you have a problem which is having to aim in a volumetric space, rather than a point. You have some chance to miss.

Another problem is that space is large. I am not going to describe it in detail... Just open up Kerbal Space Program (an absolute must-have simulator if you want to delve into orbital mechanics) and go grab an asteroid. You will soon learn a few things:

  • Any targets moving at any considerable speed related to you will only be visible (at least to the naked eye) for very short periods, and that's if they come relatively close to you. Most of the time, you would need a powerful telescope to see them. Telescopes have very narrow fields of vision by definition, so if you don't know where a target is, and you don't have a very large array of telescopes, you might be scanning forever. Which leads us to the next point:

  • Most of the time you will only know where the target is via very accurate telemetry. If someone finds out that you want to shoot them, they might hesitate in providing you with telemetry data.

And then there is an issue having to do with distance. It's kinda a catch-22:

  • If you are close enough to them that you can shoot at them with any accuracy, then you are close enough to them that they can accurately shoot at you. And you will only know that they shot at you when you've been hit.

  • If they are sufficiently far that they can change speed before being hit, then even the slightest acceleration in any direction can cause you to miss. Even if they are still in the path of the beam, they might be caught outside the focus, so damage might be reduced or even negated.

All things considered, lasers don't seem like a good space-to-space weapon - at least against targets that are not in orbit of a planet. For "stationary" stations and bases, and against targets on the surface of a planet (specially if the atmosphere is thin or absent), then they might be a good choice.

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  • $\begingroup$ At distances where the laser won't disperse into uselessness, the light speed lag is short enough to be easily dealt with. $\endgroup$ – Keith Morrison Mar 15 '18 at 22:53
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    $\begingroup$ I do have kerbal thx for the suggestion. $\endgroup$ – Efialtes Apr 1 '18 at 20:57

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