Is it because it's a game, not a simulation, that the laserships start fighting at only 20km?
I don't think it's realistic given the performance of real-life laser weapons.
Real life lasers actually have an immense difficulty reaching 20 km right now, even with the immense advances we have had. There's currently very hard trade offs between beam power and beam range. From rocketpunk on the current state of the art of lasers:
www.projectrho.com/public_html/rocket/spacegunconvent.php#id--Laser_Cannon"Industrial fiber lasers can be made very powerful. IPG recently sold a 100-fiber laser to the NADEX Laser R&D Center in Japan that can weld metal parts up to 30 centimeters thick. But that high of a power output comes at the sacrifice of the ability to focus the beam over a distance. Cutting and welding tools need to operate only centimeters from their targets, after all. The highest power from single fiber lasers with beams good enough to focus onto objects hundreds of meters or more away is much less—10 kW. Still, that’s adequate for stationary targets like unexploded ordnance left on a battlefield, because you can keep the laser trained on the explosive long enough to detonate it.
Of course, 10 kW won’t stop a speeding boat before it can deliver a bomb. The Navy laser demonstration on the USS Ponce was actually half a dozen IPG industrial fiber lasers, each rated at 5.5 kW, shot through the same telescope to form a 30-kW beam. But simply feeding the light from even more industrial fiber lasers into a bigger telescope would not produce a 100-kW beam that would retain the tight focus needed to destroy or disable fast-moving, far-off targets. The Pentagon needed a single 100-kW-class system for that. The laser would track the target’s motion, dwelling on a vulnerable spot, such as its engine or explosive payload, until the beam destroyed it.
Alas, that’s not going to happen with the existing approach. “If I could build a 100-kW laser with a single fiber, it would be great, but I can’t,” says Lockheed’s Afzal. “The scaling of a single-fiber laser to high power falls apart.” Delivering that much firepower requires new technology, he adds. The leading candidate is a way to combine the beams from many separate fiber lasers in a more controlled way than by simply firing them all through the same telescope. Two approaches looked promising."
This is an issue that is brought up in the initial blog post describing the logic of how lasers are implemented in the game, here:
childrenofadeadearth.wordpress.com/2016/07/02/the-photon-lance/"A hypothetically perfect laser will suffer from diffraction and is referred to as being Diffraction Limited. But this is not what is actually limits most actual high powered lasers in warfare.
Most high powered lasers will never even come close to being diffraction limited.
Truth is, the Beam Waist, or the minimum diameter the beam will achieve, is a more effective measurement of how damaging a laser is. A perfect laser will have a beam waist limited only by diffraction, but lasers like that don’t exist. And the greater the power of a laser, the further and further away that laser strays from being diffraction limited.
A good way to measure this is with the Beam Quality of the laser, or with the M Squared. M^2 is the beam quality factor, which can be considered a multiplier of the beam waist. So, an M^2 of 5 means the beam waist is 5 times that of a diffraction limited beam. In terms of area, this means the beam is 25 (5^2) times the area of a diffraction limited beam, or 25 times as weak. As you can see, having a M^2 even in the high single digits will yield beams a far cry from “perfect” diffraction limited beams.
In practice, it is not the pumping efficiency, nor the power supply, nor diffraction, which ultimately limits lasers. It is the beam quality factor. In the end, M^2 ends up being the number one limit on laser damage in combat.
In small lasers, M^2 close to 1 is easily achieved without issue, but in high power lasers, M^2 can easily reach into the millions if not accounted for. This is because generally, M^2 scales linearly with laser power.
Each optical component of a laser affects the M^2. In particular, using a deformable mirror to focus a laser at arbitrarily long ranges (such as from 1 km to 100 km) is measured at reducing M^2 to between 1.5 to 3. Problematic, but not exactly debilitating.
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Since our lasers are assumed to be deformable mirrors in game, its basically impossible in the current system to have a M^2 in system of less than 3. Which means the lasers in game are at least 9x worse than a theoretical "perfect" laser. Which makes them better than existing high power lasers, which have trouble with couple 100 meter ranges, but much worse than in many other sci fi which assumes merely diffraction limited lasers.
For example, atomic rocket for its example laser has a 10 meter 20 megawatt infrared laser have an intensity of about 15 mw/m at 4,000 km. This is a perfect, diffraction limited laser. 15 MW/m is also pretty close to a minimum intensity to damage things armored against lasers. With a M^2 of 3, the basic minimum in this game, the intensity of this laser at 4,000 km is only 1.6 MW/M: might be able to damage steel (a poor laser armor) under ideal circumstances. In order to get that same 15 MW/m intensity, the range of a 3 M^2 laser falls from 4,000 km to 1,300 km. And 15 MW/m is actually pretty weak: for my own laser designs, I consider truly effective ranges to be in the realm of 100 MW/M. To get that kind of power with the above laser with a m^2 of 3 is a range of a mere 500 km.
Further, remember that in this game, an output of 20 Megawatts requires an input of about 500 MW, or half a gigawatt, of electricity, since many lasers run at about 4-5% efficiency in the game. And since that power reactor is operating at something like 10% efficiency, your actually producing around 5 GW of heat. Thus, in the game a 20 Megawatt (imput) laser would only output a 1 MW beam. This reduces the 100 MW threshold to 110 km.
So, those are the two big assumptions that limit effective laser range in game
1) Laser quality, with it not really being possible to get a M^2 under 3.
2) Low efficiency of the lasers overall, thus requiring very powerful rectors to get lasers of any significant power, and thus range.
Some other things that reduce laser effectiveness, though I don't know if its modeled
3) Jitter: small vibrations, such as can be caused by all the pumping fluids, an active engine, or simply people moving arround can signifigantly degrade the effective laser power as the beam wobbles across the enemy hull rather than hovering in place. Tiny amounts of jitter can radically reduce intensity: 1 miliradian of jitter reduces the 100 MW threshold of the above laser from 110 km to 50 km. The most recent mobile laser we have information on had a jitter of 25 miliradians. Both jitter and M^2 become increasingly worse issues the longer the distance one is trying to shoot at.
4) Angle/reflection: angles spread the laser beam over a larger area, and cause more of the beam to bounce off the hull. reflective armor also reduces the damage threshold, but also require active cooling to be useful.
But, its basically from my understanding 1 and 2 that are the big drivers of laser range in the game: assumption 1 is potentially optimistic, maybe, while 2 is potentially pessimistic about achievable efficiencies of laser beams. Though 3 and 4 are, from my understanding, not modeled at all, and those can reduce laser effectiveness by 50-90% ish.
Hopefully thats a comprehensible explanation of what's going on in the system, and gives you some basis to judge the reasonableness of the assumptions.