Wednesday 2 March 2016

The Laser Problem III

We've seen the effects laser weapons have on space combat, and the consequences of deriving laser power from rocket engine power output. As promised, in this post, we'll go through a few solutions authors and game designers have to avoid cornering themselves into unfun combat systems.
There are three major methods of avoiding the Laser Problem.

-The first is to try and find a sweet spot between relative acceleration and engagement distances.
-The second is to decrease the effectiveness of lasers in the setting.
-The third is to decouple laser power from drive power.
We'll go through the advantages and disadvantages of each solution from a writing and worldbuilding standpoint.
Ah, lasers. So elegant yet so frustrating.
As anyone with worldbuilding experience would say, it's a process of iterations. You throw in a set of parameters, work out the consequences, then adjust one parameter and run the simulation or calculations again. Each 'iteration' brings you closer to the desired result.

Trying to balance relative acceleration and engagement distance is where this iterative process breaks down. Each step towards making spaceships move a significant distance relative to the effective range of lasers requires a disproportionate sacrifice from other parameters. You'll find your warships with silly engine masses, very high thrust-to-weight ratio despite low exhaust velocity and implausibly inefficient laser technology. In-universe, all it takes is one person to think outside of the box to break down the balance the author has achieved.

The first solution is possible, but trying to achieve it is not pleasant.

The second solution try leveraging multiple factors into the equation to tone down the effectiveness of lasers.

The most obvious factor is armor. If warships are armored enough to stand up to laser beams at long range, then they can approach to short ranges and start manoeuvring against each other. Unlike the balance between relative acceleration and engagement distance in the first solution, the balance between armor and laser effectiveness is much less driven by mathematics, but by in-universe design and technology. Justifying a super-material's existence is also much easier than trying to write-in a super-inefficient laser.

Another way is to complexify the laser weapon system. Laser accuracy, beam stabilization, lens warping, beam wander, waste heat management, wavelength-dependent efficiency, debris damage... all these are generally ignores or considered as minor issues when discussing laser weapons. By highlighting them or increasing their effects, you get a 'free' reduction in engagement distances. For example, a laser weapon system that has 0.05 arc-seconds of accuracy would only hit a 100x20m target 8% of the time, and even then, only hit the same spot of armor (cumulative penetration) about 0.005% of the time.

A popular idea on sfconsim-l threads a few yeas ago was trying to shoot up the focusing optics of the opponent's ship. Getting even a fraction of an incoming beam focused in the wrong direction would wreck havoc on the laser generator. This would lead to a relatively interesting contest of peek-a-boo - the lasers would have shutters that allowed outgoing lasers to be shot. An opponent would have to time their shots to try and shoot up the optics, try and whittle down the shutters, or stare down the opponent if they got the upper hand. It became complex and fun when multiple opponents were involved, and even more so if they were separated by more than 300000km (light lag effects).

Just imagine the shutters being armored plates.
The third solution is probably the most effective, and gives the greatest freedom in terms of game balancing.

The idea of decoupling laser power from rocket engine power relies on electrical energy not being readily extracted from the exhaust, or that the laser does not consume electricity.

For engines that are difficult to extract energy from, you only need to take a look at this excellent Engine List.

Examples include Orion-style nuclear pulse propulsion systems, where the exhaust cannot be 'diverted' for electric power generation, low-temperature nuclear thermal rockets like NERVA, where exhaust is not ionized and only waste heat can be utilized, or the classic chemical propulsion systems of today.

The point is not to make it impossible to extract electrical energy from the engine, but to reduce the fraction that is possible down to an acceptable percentage.

Lasers themselves can be operated without electrical power. Examples include chemical lasers, that combust fuels as 'ammunition', or bomb-pumped lasers, which are ejected and fired like missile warheads.
A real-world chemical laser, plausible for low-tech or near future SF settings
In a real worldbuilding attempt, authors would use a combination of the above three solutions to avoid the Laser Problem.

Hopefully, this has spurred you to look again at your own work, check whether you have the Problem, and explore solutions for it as described in this series of posts.

Don't hesitate to ask questions in the comments below!


  1. How about thermal issues? A drive that kicks high-energy remass out of the back sends a lot of the thermal problem with it. Trying to extract energy to power the Ravening Beam of Death might be infeasible without turning the ship into so much molten slag.

    1. Such engines exist, such as chemical or solid-core nucleat thermal engines. They have a high energy efficiency (low waste heat), but it is still possible to extract energy from them.

      A well designed Ravening Beam of Death has a cooling capacity matching the maximum power output. In most of my warship designs, and as demonstrated by the ultra-realistic game 'Children of a Dead Earth', cooling systems can end up massing much more than weapons and propulsion combined.

  2. The hitch with #3 is that it rules out most Cool Space Drives. Say goodbye to electric propulsion, either nuclear or solar. Alas, that kills the main Realistic[TM] drive for getting around the solar system on a convenient time scale. And most torch drive concepts have the same problem, orders of magnitude worse - since you pretty much need electromagnetic containment to keep from vaporizing your drive engine.

    Yes, Orion drive is still viable, but its specific impulse is not all that great. And it has ... issues - especially for general space transportation.

    My best guess for an uber laser deal breaker - and 'best' is not that great - is what Robert said: that waste heat disposal could make it difficult to aim a laser with sufficient precision. Large scale fluid (coolant) flow tends to be turbulent!

    It could work - in the sense of making uber lasers NOT work - but it feels a bit like special pleading.

    BTW, your comment on my last blog post (until today!) just led me to this blog. Great stuff!

    1. Thanks! I've been a long time reader of Rocketpunk Manifesto and I've stated several times that it was an inspiration for ToughSF!

      Nuclear thermal drives seem to me, so far, to be the solution. Solid-core for high-thrust, high-specific power propulsion (combat) and radiator-cooled gas-core for high isp, high power propulsion (interplanetary). Apparently, the latter can reach 20km/s exhaust velocity with full regenerative cooling, and up to 80km/s using radiators to cool the reactor chamber.

      On Projectrho, it is stated that anything above 100km/s exhaust velocity is wasteful for travel inside a solar system, so these may be sufficient.

  3. Thanks for the welcome!

    Gas core fission would indeed do the trick. I am not sure that handling a dense mass of U235 (plutonium?) vapor would work out in practice (have we ever done anything even kinda sorta close?). On the other hand, when we are talking about a techlevel that supports space armadas, a bit of slack is permitted. Compared to any fusion drive, mere gas core should be nada problem!

    Totally unrelated, but I gotta grump at the way Google will only send email reply notification to my gmail addy, which I don't use and never check.

  4. How would stretching out the wavelength of a laser work as defence? Could concievably be done in a setting with warpdrives, simply stretch out space a bit between you and the enemy. And what would happen with projectiles and missiles that try to traverse the stretched space?

    1. I'm not entirely sure how the physics of a warpdrive work (no-one does, or else we'd be flying to Alpha Centauri already), and I suspect that stretching space won't affect light going through it (it would return to normal once it enters the bubble), but consider this:

      How big is the warp bubble? If the laser beam's wavelength gets stretched to 10 times its original size (from 400nm to 4000nm, for example), and its divergence is multiplied by a factor ten, it would only suffer this worsened divergence over a very short distance. The effect on the spot size would be minimal, leading to a very small decrease in intensity and destructive effect.

      The laser beam would have travelled thousands of kilometers with its original divergence, only to start spreading out ten times more quickly... over the last hundred meters. The effect would be minimal!

      Projectiles and missiles would be even weirder. I suspect that subjectively, they won't feel anything different when going through stretched space. But, if they are stretched, then they'd be ripped apart down to their subatomic particles and disappear in a flash of gamma rays. A perfect shield.

    2. Was thinking it'd work like redshifting, as the wavelengths were stretched out they'd become less energetic right?

      Oh and something you forgot to mention as a possible defence (unless I missed it) is reflective surfaces. using meta-materials to basically turn the ship into a mirror or similiar to reflect light, decreasing it's power

    3. Basically, if you took 1 meter of space and stretched it to 10, it ought to increase the wavelength by 10. Then again, it mgiht be as you say, that it is compressed again when it emerges on the other side

    4. I will approach the subject of mirrors vs lasers in the next blog post, but it won't come soon...

      That's the idea I understood about stretching laser wavelengths.

  5. Would nuclear salt water drives also fall under the easy to harness energy category?

    1. Sorry, I meant nuclear salt water rockets, in particular. Also, since the mini mag Orion harnesses energy from each pulse, would this also count under the problematic laser powering drive problem?

    2. A Nuclear Salt Water Rocket can be considered as a very power nuclear thermal rocket. Energy can extracted from the waste heat it radiates at the nozzle, but compared to other rockets, it is not the most 'problematic'.

      The Mini-Mag Orion uses a magnetic nozzle to bounce away the charged particles (plasma) that each pulse accelerates. If the magnetic nozzle is used to instead slow down the particles and convert their motion into electricity, like a MagnetoHydroDynamic Generator, then it can extract a huge amount of energy from each nuclear pulse and power very large lasers with it. It is very 'problematic'.

  6. Can pulsed ntr be used as energy sorce. With some advancement it can be a low end torch drive and it can produce a lot of power. better radiators are needed like liqued droplet or dusty plasma.

    1. If you have a means of absorbing its neutrons, you can convert 90% of its output into heat. That heat can be converted into electricity with an efficiency between 10% (thermocouples) and 50% (turbines).