A blog dedicated to helping writers and worldbuilders create consistent, plausible Science Fiction.

Wednesday, 13 April 2016

LWW: Laser Weapon Web

For those who have read the previous few posts, you'll find many arguments and solutions to reducing the effectiveness of lasers. Unfun by design, they allow for combat at extreme ranges, where maneuvering becomes nsignificant and combat nothing more than a slugfest.... but what's the logical extreme?

A common adage is that to defeat your enemy, you have to know them first.
In this post, we'll design a system where lasers reign supreme, and how they would be used to maximal effect.


Laser weapons are made up of four major components:

  • Energy source
  • Laser generator
  • Waste heat management
  • Focusing array
How a focusing array might look like
Energy source covers all the various methods of powering a laser generator. Today, they are powered chemically, either through a direct thermodynamic reaction, or by burning a fuel to generate electricity. In space applications, solar power, nuclear energy or other such methods will be used.

The laser generator used depends on the laser type. These can range from modern chemical or solid-state lasers, to advanced Free Electron Lasers that integrate a particle accelerator.

Waste heat management is another complex subject, especially in the vacuum of space. It is safe to say that however effective a radiator is, being able to dissipate heat into a planetary background (atmosphere, oceans, ice) is infinitely more effective and easy to do.

Finally, the focusing array. The simplest form is a lens on a gimbal that directs and focuses the laser at a target. In space, the focusing array can be split into stages, and can even be physically separate and free floating relative to the laser generator.

As described in The Laser Problem posts, there is another set of relationships between the magnitude of the power generated by the energy source, the laser components' efficiencies and the power of the laser beam.

Space Structures

Objects in space have no weight. 
This leads to the interesting consequence that large structures do not have to hold themselves up against the force of gravity. Where a sheet of paper would collapse on the ground, it can be maintained flat in orbit without requiring additional support.
Very little in the way of struts
Solar sails, being nothing more than thin sheets of metal, would not be possible otherwise.

What does this mean for laser weapons?

The amount of power delivered at the target depends on three things, which can be summarized in the following equation:
  • PT = (P * 0.855 * Mirror^2) / ( (Distance * Wavelength)^2 )
PT is the power delivered to the target, focused on a certain area.
P is the power that reaches the focusing array. This is the energy source's power output after all inefficiencies have been factored in.
Mirror is the radius of the focusing array. 
Distance is the separation between the focusing array and the target.
Wavelength is a property of the laser beam, and depends on the type of laser generator.

For example, let's use something that the United States Navy would consider for its first laser weapon system.

United States Navy laser
P = 300kW
Mirror = 0.5m radius
Distance = 20km (20000m)
Wavelength = 700nm (700*10^-9 m)
PT = 327 MW/m^2 or 32kW/cm^2

This is actually pretty impressive. With a spot size of only 9 square centimeters, this weapon can burn through 20mm of aluminium per second.

Now let's calculate for a megawatt-level laser using a vacuum frequency.

Orbital laser
P = 10MW
Mirror = 2m radius
Distance = 1000km (1000000m)
Wavelength = 200nm (200*10^-9 m)
PT = 800MW/m^2 or 80MW/cm^2

This is a ravenous death beam that can go through 15mm per second of alumium at the extreme distance of 1000 kilometers.

So now what?

The effectiveness of a laser depends on the resources invested in each of its four components. A larger energy source means a more powerful beam. A more advanced laser generator means a shorter wavelength. 

What is of interest here is the focusing array. In space, you can use simple reflective film as your main focusing reflector. For an ultraviolet beam, a simple sheet of aluminium will suffice. Because it doesn't have to support its own weight, you can create invest in a larger focusing array with disproportionately larger returns compared increasing power input or improving efficiency.
  • To make a laser more effective, the easiest solution is to use a larger focusing array.
The energy source will eventually become impracticably large, and past a certain point, will reach scale inefficiencies. A laser generator can be tuned to produce extremely small wavelengths, but it becomes similarly hard to handle the beam. Component efficiencies can be improved, but will rapidly hit diminishing returns.

The Laser Web

The Laser Weapon Web is a weapon system that combines all the design decisions that maximize laser effectiveness.

A ground-based power station will allow us to nearly relegate waste heat management to a minor nuisance. 100 gigawatts is an outrageous output to achieve in space, since it would require hundreds of tons simply devoted to waste heat management. That power output is regularly achieved by nuclear power stations today, at a national level.

Artist's impression of laser relay satellites.
This energy is beamed using near infrared from the ground to power relays in low orbit. These relays redirect the power to a laser generator in geostationary orbit using a shorter wavelength for better focusing.

Once at the laser generator, it is converted into energy using an efficient thermal-magnetohydrodynamic generator

Overall losses could be x0.5 from the near infrared beam, x0.8 from the relays and x0.5 from the electrical generator.

Our laser weapon should try to use the shortest wavelength available, but not so short that we'd require supermaterials or inefficient processes to focus the resultant beam. A 200nm ultraviolet beam can be reliably handled and focused by alumium reflectors.

Let's say our geostationary weapon is a Free Electron laser with 60% efficiency. These numbers are what are currently obtained in laboratories, and are likely to be reproduced in a military setting. It would have to get rid of 8 GW of waste heat, which could be handled by a liquid droplet radiator operating at 1600K and with a coolant volume of 40m x 40m. It would mass 2000 tons with a power density of 10kW/kg.

A Free Electron laser

Our primary focusing array only masses 10% of the laser weapon. This is enough to build an aluminium reflector 1mm thick and 200m in diameter, with mass to spare for control mechanisms.

So what can this laser do?

Laser weapon
Efficiency from ground to reflector = 0.5*0.8*0.5*0.6 = 0.12
Laser power = 12GW
Mirror = 100m radius 
Wavelength = 200nm (200*10^-9 m)
Distance = 40000km (Low Orbit to Geostationary)
PT = 1603GW/m^2
Penetration = 35622mm/s of alumiunium, 15702mm/s of diamond
Distance = 360000km (Geostationary to Moon)
PT= 19GW/m^2
Penetration = 48mm/s of aluminium, 21mm/s of diamond

This laser is so very powerful. Using only modern-day power levels, it with vaporize anything in Earth's orbit, and will quickly punch holes through spacecraft in Lunar orbit!

However, that might not be enough. 

Our first option is to use more of the same. A larger focusing array will increase range. Increasing the surface area leads to increasing the laser's range in a 1:1 ratio.  

This means that dedicating 400 tons to the focusing array will allow it to do the same level of damage at 720000km, and dedicating 2000 tons will allow it to penetrate 48mm/ of aluminium at 3.6 million km.

However, you will encounter the problem of trying to switch targets and control adaptive optics on a focusing array that masses several thousands of tons.

The solution is distance. Remember, in space, volume is free, mass is at a premium. To make this work for you, send out a focusing array into the space between you and the target. This 're-focusing drones' can be sent out in multiples, forming nodes of a network.

LWW Configuration
In the above diagram, we can see the configuration of the simples Laser Weapon Web. The Laser Weapon emits a beam that travels a certain distance before the spot size becomes equal to that of the first Node. This becomes the node separation 'S'.
  • Node separation S = 1.64 * Lens Radius^2 / Wavelength
For our example 12 GW laser weapon, the beam has to travel 82 million kilometers before the laser spot radius becomes equal to the radius of the re-focusing node.

This node then does its job: it re-focuses the beam down the next 82 million kilometers, with losses. The losses can be minimal for a short wavelength (1 to 10%) or extreme for X-ray wavelengths (50% or more).

If we want to reach Mars, we would need 6 nodes at most. Our 99.9% reflective aluminium refocusing nodes can expect losses of only a handful of percent.

Interplanetary lasers

Our single laser weapon, with its own mass invested in re-focusing nodes, can shoot down spaceships on Mars.

If you use multiple 'chains' of refocusing nodes, you can increase the number of firing windows available. Since each node is much cheaper than a laser generator, a military space power will have hundreds of these refocusing drones all around the solar system. A single laser beam could be bounced across dozens of these drones, losing only a few pecent of its power and putting gigawatts of focused energy on the doorstep of another planet.

Strategically, these nodes are amazing.

They are cheap, recoverable, expandable and can be replenished and put into position with ease. For the cost of building one laser weapon, you can field thousands of these nodes, extending its reach to the entire solar system.

With a couple laser weapons and multiple chains of refocusing drones, you'll end up with something resembling an electrical network. Electrical power stations are placed in convenient locations and the 'transformers' convert their power into 'electricity'. This 'electricity' is channeled down long wires, regularly 'boosted' until it reaches the 'home' and is available at the flick of a switch. Replace 'transformers' with laser weapons, 'electricity' with a laser beam, 'boosters' with refocusing nodes and 'home' with the target's planetary system, and you'll see the analogy.

Lightspeed lag

One obvious argument against the laser web is lightspeed lag.

12 minutes to Mars.

If your laser crosses millions of kilometers to reach the target, would lightspeed lag cause your known position of the target to be outdated by minutes, if not hours?

Well, there is a way around this, and it continues the electrical power grid analogy.

At home, when you switch on the lights, you don' have to wait a few dozen seconds for electricity to arrive from the power station. No, the electricity is always on, with the final step of the circuit, the electrical switch being closed, being the act which completes the circuit.

A similar strategy can be employed by the laser web.

You laser weapon could be constantly on. The laser is focused down 5 nodes, but is deviated harmlessly by the final node. If a target appears, all you have to do is send a signal to the final node to correctly aim it at the target.

Since the distance between the final node and the target is much, much shorter than between the target and the laser weapon, the lightspeed lag becomes a non-issue.

This can be improved upon.

Instead of having the final node handle both re-focusing and aiming the beam, you can separate the roles into an efficient node, and a targeting spaceship. The targeting spaceship is maneuverable, armored, and carries a relatively small mirror. The final node points a half-focusedbeam at the targeting spaceship with great accuracy, since it is reporting its position in a tight feedback loop. The spaceship then finely focuses the beam onto the target at very close range. 

This set-up provides improved counters to the target's attempts at evasion, stealth or counter-attack. In terms of narrative, the targeting spaceship could be crewed, and would be the centerpiece of a 'gunboat diplomacy' that parades as a symbol of the state's military power very close to the planet's population. 

In terms of design, it combines the maneuverability of a small spaceship, the effectiveness of a local force with the overwhelming power of a 2000 ton multi-gigawatt laser weapon.

A future history 

This is a worldbuilding supplement.

A laser weapon web would not appear in a vacuum. Most likely, the individual components would serve different, peaceful roles before the technology is repurposed for military purposes. 

The electrical ground-based power station could in fact be the combined output of several modern nuclear power plants. To achieve cheaper launches, a laser-launch system is devised, leading to the creation of a near infrared array.

Near infrared is excellent at traversing the atmosphere.

With laser-launch reducing launch costs, orbital relays could be used to accelerate spacecraft into interstellar journeys. 

Eventually, it becomes cost-effective to build larger focusing arrays in space as 'booster stations' than to increase the power being delivered from the ground.

The next step, naturally, is to send along re-focusing nodes with the interplanetary spacecraft. If longer accelerations are needed, more re-focusing nodes are needed. As traffic becomes more important, it would make sense to invest in permanent booster and relay stations: this results in a propulsion network that routes power to spaceships that need it. It would be constantly 'improved' by using shorter frequencies that increase the node separation distance 'S'.

One day, the military adds a targeting spaceship at the end of a chain of relay stations. The combination of short wavelength, impressive power levels and increased focusing turns the propulsion network into a laser weapon web....


  1. Whatever happened to lasers being unfun. This is amazing.

    1. Thanks! Read the comments below to see how the LWW can be converted from a 'nuclear-style MAD weapon' to a sort of 'Maginot Line defense'.

  2. If only one side has this, then stealth missiles, or VERY large numbers of conventional missiles fired at targets from extreme ‘range’ would be the only way to survive this unless you want to go guerilla.

    If two opposing sides get their hands on such a system, then you have a number of problems for both powers. If the final focusing mirror/spacecraft find themselves without a beam due to damage further up the chain (made possible by taking shots at mirrors when they are orbiting in a different order, firing missiles at multiple arrays, forcing them to focus on those for the sake of their own survival, or simply hammering away with multiple beams at multiple targets with your own mirrors), then there would be a need for a force of spacecraft with independent generators to fill the gap.
    At the same time though, this would expose that force to an extremely powerful weapon that they might not be able to dodge. If the focussing mirror/spacecraft is armoured, then the independent generator craft might get in the first shot, but probably be able to do no more than that before being vaporized. To make things worse, this armour would not work against the laser web beam, but would be needed to prevent independent generator craft blasting them in a ‘fast draw’ battle.

    The only solution I can think of to this (other than stealth/ stealth missiles at mirrors further up the chain) is to fire light craft projectiles at the final mirror before entering the mirror’s focussing range, a projectile that would keep going long after the smaller laser has dispersed.

    Nonetheless, no matter how many perspectives I put on it, to my unmathematical mind, such a war could be over in days. A constellation of independent generator craft would only be of limited and short use, and whether they are useful or not, the first side to open up a significant number of gaps in the mirror chain wins.

    On the other hand, trying to put this with the ‘space castle’ constellation idea by Rick Robinson ad Blue Max Studios makes for some interesting opportunities for a future history involving technological progression and military revolutions.

    1. First of all, thank you for furthering the discussion with a comment :)

      Second, I'm actually quite excited to read a story set in such a setting. Having the difference in military spending between two nations exemplified by the unequal density, distribution and coverage of their laser weapon webs, and it falls on our intrepid heroes to compensate using independent, fully contained laser warships.

      Now, onto your comment.

      I see taking down an enemy laser web as necessitating a two-pronged attack. The first, and easiest to accomplish, is by attacking the 'terminal' refocusing spaceships with a massive, conventional attack, using guided missiles and your own lasers. Once they are down, the enemy's laser web is still functional, but is now limited to relatively stationary or predictably moving targets.

      Your independent fleet can now move out and attack upstream.

      The second prong is a lengthy, stealth-missile attack that penetrates the enemy's defences and targets the laser generators near home. Since they are expensive and big, taking one down will significantly impact the laser web's maximal output.

      As for attacking 'upstream' mirrors, I doubt their effectiveness beyond a temporary measure. It'll cut off downstream refocusing spaceships, but the damage can easily be circumvented by re-routing the beam. You'd have to destroy a majority proportion of the web to have an effect, and even then, the enemy can reposition the mirrors, or just wait for the next set to swing into position along their orbit.

      In all cases, you are right, an independent fleet of laser ships cannot stand up to the laser web except in temporary, local engagements. What you'd require is a hard counter, like a railgun shooting buckets of sand or some sort of cheap missile bus. The individual mirrors are unlikely to be effective in a point defense role, as it counters their optimization for range and reducing beam losses. So, scores of undefended mirrors can be taken down cheaply but a kinetics or missile ship.

      In turn, this ship has to have a clear bubble of space it can operate it, safe from the web. This can be the job of an offensive laser fleet. It takes down the nearest mirrors, opens up the web for a wave of kinetics, and moves ahead.

      As for the length of the duration... well, even the most effective laser/kinetics web-sweeper fleet can only maintain an advantage over relatively short ranges. The web will therefore be eaten away from its weakest point (the edges) up to its strongest point (the center).

      If the defenders overpower the enemy laser web, they'll ride on propulsive beams on a Brachistocrone trajectory to stare down the aggressor planet.

      If they screw up their stealth operation or have their independent fleets caught by the web early on, they'll be helpless and the web clears out all of the defender's forces in short order.

      If the forces are nearly equal, then the attacker will be able to replenish their number of refocusing drones and send them out from their planet outwards. The defenders try to destroy then as quickly as they can. If they fail, they are caught by the web and vaporized. If successful, they slowly advance up the web.

      If becomes a whole bunch more complex if both sides have a well-developed laser web, or if the web is advanced enough to have portions of it as a 'counter-web', composed of maneuverable refocusing drones...

      I've never heard of the space castle constellation idea. Could you link some info?

    2. Sorry, took some time to find it. here it is:




      I have a few questions regarding the web system you built last week. I'm not sure how useful it is, but I hope there are some possibilities there:
      I had an idea similar to the web some months ago, but never developed it much after I assumed that a nation on earth would just shoot out the first upstream mirror belonging to a rival nation. As you said in your first comment, multiple generators spread throughout various bodies would solve that. The best place for any generator though is on earth. Can you think of any way to allow hostile powers to shoot from earth without fast-drawing on each other’s nearest mirror arrays other than earth based ground action/cruise missiles?
      In the event of a web being the only one of its kind it may be spread all throughout the solar system for maximum coverage. If there are multiple powers though it may be different. The set of a web may take the form of ‘nodes’ that have extra mirrors that can be sent out at need to increase the area controlled outside the node, ensuing that mirrors aren’t too close to opposing forces at the beginning of a shooting war. Essentially these would be ‘island fortresses’ that can be expanded (and besieged) when necessary. On the other hand might they be positioned throughout the system in anticipation of a war but concentrations of enemy mirrors left clear until they can be ganged up on? Either way, slowly moving mirrors to a target like an artillery train would make for an interesting set piece as the rebel fighters attempt to destroy it.
      I thought of some ways to achieve a waxing and waning of technology for the mirrors system- if lasers are dominant, you might get forces of laserstars that send out expendable craft at different angles to distract the laser, fire out some missiles to occupy the mirror’s pds, then slash past at a speed that would be difficult for the mirror to track (though as you say in your above post, the smaller the mirror, the harder this will be to acheive- lightcraft projectiles from stand-off ranges might be better). Independent craft might try and get between mirror constellations (though not directly between for course as they will be vaporized- they would be at an oblique angle and would try to hit multiple mirrors with lightcraft missiles), working on their own to distract and do as much damage as possible 'behind the lines'. Essentially like modern paratroopers.

    3. In a missile heavy future, you just spam with different types of missiles- perhaps ones that have a mirror on to accept power from the laserstars behind, but which can still slam into the mirror/ defences if need be? Missiles coming in from multiple vectors as with the ‘Space Castle’ idea I linked to above might be useful. Thus, with missiles and lasers being more important than the other at different points, the tactics and the strategy changes. You can have much more change in your future history than the usual 'x becomes faster and more powerful'.

      If the independent craft are too valuable to risk in battle, have them take a page out the opposition’s book and fire lasers into the mirrors of small frigates (space fighters?) that are more expendable, and have them fire on the mirrors from there. (how much damage can a dispersed beam do out from 1 light second?)

      If that isn’t enough, might mining a crude chunk of ore and tossing at the opponent work? Laserstars, etc could dodge it, but a giant mirror array might be less able to do so. How much could be ablated away? Especially if an escorting force was also joining in on the offensive? The only problem would be moving a large chunk of iron ore.

      Whatever happens, the power of a web will mean that any independent spacecraft will be based far beyond their reach- mcguffin for solar system expansion?

      Finally, the key problem for independent craft manned by the heroes, is the type of craft you mentioned that has power beamed to it by a mirror. It has mass for more armour or manoeuvrability, and being at the tip of any spear it may be nigh on invincible for conventional craft to take on- its mirror would be small enough to track fast targets, and backed by a laser far more powerful than those used by its opponents, and it doesn’t need the mass heavy generator of independent craft. What disadvantages do you think such a craft will have (other than being dependent on the beam)?

      A lot of stuff there. Apologies for that. Is any of that useful?

    4. Your space-fighter design is actually what is used in the book I am currently reading, "Hegemony" by Mark Kalina.
      Capital ships have behemoth laser generators, and use them to drive 70g-capable light interceptors and throw them in the face of enemy capital ships. They are also capable of using their lasers directly at closer range, for example against enemy interceptors and warheads.
      This give them better range than lasers and self-powered warheads, and interceptors can both refocus the riding laser (mostly as defence and blinding/scorching) and carry warheads on their own.

      Interestingly, at those accelerations, they need a pilot on them to avoid light lag, and they often stray off the beam and use a high-thrust low-dV drive at those moments.
      In the book, uploaded (non-copiable) human minds are used as pilots as they are superior to AIs, but I suspect self-learning AIs would end up being superior. Though I can see other reasons for banning self-learning AI drones, a position that is defended by multiple nations in the current debates about drone regulations.

      Surprisingly, there are no laser webs described in the book. Maybe something doesn't scale well past a point, preventing interplanetary lasers? Also, while difficult and dangerous, they can use in-system FTL jumps, so maybe this can make laser webs less interesting.

      (The book itself is not the best-written, I suspect it is his first book - Marc, if you ever read this, you can improve it by avoiding past perfect as much as possible and try to cut a bit and smooth exposition.
      Still, the premise is solid, the story interesting and entertaining enough, it avoids characters - I'm looking at you, Honor Harrington - and obvious efforts were put in world-building with science-fiction hard enough for cutting glass. And it's less than one $ for good-sized book, so yeah I recommend it, and am curious about his next books.)

      Assuming no FTL, an up-scaled version could be used by a laser web, with light crafts, probably drones, using the laser for high-dV high-thrust drives and try to kill each-other by refocusing the beam (with the trade-off of cutting your main drive) or throwing missiles at each-other - or even smaller crafts, I wonder how recursive this can go...

    5. @Geoffrey S H:

      As I read it, the space 'castle' concept is the space-going version of a modern Aircraft Carrier with destroyer screen and tenders. I disagree with the proposed arrangement, because I think the author over-estimates the distances involved and under-estimates the flexibility of the weapon systems utilized, but I'll leave that discussion for another time.

      If an Earth-bound state want to fight a neighbouring nation, they would not be using laser webs, which are best used as interplanetary weapons.

      The best place to put the laser generator and power plant is actually on an airless moon, specificially, our Moon. The advantage is that the beam can be produced in weapons-grade short wavelengths right from the start, skipping the 50-75% reduction in output that comes with using an infrared beam to traverse the atmosphere, then converting its energy into a separate beam.

      The only disadvantage is that you don't have access to vast amounts of colling fluids, and it costs more to build in vacuum, but these problems can be mitigated by its close proximity to the industrial base of Earth.

      A Solar-System wide laser web is unlikely. It is much more profitable to build local webs that interconnect, much like today's railways. There's several reasons not to undertake the entire project yourself, such as incredible cost, loss of the military advantages (de-centralizing the power and laser units to compensate for the range-related losses cuts into the extreme leverage the web provides in a military scenario).

      Remember, all you need is a single state-sanctioned spaceship to turn the 'laser railway' into the 'laser weapon web'.

      With regards to the tactics you mentioned, I cannot stop thinking as to how the laser web and stealth interact.

      The main mirrors, the ones spaced out by millions of kilometers, are likely to be civilian in construction and easily tracked by everyone.
      I see two ways in which they can be used as a weapon: providing laser power to a laser-ship 'terminal', or by powering a parasitic military web.

      The 'terminal' method has the advantages of providing very little reaction time to the enemy (it's already on their doorstep) and without necessitating its own nuclear power supply and generator, it can be very cold and stealthy. It might be a sleeper ship, giving the laser web owner first-strike capability. In such a case, any standard, civilian trawler can suddenly produce a raveonous beam of death that wipes out your orbital infrastructure in minutes.

      The second scenario has a fleet of small, fast, defended mirrors placed adjacent to the civilian nodes. When a war breaks out, the laser beam is diverted from the before-last node of the civilian laser web, and into a dormant, stealthed web under military control.

      This web can evade counter-fire, is more durable, can replace losses quicker and most importantly, is much less vulnerable to the nodes closest to the target being destroyed.

      As I stated before, the real 'decapitating strike' is if your opponent activates their doomsday fleet of kinetic weapons orbiting an extremely elliptic trajectory around your home planet. Extremely cold, therefore invisible, take a long time to put into place but only require a small puff of gas to change the next fly-by into a devastating simultaneous strike on all the orbital or vacuum laser generators.

      Laser webs are the low marginal cost expansion of a laser launch system, itself developed to reduce launch costs by a factor 100 without something as fancy as Skyhooks or a space elevator.

      The disadvantages of the hero-terminal-spaceship is that it is sitting right next to the enemy. It's the equivalent of a gunboat equipped with an endless stream of nuclear missiles. Yes it can take on an entire CV group and probably win, but it is as fragile as its target. In space, close ranges mean that even the smallest lasers can rip through it front to back.

      Yes, I loved this discussion.


    6. @ Eth:

      If the behemoth laser generators of 'Hegemony' as being used as close range defenses, then the author under-estimates the power of a laser paired with a large focusing mirror.

      If human minds can be uploaded into the same hardware that AI use, then realize two things: the uploaded humans can think as fast as the AI, and they have absolute control over their own though processes. Imagine an uploaded person able to think at 100 speed. Why not live at 10 speed, and use the rest of 90 calculation cycles switching off the conscious part of their brain. They'd experience it as a series of marvelous insight and countless hours of work done instantly, much like a professional today would be able to talk about their field without putting any effort into it.

      Laser webs do not scale well with acceleration. Planets are fixed objects, with known trajectories. Wars between planets is like the Paris-gun firing on London. Trying to use laser webs on accelerating spaceships, against other spaceships, is trying to use the Paris Gun to shoot down aircraft - it just can't.

      As for how finely distributed a laser web can go... possibly microbots with reflective surfaces. But there's no point in doing that. The separation distance between the individual micro-nodes would be so small that a huge number of nodes would be requires to cross any appreciable distance without massive beam losses.

    7. The author doesn't give that many hard numbers. Ship size (up to 1km), acceleration (3g for the biggest hitters, 70 to 80g for the interceptors) and range (a few million km for direct laser range, a bit more for interceptor range). So I can't really give a complete analysis, and I'd actually be curious to read his.

      That said, close range is relative. As I said, close range is relative, measured in millions of km for direct laser fire. This is not just scorch range, but the range at which they can cause a thermal explosion that will shatter the entire hull. (They do have ablative armour, but it can deplete and won't stop the strongest attacks.)
      We are, after all, talking about death-beam-wielding singularity-powered torchships.
      They will go for kill range instead of waiting at scorch range because of heat build-up, and you'd end up out of cold and open to kill-range attack.

      I think the logic he used is this:
      - Direct laser fire is limited by mirror size (They use phased laser arrays, do we still call those mirrors?)
      - With high acceleration and random vectors, tiny ships can avoid targeting, thanks to light lag. Effective defence range decreases more than effective offence range due to smaller size (smaller mirror, smaller bomb-pumped laser warheads).
      - The bigger the interceptor, the longer the range, but the smaller it is, the harder it is to hit and the more it can accelerate per laser power. So there is an optimal size.
      - Even knowing the random move algorithms used by the mothership laser, light-lag works both ways and a manoeuvring interceptor will often have to leave the beam path. So it needs its own low-autonomy engine to keep its performances at those times. The author assumes it to be given same performances as the laser drive, which seems fair enough. Total dV is of course low, as most of the engagement the laser is still used.
      - The higher acceleration and the longer the distance, the less it will keep on beam, meaning more fuel, meaning a bigger craft. Again, there is probably an optimal ratio.

      I suspect the sweet spot is small enough so a bomb-pumped warhead has a better range than a mirror. In addition, they may pack a bigger punch for very short engagement windows, and can be multi-beam to better catch enemy interceptors.
      So while lasers are destructive enough, evading can become a significant factor at this level of energy.

    8. In this setting, ratios end up with 1km, 3g "assault-ships" with small (fighter-sized?) 75g interceptors, with smaller half-km 5g raider "lance-ships" and even smaller 150m independent 6g "swift-ships" for additional duties.
      Note that those ships are FTL-capable, and most systems are uninhabited, so they must be able to deploy after a jump with no additional network.
      The one defended system has local, less capable non-FTL warships with no interceptors, but this is because it is a less developed frontier world, and the titular Hegemony prefers to rely on its own mobile fleet than giving the locals too much military (rebellions aren't unread of). Its position isn't discussed IIRC, but I suspect it is not too close to a rival power - maybe those worlds are more heavily fortified, along with core worlds.
      But it means that fortifications aren't discussed, nor how FTL jumps influences those (ships appear and depart at Kuiper distances, and intra-system jumps are possible far enough from planets, if difficult and risky, with at best a precision of a few dozen million km, and jump cooldown is at best a hundred hours). The only reference is about lance-ships whose role is to raid heavily defended installations.
      An assumption is also that big ships have less acceleration, but proportionally more efficient reactors. They also have mass-conversion singularity drives, making them high-end torchships.

      The author assumes there is only one stage. I don't know if it is based on a fine analysis favouring this two-stages-only design, or simply an axiom. Maybe I should fire him a mail to ask him about it.
      More generally, I'd be curious about an analysis of the designs by numbers of types of stage.

      I would speculate that in this setting, fortified systems have their own superlaser-generators, powering motherships through relays themselves powering interceptors, maybe with intermediate node-fortresses.
      This would mean head-on engagement by head-limited ships would be devastating, but this could be compensated by concentration of force at an interstellar level (it is not clear how many inhabited worlds the polities have, but it could be hundreds or more).
      Lance-ships would be the smallest, fastest ships that pack enough of a punch to seriously damage stuff. It would use its high acceleration to avoid direct fire from superlasers, but their role would be to raid softer targets or tying up forces to escort duty, as I don't see a direct assault against a superlaser (or deep inside its range) working well. It would be hard to pin down, as it can use intra-system jump to escape or threaten a completely different part of the system.

      The elephant in the room, though, would be long-range relativistic attacks. Not sure how it fits, but I have always had a problem with those. They don't work for MAD like nukes do because of the years-long attack time, and I have yet to see an efficient non-handwavium countermeasure (that wouldn't also be used for even worse doom weapons, that is).

      I'm not saying the setting is flawless, mind you, but it is an interesting example of laser-dominated battlefield. In fact, the author avoids the torchship+laser problem you described in an earlier post not by cutting torchships out, but by giving them so much power they go beyond it.

      About uploaded minds, there is no talk of significant mind alteration (possibly for ethical reasons), but they are harshly selected, highly trained and do indeed run at inflated speed. Again, no hard numbers, but a second is described at a long time for an interceptor, enough for thinking split-second decisions through.

  3. Great post, quite comprehensive as well with the natural progression from a civilian 'lightway' to a military (or space terrorist) use.

    Would it be possible to incorporate photonic laser thruster sensibilities into the focusing arrays/nodes? Or is that a given?

  4. First, I think you made a typo on converting your orbital laser data to cm^2. I assume that the unit should be kW. I am also wondering why your damage assessment is reducedor the otherwise more powerful orbital laser (15 mm aliminum per second for the upgraded orbital, vs 20 mm per second for the lower PT Earth based). Was there another typo? Or had the target focal area changed?

    I have some concerns about your mirror system. Depending upon the target focal area, I might be able to accept that your 200 m mirrors won't melt (being aluminum based, and your figures that the laser can burn through 35.6 m of aluminum in one second), but I find it very difficult to believe that any targetting mirror small enough to fit on a vessel would be able to survive. In fact, at 35 m aluminum per second, I question whether or not the vessel itself could survive housing the targetting mirror. Have you run through the numbers for heat disapation to handle the inefficiencies of each mirror? For that matter, have your calculations for the aluminum damage taken into account the reflectivity of that aluminum vis a vis the wavelength?

    1. I checked again. 1MW/m^2 = 0.1kW/cm^2

      The Navy Weapon laser is less powerful, and is forced to use a longer wavelength to reduce atmospheric attenuation, but the target distance is only 20km. The orbital laser uses a shorter wavelength and is many times more powerful, but target distance is 1000km.

      For advanced dielectric mirrors, I use 99.999%+ reflectivity and ignore heat issues altogether.

      For an aluminium ultraviolet mirror, I used 99.9%. For a 12GW beam spread over a 200m diameter mirror, this translates to 381W/m^2 absorbed. If the non-reflective side of the mirror is blackened to an emissivity of 0.8, and we use a figure of 0.03 for the reflective side, then the mirror will stabilize at a temperature of pretty much exactly 300K: http://www.spectralcalc.com/blackbody_calculator/blackbody.php

      We can work it out the other way around. Aluminium loses half its strength at about 800K. This corresponds to a blackbody radiation rate of 19.3kW/m^2. In other words, the 200m mirror can survive a beam of 606GW, but this is unrealistic as 800K temperatures might damage the reflective coating.

      For a smaller mirror, we use the radius as a variable. Without active cooling, a 12GW beam with 99.9% reflectivity can be handled by a mirror with a 28 meter diameter at 800K. With a darker backside and structural support by heatsink materials, it can be reduced to 11.7m diameter at 1200K. It might even be small enough to use di-electrics affordably, with 10 times better reflectivity.

    2. I was actually refering to the PT value, which was still higher for the orbital, despite the range. However, the lower damage would be understandable if this PT is not as focused.

      Given the new data you provide (thanks for the reference, BTW), a second, related issue emerges. It would be fairly easy to counter the threat of laser attack by polishing the aluminum hull. This would obviously not be a stealth vessel. You could also use passive and active cooling systems to absorb the energy. If you want stealth, you could incorporate the suface liquid/gel armour or sandy armour instead. Alternatively, use a coat of synthetic diamond... This is highly resistent to thermal conductivity along one axis, but highly conductive on the others (given the appropriate impurities). The result is that the thermal energy is distributed along the entirety of the hull layer, without being transmitted inward.

    3. Reflective hulls are a great idea for countering very long range laser attacks, when you can still measure the attack in terms of solar intensities.

      However, the reflectivity breaks down when energy flux surpasses thermal conductivity, and heat accumulates on a tiny spot of the hull.

      The problem not mentioned here is pulsed lasers. There no way even an active cooling system can keep up with that.

    4. One feature of combat is that strategists are always revising defenses to defeat expected offenses, and revising offenses to defeat defenses. So, here is an example for a laser defense.
      The surface hull will consist of a number of extremely thin sheets of highly polished aluminum... but this is not the primary defense. The actual primary defense against the laser will be the next layer: 1 m of dense-packed cryogenic snow (artificially formed water snow cooled to -200°C, or more). At this temperature, a 1 m^3 patch of vaporised packed snow will absorb up to 3.3 GJ of energy. From there, it will continue to absorb energy as vapour, but I will get back to this later. Packed snow has a reflectivity against UV superior to 95%, meaning that the snow pack will defeat at least 66 GJ laser strikes, assuming a reliable illumination confined to 1 m^2 (as I discussed previously, this is virtually impossible at ranges greater than 100 000 km, and unlikely at ranges greater than 1000 km). This figur assumes a direct burn through, which is itself highly unlikely.
      Water, in its various forms, is a reasonably good conductor of heat (not great, but quite good, nonetheless). Instead of a laser burning straight through, the heat is going to be dissipated throughout the adjacent snow, so the resistence increase to about 80 GJ.
      In space, the first thing that happens after the outer aluminum layers are defeated is that the snow melts. In gravitational fields, the liquid water would tend to run off, but in micro-g, the melted water would instead form a film held in place by surface tension. If the surface tension DOES break with the surrounding snow, it will stay in place in the form of a big bubble of water. If THIS surface tension breaks down, it stays in place in the form of many smaller bubbles of water. In short, it will continue to act as an effective barrier until the water begins to evaporate.
      This is where things get interesting. When the water evaporates, it will send out a stream of steam... but lasers do not like steam very much, or any other particulates. This plume of vapour will continue to absorb and reflect the radiation flux from the laser. The more water evaporates, the greater the plume, and the less likelihood that the laser energy will even reach the vessel, let alone remain focused.
      This assumes armour that is not being actively renewed. If you are actively chiiling the snow pack to maintain temperature, then the coolant flow is going to increase the amount of energy that the armour can absorb.
      Another interesting thing... the vapour plume is going to have a fair amount of energy behind it, and this will effectively act as a manoeuvering thruster pushing the vessel away. This will further reduce the ability of the laser to maintain a focal point.
      Sloped armour is also useful here, because the greater the angle of incidence, the more spread out the laser is going to be. The larger spot area, the less effective the laser burn.
      I would not be too surprised if this snow pack armour system were to withstand 100 GJ laser burns.
      This sounds like a lot, but the thickness is roughly equivalent to the water filled voids used as protective armour layers on most large warships. Additionally, the recommended protection against cosmic radiation is 250 g/cm^2. This is the equivalent of 2.5 m of liquid water.

  5. Yes, the reflectivity will eventually break down, but such break downs will be largely localised.
    There is no question of being able to maintain a laser lock on "a tiny spot ofthe hull"... not if we are talking about ranges on the order of 80 million km. Keep in mind that 1 AU is only 150 million km. At 1 AU, the target vessel is at most 1 pixel on a 300m detector. At 1/2 AU it upgrades to 2 pixels, at best. You are not going to be able to SEE a tiny spot on the hull, let alone target it. You will need to be within 1 million km just to be able to target a spot on the vessel to the nearest meter. If you want to be able to do the targeting with a sensor you can actually mount on a ship, this range is going to be reduced to 100 000 km (for 35m telescope).
    Then there is the issue of actually holding the laser beam on target, which will be at least as difficult. For 1m accuracy at 100 000 km, you will need to control your laser beam with an accuracy of 1 arc millisecond. at 1 AU, you will need an accuracy of 10 arc microseconds. Don't forget that your 1 m target area will be moving at a displacement of 10 km/s. Being generous, the difficulty will be similar to your spearing a gnat with a pin... except that spearing the gnat will be much easier. It is not an easy thing to pivot a 35 m mirror with 1 arc millisecond accuracy at rates upward of 10 arc seconds per second. On a 35 m mirror, 1 arc millisecond corresponds to about 10 micrometers. A fibration as little as + or - 0.5 mm would have the beam swinging wildly on and off the vessel. And then you have to consider the reacivity of the adaptive mirror, presumably to maintain focus. Even if you would be able to accurately determine the required depth (not possible from single location), the vibration tolerance for the adaptive mirror components would be less than a nm.
    Still, at 100 000 km you have sort of a "fighting chance". The lag time between receiving the targeting data and the laser arriving on target could be as low as 1 second (unless you are receiving relay info). But at your original proposed range... no chance. The lag time will be over 8 minutes, which is long enough for evasive manoeuvres s pathetic as less than 1 mm/s^2 to defeat any tracking once they no you are there... even with your switchboard scenario. Your only chance would be a complete surprise.

  6. If the target craft is manoeuvring, the water bubble may well be thrown off the craft by acceleration, so this would be limited to slow acceleration.

    Also, wouldn't explosive heating rip cryogenic snow away? How do you avoid chunks of snow being thrown away instead of melted?

    Also, while water is great due to its abundance and ease of use, what would a dedicated military craft use, if they have access to other materials? The main factors would be how much heat can it take per mass, how strong is it against explosive heating and, possibly, how much would it reflect?

    (Which may then lead to the use of more varied weapons to deal with it, in a good old arms race)

    1. Manoeuvres will be limited under combat conditions, yes. This is not a major hardship, as the vessel is not going to outrun the laser anyway, and slow acceleration still allows for large delta-v. Most large vessels will opt for low acceleration because of the increased engine efficiency. Another consideration is that the attack will probably be rapidly paced, so there is not likely to be a lot of time for the water to drift. Finally, the actual effect depends upon the vector of acceleration vis à vis the location of the strike.

      If you DO have chunks of snow being ejected, it will work even better. Because you have a restraining barrier (granter, one that is quickly disintigrating), the only place for the chunks to go is into the path of the beam. It will be even more effective at diffusing the beam energy, and carrying the energy away from the vessel.

      Water is actually one of the top ten, if not the single best, absorbers of energy. The military actually use it extensively in their protection against torpedos, as well as missiles and shells approaching from a low angle. On most naval vessels, the uter hull is rather thin... it's purpose is actually to cause explosive shells to detonate outside the actual protection. Next, you have a void (a large gap between hull layers), then usually another thin hull and void, followied by the actual armour belt, another void, and a thin inner hull to catch spalling. The inner void stores fuel oil. The outer void stores water, used as the primary armour against ost weapons. If there is a center void, it is either partially filled with water, or left vacant.

      Yes, an actual arms race will lead to evolving strategies.