Justifying the use of directed energy weapons

Question

In my scifi setting I want a broad range of slugthrowers (bullets, smart ammo, grenades,...), exotic weapons (flame/chemthrowers, high tech blades, nano-bots,...), and directed energy weapons (heat rays, blazers, plasma rays, particle beams, plasma weapons). Since the setting is supposed to be hard-scifi, I want the weapons to be plausible from a basic physics and efficiency point of view. The slugthrowers and exotic weapons all fulfil this criteria, i.e. they are optimal for the purpose they are meant to serve.

Directed energy weapons, however, do not generally fulfil that condition. The issue is something Atomic Rockets calls Routledge's Law.

Any interesting battery material for a laser gun would be more usefully deployed as an explosive warhead.

Let me illustrate the issue by going over an example. The weapons energy source is a very small quantity of antimatter annihilated in a chamber made of a gamma-voltaic meta-material. This is the pinnacle of my settings technology, so lower tech societies might use something less energetic, but the issue remains. Assume the chamber has an efficiency of 0.7, and the laser an amazing efficiency of 0.6. Thus the weapon might deliver about 40% of the antimatter's energy to the target.

This sounds pretty decent, until one realizes that just putting the antimatter into a bullet shot at the enemy will deliver almost 100% of the energy. Even worse, modern slugthrowers deliver up to 80% of the chemical energy as kinetic energy to their target.

How can I justify that directed energy weapons are not just specialist tools fore nice applications, but can compete with slugthrowers, so that picking between a slugthrower and a directed energy weapon is more a matter of personal preference than anything else?

Answer 1

• Laser beams have straight trajectories in anything other than totally unreasonable gravitational fields (where you won't be using bullets, that for sure). This makes them very easy to aim.
• Lasers beams propagate at the speed of light. This will be substantially faster than any projectile weapon, and a reasonable amount faster than any particle beam, too.
• You cannot make armour out of regular, solid matter that will protect you from a high energy laser pulse. Even fancy handwavium "thermal superconductors", even assuming such things could exist above the temperature of liquid helium, will be vulnerable to ionisation because it isn't a thermal effect. Multiphoton ionisation from longer wavelengths and single photon ionisation from shorter ones will damage it.
• The business end of a laser is merely a blob of optic elements that can be re-oriented without the need to slew the whole weapon around. Even if the bulk of the laser is huge and heavy, the emitting part can be a lightweight hollow sphere that can be rotated pretty swiftly. Tracking fast moving targets, striking multiple targets or simply striking that one guy who managed to get round behind you are all much easier when you don't have to turn your entire gun barrel to face towards the enemy.
• Laser beams cannot be practically intercepted. Given the above advantages, projectiles are at risk of being simply shot out of the sky by a sufficiently capable point defense system. For man-portable weapons there's a limit to how much acceleration you can apply (and hence to the maximum velocity of the projectile) given material limits and electrical arcing from railguns and timing issues with coilguns, so you can't just use a faster slug thrower to overcome this.
• Defocussed laser beams past their killing range become rapidly safe. The same cannot be said of dumb projectiles (especially in a vacuum, especially especially in zero-g) and definitely cannot be said of explosive warheads.
• A powerful laser can also be used, via a slightly different set of optics, to ionise an air channel and combined with a suitable electrical discharge source be used as an electrolaser. Yes, you can set your lasers to stun.
• Visible light lasers, especially ones in the blue-green region, have reasonable attentuation lengths in water, so you can potentially shoot targets that are underwater or if you are swimming, shoot at targets on the surface. Because the air/water interface bends the laser beam, you don't need to adjust your aim to acocunt for diffraction. Just point and shoot.
• Lasers are pretty precise... you'll blast a hole through your target, and maybe set it on fire, but there's not too much collateral damage. You can use a laser at close quarters, or hit just one person in a group, or zap something without destroying the room, vehicle or building it is in. Grenades and rockets are not necessarily so precise.
• One laser pulse from a specific gun is much like another from the same gun. The same can't be said for projectiles, which may well have manufacturing tolerances. Hence the need for careful production of rounds intended for high precision work, like sniping.
• Near-IR lasers can be made invisible in air, produce no muzzle flash and depending on your laser's cooling systems, don't produce any noise on firing either. Unlike "silenced" projectiles you don't have to make compromises on killing power (by using subsonic ammunition) or accuracy (by increasing time-to-impact due to slower round) to use silent lasers. Misses against a distant background may never even be noticed by the target.
• It is possible to use an optical system that lets you switch between two wavelengths, say visible for long range non-stealthy work, and near-IR for close range stealthy operations. Switching to stealth mode compromises range only. The switched element could be rapidly moved in an out of place, and doesn't have a particularly limited lifespan or require a change in ammunition.
• (courtesy of Skyler) Lasers have no recoil. Obviously not all projectile weapons do, but in general they are likely to if only because fully recoil compensating systems may be expensive, bulky, delicate, etc.
• And relatedly: compensating for operator movement and shaking is easier with lasers because you can use stabilised optics which are much more compact and convenient than they projectile weapon equivalents.

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Also note:

• There's more to energy weapon power sources than just batteries. Weapons that require a team or vehicle to move can use generators and capacitors. If your engineers are clever enough, they could make you a smart robotic cable like a superconducting millipede that follows you from a power source, reconnects itself after being severed, navigates around obstacles and hides in cracks and so on. With that sort of thing at hand, you can shoot an awful lot without having to worry unduly about your supplies of bulky or heavy ammunition.
• Range may be limited by the size of your emitting element, but there's nothing to stop you using additional reflecting and focussing elements forwards of your position, perhaps mounted on a drone, aircraft or even a satellite. These could be used to shoot round obstacles or even over the horizon.
• Whilst range is more limited than projectiles, remember that small arms aren't used at greatly extended ranges. Sure, with a good rifle you can get kills at 2-3km in Afghanistan, but in wooded or urban areas lines of sight are seldom that long. Also, a mid-wavelength visible light laser with a 10cm aperture could certainly operate at those ranges, and you wouldn't need to account for windage, bullet drop or target movement.
• Dust, smoke, rain and fog are the real things that'll kill laser performance in practical use, not range.

Answer 2

Lasers can be armor-penetrating bypassing

Slugs energy goes into the outer layer of armor first. Armor-penetrating bullets are specially designed to get through armor better, but the fact remains that they have to destroy the armor before they can hurt the target.

An x-ray or gamma laser is different. Yes, some of the energy is going to be deposited in the armor, but some will go right through (for more info, look up "x-ray scattering length"). Modern-day ballistic armor is made of Kevlar fiber and ceramic plates, which x-rays can penetrate very effectively. Vehicles, robots, and fixed installations could be hardened with steel, lead, or tungsten armor, but humans aren't going to want to carry that around.

Lasers can be remotely powered

In present-day society, you can generally find a power outlet anywhere in urbanized areas. In a near-future setting, it's not crazy to think that all human-inhabited areas would be built with wireless power transmission. We've already got wireless phone charging, and R&D systems that can do wireless transmission for about the size of a room, so either build that out as infrastructure everywhere or extend the technology to some sort of longer-range, even satellite-based system. Now laser guns have infinite ammo!

Lasers can hold more shots

Routlege's Law ignores the concept of overkill. If I'm trying to kill one person at a time, he's just as dead with a tiny hole drilled into his skull as he is if completely atomized. So what if delivering the antimatter directly to the target is more energy-efficient? In order to deliver it, I have to package it in some sort of bullet casing, and then propel that bullet with either chemical or electromagnetic (i.e. railgun) means, which means I have cool explosive bullets, but I'm limited to carrying similar amounts as boring lead slugs. You can't make the bullets too small or light or the range will be reduced too much by air resistance. Conversely, if I use the volume and mass I save by NOT having bullet casings, I can carry WAY more antimatter, and therefore have way more shots.

Answer 3

Precision

Lobbing the battery as a grenade may be more effective in terms of energy transfer to the target, but it also has a habit of transferring that energy into everything around the target. That doesn't work well if there's something in the vicinity you want to preserve, such as a hostage, or a building, or depending on your battery capacity, the city.

Firing a laser might transfer only 40% of the energy to the target, but it transfers it to only the target.

Answer 4

Any interesting battery material for a laser gun would be more usefully deployed as an explosive warhead.

I don't think this should be treated as a universal assumption. After all, the gunpowder or cordite used in modern-day firearms and artillery pieces is plenty interesting, but it's hardly a good material for an explosive warhead -- and even artillery pieces that fire high explosive shells are reliant on it!

I think that the assumption that the power source is antimatter is part of the problem. Besides just being an extremely energetic and rather dangerously unstable fuel, it's also much more explodey than some other options. Contrast things like nuclear fusion, where getting it to explode is difficult (the exact design of the H-bomb is apparently extremely tricky to get right, and we haven't managed to get reactors to work yet) but a large amount of power can still be developed.

Also, "energy delivered to the target" is only one of many figures of merit you need to consider -- you also need to consider aiming (lasers and relativistic particle beams are good for hitting moving targets, but lasers diffract over extremely long distances, for example, projectiles can be made homing but beams cannot), focus (splash damage is great when you want it, and REALLY BAD when you don't), energy-to-target transfer (overpenetration wastes energy, causes collateral damage, and is probably all but guaranteed for overpowered weapons), the ability to breach various defenses, and the ability for an unprotected shooter to survive the muzzle flash of their own weapon.

I don't know if you're likely to get it down to the level of "personal preference", but if you retreat from the rather dangerous antimatter power cells to, say, some form of microfusion or microfission, or perhaps some kind of weird, futuristic electrodynamic technology that isn't so unstable, and you'll both have safer ammunition, but your comparison will make sense.

Answer 5

International laws and agreements

In real life there are lots of ways of bringing an enemy to their knees, but they are unnaceptable due to humanitary reasons. See the UN's Convention on Certain Conventional Weapons for banished weapons and Protocol I of the Geneva convention for banished practices. You can't, for example:

• Blind people with lasers;
• Bomb dams;
• Set neighbourhoods on fire;
• Rain napalm on people;
• Generally nuke enemies around.

Well you can... But UN/NATO will hate you, and when your dictatorship is over and you get caught you may be sued for war crimes.

Same thing with your energy weapons. The non-directed variant is just too efficient, so just like chlorine gas it is banned internationally. You are left with the less efficient option.

Answer 6

In hard-scifi you can't.

And reason is not only "Routledge's Law", but range. Laser weapon has very limited range compared with slugs. You can shoot slugs from Moon to Earth with common modern howitzer and case some damage (to city-size target), but you will never be able to do it with reasnoble laser even utilizing futuristic technology.

It doesn't means that lasers are usless. But they are exactly a niche weapon, perfect "specialist tools fore nice applications".

UPD: some conter arguments for "pew-pew" adepts:

While lasers indeed faster and easer to handle, to deliver damage you need to hold your aim for some time. And it is hard to impossible. While slug hits or not as whole. Lets get some numbers.

Distance 2Mm (2000 km). Laser has "ray power" of 100kW (its a 1-10Mw of power consumtion), apperture of 1m, wavelength about 100nm (10e-7 meters - ultraviolet) - it is a very good laser! At that distance ray would be 1 + (2*10e6)*(2*10e-7/1) = 1.4 m, ≈ 1.54 m^2 across).

To melt 2mm aluminium plate (5.4 ≈ 5.5 kg/m^2), we would need to hold energy income at any point for ( 390 kJ/kg * 5.5 kg/m^2 )/( 100 kJ/s * 1.54 m^2) ≈ 14 s (even times longer - I neglected reflaction, energy dissapation and heating to 660°C).

It means that laser beam need to have to follow target angular speed with deviation if no more than (1.4 m / 2e6 m) / 14 s = 5e-8 rad/s ≈ 3e-6 °/s (best astronomical instruments beats that by orders, but there targets and platform are much mor stable than battleship), or target plate have relative acceleration (including gravity and centrifugal one) less than 1.5 cm/s^2 (0.01g). Crew running around in panic could create more than that :)

There is another problem with aiming: our aiming equipment has much larger wavelenths and smaller appertures, than that of laser, and thus much less accurate. For example, to achive same accuracy as abovementioned laser in visual specter (including red colors) you need targeting equipment with 6-8m apperture - quite a huge telescope even by surface standarts (and no, 10m base coincidence rangefinder would not help: apperture determines the size of a minimum point, a "pixel size").

And for active protection: 14 seconds - is a lot of time to apply countermeasures (gas, or just turn other side). And more of that: you need only 2mm of non-activly cooled alluminium to protect you against battleship class laser for that long time. And 10mm steel plate would be impannetratable by that laser: laser would melt faster than this plate (and no one would sit for half an hour under laser beam)

So no laser would ever compete shrapnel gun at "broadside baragge" in hard-science settings!

Answer 7

I'm surprised a real world system that is currently being used hasn't yet been discussed.

Lockheed Martin has developed a laser system to overheat drones and missiles to thus shooting them out of the sky. A laser is an ideal system for this since it can track targets with erratic paths much better than conventional slug throwers, it can also frequently disarm missiles so they don't set off their ordinance.

The system is cheeper per use then systems like the patriot missiles and are usually mounted to large ships are powered by large fission reactors that wouldn't be feasible to launch at small objects such as drones and missiles.

Here is a video of a modern directed energy weapon in use, just imagine what a weapon set in a distant future world could do! https://www.youtube.com/watch?v=XH6NIazR5pA

Answer 8

In the second Ringworld novel, Ringworld Engineers, Larry Niven gave the engineers of the title a drastically large energy source, but one that was ordinarily very diffuse. He gave them the ability to manipulate solar flares, and to cause them to lase in the x-ray region. So the result was a fairly tight x-ray beam with an energy of many Gigawatts, that could in principle fire for hours. Probably it was originally intended to kill off any last few stray meteors the engineers missed. But as a weapon against potential invaders, it was reasonably effective.

An x-ray laser is pretty much automatically a weapon, even if it has other uses. It requires some quite special technology to be able to resist it. Ordinary matter of pretty much any type will ionize and so lose its structure. So any enemy without that special tech is going to fry if hit by it. As well, even to be aware there is something going on before it is going onto you, you need to have some pretty advanced sensor equipment. Of course, the drama of the novels required that a few of the potential invaders had the tech to resist it...

In an answer to another question (which I can't seem to find just now) I pointed out that a Dyson sphere would give you control of the entire stellar wind from a star. In principle that would mean you could control a huge swath of high energy protons which would be a formidable weapon if concentrated in a beam.

In both the Ringworld and a Dyson sphere case, throwing mass as part of a weapon means you lose the mass, and you now have that mass moving rapidly through your system. If you hit your enemy there may be shrapnel that goes in hard to predict directions, potentially hitting your home. And you don't want to give up any mass since you have used all the local mass to make that home.

So, when you have some drastically large energy source available already, and you are able to manipulate it accurately, possibly you can make an energy beam weapon that people will take notice of. If you are potentially facing many or very large enemies, and you don't want to throw away mass, possibly you can make good use of a beam weapon.

Answer 9

If you take a look at the yield of annihilating a gram of antimatter, it releases ~2*10¹⁴J, roughly triple the yield of Little Boy (see https://en.wikipedia.org/wiki/Orders_of_magnitude_(energy)#1012_to_1017_J). So lobbing antimatter bullets is like lobbing small nukes. This poses a couple of problems if you are not aiming for all-out annihilation of the target:

• Radiation byproducts
• Destroying the eveything you are trying to conquer

Another thing to keep in mind for a hard-science story is that the payload has to annihilate on impact, but not during the acceleration phase. This creates a difficult problem, and a great possibility for unintentional self-destruction. I am assuming that you have a system for containing the antimatter at "normal" accelerations.

If you have a system that can control the release of the energy to acceptable levels, and still deliver with 40% efficiency (at least at the source), you are in business. You can sustain a rate of annihilation sufficient to conquer planets almost indefinitely with a relatively small amount of antimatter. Most importantly, you can do it in a way that minimizes the risk of vaporizing yourself.

Answer 10

First things first, the real difficulty in creating a viable laser weapon is not energy density as many people claim but rather power density - we already have batteries that can store half a million joules in one kilogram, the problem is we can only get those batteries to provide a trickle of a few joules per second. Capacitors on the other hand cannot store as much energy, but they can pump out that energy at a much faster rate, so that's what we really need.

It is popular to claim that a magnetic accelerator gun is easier to achieve than a pulse laser gun, but this isn't really true since they also rely on electrical input, and any capacitor that can provide energy at the required rate to make a practical gauss pistol/rifle will also be able to provide energy at the required rate to make a practical laser pistol/rifle.

In my scifi setting, body armour made from fullerene mesh and diamondoid laminate is virtually impervious to traditional firearms, and lasers have been miniaturised to the point where interplanetary age soldiers have point-defence beam projectors built into their suits. The combination of ultra-durable armour and miniature point-defences has made laser weapons preferable.

1. they cannot be intercepted the same way a firearm slug can be intercepted
2. even if they fail to penetrate armour entirely, they still cause ablation
3. they require only a power supply, unlike gauss guns which require both a power supply AND a slug/needle supply
4. they are totally recoilless, pinpoint precise, and very easy to learn to shoot
5. the beam can be defocused and the power level reduced so as to minimize overpenetration inside fragile habitats, and even to create a "stun" effect rather than a lethal drilling beam
6. you can shoot them on airless low-gravity moons and asteroids and without creating the same kind of persistent navigational hazard that slugthrowers make

of course, the effectiveness of lasers can be reduced by the use of prismatic aerosol and handwavium "reflec" armour (not actually reflective, but rather diffuses the energy over a wider surface area to protect the material beneath), so gauss guns are still popular but honestly, Routledge's "law" is just plain silly, the explosive batter-bullet is easily countered by personal point-defence lasers, and the fact that the 1kg beltpack powering my laser pistol can hold several hundred shots while yonder space pirate's slugthrower is limited to a couple dozen at the most =P

Answer 11

If you're looking at purely lethal options... ballistics would probably be better then energy weapons.

But, energy weapons have more applications then lethal application.

• microwave guns that heat targets uncomfortably for crowd control / dispersal

• sound guns that create high pitch noise .. again, for crowd control / dispersal (sound force can be used to penetrate and destroy solids, but, I think that would require an amount of energy that would best be used some other way.)

• emp weapons will fry unprotected electronics.. so if someone has a pacemaker .. poof. If someone is driving an eletronic-primary vehicle .. poof.

• in the Mass Effect series, guns were ballistic, but used energy to accelerate small fragments of metal at railgun speeds.. so, you could have hybrid weapons that require energy. Depending on how high tech your story is, these could be small Mass Effect type weapons, or massive prototype man-portable railguns (a la Fallout's Gauss Rifle)

• stun guns are an obvious energy weapon designed to incapacitate / crowd control. You could have energy weapons shooting flechettes with their own energy sources that shotgun-disperse into crowds, penetrate, and create shocks to pacify crowds. Or, have a simple one-shot stun gun. Or a multi shot stun gun that has multiple tether-attached spike lines, allowing multiple shots to stun people before needing reload.

Answer 12

Since it is sci fi, i just want to add that lasers are good anti-spaceship weapons.

• propagating at the speed of light they are hard to evade. The trajectory of a hard sci fi spaceship is difficult to modify and easy to forecast.
• the power source is in the ship itself, any hard sci fi ship worth his salt having some kind of fission, fusion or antimatter reactor to power its engine.
• can't be deflected, decoyed or destroyed by a coil gun like a missile
• very long range thanks to the vacuum of space. Diffraction will reduce the power to the square of distance, but also make the beam larger, facilitating your aim and preventing evasion.
• no ammo to take care of. As long as your power source goes, you can fire. The weight of ammo is a crucial factor in realistic fighting spaceships.

The goal is not to destroy the ship in a few shots, but to keep your aim long enough to build up more heat than its radiators can radiate and cook the passengers alive. Time consuming, but the above pros balance this con. Ideally, destroy the radiators with more conventional missiles or coil gun before aiming the laser. If the enemy ship folds its radiator to protect them, all the better: keep roasting. If it deploys them, all the better: more surface for you to aim your photons at.

Answer 13

Cost

Your gamma-voltaic meta-material is expensive. So you'd rather your soldiers didn't just throw the batteries at the enemy unless they absolutely had to, as you don't have the budget to keep replacing every soliders battery after every battle.

Answer 14

Newton's 3rd law

If you use the battery material to propel slugs, 50% of the force are lost to recoil. Additional energy is lost in heat/radiation, friction etc.
If you deploy the battery material as explosive, the energy is emitted radial. This is great if the bomb is inside a head, house or mountain but to focus the energy on a point you would need a shaped charge. However the shaped charge would suffer the same problem: 50% of the force generated are lost in the opposide direction...

Tl;dr: The energy conversion efficiency of the laser might be lower at the first glance, but only if the warhead explodes inside of your target.