Friday, 4 March 2016

Electric Cannons and Kinetic Impactors II

In this post, we've go through and discuss a list of options for the scifi writer or worldbuilder to make electric cannons as useful, if not more so, than other weapon systems. Both advantages, disadvantages and predicted consequences will be looked at.

The options will be numbered for easier reference and discussion in the comments, when they arrive.
Go big or go home.
Projectile Velocity

Already mentioned in the previous post, you have the option of making projectiles go faster. The same railgun example propelling a 10kg projectile to 50km/s could plausibly push a 100g projectile to 500km/s without increased expenditure of energy or waste heat. A million km can be crossed in 33 minutes, a hundred thousand km in barely over 3 minutes.

This solves the problem of time-to-target, and carries no disadvantage in terms of damage on impact. It might also increase your ammunition capacity. More likely, you will need to use a coilgun instead of a railgun to achieve those velocities. If you do more research into coilguns, you'll find there are limits to how much you can accelerate the projectile over one meter length of cannon. This pretty much relates to the strength of the magnetic field imparting an accelerating force, and that is both a consequence of how much current you can force through your electromagnet, and how strong your bracing is to stop the coils from exploding outwards. These, however, are mechanical and technical issues that are easy to hang a Lampshade on during worldbuilding.

They also give you a spectacular failure mode if you try to overload the cannon. 

Mass per meter

Reducing railgun/coilgun kg-per-meter and capacitor kg-per-kW is another option. Both are critical measurements for determining how massive an electric cannon will be.

In the previous post's example, a figure of 400kg per meter of railgun was used, taken from a DARPA project estimate. A science fictional setting would have no trouble explaining that the technology has improved that figure to 100, 50, 20kg/m. Just pay attention that at the lower end, those figures get ridiculous unless you use super-materials. 20kg/m is what a thin-walled tube of aluminium would weigh.
4MW capacitor bank in California, containing 86GJ
Capacitors are a more complex technology. They actually have two main characteristics, and several minor ones. The main characteristsics are how much energy they can hold ready, and fast they can expend it. Given a mass, this gives a kJ/kg and kW/kg rating. Today, 'supercapacitors' are capable of astounding energy discharge rates per kg (up to 10kW per kg) but terrible energy capacities (100MJ/kg). Batteries are the opposite, with high capacity and low discharge.

The solution, once again, is supermaterials. Graphene-based nanocapacitors or the maximal theoretical performance of lithium batteries promise good figures for both characteristic.

One universal disadvantage of increasing your capacitor's energy capacity is that they tend to explode when damaged, and increasing energy discharge means that even the slightest resistance leads to a massive dump of waste heat into a fragile system.

To finalize this option, you can also increase the railgun's competitiveness by increasing the mass of other weapons systems. For example, lasers can measure their mass efficiency as kW output per kg. Reducing that number while increasing that of electric cannons would make an in-universe spaceship think twice before dismissing the latter as a valid option for warships.

Area denial

Here, the railgun forgoes trying to shoot down the craft itself. That would require that the projectile both cross the huge distances that separate attacking and defending spacecraft, but also do it quickly enough so that it hits the target before it can accelerate out of the way.
Streams of hard-to-detect projectile hurtling through the void. You might want to avoid them.
Area denial relies on the fact that the target spacecraft is uncertain about when you fired your railgun, and what velocity the projectiles are, but does know in which direction you fired. This creates 'corridors' in space where your projectiles can be, and that the target spacecraft will have to evacuate until it is reasonably certain that the projectile have passed.

By varying the velocity of your shots or working in conjunction with a second or third railgun, you can eventually cover all routes of escape from these 'corridors', catching and therefore destroying the target.

The advantages of this option are multiple. It relies heavily on tactical choices and group work, which are critical to exciting, meaningful combat. You can write it as slow moving chess against walls of incoming projectiles, or a frenetic dash through spaces between these corridors. It also removes a lot of constraints from the railgun, such as the requirement to fire high velocity projectiles or to be light enough to directly compete against lasers or missiles.

There are some disadvantages, though. It assumes that the projectiles, after firing, are hard to detect. This might not always be the case. It is also slow. The railgun ships trying to close the trap with slow-moving streams of projectiles can have their work interrupted by laser or missile fire. This would leave gaps for the target to escape through. Finally, it would always consume large amounts of ammunition. Railgun-ships would probably only have a handful of attempts at a trap before running out of ammo.

Bullet Dance

An alternative on the theme of indirect fire. Instead of trying to fully close a trap with criss-crossing streams of projectiles, all you try to do is make the defending craft manoeuvre long enough to expend all of its propellant.



I'll save you the embarrassment of having a gif of a cowboy dancing in front of a pair of revolvers, but the term 'bullet dance' comes from a trope in Wild West movies where a cowboy shoots at the feet of a bandit, forcing them to jump repetitively.



This is another option that is sensitive to the technology of the setting. If the target has a high-exhaust-velocity rocket engine, then they can accelerate for days, if not weeks. If they decide to move in at an otherwise suicidal velocity, they might cross the distance from engagement range to deadly proximity before they are forced to expend any consequent amount of propellant. It also assumes that the target is unsure about the position of the projectiles or cannot deflect them before they arrive.



Special ammunition


So far, we've considered the projectile to be some sort of inert impactor. It needed to be launched at astounding velocities , or in incredible numbers, to expect to hit a spacecraft thousands of kilometers away.


We use special ammunition instead. One option is guided projectiles that can track the target and ensure a hit unless shot down, and the other -sand- is the logical extreme of the 'numerous projectiles' method.

One of the rare examples where current technology has caught up to what authors consider futuristic.
So, a guided projectile. Uses onboard or linked sensor information to target the enemy spacecraft, and then onboard propulsion matches the enemy's acceleration until it hits. There are three 'levels' of guided projectile:
  • Guided warhead
  • Railgun-launched missile
  • Missile 
The distinction lies in how much they add to their initial velocity during transit. A guided warhead maintains a constant velocity after launch, only moving up/down/left/right to match the target's position. A pure missile starts out at zero relative velocity and accelerates on its own. The railgun-launched missile is boosted to an initial velocity, then uses its own engines to increase that velocity. The balance between these three types depends on the rocket engine technology in the setting.

If the target spacecraft has an engine that is equally effective when miniaturized, such as a chemical rocket or solid nuclear thermal rocket, then the pure missile configuration is best.

If the target spacecraft has an engine that is not effective or not possible in small sizes, and it has low acceleration, then the guided warhead is most effective, since the target only moves by a little during the projectile's transit. An example would be the target spacecraft running on a nuclear-electric engine with low thrust.

Anything in between favours the railgun-launched missile, that has a less powerful engine than the target spacecraft, therefore requiring an initial boost, but can match the acceleration afterwards with an onboard engine. An example would be some sort of complex nuclear engine that has moderately good acceleration, but can be outrun in short distances by simple solid rocket booster.

In that last case, you can justify the use of a railgun. Just be careful to include the effects of acceleration on the missile - you'd favour solid boosters over liquid-fuelled boosters, for example.

Now, onto sand.
This can be recreated intentionally.
Sand as ammunition is a complex subject that I would dedicate a post to in the future, but for now, understand that it tries to cover large volumes of space by reducing the projectile size to centimeter-to-millimeter diameter particles. Trying to do too much with the sand is unlikely to be effective, such as trying to go through thick armor or cover the entire disk of space a target can reach while accelerating after the sand is fired. More modest objectives are likely, such as forcing laser mirrors to be covered against particles to give more time for your missiles to reach the target or restricting the directions your target can accelerate in by threatening to perforate their propellant tanks.

Efficiency

It was mentioned in (2) that railguns can be rendered more competitive by increasing their mass efficiency compared to lasers. There are other techniques for doing this.
For illustration purposes.
One is to tweak the efficiency of the weapon systems. For something like a coilgun, which is nothing more than an electric engine strung out into a straight line, efficiencies over 90% are easily justifiable. High efficiency means less waste heat, which translates into smaller, lighter waste heat management systems. A laser on the other hand, especially low-wavelength laser generators tuned for extreme range, can be written as having lower efficiency, requiring massive waste heat systems, or maybe restricting how long it can fire.

Another option is compactness. While it might not matter to craft at the limits of each other's engagement range, spacecraft that have to change direction and burn at full power away from incoming projectiles will be sensitive to the size requirements of their weapon systems. Electric cannons are usually dense, and unless required to produce incredible velocities, they are fairly short and narrow. A laser generator, on the other hand, would benefit from the same technologies and can be made large and bulky. A Free Electron laser, for example, might need extremely long and fragile loops of particle accelerator to provide the necessary energy to the electrons. A railgun-equipped spacecraft might end up being smaller and easier to maneuver, and have less surface area that requires armoring.  

Comparing two weapon systems, where one can hit instantaneously at extreme range, and the other takes hours, is an easy, trivial affair. Adding complications and evening the playing field with waste heat management, size issues and other factors makes the choices a much more interesting affair.

Improvised weapons

One way to include railguns into the setting relies on the narrative. If a faction in the war does not have a dedicated military, or has not yet developed high efficiency, high-power lasers, then it might decide to convert its industrial-use mass accelerators into weapons. This is accomplished simply by pointing them in another direction. 

This way, you justify their use as aweapons not because it wasn't possible, but because they did not have other options.

Utility instead of weaponry

If the technology of your setting always ends up placing lasers as the incontestable weapon of choice, you might have to resolve eliminating electric cannons from the list of possible weapons.

However, you can still keep them in the setting.

To do this, you will have to find ways to employ them as mass accelerators. You can give them roles such as mine-laying, launching drones or sensor relays, as flak-cannons for defense against missiles, for placing projectiles into the orbits of target craft  (over-the-horizon/ballistic shooting).
Rods from God.
Another utility role railguns might fill is orbital bombardment. As you might now, you can't just 'drop' something from orbit. You have to de-orbit it first, that is, impart negative velocity until its orbit curves down and ends up hitting the surface.

In a setting where space warships are equipped with short-wavelength lasers optimized for extreme range, they might find their beams unable to traverse the atmosphere and their missiles too fragile to survive atmospheric entry. The solution in that case would be something that instantly kills orbital velocity and drops a dense projectile from the sky... this is kinetic orbital bombardment.

Armor

The final option on this list is the most subjective. It deals with how spaceships look, after all. 

This option is an extension on the compact railguns/bulky lasers from (7). Basically, electric cannons can be enclosed in lighter armor because of their lower surface area. They also only need a small hole to fire through. Lasers, on the other hand, would require both more armor and a very large hole to be filled in by the focusing lens or mirror.
Haha.... no broadsides in space.
If the orbits the battle takes place is full of debris, projectiles and railgun sand, then every spacecraft needs armor. Laser warships are unable to fire for long without something smashing through their focusing mechanisms. In this sort of scenario, whether coincidental or purposely generated by one side or the other, railguns have the advantage.

Now then, please do realize that the best 'option' is several. Use a combination of the above for maximal effect.

Thank you for reading!  

11 comments:

  1. Interestingly, the UNSC in Halo opts for mass over speed. For example, the Frigate-class magnetic accelerator cannon launches a 600-ton slug at 30 kilometers per second for an impact force equivalent to 64 kilotons of TNT.

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    1. That would be the correct approach if your targets are both very close and accelerate slowly.... basically, star wars combat.

      In reality, having a 6 ton projectile travel at 300km/s is equally devastating and can reliably hit targets much further away.

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    2. I was gonna say it's because their ships are heavily armored. Such as the UNSC's cruisers are equipped with 6 feet of Titanium.

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    3. The dreadnought-class mass drivers in Mass Effect launch a 20 kilogram slug at 4,025 kilometers per second.

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    4. Those are extremely overpowered for their targets.

      A 2m deep plate of titanium armor can be blown through in the most inefficient way possible with a 1560MJ impact. That energy is achieved by a 3kg projectile travelling at 30km/s.

      The Mass Effect mass driver requires 40kt (nuclear yield) of energy, if 100% efficient...

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    5. Hahaha, the SuperMACs are even more overkill, launching a 3,000 ton slug at 12,000 kilometers per second.

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    6. Matter Beam: could you site the assumptions for your figure of 1560MJ, above? Your projectile has a momentum of 90 000 kg*m/s, so I was wondering where the 17 000+ m/s component was coming from. Not doubting, just curious.

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    7. Nothing to do with momentum.

      2m deep crater assumed hemispherical: 16.8m^3 -> 75.6 tons
      Heat of titanium vaporization: 425kj/mol -> 8.89MJ/kg
      Efficiency: 50% (not all of the kinetic temperature is converted into heat, and none of it excavates titanium through mechanical means)

      Energy required: 1344GJ
      You are right :/
      I made a thousand-fold conversion error!

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  2. What about induction heating of the magnetic fields which produces eddy currents in the projectile, even if the projectile was a superconductor, it would reach its critical current when having magnetic field in order of accelerating something to over 100km/s. Also wouldn't the magnetic field itself be extremely dangerous to nearby electronics? Also Have you thought about quench guns? because you mentioned 90% efficient coilguns, which are to my knowledge only possible through Superconductive Electromagnets.
    Few month ago I also considered Rail Missiles, but thinking about the extreme accelerations required to achieve decent speeds in under 10km of barrel length.
    It would destroy nearly all electronics by shear force.
    Sincerely yours.

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    1. A realistic, short coilgun will require a few hundred Teslas in field strength to accelerate projectiles to 100km/s+ within a hundred meters.

      However, there are tricks to reduce this requirement.

      A longer barrel needs less magnetic field strength. A sabot can be used to both increase the projectile diameter (the barrel length depends on volume).

      An important point I made is that you can still make 'slow' railguns and coilguns useful. So if you find it difficult to push projectiles to such velocities, use staged projectiles.

      The Excalibur rounds survived 40000Gs of force during launch. Future electronics might survive even higher forces. The real fragility of these projectiles is in the propulsion system meant to allow them to track and intercept their targets. Solid propellants are best for this purpose, but have poor Isp.

      However, a mostly inert laser ablative projectile propelled to a good initial velocity by coilgun could be the solution.

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  3. This comment has been removed by the author.

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