# Justification for Gatling type Coil Gun

From what I understand, the reason modern day Gatling guns exist, is that to achieve the high rate of fire, the heat from the rounds passing through would severely damage a single barrel in a short time. Thus we have multiple barrels, rotating at a high rate.

While starting design of a new space ship model, I thought that a Gatling style turret would look really cool on it. However, the navy in this universe uses as their primary armament coil guns. Energy weapons are used, but not nearly as much, the idea is that they can use multiple different types of munitions with the coil guns.

With lasers, or rails, I can see a Gatling style being useful. But with (effectively) unlimited energy, and no contact or heating (I am assuming superconductors can be used for the coils) what, if any, justification is there for a rotating, multi barreled gun turret?

EDIT for clarity; This would be in a space setting, so, vacuum etc.

• I think mechanical rotating barrels are things of the past but circularly polarized laser pulse isn't fun either so by spinning the mirrors at relativistic speed we can create energy from vacuum (the zero point energy or vacuum energy is infinite relatively put) I know there are some engineering issues involved and how to harvest sufficient power to boil a cup of coffee by any significant amount of heat... any way you got to admit its kind of cool! b4 i forget welcome to worldbuilding... – user6760 Oct 18 '16 at 15:30
• Ill admit, you lost me pretty good there! But thank you! :) – Nick Oct 18 '16 at 17:37
• @user6760 «by spinning the mirrors at relativistic speed we can create energy from vacuum» huh? – JDługosz Oct 18 '16 at 20:08
• @JDlugosz: I'm referring to dynamic casimir effect like the hawking radiation theorized to allow black hole to have a temperature OK not related but the acceleration here is the gravity instead of mirror alright I just lost myself pretty good too! – user6760 Oct 19 '16 at 0:26

Whoa whoa whoa, did I just hear a 'no heating' in there? I see these coil guns have a thermodynamics compensator!

The second law of thermodynamics states that entropy is always increasing, or from a practical engineering point of view, every process generates waste heat. In a coil gun the waste heat will come from resistance to the current in the coils according to $P = I^2R$. Since the current term is quadratic, even with a very low resistance, a very high current (i.e. a very powerful Gauss gun) would still produce appreciable waste heat.

I couldn't find a lot of hard numbers on energy efficiency for coil guns. Rail guns seem to be more popular with governments (specifically the US Navy) for funding. Rail guns, to be sure, have huge cooling problems, but I can't find similar statements about coil guns. Here is a possibly legitimate website talking about trying to get coilgun electrical efficiency up to 26%. That means that 26% of the electric current drawn is converted to projectile kinetic energy; which implies that the other 74% becomes entropy.

So, in conclusion, the coilguns have heat problems too. So for a certain application where high rate of fire in short bursts is desired (point defense weapons, specifically), a Gatling gun design has advantages over active cooling due to its low cost and mechanical reliability.

Edit for more maths

Greybody power radiation from the Stefan-Boltzmann law is $$P = A\epsilon\sigma T^4$$ where $P$ is the radiation power in watts; $A$ is effective surface area (i.e. facing away from the object so emissions are not reabsorbed); $\epsilon$ is emissivity; $\sigma$ is the Stefan-Boltzmann constant; and $T$ is temperature.

Since we want maximum heat disspiation at high temp, assume we can find a gunsteel alloy with emissivity of 0.9 and excellent strength at the desired operating temperatures of no more than 1200$^\circ$K. Assume effective surface area of 1 m$^2$ and we get $$P = 1 \text{m}^2 \cdot 0.9 \cdot 5.67\times10^{-8} \frac{\text{W}}{\text{m}^2\cdot\text{K}^4}\cdot (1200)^4 \text{K}^4 = 105 \text{kW}.$$

Assume that our gauss gun is 80% efficient so 105 kW of waste heat correlates to 26 kJ of muzzle energy per second. If you fired a 0.1 kg projectile (twice the mass of a .50 bullet) you would get muzzle velocity of $26000 J = \frac{1}{2} 0.1\text{hg} \cdot v^2$, $v = 721 \frac{\text{m}}{\text{s}}$.

So it is feasible to cool by radiation a 6 barrels * 1 round / sec * 60 sec / min = 360 rounds/min, 26kJ muzzle velocity Gatling gun.

• I just made a quick edit, as Im not sure it would have been caught before, but this is meant for space based combat, do you think there would still be appreciable cooling benefits from this? – Nick Oct 18 '16 at 14:51
• There is still heat dissipation through blackbody radiation in space. This can amount to a lot of energy lost if the barrels are designed to operate at very high temperature. – kingledion Oct 18 '16 at 16:01
• back of the envelope, wanting about a 50k range, using the KE i derived from that....Im looking at about 10 times the melting point of steel, per shot! May need to be re thinking some numbers... – Nick Oct 18 '16 at 17:28
• @Nick In space there is really no such thing as range, a projectile that starts moving, keeps moving. I added some numbers for calculating how powerful your weapon could be with just radiation to space. Not super powerful, but with some engineering and adding radiators to increase surface area you could probably match the performance of current Gatling guns like the Vulcan or CIWS (which is, incidentally, internally cooled). – kingledion Oct 18 '16 at 18:25
• I more meant an effective range. As in, fairly short travel time to improve hit chance. Thank you for the additional math, I still have a ways to go to make ammends between what I want and feasability... – Nick Oct 18 '16 at 18:45

Another reason for Gatling style guns has to do with the increased rate of fire, rather than heating issues as you identified.

Loading projectiles into the barrel and firing them takes some amount of time, even if very very fast. A multi barrel system allows one barrel to be firing while another barrel is finishing reloading this allows for higher overall rates of fire than a single barrel could produce.

This would apply to any projectile propulsion technique, including coil guns.

The advantages of connecting them into a single gun rather than have multiple single barrel guns include only needing one aiming/guidance tracking system as well as taking up less physical space.

Josh King described the reason for a Gatling gun's high rate of fire, but the second reason (dumping excess heat) really only works inside an atmosphere, where the barrel can be cooled by convection as well as radiation.

In the space environment, cooling is handled by radiation, and the best way to do that is to pump coolant through the barrel and to a large radiating surface.

Atomic Rockets has a section of coilguns on the "Conventional Weapons" page, which includes many of the equations you will need to calculate the efficiency and power of the weapons, which should also allow you to calculate efficiencies and heat loads.

Short answer is a coilgun should be a single barrel weapon connected to a large radiating surface. If you really need an extreme rate of fire putting multiple barrels together in the manner of Leonardo da Vinci's multi barrel cannon and firing them in rapid sequence