![]() Obviously, your story, your hands to wave, but this stuff is important to remember if you want a hard-ish scifi setting. You cannot get anything better unless you use magical materials, or magical ultra-efficient coilguns (and this example is already quite magical). ![]() That's a bit better, but remember: this is a maximum, regardless of anything else like barrel length, or field strength, or projectile weight. We can now have a maximum muzzle energy of more like 1.2GJ, which for a 200g round gives us a muzzle velocity of nearly 110km/s. We'll give your coilgun an efficiency of 99.9%, and half the wastage goes to heating up the projectile again. You can now heat it up by more like 3MJ/kg instead of 900kJ/kg. Lets imagine your projectile now has a SHC of more like 1kJ/KgK, and a melting point of 3000K. Non-superconducting projectiles will be subject to more heating!) superconducting projectiles musn't be heated above their critical temperature, for example, and that'll be a lot lower than their melting point: ~300K for a scifi "room temperature" superconductor, for example. (Note that there will be other limits to projectile temperature. A bigger projectile won't help you you need one with a higher melting point, a larger heat capacity, and a more efficient coilgun to get the speeds you want. This is obviously bad for your hopes and dreams. Working backwards, this gives us a maximum kinetic energy of ~35MJ and so a maximum muzzle velocity of 18km/s, regardless of anything else about our gun. If we magically start it off at 0K, we can see that it can absorb no more than 180kJ. If we have a 200g projectile with the specific heat capacity of steel (500J/KgK) and the melting point of steel (say, 1800K) we can see that the projectile's temperature will raise by 1K for every 100J of wasted energy. Lets say that our coilgun is 99% efficient, and of the 1% of the energy that it wastes, half of it goes into the projectile. our coilgun will inevitably cause the projectile to heat up as a result of induced currents, and if we heat it up to melting point the projectile will disintegrate making it much less useful as a weapon or worse: it'll explode, taking a chunk of your ship with it. The first thing we care about is efficiency. There were some errors in the barrel length calculation for the earlier draft of this answer, which have since been corrected. You can assume that any figures this gives are over-optimistic, but it isn't a bad upper-bound for coilgun performance. For a rough idea, you can use Luke Campbell's approximation for coilgun performance, which basically imagines the gun as a tube of energy in the form of the magnetic field, which is swept out by a projectile that turns it into kinetic energy.
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