If we have hypothetical versions of these devices made of magical lightweight inexpensive materials which method of propulsion is more efficient, if either, and why?
Clearly, in the real world a simple rail gun is much lighter, less expensive, and physically simple compared to a simple coil gun. The coil gun requires multiple independent solenoids that have to be switched on and off with proper timing for a projectile to accelerate as it passed through the series of coils. A rail gun needs just two parallel conductive rails that are connected by a conductive projectile sliding along the rails.
We could simplify this to a coil gun with a single coil and a relatively short rail gun to eliminate structural and heat problems and the propose equivalent resistance in their conductors.
So forgetting about the actual materials and the efficiency of the switching electronics of a coil gun. In hypothetical ideal configurations is one more efficient than the other in terms of electrical energy required to accelerate a unit of mass to a specific speed?
I’ve seen a some generic statements that rail guns are more energy efficient without explanation of why. I suppose more directly I’m asking if there is some basic difference in efficiency in the application of these two forms of electromotive force.
The efficiency of the switching and timing circuitry in the coil gun is a non-issue. With modern electronics, that’s trivial.
The dominant sources of inefficiency will be resistive heating in the rail gun, and a burst of electromagnetic radiation for the coil gun. Though I don’t know off the top of my head which would be worse.
Without doing the math, I’d expect that inductive energy would be significant in the coilgun. That has to be dumped somehow as the projectile passes by. You could probably transfer the energy to a capacitor to recover it, though.
That came to mind and I’ve been looking to see if that technique was used in any linear motors. Not found anything so far. Many designs use a permanent magnets in the stator do a small ‘rotor’ (the moving part) can use a small number of coils (as few as one).
Seems like ideally, you’d actually just dump the energy into the next coil in line. Might need to go through a capacitor first. I haven’t looked at these types of circuits at all (and my EE knowledge is pretty limited in the first place), but I know that you can’t perfectly transfer energy directly from one capacitor to another–there is an inevitable inefficiency in the process. By symmetry, I suspect the same is true of inductors. But L->C->L with appropriate switching elements may work. You’d also need to top off each new coil in sequence to account for resistance and the back-EMF from the projectile.
Things like this are best though as all reactive components and then you spend your time worrying about phase angles - which quickly gets you working with complex numbers and the complex plain. For power delivery EEs this is their life.
With modern switching devices you can perform near arbitrary manipulation in this space. Does require arbitrarily large amounts of switching.
But there is no excuse not to recover energy sitting in a reactive component. Your losses should only be copper, iron and what is delivered into the projectile’s motion.
Minimizing resistive losses is usually accomplished via a brute force approach: lowering the temperature, using silver, silver plating, or a super conductor, bigger cross-sectional area, fancy geometries w/ lots of surface area to reduce skin effect, etc.
A bigger problem is minimizing losses due to dielectric heating (capacitive) and core losses (inductive). Those are specialized areas of research and development; it takes finesse to address those issues. I know next to nothing about that stuff; it’s pretty esoteric.
I still think there’s likely to be a lot of loss to radiation, in the coil gun at least. That’s energy that you can’t just recover from the inductors, because it’s moving away from your device at the speed of light.