In this paper, it describes a proposed cannon using a linear track with a series of large superconducting magnets.
The cannon’s advantages include near 100% efficiency on the energy transfer, minimal wear when each shot is fired, and the example designs have muzzle energies measured in gigajoules.
The main disadvantage seems to be that it is extremely long and heavy, versus a railgun, and is far too large to mount as a weapon on any military platform yet built.
Each magnetic is energized to just below the capacity limits for the material it is made from. Projectile gets attracted to the line of magnets, and when it passes through each one the magnet is “quenched” instantly, open-circuiting the magnet.
How would you do this, physically? You cannot use power control semiconductors in a superconducting ring circuit. I think a mechanical switch would be far too slow and it would arc.
A quench happens when the superconductor stops being a super conductor. It isn’t so much a matter of it being hard to cause a quench, as it being hard to stop the damn things quenching. Superconductors have the unfortunate property that they have a very very low thermal coefficient, an extraordinary small amount of energy will cause them to heat up enough to stop superconducting. Even the energy created by a small shift in the windings is enough to do this. Once any small part of the windings stops being a superconductor it starts to heat up and pretty much at the speed of sound in the material a wave of energy whips around the coil, and the entire thing catastrophically ceases to superconduct. A big part of superconducting magnet engineering is in in the mechanical design to stop this happening.
I suspect that no additional effort is required in this gun design - the magnets probably cheerfully quench all of their own accord as the projectile passes. I could certainly imagine a design tweaked so that this is what happens. The mechanical stress on the magnet as the projectile flies past is likely to create quite a bit of strain in the coil, and away it goes. Then you just need to tune the mechanics so the wave of quench propagates at the needed rate.
On the other hand, “minimal wear when each shot is fired” is almost certainly false, as is “near 100% efficiency on the energy transfer”. Magnet quenching tends to be a rather… messy… process, in its practical effects.
Indeed. The paper seems to be written by people who have never dealt with cryogenic magnets. It misses as few very nasty issues.
The railgun design seems to assume that a quenched magnet is open circuit.
When the magnet quenches it does not magically become open circuit - it merely reduces to a normal resistive conductor. As the magnetic field collapses current continues to flow in the coil, albeit at a much lower level. But it is flowing in a resistive material and continues to heat it up. You end up with a far from cool magnet, and the process can cause thermal expansion and mechanical shock that damages the superconductor. Large magnets are typically rated for a number of quenches before they fail. This number can be depressingly low (tens). My fisrt encounter with a quench was on an MRI scanner built in the 80’s, and it failed after the first quench, costing the vendor over $1million to sort out. The railgun design is cooled with supercritical helium. It isn’t mentioned that this is going to need recycling and cooling, and that the energy deposited in the (now resistive) magnet coils needs to be removed by this helium.
A cycle time of two hours is fanciful. Cooling the magnets down again, and then getting into the rather tricky process of re-energising the magnets is not a two hour job.
ETA - to be honest, this paper did not strike me as a particularly great bit of work, indeed the overall work done was about the level I would expect from a summer student. It was written in the late 80’s, I don’t think there was any understanding on how to control a quench at this time. When the LHC magnets quenched it boiled the helium. The energy has to go somewhere.
(While I have a general aversion to using someone’s credentials to justify an argument, in this case I’m not sure what credentials one would even be using.)
On this point at least, technology has partly caught up. We now have superconductors that can be cooled with mere liquid nitrogen, which is much less expensive and thus less imperative that it be salvaged for re-use.
And as an aside, this is not a railgun. It’s more similar to (but still not quite the same as) a coilgun. A true railgun has two long parallel conductors (the rails) with the projectile between them completing a circuit, and with a strong magnetic field perpendicular to the plane of the circuit.
The superconductors that can be cooled with liquid nitrogen are not very amenable to being made into a wire and coiled up into an electromagnet. Nearly all commercial superconducting electromagnets still require liquid helium cooling.
Agreed. I’ve seen an NMR quench and would prefer to never see one again. I have no idea what a dewar of liquid helium costs commercially, but apparently the University of Illinois will sell you it at $14 a liter, which is actually a lot cheaper than I expected. A magnet quench would require a massive expansion of gas and there would be real issues of making sure that the operators didn’t asphyxiate. The expansion ratio for liquid helium is 1 to 757. If there’s a liquid nitrogen blanket on that, that will be another expansion ratio of 1 to 694. That will kill you quick if you can’t get out of the inert atmosphere.
Quench is the wrong word. They want the field to go to zero, instantly, as it clearly says in the paper.
If you had a large mechanical switch in the middle of the circuit, the switch made of the same superconducting material as the rest of the magnet, could you open the switch using explosives or something to instantly cut the circuit?
There must be a way to deal with this. Theory is the projectile will gain the energy that was in the magnet - so the electric field in the magnet should be zero afterwards, with no energy left to boil the coolant.
An old rule of thumb was liquid Nitrogen costs about the same as milk, and liquid Helium costs about the same as whisky. With the current shenanigans with Helium it could be a lot worse. $14 is indeed pretty cheap (unless you are a very cheap drunk.)
The big problem with the lunar coilgun is that there is a lot of energy being dissipated as heat in the quenching coils that doesn’t seem to be being considered. The idea of flowing supercritical Helium gas as a coolant avoids boiling large amounts of Helium, but still avoids the question of exactly how the now hot magnet assemblies are taken back down to cryogenic temperatures with any speed or efficiency. Given the energy cost of such cooling, and the time needed, it doesn’t sound practical. The whole point of the design is to avoid consumables, due to the insane cost of lofting the mass to the moon, so recycling the Helium is required. As would Nitrogen and other intermediate coolants.
Well explosively blowing a link means it is probably a one time device. The shock will induce a conventional quench anyway. I’m pretty sure they do mean quench. After all, they used exactly that word, and when the original idea was mooted (in 1978) they should have at least understood what it meant.
Superconducting magnets are not simple beasts. They have all sorts of curious properties, and require very careful feeding. Energising a superconducting magnet involves heating a small segment of the magnet coil so it ceases to superconduct, and thus is relatively open circuit to the rest of the coil, then you attach the power, and power up the magnet, then very carefully, cool the segment down again, then remove the power. Normal de-energisation is the reverse, and done very carefully. So far the paper makes no useful contributions to how the magnet is to be constructed or managed, and I feel the whole idea has been worked up in something of a practicality vacuum, possibly by people that had little to no practical knowledge of real superconductors.
If you read the first few paragraphs, the way this gun works is you put exactly enough energy in each superconducting coil that the electric field will reach zero when the armature passes through the center of the magnet.
At the instant the field reaches zero, THEN you quench.
There’s no energy in the magnet when it has a field of zero, and by “quench” I mean you just heat up a segment of the magnet so it is no longer a superconductor, or you ground one part of the magnet to another or something. No dramatic boiloff.
This means it will not have the barrels current reverse as the projectile moves away from this magnet.
It would be almost 100% efficient, and very little energy would leak into the coolant. (by 100% I mean that with this method all the energy that was in the magnet is now in the projectile. You do need to supply energy for the switches, the heaters, the control system, etc etc.)
