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I am currently tinkering with a railgun experiment, and I was thinking of using a liquid nitrogen loop to cool the rails down to very low resistance, and inject some nitrogen into the barrel between the rails, to remove some oxygen and prevent corrosion, and excessive arcing. I would run the liquid nitrogen through a hole down the length of the rail, and into the barrel through much smaller holes connecting to the main line in the rail. The nitrogen would go from the tank through a pump, through one rail, into a tubing to the other rail and back to the tank.

Now I know nitrogen is magnetically transparent, and being a mostly inert gas doesn't run the chance of reacting unexpectedly. Some things I am unsure about:

  1. I have looked around but it seems there are not a lot of places that sell tubing and fittings for liquid nitrogen. Is there some place I can browse to look for standard size fittings and tubing, and a small pump to carry liquid nitrogen through my system? The other difficulty is that I need to keep fittings and tubings to magnetically transparent non conductive materials, otherwise the magnetic field while firing (let's say 10 kJ) will mangle them. I am guessing there will be some custom fabrication involved, but still an idea of the basic sizes will help me make some design decisions.

  2. At what pressure is it safe to carry liquid nitrogen through a system like this? Obviously the pressure will not be very high, but since I need to bleed some gas into the barrel, I need to set a target main line pressure to calculate injector channels size.

  3. Is there an inherent danger I am overlooking with running an inert gas through a rail that will conduct a 1 ms pulse of 10 kJ? Obviously it won't ignite or explode, but is there a chance it will ionize at those relatively low energies?

Edit:

After some more looking, it seems PTFE (Teflon) is the right material for the job, but its stability under a high magnetic field is unclear. Is there a better material for this?

Edit 2:

This is a crude cross section of the proposed arrangement. Orange is the copper rails, dark grey is PTFE, light grey is payload.

enter image description here

Edit 3:

There has been concern about the heat dissipation in the system, and I should mention that the system is fed power through some large inductors that should dissipate the bulk of the heat. They are in place to stretch out the pulse in the first place. The dissipation in the rails should be very, very low, considering oxygen-less copper at 77K has an abysmally low resistance. The coils will probably also get the nitrogen treatment.

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    $\begingroup$ If there is contact between the rail and the projectile, you may want to explore the effect of such a low temperature on friction between them, and such problems as galling or icing. $\endgroup$ – Brian Drummond Oct 12 '16 at 10:53
  • $\begingroup$ UHMWPE and 316 Stainless Steel are both suitable for use in cryogenics from what I understand. There are some significant differences between them, but they may be useful for different respective components. $\endgroup$ – wwarriner Oct 14 '16 at 20:47
  • $\begingroup$ Also your question is too broad, it would be helpful if you could focus on one question at a time. I would edit this down to one of the questions, then create a new question for each additional issue you are working to resolve. $\endgroup$ – wwarriner Oct 14 '16 at 20:48
  • $\begingroup$ UHMWPE becomes brittle at -150C, which is a problem, and steel is magnetic, and will get torqued a LOT by the magnetic field... $\endgroup$ – Drunken Code Monkey Oct 14 '16 at 22:13
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Regarding questions 2 and 3, I think you need to look deeper in the chemistry, reactivity, and heat transfer characteristics of Nitrogen.

1) It is not a noble gas. It is reactive, and you benefit from this reactivity each time you eat a plant protein.

2) I have not designed a railgun and I don't have much sense of the thermal inefficiencies here. But I think it would be a good idea to consider what kind of heat your launch pulse is going to transfer to the liquid nitrogen in the system. Depending on the temperature of the nitrogen (presumably just at -196 C, i.e. boiling point) then (some?) nitrogen in the system is likely to "flash" into its gaseous form during the launch, and pressure will go up considerably. (Assuming atmospheric pressure and ideal gas behaviour, it will take up ~700x the volume as soon as it gasifies.) Obviously you'd want to make sure the sudden gas pressure doesn't immediately blow up all the tubing, or the barrel.

Some aspects of this system will be similar to a water-cooling system in a foundry or a power plant, and there will be much more engineering experience with that coolant. You're just dealing with something 300 C colder.

Possibly you've already modelled out this thermal stuff but if so, mention it in the question to prevent us from worrying. :)

Your project sounds like fun, good luck!

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  • $\begingroup$ 1) Yes, what I meant to write was "mostly inert", I edited the question. 2) I don't expect the rails to produce enough heat for a very big increase in pressure, but if it does, the rails will have small channels between the center nitrogen channel and the barrel, which ultimately exhausts in free air, which should alleviate any sudden expansion (the nitrogen will simply expand in the barrel and exhaust at the tip). I will add a crude cross section in the question to help with the imagery. $\endgroup$ – Drunken Code Monkey Oct 12 '16 at 3:24
  • $\begingroup$ And liquid nitrogen is only -196 C ;) $\endgroup$ – Drunken Code Monkey Oct 12 '16 at 4:12
  • $\begingroup$ I like edits 2 and 3 very much, DCM. Can you ensure that not much flex is needed to the various tubes and fittings, also? Because sudden cooling to this temperature range is very likely to make polymers brittle, as I'm sure you know. The first test for any candidate material in the whole cooled assembly should be to immerse it in nitrogen until the fluid stops boiling, then tap it firmly on a piece of steel or something. I'd never seen latex shatter before. It's pretty dramatic. $\endgroup$ – Tai Viinikka Oct 14 '16 at 4:06
  • $\begingroup$ PTFE is THE most flexible polymer in the world, and stays flexible down to -240C. I did do a lot of searching for this, because I knew I couldn't use any magnetic materials, as the magnetic field would crush them instantly. There seems to be very few materials that are magnetically transparent, electrically non-conductive, and stable over wide ranges of temperature. If you have other alternatives in mind, let me know, but Teflon seems the way to go... $\endgroup$ – Drunken Code Monkey Oct 14 '16 at 4:49

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