h a l f b a k e r yThe embarrassing drunkard uncle of invention.
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Why am I fascinated by railguns? Probably because I still don't really understand how they work. On reading (again) my favorite railgun page, I was pondering the mysteries of projectile composition. The projectile must be conductive, and must be heat resistant because of "ohmic heating". It must
not melt, as apparently projectiles want to spotweld themselves to the rails. On the linked page the author addressed this using a copper/carbon composite.
A google will find various folks bandying about superconducting railguns. It seems like a fine idea except the rails must be strong, and big. Superconductors must be very cold, which is hard to do with with big things like the rails, and are generally not that strong - exotic ceramics etc.
Why not make the projectile a superconductor? A superconductor should not experience ohmic heating, bypassing several problems. Projectiles are small, and so easier to keep cold. Finally, physical strength is not a necessity for a projectile: all that matters is mass and velocity. One could make it out of frozen mercury cooled in liquid helium (if you have some of that lying around near your railgun). For a space-based weapon, the supercold temperatures should be less of a problem. I googled around but this idea is original as far as I can tell.
Railgun page
http://www.powerlabs.org/railgun2.htm
A fine railgun webpage. It is amazing how little there is about working railguns on the web. A lot of gamer links. No doubt most real railgun research is classified. Spooky railgun researchers, if you use the above idea, please only use it to do good. [bungston, Jan 18 2006]
More railgum stuff from the same author
http://www.powerlabs.org/railgun.htm
Good nuts and bolts. [bungston, Jan 18 2006]
Superconductor Niobium/Tin current density example
http://hyperphysics...e/solids/scex2.html
This one is 200,000A per cm2 - actually, not bad at all... [Ling, Jan 18 2006]
Graphite Electrodes
http://www.graftech...te+Electrode%3f.htm
See the bottom of the page for link to a brochure, too [Ling, Jan 20 2006]
Iterative space railgun
Iterative_20space_20railgun
railguns railguns yah yah yah. railguns railguns yah yah yah. ooo-oooo! I like railguns. [bungston, Mar 27 2010]
[link]
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The idea of making the projectile superconductive never occured to me. Mwhah Hahh Ha Ha! |
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A "summer" employee at a place I worked with almost 10 years ago said he built one of these (not superconducting). I didn't really believe him until I asked his dad about it. Knowing his dad, I couldn't doubt it. |
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I wish I had the time to learn how to and construct one. |
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[bungston], I think that super-conductors have a maximum allowed current density (if I can remember correctly). I'm not sure what happens above that current density, but I assume that they aren't super-conductors any longer. |
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I was considering the problem that powerlabs mentioned: arcing of the Copper rails. There is another material that might be considered, and that is Graphite. |
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1. Low friction
2. Excellent conductor (not as good as copper, but close)
3. Very good thermal characteristics (stronger when hotter!) |
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It is used in electric arc furnaces, and withstands continuous arcing at 60,000Amps with a 20inch diameter electrode, only wearing at a rate of 117kg per hour each electrode. |
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I'll try to find more information via a link. |
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Not exactly addressing the topic, but would a solenoid technically qualify as a railgun? |
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I think arthur feidler of the boston pops is a superconductor. |
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"solenoid..."
Would technicaly qualify as a 1 stage coilgun with a retained projectile. |
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"maxium current in superconductor"
Yeah, they do. (or is it just maxium magnetic feild? Doesn't really matter - currents in a wire create magnetic feilds) I don't know what it is... so the feasability of this is still unknown. |
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"frozen mercury"
Lead superconducts at a temperature of 7K, compared to mercury's 4.2K. |
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[bungston], have I oversimplified this in my mind, or has the concept drifted by w/o much comment like a heavy cloud? |
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It is the latter. The HB serves a useful function for me - by depositing my heavy clouds, I no longer need to mull them over myself. At least not as much. |
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I'm confused. How exactly are you going to keep the projectile cold while it's travelling through these coils? The projectile has low mass, and cannot be connected to any exterior heat sinks or refrigeration lines, and obviously, it gets as much energy transferred to it as the rails get transferred to them. I'd be surprised to find out that the projectile is not sizzling hot when it leaves the "barrel," and unless you refrigerate the entire barrel... and add a powerful blower to transfer the heat of the projectile to the barrel with the added friction necessary to do so, there doesn't seem to be any way to keep the projectile cool. |
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I'd also imagine that the stresses applied to the barrel would also be applied to the projectile, though I admit most of this force would likely be compression... How do most superconductors stand up against crusing forces? |
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1. The projectile starts cold. It cannot
be kept cold. It only needs to be cold
during the instant it traverses the rails. |
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2. Transferred energy: yes. Electrical
resistance causes some of this energy
to heat the projectile. Superconductors
have no electrical resistance and so
should not get hot. |
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3. Maximum allowed current density.
This is probably why this idea wont
work. Probably. I do not know enough
about such things to figure out how to
match current required by a railgun
with current permitted thru a
superconductor. |
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I was thinking about graphite rails, as suggested by Ling up there. They could be augmented with ceramic so they did not crack. In fact, the rails themselves could be ceramic and a fresh coat of graphite brushed onto them between firings. |
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A good test projectile would also be graphite. For tabletop tests, one could use pencil leads. That would mean a nice little railgun. |
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It sounds a lot like the "Mercury Bow" from Oni. |
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Essentially, it's a rail/coilgun (they don't explain) that uses a sliver of supercooled mercury as its projectile. |
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I found the Mercury Bow on Wikipedia. It is from a computer game. It looks like they like the idea of such a projectile causing mercury poisoning to a human target, not so much the superconductivity of mercury (which as pointed out above could be accomplished with lead). Given the low toxicity of metallic mercury and the wound a railgun would produce I do not think poisoning would be much of a concern. A high velocity mercury bullet (more generally) would have the advantage of probably fragmenting in flight or on impact and so transferring more kinetic energy to a soft target, as opposed to a more solid high velocity bullet that would probably travel right through a body and out the other side. |
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This idea was not really posted with the notion of using a railgun as an antipersonnel weapon - ultrahigh velocity projectiles are not much of an advantage in that context. |
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still subject to ohmic heating after it leaves the "barrel". Graphite would be a better choice of projectile in my opinion. Lead WOULD strke me as being a more efficient material as well actually. |
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A ceramic might hold up well though and could be jacketed with something? I know some munitions jacket the rear to prevent ohmic heating from causing problems. I would concern myself more with permeability and less fancy superconductors myself? |
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The entire projectile need not be made of a superconductor, just the shell or sabot. |
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The metallic superconductors are averse to external magnetic fields, so you'd need 3rd type superconductors, which have the unseemly problem of being a near isolator when not being a superconductor. High currents and high external magnetic fields are cumulative as aproblem, so the projectile might switch to isolator mode while a Megaampere is going through it, which makes for a fireball of epic proportions. |
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If the "rails" are fixed to the projectile, why would the projectile move relative to them? Or maybe _how_ could it move relative to them? |
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here is Ling on the topic, pasted from the linked Iterative railgun idea. |
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/Plus, of course, that it wouldn't work if the current only existed in the projectile (the reaction is from the current in the rail vs. the current in the projectile...).
— Ling, Mar 27 2010 / |
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