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Electromagnetic Projectile Launcher Efficiencies

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kjjohn

Fri Aug 13 2010, 05:04AM

Registered Member #2985 Joined: Fri Jul 09 2010, 04:05PM
Location: Chippewa Falls, WI
Posts: 9

I have learned a lot about advanced electronics over the past couple months, and I think I would like to start a project. I want to make something with a considerable amount of power (and by considerable, I mean at least twice as power as a .22LR). I don't want to make a coilgun, as I have already done a lot of research about them, and they are simply not powerful enough due to their low efficiency. So, I am choosing between a railgun and an ETG. I want to know if a railgun or ETG will be more efficient than a coilgun, and if it will be practical to make a somewhat portable one with my power requirement. I am leaning towards an ETG, as I recently came up with an idea for using silicon carbide ceramic as a barrel liner, but I got no replies on the thread about it.

HVgeek

Fri Aug 13 2010, 10:31AM

Registered Member #2998 Joined: Tue Jul 13 2010, 08:34PM
Location: Swedish forests.
Posts: 26

Well, unadvanced railguns of the common hobbyinst type usually never exceed about 1% efficiency. Coilguns might get you up to 7 or 8 %. ETG:s I have no dea, but doubt it would be any better than coilguns.

The thing about a railgun is that it has no theoretical LIMIT to it's speed. So if you have enough capacitors you could fling any object at any speed up to 299 792 km/s.

A coilgun has a higher efficiency, but the muzzle velocity is limited by the equilibrium of the air resistance and the magnetic attraction at saturation for the magnetic projectile.

An ETG would have the limit of the speed of sound in the barrel, just like a regular firearm.

So, if you want power, make a railgun. If you want efficiency and portability, make a coilgun.

You can series-augment railguns for much higher efficiencies, like military -operated railguns do if you have the time, will and engineering skills for it This could get you up to about (i'm only guessing) 5% efficiency practically, but it's alot of extra work.

DYI

Wed Aug 25 2010, 04:25AM

Registered Member #1917 Joined: Fri Jan 09 2009, 02:38AM
Location:
Posts: 62

Railguns tend to want relatively high energies before they get very efficient, making them impractical for portable use. Also, without some downright magical materials, they suffer from just as many limitations as ETGs do. Efficiency can reach at least 50%, but the best amateur designs are closer to 15%, if I remember correctly.

Coilguns can be very efficient and reach very high speeds, it is only reluctance coilguns which are a slow, inefficient, dead-end technology. Serious designs for inductance coilguns exceeding 8km/s with heavy projectiles have been published, and there is no fundamental lower limit in terms of energy as with railguns. A member of this forum built a small coilgun reaching 300m/s (and with appreciable muzzle energy), which could likely be made relatively portable.

ETGs can reach very high speeds and efficiencies (20% efficiency and >2000m/s, in my case) with access to high voltage capacitors and basic machine tools, but reloading time is terrible, as the capillary tubes need to be replaced every shot (and the seals need to be VERY good). If you happen to have a CNC lathe, cartridges with pre-loaded capillary tubes, fuse wires and propellant could alleviate that issue. Barrel erosion in ETGs is a major issue if one is attempting to fire full bore steel slugs in excess of 1.5km/s. As one would expect, barrel erosion in the ETG equivalent of a 9mm pistol is not necessarily more of an issue than it is in the pistol, and it may be less so with careful propellant configuration.

As such, I'd suggest a coilgun if you're inclined towards the electronics required for timing, as it will be the most efficient, easiest to use option. If you have a CNC lathe (or a manual lathe and lots of patience) and aren't much for the fiddly electronics part, an ETG is the way to go. I advise against a railgun. You're just not going to be able to lug around an efficient one.

kjjohn

Sun Aug 29 2010, 02:57AM

Registered Member #2985 Joined: Fri Jul 09 2010, 04:05PM
Location: Chippewa Falls, WI
Posts: 9

I don't quite understand how an inductance coilgun works. Again, I am very much a beginner to this. My idea would be to make something that would fit inside a large rifle-sized package, and an inductance coilgun would definitely be an option. My goal is to achieve velocities and muzzle energies somewhat comparable to powder firearms.

As for ETGs, would my ceramic liner idea help with corrosion? Or would that be considered a capillary tube which would have to be replaced every shot?

GhostNull

Sun Aug 29 2010, 05:53AM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

Induction coilguns:

This site also has a wealth of information on coilguns in general

kjjohn

Sun Aug 29 2010, 06:12AM

Registered Member #2985 Joined: Fri Jul 09 2010, 04:05PM
Location: Chippewa Falls, WI
Posts: 9

Thanks for that link, very helpful. However, I still have a few questions. Would I be able to get a reasonable amount of muzzle energy (>600ft/lbs) with, say, a 1kJ cap bank, and assuming optimum number of stages and coil design?

ramses

Sun Aug 29 2010, 05:53PM

Registered Member #1208 Joined: Thu Jan 03 2008, 05:30PM
Location: Chesterland, OH
Posts: 154

If you do, please invite me to your nobel peace prize ceremony. 81% efficiency would be amazing!

If you want that much KE, and are okay with low velocities, you need to go with a conventional gas powered cannon.

kjjohn

Sun Aug 29 2010, 09:45PM

Registered Member #2985 Joined: Fri Jul 09 2010, 04:05PM
Location: Chippewa Falls, WI
Posts: 9

I am not looking to make anything like a cannon. I have made potato guns before, and, although impressive, they simply don't compare to real firearms. Short of making real firearms, I would like to create something with lots of power, but with much higher energy density than a potato gun, and performance similar to a real firearm. This makes me think an ETG would be a better fit
.

Turkey9

Mon Aug 30 2010, 01:48AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

You're not going to get anything comparable with powder firearms with electric means. You may get muzzle energy that comes close to a small firearm, but that would require a device that is barely portable at best. These devices are more about exploring the physics and electronics behind the phenomenon than creating a device that can be used for sport.

DYI

Mon Aug 30 2010, 03:03PM

Registered Member #1917 Joined: Fri Jan 09 2009, 02:38AM
Location:
Posts: 62

John, if you want energies comparable to a firearm in a portable package, why not use some variation of SpudBlaster's spool valve piston hybrid design? It can fire several rounds per minute at ~6kJ muzzle energy, and he's managed to bring it up to 11X without any parts that can't be purchased in hardware stores. It is handheld when firing, and not exceptionally heavy.

What you request is, however, possible with an ETG. Working from your minimum specified energy of 800J and assuming 15% efficiency (I've reached 20%, with much lower input energies), you'll need a 6kJ capacitor, at the very minimum (preferably larger, because high energy density capacitors have short lifespans without pushing them to their limits, and they're horribly expensive). General Atomics' CMX series capacitors go as high as 3J/cc. They could, in theory, build you a 6kJ capacitor that occupies roughly 2L of space and weighs under ten pounds. Doing so may cost over $10000 and have a lead time exceeding half a year, but it could be done (assuming that you could convince them to sell to you - they're very picky about their customers). More realistically, you could get something at around 1J/cc which would still be of manageable size, and weigh 20-30 pounds. Now you need a charging supply, a power source (capable of charging to 6kJ in a reasonable time frame), a switch (a hydrogen thyratron or a trigatron would be ideal, as far as size is concerned), transmission lines to the gun, the gun itself, cartridges, and the electronics for controlling the trigger and charging.

If you were lucky with sourcing the parts, and used a modification of my existing chamber design, you could probably build that for between ten and twenty thousand dollars. It isn't comparable to firearms in that it is heavier, and requires a backpack, but I'm very confident that it could reach the energy you're looking for, and more, in a package which you could conceivably lug around for a few hours without collapsing (if you're in good shape).

Also, as I stated before, barrel erosion is related to energy density. If you're getting the energy density of a pistol, and using a suitable amount of working fluid to cool the plasma, barrel erosion will be comparable to that of a pistol, or less. The bottom line is that the more energy you try to get for a given barrel volume, the faster the barrel will erode. If you have a rifle sized barrel and are getting the muzzle energy of a large pistol (and assuming that you use enough working fluid to cool the plasma before it hits the barrel), you should see less barrel erosion than you do in the large pistol. If you build this properly, a single barrel will last for thousands of shots.

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