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Railgun Switch Idea

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LAN_403

Forty

Thu May 26 2011, 10:31PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

no that's a great idea inducktion. but if you halved the power for each rail, with the goal of having the power be combined through the projectile, then you'll still have the same spot welding problems with the stationary projectile.

Electroholic

Fri May 27 2011, 09:49AM

Registered Member #191 Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720

Inducktion wrote ...

Is my idea not any good then? No one's said anything about it...

IMHO, it is a complete waste of time.
The two switch solution you proposed will have the two switches in a series connection. Increasing the number of high current connections and complexity, not to mention the effects on total ESR and ESL.
And then there is the problem with precisely synchronizing the turnon.
All in all, I think if you already have a switch suitable for railgun use, it would be much easier to double its voltage rating than to use two in series, and try to get the whole mess to work.

Fraggle

Fri May 27 2011, 02:09PM

Registered Member #1526 Joined: Mon Jun 09 2008, 12:56AM
Location: UK
Posts: 216

big5824 wrote ...

Going from what iv learnt from the past 3 years of browsing, id say that amount of energy with a stationary projectile is going to cause too much heating at the area where it meets the rails, I can see it just turning into a glorified arc welder. An injector would spread the heat out more evenly, and it could probably get the projectile into tighter fitting rails than by hand (just a guess)

If you think carefully about it you`ll see that it`s really the other way round - if you inject the projectile then only a small fracton of the contact area will conduct a lot of the energy, increasing the chance of welding etc...

klugesmith

Fri May 27 2011, 03:41PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714

Fraggle wrote ...
If you think carefully about it you`ll see that it`s really the other way round - if you inject the projectile then only a small fracton of the contact area will conduct a lot of the energy, increasing the chance of welding etc...

I respectfully disagree. The ideal railgun circuit is not a RC network but a RLC (with time-varying rail-loop RLC and electric motor back-EMF).
The current begins to ramp up when the circuit is closed. In a good design, the energy delivery should be spread out over the entire time the projectile is between the rails. Like propellant "powder" grains in a gun, sized for a burn rate that matches the projectile time in barrel.

Electroholic

Sat May 28 2011, 03:03AM

Registered Member #191 Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720

I think the injector system started as a solution to the railgun switching problem. it is nothing but an air actuated mechanical switch, which has its own drawbacks. Namely the amount of contact area available at switch on. Which is usually solved by increasing injection velocity, and increasing ESL to slow the dA/dt.

Also the welding problem is nothing but a myth. Most amateur railguns I've seen have too small a capacitor bank to work properly, not to mention sloppy construction.

Fraggle

Mon May 30 2011, 12:32AM

Registered Member #1526 Joined: Mon Jun 09 2008, 12:56AM
Location: UK
Posts: 216

Klugesmith wrote ...

I respectfully disagree. The ideal railgun circuit is not a RC network but a RLC (with time-varying rail-loop RLC and electric motor back-EMF).
The current begins to ramp up when the circuit is closed. In a good design, the energy delivery should be spread out over the entire time the projectile is between the rails. Like propellant "powder" grains in a gun, sized for a burn rate that matches the projectile time in barrel.

I can`t argue with that but the inductance is very low right? I`m betting an injector is more trouble than it`s worth to get right. I`ve never actually made a railgun though!

Navrit Bal

Mon May 30 2011, 01:11PM

Registered Member #3323 Joined: Sun Oct 17 2010, 03:19PM
Location: West Midlands, UK
Posts: 116

With my pretty pathetic attempt at making a railgun a while ago now, I didn't have an injection system, the projectile was stationary and it DID weld itself to the rails multiple times, although the plasma generated by that was propelled in the direction expected. So, Electroholic I respectfully disagree that the welding in railguns is a myth... unless I have misunderstood something of course :D

Electroholic

Tue May 31 2011, 03:02AM

Registered Member #191 Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720

I admit, "myth" might not have been the best word to describe it. In fact it happens a lot in amateur built guns.

I guess what i meant was that, the statement "welding will occur unless you have injector" is a myth.

I'm just going to leave this old thread here.

rp181

Tue May 31 2011, 04:15AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

Welding only means you do not have enough current. It takes a lot to achieve the diamagnetic affect.
It is more efficient to start the projectiles in the rail, but is simply not economical for hobbyist.

The myth of an injector preventing welding is simply due to powerlabs. If this is the case, why doesn't the CO2 shielding gas in MIG welding prevent welding? The magnitude of energy is very different.
If you keep resistance and inductance low, the current would actually be done flowing before the projectile was fully in the rails, but it is not an ideal world. The projectile loses contact (more likley as you have to make it looser to allow the injector to work), hence the plasma:

Now you can do this outside of the bore through a switch, or inside. The bore is already built to withstand the forces; you just make the most of the plasma to propel your projectile. This also becomes useful with the winged design, to prevent further vaporization. You can't do this outside.
After my railgun shots, it was clear the wings had done their job. It was also clear there was large palsma production in front of the projectile, but what are you gonna do.
The link posted above me is very good. Coincidently his bore design is similar to mine, though he used GP03 and i used G11. I did stress analysis of this design:

This was designed to withstand 120,000 pounds of repulsive force between the rails and 60,000psi of bore pressure.

Kluge: You should not intentionally lengthen the pulse time, it simply wastes energy. Rather, the system should be designed around this.

Sorry for deviating a bit...

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