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New Coilgun Project

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Saz43

Wed Sept 26 2012, 05:39PM

Registered Member #1525 Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294

Just apply a little imagination...

Hanzie

Wed Sept 26 2012, 06:38PM

Registered Member #6534 Joined: Sat Sept 08 2012, 12:22PM
Location: The Netherlands
Posts: 32

Aha, nice big LiPo's. xD
Are they standard 6s RC-grade batteries? I'm planning on starting a build soon, and I think I can just go with the same type of batteries I use on my RC-planes. Would that work or do you need a higher discharge rate for that? 2.7 Ah at +/- 12 volts (35C discharge) should probably be enough to power a simple transformer me thinks, but I'm not sure if charger circuits pull any strange tricks with the currents while charging.
Hanzie

Saz43

Wed Sept 26 2012, 07:36PM

Registered Member #1525 Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294

Nah LiPos will do the trick. 35C times 2.7Ah= 94.5A which will be more than enough to power even a huge transformer. The batteries I'm using here are 3.6Ah, 6S racing packs with 100C burst rates since my coils are direct drive. The batteries are in series, so the coils are getting about 50V at around 300A.

Saz43

Sat Sept 29 2012, 04:39AM

Registered Member #1525 Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294

First circuit board for the final build! This board is 1 of 8 for each stage. It has 4 diodes in series to commutate coil switch-off current, along with a gate resistor and a resistor for an LED that lights as long as the coil is triggered (if you watch closely and don't blink, you can actually see the LEDs race down the barrel during a shot).

Hanzie

Sat Sept 29 2012, 02:34PM

Registered Member #6534 Joined: Sat Sept 08 2012, 12:22PM
Location: The Netherlands
Posts: 32

Ah, so LiPo's should work then. Thanks! I have quite some laying around here...

Saz43

Tue Oct 09 2012, 02:05AM

Registered Member #1525 Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294

This week I made lots of progress on the IGBT+diodes housing, checkit.

Yandersen

Wed Oct 10 2012, 10:38AM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

Looking at the assembly You build I guess no any changes in the design could be done now. Howether I'm still interested why didn't You tried shorter thicker coils and shorter projectile this time? I have a feeling that 16 half-size coils could have much more potential than 8 on the picture. Taking into account that You powering this gauss directly from battery I would recommend to consider much smaller mass for acceleration, otherwise You risking to have a dead design (unless You have already tried at least single stage proved to be powerfull enough).

Howether, the assembly has the same mesmerizingly perfect quality, keep a good job! :)

Saz43

Wed Oct 10 2012, 07:02PM

Registered Member #1525 Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294

Yandersen wrote ...

Howether I'm still interested why didn't You tried shorter thicker coils and shorter projectile this time? I have a feeling that 16 half-size coils could have much more potential than 8 on the picture.

This is one of the most frequent comments I get on my coilguns. A lighter projectile won't improve performance- it will actually worsen performance in terms of muzzle energy. It's counterintuitive, but it actualy requires more power to accelerate a lighter mass to a given kinetic energy over a given distance than it does a heavier projectile. Think of it this way: the coils more or less provide constant power. A light projectile picks up speed faster, and exits the accelerator more quickly- allowing less time for it to absorb a high total energy. If I load a ligther projectile, it will have lower muzzle energy and less smashing power, which is necessary for a cool gun.

Furthermore, a major attribute of a good design is manufacturability- it's already been a nightmare winding 8 coils, forming 9 powdered iron endpieces, soldering 8 circuit boards and paying for 8 IGBTs- could you imagine doubling that number? Plus, 16 IGBTs and 64 diodes would never fit inside the gun. The project might never get done.

wrote ...

Taking into account that You powering this gauss directly from battery I would recommend to consider much smaller mass for acceleration, otherwise You risking to have a dead design (unless You have already tried at least single stage proved to be powerfull enough).

There's no risk of that! Check page 1 of this thread for the performance data of the first 7 stages.

wrote ...

Howether, the assembly has the same mesmerizingly perfect quality, keep a good job! :)

Thanks!!

Yandersen

Wed Oct 10 2012, 08:34PM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

Sorry, Saz, but I must resist about shorter coils. The energy transfer efficiency of shorter coils is higher because short thick coils have higher gradient of a magnetic field - this gives higher acceleration. Back to the simple principle of coilgun operation: every magnetic dipole of an iron bullet inside a magnetic field aligns in the direction of a magnetic lines, but PULLED in the direction where a magnetic field is higher. In the middle of your long coil bullet is pulled closer to the walls of a barrel, so this part of a coil just dessipates energy, while the pull is achieved only by the front end of a coil through which bullet enters. To prove and visualize it, energize one of your coils with a few Amps (AA battery, for example) and drag a little piece of iron through it via a plastic rod to check the difference in pull force in different parts of a coil. Mention how comparably low pull force is in the middle of a coil. Now take into account that your bullet is actually a mesh of such particles.

If you want to wind The Best Coil, you must take this into account: every turn of each layer lowers the pull force, but extends it's effect length-wise; every layer highers the pull force, but each subsequent layer has less effect and contain more wire. So when you choosed the length of the coil (the shorter the better), you must add layers up until the effect of each next layer wound will become negligible. In practice such coil looks really thick - like triple of thickness of a barrel.
In other words, many short coils is better than one long efficiency wise. The bullet may still be long one, but in this case single coil right in front of the bullet must be energized up until bullet enters the next one - one coil at a time, creating a concentrated magnetic field right in front of a bullet tip, pulling it inside via highest gradient possible.

But long bullets are better than short ones, penetration force-wise, because it is pulse-related, but not energy-related. In other words, assuming diameter is the same, longer bullet with lower speed penetrates the target deeper than shorter speedy one, which has more kinetic energy.

Yandersen

Wed Oct 10 2012, 08:38PM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

And one question about IGBT you are using for energizing your coils: aren't they latch in ON state at such currents?!

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