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Registered Member #4659
Joined: Sun Apr 29 2012, 06:14PM
Location:
Posts: 158
So, shorter projectiles are better, and the coil should always be as long as the projectile, so those should be short also. Taking this concept to its logical extension, projectiles should be supershort coin-shaped projectiles shorter than their diameter.
Ignoring the mechanical issues with this (projectile might jam in barrel, projectile is not stable in flight, etc), is there any magnetic reason projectiles should not be this short?
No, the shorter the better. But consider timing issue in addition to mechanical problems. Actually, the longer the projectile the more kinetic energy it will achieve by the same coil with the same pulse and energy. Howhether, the longer the projectile the more time it takes to pass a coil. While coil is in current, it dessipates energy. So here is a tradeoff: more iron grabs more energy, while shorter one accelerates faster, so coil dissipates less. All those tradeoffs will become a funny thing of the past when high-temp superconducting materials will become available so coils will stop dissipate while projectile passes through. But by now, consider heat losses as the main factor. So make shortest projectile possible to minimize heat losses during the acceleration. One more thing: even if projectile is so short that can no longer be shortened (length is equal to diameter) it may still be a little efficiency better to keep making the coil even shorter.
Registered Member #4659
Joined: Sun Apr 29 2012, 06:14PM
Location:
Posts: 158
Yandersen wrote ...
No, the shorter the better. But consider timing issue in addition to mechanical problems. Actually, the longer the projectile the more kinetic energy it will achieve by the same coil with the same pulse and energy. Howhether, the longer the projectile the more time it takes to pass a coil. While coil is in current, it dessipates energy. So here is a tradeoff: more iron grabs more energy, while shorter one accelerates faster, so coil dissipates less. All those tradeoffs will become a funny thing of the past when high-temp superconducting materials will become available so coils will stop dissipate while projectile passes through. But by now, consider heat losses as the main factor. So make shortest projectile possible to minimize heat losses during the acceleration. One more thing: even if projectile is so short that can no longer be shortened (length is equal to diameter) it may still be a little efficiency better to keep making the coil even shorter.
So coin shape it is, then. I had an idea for solving the stability/jamming shortness problem - you could have a projectile with a coin-shaped iron "head" with a longer wood/cardboard-tube section in the back. So the active (iron) part is short like the coil, but it's physically longer.
But for more than one stage, you'd probably need a timing system that could differentiate between the metal and wood/cardboard parts of the projectile....
Heh, good luck. I will be playing with equal diameter to length ratio and will not go any further. Just because I have some practice. I would like to mension that acceleration of 3000g or around projectile may be experienced in a peak during the acceleration... Compose it from wood and an iron coin... Ha. :| Well, at least you a trying to think about what you are going to do, instead of just copying what other monkeys do. ;)
Registered Member #1525
Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294
I have yet to hear a single good explanation for why short coils and short projectiles are better than long ones... just "trust me, I have experience".
Let's assume two coils have identical inductance, number of turns, length of wire, and receive the same amount of power. One coil is short and thick, one is long and thin. Both have projectiles matched to coil length.
True: The short thick coil will have a stronger gradient in the magnetic field, and will produce a stronger force when integrated along the projectile than will the long coil.
False: This will result in a higher exit kinetic energy.
Why: It is a fact that Energy = Force times Distance. The long coil produces less force, but over a longer distance. In this case the end result is the same. Claiming that the shorter coil imparts more energy is the mathematical equivalent of saying a rectangle of height 3 and length 2 is bigger than rectangle of height 2 and length 3 because it's taller.
The actual factors that increase efficiency are higher initial velocity, more inductance (flux linkage), more turns, and lower current. The primary cause of low efficiency is suckback, which is more of a switching issue.
Registered Member #8497
Joined: Tue Dec 04 2012, 06:24PM
Location:
Posts: 74
I should probably introduce myself. I've been lurking on this site for quite a while now - but never formally registered. So technically this is my first post.
Yandersen - As far as theory goes - your among the top 3 users who seems to know the physics behind CG's.
Can you explain why you wouldn't want your armature (projectile) exactly half as long as the coil for a reluctance-type accelerator?
As I understand it, the armature is accelerated until the center-point of the armature reaches the center-point of the coil. Past this point, the magnetic field acts as an opposing force to the armature. Therefore, unless you can reverse the polarity of the field ( reverse the current direction ) exactly center-on-center, the armature should be exactly half the length of the coil?
Saz, if you don't believe me, would you trust FEMM simulations? If so, try two cases which you think would be fairly identical for you and see the difference. So far, I'm the first one who squezed 31% of efficiency from the CG' coil and build a coilgun with a dozen controversal differences from what you guys used to build, achieving 20% of average efficiency. Did you actually ever tried short coils, or you just afraid or too stubborn to step aside from common path of mistakes? :) Or it is just hard to do a triggering for many short coils? Well, this one I can understand - I'm as lazy as noone else is... :)
Maxwell, truly, I've understood how it all work only after I've finished my first shooty-thing just recently. I completely did not remembered even A=F*s formulas from school, learned electronics from a little push from my dad, datasheets for the parts we'd bought, touching all points in the circuits with my fancy oscilloscope discovering why and how everything burns on my circuits; electrodynamics were enlighted to me by FEMM; and thanks to wikipedia for it's existance. So basically, I'm an uneducated curious amateur who likes to touch wires instead of touching girls in that age, and likes to think about interesting things while sweeping the floors at Wendy's. The result of all this - see my signature at the end of a post. I did not read a single book about electrodynamics, so I truly do not *know* the physics behind CG's - instead, I just *understand* how it works on the instincts level, based on MY OWN exp instead of believing woodo words from schoolbooks. What I can tell about reluctance-type accelerator? The best efficiency will be achieved, IMHO, without any iron around, with coin-flat spirally-winded 1-layer coil and a plastic dome with a penny glued to it's bottom as a projectile which initially lays flat on the coil. I would also recommend to put a plastic rod sticking from a center of a coil as a rail to direct projectile's fly direction (drill a hole trough the projectile's plastic tip and a coin to slide projectile down that rod throgh the front end of the gun down to the coil's end). The cap you have to use is non-polar, of course. Commutation element is a problem point, of course - reluctance coilgun consumes much more current but in a very short pulse, comparing to iron pulling one. Make sure charger is disconnected from a coil before shot - repolarization of a cap will burn it out. You can start from few dozens of joules and use bidirectional thyristor, preferably optically-triggered if you can find one or just use some big mechanic switch or relay. As I've tried reluctance coilgun, it did not impressed me as much as iron sucking one. The only benefit I see is an efficiency at kJ energies which is compared, of course, with a single-staged usual coilguns. I prefer to distribute energy between more stages, exploiting the efficiency of a regular coilgun (with short coils, of course, haha).
Saz, I've really tired of arguing with you about your lovely dick-length coils. All you are saying is technically right. But there are so much more points you ommit! The problem is you will never fully understand how CG works untill you try to build one on non-polar caps, which is basically is an LC oscillator and check every thing with oscilloscope and multimeter. In such coilgun type everything is clear: pulse time is, recuperated energy is an energy unused, energy difference is everything that dissipated or gone into a projectile. You can determine suckback via optical gate. Investigate effects of wire diameter, cap's capacitance and voltage on recuperation energy, kinetic energy and pulse time. You may just use FEMM to compare the inductance difference between two coils of the same wire length winded in different shapes, see the difference in pull force also. Imagine half-coil has force double as much as the long thinner coil. A=F*s, so in result the same energy projectile will achieve, but less energy will be dissipated due to shorter pulse time. Consider at least this rough comparison. The higher efficiency of subsequent stages is achieved only due to shorter time of energy dissipation while the increase in K is the same - if all coils are of the same size, they have equal power dissipation rate (in a recuperational LC design, of course - demphered ones are undescribable). Saz, just keep building your guns wrong way - at least they look cool on youtube. Just don't mention the tech parameters - keep this in secret. :)
Registered Member #1525
Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294
Yandersen wrote ...
Saz, if you don't believe me, would you trust FEMM simulations? If so, try two cases which you think would be fairly identical for you and see the difference.
Correct me if I'm wrong, but FEMM gives you a static simulation. To get a measurement of KE you would need a dynamic simulation that can integrate over time.
wrote ...
So far, I'm the first one who squezed 31% of efficiency from the CG' coil and build a coilgun with a dozen controversal differences from what you guys used to build, achieving 20% of average efficiency.
Your coilgun rocks. It's the first interesting coilgun that's been built around here in a long time (except one Zero made) and it's inspired me. But I wouldn't call it controversial. Coilgunners have been using diagonal half-bridges for a long time to recover inductive energy. It's the same idea you have but done in a way that works with polarized caps, which we tend to have easier access too.
wrote ...
Did you actually ever tried short coils, or you just afraid or too stubborn to step aside from common path of mistakes? :)
My long coils are not a mistake, the length was set for a specific kinetic energy output. As you said correctly earlier, a heavier projectile will pick up more kinetic energy than a light one. By a back-of-the-envelope calculation, my 15kW, 0.4m long coilgun should accelerate my 12.3g projectile to 8.21J KE. The actual KE is about 10J so that gives you an idea of the accuracy. If I half the length of my projectile, I half the weight to 6.2g, which means it will only gain about 6.5J.
What will impress people about a full auto coilgun? Surely it's ROF and KE. Thus efficiency was never a requirement for my design. If it was, I would build one like yours.
wrote ...
based on MY OWN exp instead of believing woodo words from schoolbooks.
You have to be born with smarts but education makes you smarter. Newton went to Cambridge, Tesla never missed a lecture at Austrian Polytechnic, Goddard got his PhD from Clark University, and Von Braun got his from the Berlin Institute of Technology. For the good of your own mind, and for our world that can benefit from it, I hope you consider getting a masters in Physics or EE.
wrote ...
The problem is you will never fully understand how CG works untill you try to build one on non-polar caps, which is basically is an LC oscillator and check every thing with oscilloscope and multimeter.
I already know what happens in an LC oscillator, thanks to those silly textbooks!
wrote ...
You may just use FEMM to compare the inductance difference between two coils of the same wire length winded in different shapes, see the difference in pull force also. Imagine half-coil has force double as much as the long thinner coil. A=F*s, so in result the same energy projectile will achieve
Yes! I totally agree, I'm glad we've been able to settle on that point.
wrote ...
but less energy will be dissipated due to shorter pulse time. Consider at least this rough comparison.
I have to think about this one for a bit. See, I'm not closed minded!
wrote ...
Saz, just keep building your guns wrong way - at least they look cool on youtube. Just don't mention the tech parameters - keep this in secret. :)
It also looks cool in The Dark Knight Rises. My tech parameters have always been posted in the description of the video for everyone to see :)
Registered Member #2939
Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615
FEMM supports Lua scripting, so it is possible to set up the problem such that a series of static simulations can be done, with data output to a text file, which is simple to analyse with Excel or similar.
"Correct me if I'm wrong, but FEMM gives you a static simulation." Unless you don't know how to use scripts. I wrote a simulator for my recuperational gauss which shows how it is dynamically happen in a progressive slideshow way. The difference between the simulation and reality is not that much, but FEMM is little more optimistic than reality, of course. I assume, most of incorrelation happened because of an induction current losses in an external iron, which I don't know how to simulate, as well as friction.
"Coilgunners have been using diagonal half-bridges for a long time to recover inductive energy. It's the same idea you have but done in a way that works with polarized caps, which we tend to have easier access too." Tell you a little secret: I've just invented the way to build my recuperational gauss using polar caps. It also allows to control pulse time. I was so inspired by halfbridge design idea (which I didn't know a week ago!) that I spend few days braking my mind trying to find the way to make it work with SCRs instead of IGBTs. Today, finally, I've got a success with the following schematic: To start the pulse, trigger SCR2 and SCR3 simultaneously. Once the cap repolarizes to just -2V (acceptable for polar caps, right?), SCR2 closes and current starts looping inside D1-D2-D3-L1-SCR3 circuit up until you trigger SCR1 which will redirect current back to the cap causing SCR3 to close. Ingenious, isn't it? You owe me the applause, guys. ;) And if you will decide to build a coilgun using my "SCR halfbridge" make sure to give me credits! Oh, to trigger SCR I've used little transformers on a ferrite rings. Easy. But so far, I've tested this on cap charged to 18V, (5V returned in a 600us triggering pulse time) of 60uF, which is my non-polar cap. I'm going to increase the voltage, put a polar cap, and hope it will still work (why not?). All I'm afraid of is that electrolithic cap may not hold negative charge of -2V... What do you guys think? I don't have much exp using electrolithics this way so to be sure... :(
"For the good of your own mind, and for our world that can benefit from it, I hope you consider getting a masters in Physics or EE." Sure, just donate me some, cus all my salary goes on appartment rent and food, with just a little extra for a stuff for home science (fucking capitalistic people-squizing world). :)
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Projectiles (and coils) should be short - but how short can you go?
