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General linear motor / mass launcher design rant

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OZZY

Mon Jul 09 2007, 08:06PM

Registered Member #511 Joined: Sat Feb 10 2007, 11:36AM
Location: Somerset UK
Posts: 55

Joe`s last post has got me thinking, what would it take to build a 300m/s coilgun. Here are some numbers to think about. I have a single stage coilgun which fires a 4.76g projectile at 29m/s, how many stages do I need to reach 300m/s. The coil is 25mm long and the projectile starts off 2mm into the coil so it accelerates from 0 to 29m/s in 23mm. This gives an acceleration of 18,300m/s^2 or 1860G

The average force on the projectile is 87N.

If we assume constant acceleration then to reach 300m/s requires an active barrel length of 2.46m and 98 stages

if there are gaps between the coils then the barrel will be much longer! The first stage uses 78J of energy in a pulse 1.6ms long, the average power consumption is 48,750W or 48.75KW. The final stage will need a pulsetime of 0.083ms or 83us, if it also uses 78J then the average power for the final stage is 936KW

and the total stored energy to fire the gun is 7.6KJ.

However all is not lost, these numbers are based on efficiency of 2.6% and joe is right the later stages will have higher efficiency than the first stage. So how does the efficiency effect our multi-stage coilgun, first lets assume that the efficiency will average out over the large number of stages so we only need to consider the overall efficiency. The total energy is inversely proportional to the efficiency so if we double the efficiency to 5.2% we only need 3.8KJ.

But if we try to use 49 stages of 78J each that means the acceleration must be double what we had before, somehow I think its unlikely that we can have double the efficiency and double the acceleration in one gun. Its more probable that we will end up with 98 stages of 39J each.

Do not give up all we need is:-
Higher efficiency,
Higher acceleration,
Switches that can handle massive peak powers,
Energy storage that can provide massive peak power,
Timing accurate to a few microseconds.

All of these have been achieved by professional researchers, if they can do it then we can do it.

...

Mon Jul 09 2007, 11:07PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

You know, if we really wanted to we could probably manage a gun that size. For the first few stages you could get away with a 50J 400V cap and mid sized SCR (only needs to handle 100Apk), then in the mid stages use less turns and bigger switches, and at the end we would probably need about 1kV worth of electrolytics and brick/puck SCR's... But I bet the silicon and caps could be had for under $1000. Although tuning the timing would be pretty fun... Anyone have a basement 10m long

BTW, 1234th post

Dave Marshall

Mon Jul 09 2007, 11:23PM

Registered Member #16 Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554

I believe the last two posts really struck at the heart of the issue. I came to the conclusion years ago that given the present state of technology, a coilgun that approaches deflagration powered propulsion in velocity and physical size is simply not possible, even with the massive budgets available to government and education. The amount of energy that must be applied to the projectile in such a small time frame simply precludes it. Even with a huge budget, you're still bound by the physical limitations of the projectile material.

Essentially, once you exceed a certain energy input in an attempt to reach some insane velocity in a compact package, you stop shooting a metal projectile and start shooting a molten blob, and at an attrocious efficiency to boot. You go from coilgun to an expensive and very complex Linear Induction Furnace :)

I don't believe that there is any real potential for coilguns to become practical man-portable weapons. It is just counter to their physical nature. I do think they present some very interesting potential for either stationary, large scale weaponry, or more likely as a means of transportation.

I see the three biggest issues in current amateur coilgun efficiency being:

1) Use of SCR switching. They are intrinsically inaccurate and inefficient, but they're cheap and easy to use, ensuring that they'll stick around for those not hell bent on efficiency.

2) Optical triggering. Similar to the above issue, optical triggering is extremely rigid, requiring modification of the barrel to facilitate a change in trigger position.

3) Capacitor based power supplies. The combination of capacitors and SCRs cause suckback, and even with switching like IGBTs, a majority of the energy is expended before the projectile is in the ideal position with maximum coupling for best energy transfer.

The 'many small stages' approach helps to counter #1 and #3. With small stages a 300V 100A peak could be made to work, meaning that mains becomes a feasible source. Switching also becomes much more affordable. IGBTs in that range are very inexpensive, and with some creativity one large IGBT could even switch more than one stage.

The triggering thing has plagued amateurs for a long time. Every time you move an optical trigger, you have to drill a new hole in the barrel. If you change coil dimensions or location, you have to move the trigger. This lack of flexibility keeps people from tweaking as effectively as they might with a better trigger. People have had some success with ultrasonics, and very limited success with induced voltage/magnetic triggering, but these systems are usually pretty complex. and dislike being close to a giant electromagnet.

I think I might have this part licked. I'm deployed right now, so I can't really work on the specifics, but its basically an oscillator, a variable band pass filter, and a comparator to detect the presence of a projectile with a small coil of wire. The sensor coil could even be placed directly under (or possibly even over) the propulsion coil. If you want to move the trigger, you just slide it up or down the barrel to the right place.

I expect my 30 stage mains powered CG to be atleast 1.5m long. Not portable, but I really hope to reach atleast 10% efficiency.

Dave

Steve Conner

Tue Jul 10 2007, 09:36AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I agree with the above. An optimized coilgun would end up being just an unrolled motor. Either a switched reluctance motor or an induction motor, depending. Classical motor theory would apply, so there would be a lot of laminated iron everywhere to minimize airgaps. Flux density would have to be kept below saturation of the iron. The control electronics would end up essentially being an inverter variable speed drive that tracked the acceleration of the projectile.

This would solve the efficiency problem, but it doesn't solve the peak power problem. When all's said and done, the combustion gun just works so much better that it's pointless to develop coilguns as man-portable weapons. If I had to go into battle with a home-made weapon, I'd choose the potato cannon over the coilgun any day

BTW, I think Thomas Rapp has shot things at 300m/s with an induction coilgun running off 4kV and using triggered gap switches. It still only has about the power of a $150 air rifle, in a package the size and weight of two microwave ovens.

Dr. Shark

Tue Jul 10 2007, 04:37PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

I agree with the above, but I am (not jet) convinced that electromagnetic guns are just a waste of time. Obviously it would be silly to try and construct a handheld coilgun, simply because the amount of energy stored in even one gramm of gunpowder (~4kJ?) would require heavy and bulky capacitors to match, and probably a small portable diesel generator to keep them charged

I am not even saying that EM guns may ever serve a useful porpose in military applications or elsewhere - all I am saying is that they might be fun to play with for those of us who choose not to play with "real guns" for whatever reasons but still wish to make "things go fast".

I also agree that coilguns will never be as efficient as regular motors for the various reasons pointed out above, but I do think that 10% could be a reasonable efficiency goal for an induction coilgun given some careful design and understanding of the matter.
Speaking of understanding, I am quite far from it. I have come across a paper "A coilgun design primer"

from IEEE Transactions on Magnetics that makes the induction CG seem quite a bit more complex than one would naively think.

Aparently the armature is only magnetized once by the dB/dt it sees in the first stage, but in the following stages the armature sees a constant field and the armature field decays away. This requires the field in the driving coils to move with a "slip velocity" relative to the projectile, in analogy to a normal induction motor, to keep the armature excited.

I am not sure if this consideration applies in coilguns such as Thomas Rapp's design, because the stages are spaced far apart, so the armature might be remagnetized in each of the stages.

Anyway, the only way to find out is to try, err, discuss and try again. Time allowing, I will try to build a coilgun similar to the one Thomas has made, I will start a new thread to discuss the design choices. There is not a lot of information on amateur induction coilguns out there, so some experimentation is certainly in order (unless you can prove me wrong)!

Barry

Sat Jul 28 2007, 05:05PM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301

What about induction launchers where the armature is several turns of shorted coil?

Although a hollow tube is easy to make by hacking a hunk of copper pipe (note wall thickness should be just a little more than skin depth at your frequency of interest), a multi-turn coil would have proportionally higher force. Right? Or have I been breathing too many solder fumes again.

Cheers, Barry
PS - What a great topic for a thread.

Bjørn

Sat Jul 28 2007, 05:46PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

My current project is a multiple stage (20-30 stages) mains powered system. The idea is to use many smaller stages and a massive projectile to eliminate saturation, as well as suck back and the poor coupling that arises with a very high velocity projectile.

This was the methods used by the first coil guns more than 100 years ago. The stages were switched by the projectile and the design used the magnetic fields themselves to blow out any arcs. After a few years and guns that could launch projectiles with 3kJ of kinetic energy the whole idea was abandoned because the efficiency dropped a lot at high velocities.

The first one to reach 3kJ if kinetic energy can congratulate himself by reaching the 1903 level of performance. Maybe we should set up a prize to the first person that can repeat the performance.

Chris

Wed Aug 22 2007, 01:05AM

Registered Member #8 Joined: Thu Feb 02 2006, 04:34AM
Location: Harlowton, MT, United States
Posts: 214

From my tangential interest in RC planes and their motors I gather that the power density of a motor is roughly proportional to the speed it is operating at. The torque remains constant throughout the speed range, so the power goes up linearly. Small outrunner brushless motors seem to operate happily at speeds as high as 60.000rpms, corresponding to 100Hz mechanically or 1kHz electrically since they tend to have around 10 poles.

Actually, the torque of a typical reluctance motor is greatest when it is completely stalled. The torque decreases almost linearly (with a permanent magnet motor, and not quite so linearly but still more or less with other arrangements) as rpm increases. This results in a peak power and thus efficiency at a certain point of a bell curve. The efficiency then peters out to nothing as the rpm gets extremely high. This is analogous to the problem of coilguns doing the same. Very high speed motors tend to be small in size for their power in the same sense that high frequency transformers are small for their power (it does not take as much flux to transfer the same power at higher frequency) but they do suffer in efficiency (large slow motors, as well as large slow transformers, are more efficient). If we want to be making high speed coilguns, perhaps we should be switching to ferrite cores, like those found in RF transformers.

Like induction motors suffer limitations that keep them in similar bounds to reluctance motors, induction coilguns are (as some have pointed out) not some saving grace to coilgunology. They too have their limitations, mainly that of inductive heating. Attached below are some images of an overpowered projectile from my LDC (lateral disc cannon) which was a large two-stage reconnection gun (a type of induction launcher). The gun did not even achieve high performance when fired at lower power levels (it was impressive because the projectiles could efficiently cleave targets, but it did not break 3% efficiency and had a muzzle energy comparable to a .22 short), and this ~1000lb contraption (which I spent WAY too much money on) would not have been able to achieve the ballistics of an 8oz handgun even at its fullest power (with thicker heavy copper projectiles).

The first one to reach 3kJ if kinetic energy can congratulate himself by reaching the 1903 level of performance. Maybe we should set up a prize to the first person that can repeat the performance.

That's nothing. 1000 years ago, people were making their own rocket motors, which were just as good or better than most amateur attempts today. Also, 1000 years ago people made their own copper, brass, bronze, and even iron from ore they picked up along rivers and stuff, and then forged useful implements from it. Few people today would be able to accomplish anything close to that even when fully equipped with modern torches and other tools. My point is, ANY sort of technological achievement (even one thousands of years old) is far beyond the capability of a typical person, and always will be. Most people are woefully incapable. The people that made the 3kJ coilgun in 1903 knew a hell of a lot more about what they were doing than pretty much anyone here, and I'm sure they had a lot more money. They would no doubt be equivalent to the people that develop multi-MJ railguns today, and I'm sure that's what they would be doing if they lived now. I might also add that a coilgun of 3kJ muzzle energy would be trivial for most of us here if we had a large enough cap bank, so basically you are trying to equate funding to intelligence.

Aparently the armature is only magnetized once by the dB/dt it sees in the first stage, but in the following stages the armature sees a constant field and the armature field decays away. This requires the field in the driving coils to move with a "slip velocity" relative to the projectile, in analogy to a normal induction motor, to keep the armature excited.

This is the key to efficient induction motors (not necessarily reluctance motors, though). Someone should make an inductance coilgun with like 200 very small coils that are smaller than the projectile, that fire repeatedly at a forward rate significantly faster than the velocity of the projectile.

OZZY

Wed Aug 22 2007, 08:32PM

Registered Member #511 Joined: Sat Feb 10 2007, 11:36AM
Location: Somerset UK
Posts: 55

Coilguns are one of those devices that are relatively easy to build but understanding how they actually work can be complex and mysterious. Because of this people look for analogies in things that are more familiar. Transformers and electric motors have coils, iron cores and rapidly changing currents so people try to understand the coilgun in terms of what information they can find about these devices. There is virtually no mainstream literature about coilguns but the books written about electric motors could fill a house.

This approach will ultimately be mis-leading, to say that a coilgun is like an electric motor is the same as saying that a firearm is like a car engine. Think about it.... no I really mean it! Think about it....

In case anyone is interested, the firearm predates the internal combustion engine by 100s of years. The modern firearm is a highly developed device that has remained virtually unchanged for the last 100 years. The internal combustion engine is still improving at an incredible rate. Do you think that it`s fair to compare the performance of experimental coilguns to the mature technology of firearms?

Now you see I`ve used two analogies already and I haven`t even got started!

Most electric motors and all transformers are A.C. devices, most coilguns are D.C. People equate the switching on and off of coil current to the frequency of these A.C. devices but what is actually happening is very different. Lets concentrate on reluctance coilguns for now. The force exerted on a piece of iron by a magnetic field is a static force, it does not rely on a changing field to create the force.

Experiment 1. Hold your projectile so it is just into the coil and then connect the coil to a 12V battery. You will feel the force (just like Luke Skywalker). The force field is static in time but not constant with position, if you move the projectile the force will change. Static is not the same as constant. WARNING!! I have not actually tried this but I suspect that the coil will heat up very rapidly and the current may be enough to destroy the battery, work it out first!

The point I am trying to make is that a reluctance coilgun works on D.C. power. The perfect coilgun would be one that can create a magnetic field with constant strength that is moving forward down the barrel. If the field is moving at constant speed then the projectile will sit in the centre of the field to minimize field energy. The projectile will experience zero force and will travel at constant velocity. If the magnetic field is accelerating and can be made to keep ahead of the projectile by a constant distance then the projectile will experience a constant force and hence a constant acceleration.

Relativity tells us that our understanding of a situation will depend on our own point of view. Consider the coilgun described above from the point of view of the projectile. The projectile "sees" a constant magnetic field, constant force and constant acceleration, it will have no eddy currents and no hysteresis losses because nothing is changing.

Now think about how much more exiting this situation looks from the point of view of a stationary coil?

Now think about how it looks from the point of view of the target?

There is a very good and very long thread in the archives titled "coilgun efficiency".

ramses

Tue Jan 08 2008, 10:36PM

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

just my 2 cents:

design the coil with an under dampened coil (a lot under dampened) and let it ring down with the capacitor bank (good bye electrolytic capacitors, or hello high voltage, uber high current diodes)

that would provide some AC, which might help with projectile saturation( the second half of the first wave would have to first un-saturate the projectile, then saturate it the other way. obviously this would also mean high voltage, high current IGBTs or trigatrons.

you could try to use plasma as a projectile, but that would over-complicate things a lot. think plasma is only attracted to one pole of a magnet, (or the poles in general) wouldn't that give twice the effective length for acceleration with a given coil length? that would mean longer pulse width, easier (a little) switching, and the plasma cool factor.

if all that was wrong, i am sorry. at least you can't saturate an induction launcher( i don't think)

ramses

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