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Ring launcher optimum oscillator frequency

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MRMILSTAR

Fri Feb 15 2019, 04:11PM

Registered Member #62119 Joined: Sun Feb 04 2018, 04:59AM
Location: Cedar Rapids, Iowa
Posts: 136

I am going to build a ring launcher. I want to re-use the pulse capacitor on my quarter shrinker and other ancillary circuitry to avoid replicating equipment. The capacitor is huge and weighs about 180 pounds. It is an Aerovox pulse capacitor measuring 100 uF at a maximum of 22 KV. Fully charged, this monster can output 24 KJ. Obviously for my ring launcher I am not going to come any where close to fully charging this capacitor to avoid total destruction of the ring launcher. Maybe a few KV.

Given that I would like to re-use this capacitor and my other ancillary quarter shrinker circuitry, what would be the "best" oscillation frequency to use when triggered? This frequency will obviously determine the number of turns required for the launching inductor. Is there an optimum oscillation frequency? The primary projectile I will be launching will probably be old hard drive platters.

klugesmith

Fri Feb 15 2019, 11:25PM

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

From the design frequency, you want to figure the inductance and thus turns count, right?
Why not start by making 2 coils with substantially different turns count, and compare the launching performance? That will guide you toward an optimal design.

I would explore the turns-count sensitivity, and optimum value, at a low stored-energy setting. Tens or hundreds of volts. Measure ultimate height of platter in inches, instead of measuring size of crater in your ceiling.

Then crank up the energy, using your assortment of coils, and see how the optimum frequency shifts.

The system is basically linear, at a fixed frequency and damping ratio, with output exactly proportional to input. (Frequency and damping ratio with and without platter will be different). It begins to go nonlinear when platter moves substantially, or conductors become very hot, during the main part of the current pulse. I bet the geometry change will kick in long before the temperature coefficient effect, unless you have the platter physically driving a brick or something.

Did you ever tell us about your quarter shrinker, beyond the planning stage?

MRMILSTAR

Sat Feb 16 2019, 04:20AM

Registered Member #62119 Joined: Sun Feb 04 2018, 04:59AM
Location: Cedar Rapids, Iowa
Posts: 136

I am in the process of building my quarter shrinker now. It is my winter project. I have most of the parts. I am currently working on the high energy switch which is the most difficult item to build to handle this amount of energy safely and reliably. This will be the most dangerous device that I have built with the capability of being charged to 24 KJ at its maximum charge voltage of 22 KV. However I never expect to use that amount of energy. It would obliterate the coin.

I am asking about the ring launcher now so that I can easily incorporate it into the quarter shrinker architecture in the future. I may have to use a direct-contact type of switch for the lower-energy ring launcher because the minimum separation between electrodes for the quarter shrinker switch will be about 1 mm to prevent the electrodes from welding together. This is acceptable for the quarter shrinker because it will be operating at greater than 6 KV. I cannot get reliable firing at only a few kilovolts for the ring launcher with an air gap switch.

3 KV would correspond to 450 joules with my capacitor. If the ring launcher coil can take this without breaking up, using 3 KV/mm as the dielectric strength of air for flat plates, I may be able to use the same air gap switch for the quarter shrinker and ring launcher. I have seen ring launchers with 1500 joules and the coil stayed intact, but these all seem to have been built with electrolytic capacitors which don't have a high discharge rate. Since I am using an Aerovox energy discharge capacitor, which has an extremely fast discharge time, I don't think I can make reliable performance comparisons between a launcher using electrolytic capacitors and one built with an energy discharge capacitor.

By the way, this ring launcher will be operated outdoors as will the quarter shrinker.

Sulaiman

Sat Feb 16 2019, 10:52AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

A thin flat disk sounds very un-aerodynamic,
short lengths of copper pipe/tube may be more suitable for launching ?

MRMILSTAR

Sat Feb 16 2019, 03:57PM

Registered Member #62119 Joined: Sun Feb 04 2018, 04:59AM
Location: Cedar Rapids, Iowa
Posts: 136

True enough that the flat disk has terrible aerodynamics. For high-power launchers, this may be an advantage. I have seen videos of high-power launchers shooting these disks upwards of 50 feet even with the terrible aerodynamics. Since they are flat, they tumble when coming down which slows their speed. I really don't want anything dangerous coming down on my or anyone else's head. The disk drive platters are made of aluminum and are readily available and I have a big stack of them. Being 5 inches in diameter, they are also pretty visible in the air especially if I paint them fluorescent orange or green. I can always try a different projectile later if I can figure out a way to make them somewhat safe when falling.

Sulaiman

Sat Feb 16 2019, 07:55PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I mentioned the aerodyamics because the projectile size and geometry affects launcher design,
but you are right, I had not considered the projectile return, (and visibility),
hdd platters are a much better choice than I realised..

So, I guess that my idea of using razor blades as fins is also not very useful ;(

klugesmith

Sun Feb 17 2019, 04:21AM

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

No need to make a direct-contact switch.

The firing switch for my can crusher has shiny spherical electrode faces (approx 1 cm radius of curvature). Minimum separation is set by a limit screw, so they could be allowed to touch. But a finite minimum air gap works fine at 1000 volts, which is 26 joules on my capacitor and would be 50 joules on yours, and can visibly indent a can (barely). When the open-circuit voltage is higher, the switch fires earlier (no need to adjust the minimum gap). My moving contact is attached to the short arm of a lever. The long arm is moved by pulling a lanyard made of non-conductive string.

I hope you are not planning to waste effort on some kind of electrically triggered spark gap, even for the quarter shrinker. More complication = less safety and reliability. Bert Hickman uses a mechanically operated spark gap. I think the Hackerbot guys used a string to remove a block that held a slightly glorified clothespin open. An unenclosed spark-gap switch can make a firecracker-like bang when it goes off.

Back to frequency -- why don't you start by figuring a lower bound, by yourself? What's the time scale for platter departing from coil, in the 1500 joule launcher you mentioned? How does that compare to the first cycle of damped oscillation, for some hypothetical coils with various inductances and realistic values for resistance? You could use Barry's Coilgun Calculator. To keep it simple, omit the inductively coupled platter from the effective L and R model.

MRMILSTAR

Sun Feb 17 2019, 04:39AM

Registered Member #62119 Joined: Sun Feb 04 2018, 04:59AM
Location: Cedar Rapids, Iowa
Posts: 136

I am sticking with "simple". I am basing my spark gap switch on Bert Hickman's design. I have been in communication with him. With careful construction, my objective is to allow the two electrodes to get as close as 1 mm while maintaining good parallelism of the electrode faces. This will allow me to trigger at fairly low voltages and maintain even wear across the electrode faces. I am also using simple fool-proof and safe string lanyards to operate the various switches.

I am going to test my flat-plate spark switch to see how low a voltage I can reliably trigger. Maybe I won't need a separate switch for the low voltage ring launcher. If that doesn't work out, I may have to experiment with your ball bearing spark gap switch for low voltage operation.

I did some computations today to figure out how thick my conductors need to be for the quarter shrinker. Using my 100 uF capacitor and assuming a 10-turn, 1 inch diameter work coil, the resonant frequency came out to be about 8500 Hz. The skin depth for copper at that frequency came out to be about 0.7 mm. This tells me that my conductors have to be at least 1.4 mm in thickness, assuming flat-bar conductors.

Sulaiman

Sun Feb 17 2019, 11:20AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

two tiny details;

if skin depth os 0.7mm then you need at least 3x 0.7 = 2.1mm thickness for the approximations to be valid.

'string' to me implies multi-strand which can retain (electrically conductive) moisture,
use mono-filament fishing line or similar.

Do you already have earplugs and ear defenders (and goggles/face shield just in case..)

MRMILSTAR

Sun Feb 17 2019, 03:45PM

Registered Member #62119 Joined: Sun Feb 04 2018, 04:59AM
Location: Cedar Rapids, Iowa
Posts: 136

I realize this sounds like a shotgun going off so, yes, ear protection will be mandatory. Good suggestion about the mono-filament line and skin depth.

I am still pondering the blast cage. Initially I was going to use a 12" x 12" x 12" section of of 1/4" thick steel square tubing, however this weighs about 40 pounds and is conductive which complicates bus bar entry and exit points. This might be overkill. I am also considering building a blast cage from 1/4" thick G10 joined at the corners by angle iron. This would be a lot lighter and is non-conductive. It would also be easier to cut. I have seen blast cages made from thinner aluminum for shrinkers with similar power and that seems to hold up. Any thoughts about the blast cage?

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