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dan

Wed Mar 29 2006, 11:06PM

Registered Member #223 Joined: Mon Feb 20 2006, 06:42PM
Location: Ottawa, Ontario, Canada
Posts: 125

I plan to build a disk launcher that will launch a disk that is 6 13/32'' in diameter and just shy of a 1/2'' thick. The disk was machined from the bottom of an old aluminum nitrogen cylinder that failed its hydro test. There is a hole in the center where a shaft will be inserted so the disk can be spun up before it is launched for better stability. Is a 1/2'' too thick? I think thicker is probably better because more current can flow through it. (less resistance the thicker it is). However I think after a point that the additional thickness would just add more weight without any extra gain. I have not measured its weight yet but I have calculated it to be ~1.57LBS if my math is correct. And it feels about right.

I plan to use my 1050v @ 2250uf (2x3 grid of 1500uf @ 450v (525 surge) capacitors). This bank is probably way too small for a disk this large. Even pushing the capacitors voltage ratings to their surge rating the bank will only store 1.2kJ. I could reduce the voltage so I can parallel all the capacitors and add one of my larger caps to the bank (6*1500uf + 1*5600uf = 14,600uf @ 450v). I can push them to the 525v at which point the new bank would store ~2kJ. Which option would be better? 1050V@ 2250uf or 525V @ 14,600uf. I still think going for the 1050v @ 2250uf would be best since the higher voltage would allow for faster discharge time and probably much better efficiency.

The Coil will probably be 20 turns or so of 10awg wire. This is the largest wire I currently have available to me so maybe I should try paralleling two coils.

Switching will be rather difficult.. I fried my big 300A 3/4'' stud SCR

so I will have to resort to a high current relay or a triggered spark gap. Both of which will be really lossy. I have 1LBS of mercury metal so maybe I can make a high current tilt switch. There would be no contacts to ware out and switching times would probably be improved since there would be minimal arcing. Although I'm not sure how this liquid conductor will act from the magnetic field generated by the huge currents flowing through it. (Iâ€™m betting it would explode..)

edit: assuming a 5% efficiency (probably a bit high) 1.2kJ would translate to 60J to the disk..
so:
V = sqrt(E/(0.5m))
where:
E = energy into object
m = mass in kilograms
V = Velocity in m/s

V = sqrt(60J/(0.5*0.712Kg))
V =~ 13 m/s

Which is about 47km/h

Rather disappointing actually. I could probably throw it faster. (Unless I made an error rearranging the formula to isolate for V.) Then again I still probably wouldn't want to get hit by a pound and an half of aluminum traveling at 47km/h.

Any tips or corrections of my design would be greatly appreciated.

FastMHz

Wed Mar 29 2006, 11:19PM

Registered Member #179 Joined: Thu Feb 16 2006, 02:08AM
Location: Hagerstown, Maryland - Close to Prime Outlets
Posts: 287

That's a mighty sized platter there...I'm about to cut out a 1/4" thick one to fire on my setup to see what difference it makes. But I don't think 1.2kj is enough to move something that heavy really fast...I've gotten near 10% efficiencies on my rig here. But that shouldn't stop you from trying and see what happens

Quantum Singularity

Thu Mar 30 2006, 12:36AM

Registered Member #158 Joined: Sun Feb 12 2006, 09:53PM
Location: Central Ohio
Posts: 282

Yeah thats pretty darn heavy! But I would try it none the less. You can easily place other objects on the coil so its not limited to just that one heavy disk. FastMHZ has shown the thicker disks gave him better efficiency. I think 1/2" thick is past the point of diminishing returns though. That all being said I launched a similarly weighted projectile with only 1/8" thick platter with ~1.5kJ and it went up pretty high. Maybe 50 feet I dont really remember. That was with a spark gap / discharge rod trigger and a 10kV 120uF cap. ~5kV charge on the cap. Coil was 7 turns of #8 wire from Lowes or Home Depot. Didnt really tune the coil, didnt see any performance difference from changing it somewhat drasticaly. I was using a 5" hard drive platter, the extra weight was from a piece of 2x4 wood I set atop the disk. Was probably a little less than a foot long. Not sure how much it weighed but I am guessing around one pound.

It didnt go quite as high but it was just as fun seing it launch that much weight. Unless you have set at certain goal it shouldnt really matter if it doesnt go super fast, so long as it works.

Here is a pic (sorry for the poor setting - its from a long while ago). You can see the disk warped around the contour of the 2x4 that was placed on it. It also indented the 2x4 in the shape of a half circle on each side were the disk slapped the side of the 2x4, although thats not all that clearly visible. Good luck, have fun, and be safe!

ragnar

Thu Mar 30 2006, 06:00AM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

If you have to sacrifice your principles and the thick disc is too hard, try spinning up hard drive platters to a few thousand RPM then thwacking them with a tennis raquet. Very entertaining splatting them across a field and trying to catch.. they tend to bounce off everything and keep flying, stabilised of course like a gyroscope.

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