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Lem0nHead

Mon Apr 27 2009, 03:30AM

Registered Member #2096 Joined: Mon Apr 27 2009, 03:08AM
Location:
Posts: 18

hello
I want to throw a tennis ball about 4m away (first kick) launching it from 30Âº inclination
if my calculation is not wrong, I need the ball (which weights about 60g) to be throw around 7m/s... what means around 1500J (in a 45Âº inclination 1100J may be enough)
but I guess I'd need overkill capacitors, wouldn't I? anyone can make a guess based on prior experiences?

maybe the coil gun principle wouldn't be the most efficient for doing that? (another option I had would be to use motor+gear reduction to pull a spring, that would be way more straight-forward, but way less cool :))

another thing I was considering would be to (since I'm not throwing the projectile, just making it hit something and come back by gravity), I could use a electromagnet + permanent magnet
something like

[electromagnet] [magnet] (very close)
then I turn the electromagnet on repelling the magnet and maybe I get a good impact, but since it's 45Âº inclined, the gravity would be my biggest enemy (as always)

[Edit: Double post]

...

Mon Apr 27 2009, 03:54AM

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

Double check your units, a joule is a kg*m^2s^-2, so you only need about 1.5j to do what you want which should make your design a fair bit easier. I would use a solenoid with a heavy rod (maybe 3" long by 1" diameter) that gets sucked through the coil to launch the ball (make sure to have some kind of stop to keep the rod in the launcher!)

Weston

Mon Apr 27 2009, 03:58AM

Registered Member #1316 Joined: Thu Feb 14 2008, 03:35AM
Location: Cambridge, MA
Posts: 365

Coil guns are only about 5% efficient at maximum. That means you would need a 45000000J capacitor bank (correct me if I am wrong). A motor/spring system or a compressed air launcher (like a potato gun) would be much more viable option.

Lem0nHead

Mon Apr 27 2009, 04:24AM

Registered Member #2096 Joined: Mon Apr 27 2009, 03:08AM
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Posts: 18

oh, that really makes way more sense
so 25J probably should be enough

I'm thinking about a 50g projectile (little less than the tennis ball) rod, similar to what you said
supposing projectile density around 5g/cm3, I can get it with a 1cm diameter x 3cm length or 0,5cm and 6cm length, for example
anyone knows if it makes a difference on the efficiency?

I guess longer projectiles makes it easier, because I don't risk losing acceleration when it reaches the center of the coil

Shaun

Mon Apr 27 2009, 07:21AM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

I think this project should be very simple. You need barely any energy at all, in fact it could probably be done using a bunch of camera flash caps and a toggle switch to fire the bank.

If you choose to buy one mid-size capacitor and SCR off Ebay (say ~400V 2500uF), even a poorly designed system will be more than capable of doing what you want. You may even have trouble keeping it from going too far! These parts should be very cheap too.

Lem0nHead

Mon Apr 27 2009, 08:48AM

Registered Member #2096 Joined: Mon Apr 27 2009, 03:08AM
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Posts: 18

I'm in doubt if I make it portable or not
using a sealed battery would make design simpler to charge the capacitor (and, of course, has the advantage of the portability)
I can find a small/cheap battery with 12V, 7Ah
any ideas on how to find out how many shoots it can make?

also, from what I was reading about other projects, it seems 18 AWT wire, 30mm length and around 5 layers should be fine for this coil... I was also thinking about a 470uF @ 250V (30J)

does it seems efficient?

hotcrazyfruit

Mon Apr 27 2009, 11:41AM

banned on 5/26/2009
Registered Member #1877 Joined: Mon Dec 22 2008, 02:03AM
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Posts: 147

Weston wrote ...

Coil guns are only about 5% efficient at maximum. That means you would need a 45000000J capacitor bank (correct me if I am wrong). A motor/spring system or a compressed air launcher (like a potato gun) would be much more viable option.

considering thats half the power the navy has for their rail gun, yea its likely wrong. but lemonhead made a mistake in his initial calculation anyhow, which is probably what made your energy so high.
[edit] coil guns aren't very efficient at all. compressed air, or even a mechanical system would be more efficient.

Sulaiman

Mon Apr 27 2009, 07:53PM

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

If you're going to use a 12V 7AH battery I suggest you consider a low resistance coil switched by a 'beefy' transistor .. directly from the battery.
(no inverter/high voltage/capacitor)

Put a big fat diode across the coil (cathode towards battery positive) to
- prevent overvolting the transistor due to inductive 'kickback'
- increase efficiency (short ON pulse)

polop

Tue Apr 28 2009, 05:08PM

Registered Member #1537 Joined: Thu Jun 12 2008, 06:44PM
Location:
Posts: 51

if you are going with a direct battery idea i would highly recommend the use of lipo batteries small and light a decent sized 4 cell battery is capable of supplying 12v (volts dropped from 16.8) at over 100 amps for short periods, using them in this way is unlikely to damage them though lead acid batteries would not like the abuse. look at hobbycity for cheap lipos and chargers.

Plasma Lover

Tue Apr 28 2009, 06:10PM

Registered Member #1911 Joined: Mon Jan 05 2009, 06:30PM
Location: Salem, Oregon, USA
Posts: 165

polop wrote ...

if you are going with a direct battery idea i would highly recommend the use of lipo batteries small and light a decent sized 4 cell battery is capable of supplying 12v (volts dropped from 16.8) at over 100 amps for short periods, using them in this way is unlikely to damage them though lead acid batteries would not like the abuse. look at hobbycity for cheap lipos and chargers.

The unlikely to damage them part worries me. I've heard horror stories about the lithium polymer cells. If you do decide to use these, be _EXTRA_ careful.

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