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4hv.org :: Forums :: Electromagnetic Projectile Accelerators
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using barrys calcs

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JoeDarkon
Sun May 27 2007, 05:17PM Print
JoeDarkon Registered Member #485 Joined: Sun Jul 09 2006, 01:14AM
Location: San Francisco
Posts: 36
i am trying to use barrys RLC calc... he says

"Your goal in this little game is to find a combination of values which result in maximum peak coil current, and which also dies away to near zero in about the time it takes the projectile to pass the mid-point of your coil"

my question is how do i figure out how long it takes for my projectile to reach the midpoint
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Simon
Mon May 28 2007, 12:29AM
Simon Registered Member #32 Joined: Sat Feb 04 2006, 08:58AM
Location: Australia
Posts: 549
Unfortunately (or fortunately, depending on your view on coilgunning), there's no simple equation that can give you the answer.

Anything you can realistically do is a ballpark guess.

You can guess by researching other coilguns.

You can guess by estimating the muzzle velocity of your projectile and working out how long it'll take to reach the midpoint of the coil at half that velocity. (You can estimate the velocity by guessing the efficiency - say, something between 1% and 10%.)

You can can try Bill's models, which are sophisticated and impressive. Remember, though, that coilgun modelling is an extremely slippery business.
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Barry
Sat Jul 28 2007, 05:46PM
Barry Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301
JoeDarkon wrote ...

my question is how do i figure out how long it takes for my projectile to reach the midpoint

The trick here is to estimate the time in the barrel. Here is an extremely crude (but useful) approximation.

Let's assume the projectile is under constant acceleration, so we can start with the classic equation: d = 1/2 a t^2

Re-arrange it to solve for time, and also substitute in a = v/t.

As a result: t = 2d/v , where t = time in seconds, d = distance, v=exit velocity

For some examples using a two-inch coil:
  • At 50 fps: t = (2)(0.0508 m)/(15.24 m/s) = 6.67 ms
  • At 200 fps: t = (2)(0.0508 m)/(60.96 m/s) = 1.67 ms

Hopefully this will get you started. ~~Barry~~
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