Page 32 - 15 Dangerously Mad Projects for the Evil Genius

Chapter 1 ■ Coil Gun 13

To do this, we can use the formula:

2
E (mv )/2
Where m is the mass of the projectile in
kilograms, and v is the velocity in meters per
second.
My projectiles weigh approximately 0.3 g and
the best velocity I got was 15m/s. This velocity is
actually the average velocity of the projectile’s
flight to the test target, which will be a little less
than the muzzle velocity, but it is probably close
enough not to matter much.
So, the energy transferred from the capacitors to
Figure 1-23 Close-up of the end stop
the projectile is:
(0.0003 15 15)/2, or 0.033
Theory This means that the efficiency of our coil gun is:

0.033/23, or 0.14 percent
Coil guns are remarkably inefficient. You are doing
well if 1 or 2 percent of the energy in the Not brilliant. So where does all the energy go?
capacitors is converted into kinetic energy of the Some of the energy is lost because the projectile
projectile. If you can weigh your projectile, or is first pulled into the coil, and then if the magnetic
perhaps weigh a few of the nails it is made from, field has not disappeared before it passes the center
and then do the math, you can calculate the of the coil, it will be pulled back into the coil,
efficiency. slowing it down. The answer to this is to shorten
The energy stored in the capacitor is calculated the pulse, which can be done in two ways:
by the formula: ■ Use fewer capacitors (but this will reduce the
2
E (CV )/2 input energy).
Where E is the energy in joules, C is the ■ Use less turns of wire for the coil (but this will
capacitance in farads, and V is the voltage. increase the maximum current, potentially
destroying the SCR).
So, for our arrangement of capacitors, the
energy available for each firing is: The geometry of the coil and the type and size
of the projectile, as well as the material it is made
(0.0376 352) / 2 23 joules
from, all affect performance. Getting the most out
This is similar in energy to the kinetic energy in
of your coil gun is very much a matter of trial and
a .177 air gun. So if we had 100 percent efficiency,
error and reading about other people’s trial and
our coil gun would be really quite dangerous.
error.
So, now that we know that the “input” energy is
The Internet offers some great resources on this.
23 joules, let’s see how many joules of kinetic
The Wikipedia entry for coil guns is a great
energy there are in the projectile.
starting point, as is www.coilgun.info.