Barry's Coilguns

Estimating Kinetic Energy

We want to know the expected kinetic energy of the projectile to estimate the velocity of various projectiles.

Kinetic Energy

Photo of nails, coil and SCRNote that potential energy PE = CV2/2 (where C is farads, and V is voltage)

Also note kinetic energy KE = mv2/2 (where m is kilograms and v is meters/sec)

In coilguns, KE is related to PE by an efficiency factor 'e' defined as: e = (KE)/(PE)

Set the kinetic energy equal to the efficiency multiplied by potential energy, and solve for velocity:

Equations to solve for velocity from voltage and capacitance

Assuming typical values for this coilgun:

  • e = 0.01 - 0.02 (e.g. 1 - 2%) based on experience at lower energies
  • C = 12,000 uF
  • V = 100 - 190 volts
  • m = 0.02163 or 0.04301 kg (2 to 4 grams, e.g. mass of some known finishing nails
  • Note: 1 m/s = 3.281 fps

At minimum speed:

  • e = 0.01, C = 0.012 F, m = 0.04301 kg, V = 100v, which yields v = 5.2 m/s = 17.1 fps

At maximum speed:

  • e = 0.02, C = 0.012 F = 0.02163 kg, V = 190v, which yields v = 20.0 m/s = 65.7 fps
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