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EVBEC stands for "Energy & Volume Based Estimator for Combustions", and is intended to calculate the muzzle velocity of combustion launchers. The latest version of the spreadsheet (V 1.5) can be downloaded from spudfiles here A web-based version written in javascript, which is significantly more user friendly, is hosted here.

Physics model

EVBEC assumes the following is true:

  • Energy produced from combustion is directly proportional to chamber volume TIMES the chamber pressure in absolute terms.
  • The portion of this energy utilized in a launcher is directly related to the Chamber to barrel ratio.
  • The latke tests are accurate.
  • The amount of energy required to displace the air in the barrel is proportional to the atmospheric pressure*volume.
  • The amount of energy 'stolen' by the velocity of the gases originally in the barrel can be found by mass*(average velocity), and that this can be modeled as adding their effective mass to the projectile's mass.
  • The amount of energy 'stolen' by the velocity of the propellant gases can be is equal to their masses times their average velocities at projectile exit, and that this theft can be modeled as adding their effective mass to the projectile's mass.

To calculate the velocities, EVBEC uses a 2 step process: 0) Take the muzzle velocity of the round found in the latke C:B test at the same ratio
1) We find the energy of the test projectile, add the energy required to displace the air in the barrel, multiply this by the compression ratio, subtract the displacement energy, and convert it back into a velocity.
2) We find the ratio of the modeled launcher's chamber size to the test launcher's, multiply this by the ratio of the test launcher's projectile mass (including effective propellant masses) to the test launcher's projectile mass (including their effective propellant masses).

differences between spreadsheet and javascript versions

In the spreadsheet, the velocities are calculated for all the ratios and barrel diameters tested by latke, and the proper ratio to use to find the velocity is indicated on a graph. This uses the actual dataset.

In the javascript version, the velocity is calculated using a curve fit of the 3/4" data, and the velocity is output as a number in a cell.

limits of the physics model

The physics model does not account for the following:

  • Differing burn times
  • Differing amounts of heat loss
  • Evaporation of tater juices
  • Differing amounts of friction
  • Variations in efficiency caused by different projectile masses. (which is probably why the 1.5" data is "out there")
  • Odd effects caused by launcher design (ie; breach restrictions).


Generic directions for use are provided in the documentation of the spreadsheet, and a detailed article on modeling combustion spudguns, mostly with EVBEC spreadsheet, is provided here. The javascript version has many tooltips and is fairly self explanatory.