Electromagnetic Pistol: CS-P01A

Energy Requirements

The specifications require a muzzle energy of 4.0J which equates to a muzzle speed of about 25m/s using a solid test projectile. The dimensions of the energy source are limited by the general dimensions of the gun. After drawing up a basic layout I decided that the stage-1 capacitor dimensions would be limited to a length of about 100mm and a diameter of 70mm. The 2nd and 3rd stage capacitors, being required to produce much shorter pulse widths, would have smaller capacitance and hence smaller diameters. Capacitors generally come in 'standard' can sizes so this allowed me to draw up a 'short list' of possible voltage and capacitance combinations from a few manufacturer's product lines.

The actual stored energy required for each stage is based on the assumption that the efficiencies are going to be similar to those using a battery source. Looking at the battery driven accelerator performance we can see that the 1st, 2nd and 3rd stages have overall efficiencies of 2.7%, 11%, and 12% respectively. Although the current is under optical closed loop control we need to ensure that the pulse maintains a large current long enough to adequately influence the projectile. The pulse behaviour is governed by the circuit component values so Barry's RLC simulator was very useful in gauging the approximate behaviour of the system. The choice of capacitors for preliminary testing were as follows:

 Stage Capacitor Energy Stored Estimated Energy Transfer 1 63V / 33000uF 50J@55V 1.35J 2 100V / 6800uF 27J@90V 2.97J 3 100V / 6800uF 27J@90V 3.24J

Table 1. Approximate capacitor energy transfer.

Clearly the total estimated energy transfer (7.56 J) is almost double the specified value (4.0 J). This is partly to account for the uncertainties in the actual performance, but it also means that the muzzle energy/velocity could be quite a bit larger than I originally hoped for :).

Fig 1 shows the choice of capacitors for preliminary testing. Each capacitor is 105mm in length. The stage-1 capacitor has a diameter of 66mm and the others have diameters of 36mm.

Fig 1. Test capacitors.

The final choice for the capacitors is given in table 2. (actual energy in parentheses).

 Stage Capacitor Energy Stored Estimated Energy Transfer 1 100V / 22000uF 93J@92V 2.51 (2.14)J 2 100V / 4700uF 19J@92V 2.19 (1.77)J 3 100V / 4700uF 19J@92V 2.39 (1.83)J

From these data it is clear that the battery driven performance is a 'reasonable' indicator of the capacitor driven performance, given the simplicity of the analysis.