Launch Position Effect

 

This set of experiments looks at how the launch start position affects the muzzle velocity and corresponding efficiency.

Firstly let us define the 'launch position '. Refering to fig 1 we define the launch position as the distance between the leading face of the projectile and the rear face of the coil. The sign convention for delta is such that when the leading face of the projectile is outside the rear of the coil, delta is negative.

Fig 1.

The energy source used in these experiments comprises several different capacitors as detailed in table 1. Note that the third capacitor in these experiments consists of two of the 6,800 uF capacitors connected in parallel. The coil employed throughout the tests is detailed in table 2. The test setup is shown diagrammatically in fig 2. The delta-peak trigger uses the optical sensor to control the turn off point while allowing the launch position to be varied. The power MOSFET module consists of eight IRF3415 devices connected in parallel giving a pulsed current capability of around 800 A (based on a half-sine pulse of ~4 ms). These are switched via an MC34152 MOSFET driver. A 10 m current sensor is connected in series with the discharge current path allowing the coil current to be recorded up to the point of turn off.

 

Fig 2. General test setup.

 

It should be noted that the positioning of the current sensor causes a reduction in the MOSFET G-S voltage proportional to the sensor current (10_mV/A). The nominal G-S drive voltage is therefore set high enough such that the minimum G-S voltage is at least 10 V. The MC34152 can be operated with supply voltages up to 18 V. The sensor current isn't a parameter of particular interest, but the current traces do provide information on the pulse width. The reason for placing the current sensor in this location is that 'accidents' are less likely when the measuring equipement is connected to the system ground potential. The current sensor also reduces the maxumum possible efficiency since it comprises a significant fraction of the overall circuit resistance.

 

Manufacturer
Panasonic
BHC-Aerovox
BHC-Aerovox
Type
GAA
ALS-40
ALS-30
Can Size
77 Dia x 106
66 Dia x 106
36 Dia x 106
Rated Voltage (V)
50
63
100
Capacitance (F)
150,000 u
33,000 u
6,800 u
ESR (100 Hz) ()
4.7 m
8 m
23 m

Table 1. Capacitors used in tests.

 

Length
Ro
Ri
Turns
Res
Ind
27
18
7
1.8
~80
~45 m
~76 uH

Table 2. Coil Parameters.

 

The natural response of the system for each capacitor is shown in fig 3.

 

Fig 3. RLC behaviour with different capacitors.

 

Results:

150,000 uF

33,000 uF

13600 uF

Conclusions

 

Last Modified: 13 Feb 2004

 

 

  
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