This was the second coil tested, smaller than the first coil
tested by 13 turns.
Coil of 84 Turns
See the Projectiles page for details of these projectiles.
Measurements
The raw measurements are shown in red in the table
below, using horizontal ballistics measurements.
The symbol "" indicates that it was not attempted, and "dnf"
means "did not fire".
Projectile:
A
C
D
E
F
G
Length:
3.5"
2.5"
2"
1.75"
1.5"
1.25"
Mass:
4.301 g
2.163 g
1.464 g
0.8722 g
0.7345 g
0.4421 g
Potential
energy
(joules)
Charge
(volts)
Horiz
Distance
(inches)
Horiz
Distance
(inches)
Horiz
Distance
(inches)
Horiz
Distance
(inches)
Horiz
Distance
(inches)
Horiz
Distance
(inches)
0.6 J
10 v
dnf





2.4
20
40





5.4
30
69





9.6
40
96





15.0
50
115
144
155
157
159
190
Velocity
Again using the equations for horizontal ballistics and
the raw data, the velocity is calculated for each of the measurements above and is shown
in red.
Projectile:
A
C
D
E
F
G
Potential
energy
(joules)
Charge
(volts)
Velocity
(m/s)
Velocity
(m/s)
Velocity
(m/s)
Velocity
(m/s)
Velocity
(m/s)
Velocity
(m/s)
0.6 J
10 v
0.0





2.4
20
2.289





5.4
30
3.949





9.6
40
5.494





15.0
50
6.581
8.241
8.871
8.985
9.100
10.874
Efficiency
Calculate efficiency as shown in red.
Projectile:
A
C
D
E
F
G
Potential
energy
(joules)
Charge
(volts)
Efficiency
(%)
Efficiency
(%)
Efficiency
(%)
Efficiency
(%)
Efficiency
(%)
Efficiency
(%)
0.6 J
10 v
0.0 %





2.4
20
0.5





5.4
30
0.6





9.6
40
0.7





15.0
50
0.6
0.5
0.4
0.2
0.2
0.2
Graphical Results
Coil Analysis
This coil with 84 turns measured 6 ms halfcycle waveform (an oscilloscope
image is not available).
The 6 ms timing is a good result compared to predictions from using the approximate aircore
inductor model. This confirms our approach (applying a percentage change from aircore inductance
to an ironcore model) is useful.
How Does This Coil Need to Change?
The speeds and efficiencies are still very low. We wish to reduce the time from 6 ms by 33%,
to reach 4 ms discharge time.
We will again apply a percentage change from an aircore approximation to our ironcore physical
coil.
Using the Java Inductor Simulator again with values
of 12 AWG, ID=6mm, OD=60mm, len=16mm, we find that our inductance would be L = 118
uH for an aircore coil.
We want a 1/3 smaller LC time constant (i.e. 6 ms becomes 4 ms)
The new coil in air should therefore have L = 52.4 uH
The Java Inductor Sim suggests a 70 turn coil will have the desired inductance.
Therefore, removing 14 turns should result in a 4 ms discharge time.
Damping Resistor
Note the damping resistor will likely need to be changed. As the LC time constant becomes smaller,
the resistor also should be reduced to remain at the criticallydamped point. Without any resistor
change, the tuned circuit will be overdamped and the waveform is lower and longer than desired.
We also found an unexpected and fascinating behavior of this resistor's construction, which
required a slight redesign as described in the next page about damping
resistor construction.