Be aware that the equipment used can literally "splatter flesh and
bone," to quote Bill Beatty.
If that doesn't speak to you as a strong safety message, I don't know what will. Basically
my safety etiquette was simple: distance. Stay away from the capacitor, don't ever use a handheld
object to fire the gun- that means you are probably too close. Hearing protection and eye protection are
mandatory for those of us who like to hear and see things. Bleed the capacitor, and use a
shorting strap when it is not in use.
Download Entire Paper (Word97, zipped)
the theory of railgun operation. F is a force called the Lorenz
force that acts on moving charge (current, I)
in the presence of a magnetic field B.
This force is responsible for accelerating a conductive projectile between
the rails of the gun. There aren't many ways to get a usefully high
current of several tens or hundreds of kiloamps. Capacitor discharge
is perhaps the easiest and cheapest way.
Here's the experimental
setup in an early phase of testing. A 15 kV neon sign transformer
is used to charge a 50.1 microfarad, 30 kVDC Maxwell pulse capacitor from
Sales of Nebraska via a microwave-oven diode rectifier. A spark-gap
switch remotely fires the gun. The spark is initiated by a laboratory
The copy is not
too clear, but this is the circuit of both the gun and the charging supply.
The rectifiers and divider resistors were immersed in oil for insulation.
The voltmeter is analog (if you use digital equipment near a railgun, have
a wastebasket handy for the charred consequences) and can directly measure
up to 6 kV. Combined with the variac, the supply can charge the capacitor
nearly to its energy limit of about 20 kilojoules- remember that it will
charge to the peak voltage of the transformer AC cycle.
From this slide
you can see the operation of the gun from an energy standpoint: electric
potential energy is stored in the capacitor, and upon firing, is transferred
to the motion of the projectile. The effiency is defined as the ratio
of these two measured quantities as shown. So how do we measure these
Well, here was
an early attempt to measure projectile velocity. Two things are readily
apparent: even at 900 joules, aluminum projectiles break up and ignite.
Furthermore, the ballistic pendulum (a piece of PVC pipe with clay inside)
was not too good at catching the projectile and swinging up like an ideal
ballistic pendulum- and it tended to not stay in one piece either, especially
at higher energies. The ballistic pendulum is a good idea because
it allows us to look at the projectile's momentum indirectly. A small,
fast projectile is much too hard to keep track of by itself. But
the idea needed refinement.
Shown here is
the arrangement I ultimately used to measure the projectile's muzzle velocity.
A video camera with zoom feature can operate safely from a distance without
hazard of destructive EMP. The ballistic pendulum idea is still in
use, but in a slightly different form.
made with the above technique. Analyzing successive frames of the
video with software yields both the distance traveled by the "ballistic
bottle" between frames and the time between frames- exactly what we needed
to know to find V12 above.
Here are some
results using the above methods. One interesting experiment I did
was trying to improve gun efficiency by putting in an electrolyte (like
moist playdough) behind the projectile. My reasoning was that this
electrolyte would be exploded by the heavy current through it, providing
some initial momentum to the projectile. The most inefficient part
of the firing cycle is accelerating the projectile while at rest or very
low velocity. But as you can see, there's not a big difference in
Let's be honest,
this is why you came to this page- you wanted to see the thing actually
fire. This is a relatively low-energy firing with a styrofoam cup
placed in front of the muzzle. It's hard to see, and that's because
it's in a good many pieces which can be seen exiting the vicinity at high
velocity. The "puff" at right is light from the triggered spark gap,
which has just fired.
of more than ten kilojoules. Note how electromagnetic interference
has garbled the picture somewhat. The long white spray of hot ejecta
is passing through a styrofoam cup once again, whose remains can be seen
up above the spray. The triggered gap is visible at the right.
to Download my Paper (MS Word97, zipped, 50 kB)
Visit Guilford College in Greensboro,
NCUR (National Council for Undergraduate Research)
Surplus Sales of Nebraska, where you can purchase Maxwell capacitors at discount.
to Return Home
E-mail Carl Willis