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"Searching for Higher Efficiency"

The images below are taken from my viewgraphs for a presentation to the National
Council for Undergraduate Research (NCUR) 2000.  Although higher efficiencies were
obtained subsequent to the paper's submission, the problem remains of how to make this
simple linear DC motor more efficient.  But I did learn a great deal from the project, which
was my first formal and funded research at Guilford College.  I was able to use a simple method
and simple physics to find the data, despite complications like EMP and a slow purchasing
office (oops, I didn't say that!)

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.

Slide Show                       Download Entire Paper (Word97, zipped)                    Links

Here's 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 Surplus Sales of Nebraska via a microwave-oven diode rectifier.  A spark-gap switch remotely fires the gun.  The spark is initiated by a laboratory Oudin coil.

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 numbers?...

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.

A photograph 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 results.

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.

A high-energy firing 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.

Click Here to Download my Paper (MS Word97, zipped, 50 kB)


Visit Guilford College in Greensboro, North Carolina
NCUR (National Council for Undergraduate Research)
Surplus Sales of Nebraska, where you can purchase Maxwell capacitors at discount.

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