Firing a compulsator-powered rail gun is a delicate undertaking. The basic idea is to tap the power signal at the desired voltage. This can be performed in several ways, with the simplest being using a shaft encoder to sense the position of the rotor in the stator and extrapolate this to a voltage. In a properly designed compulsator/rail gun design, you would fire at or near peak signal voltage at all times. However, if mission parameters call for projectiles of various mass, g sensitivity, or melting points the ability to vary the output voltage by tapping the signal at various points in the waveform will be desirable.
As with a simple generator, the output across the compulsator terminals will be a voltage signal that varies in frequency according to the number of stator poles and shaft RPM. The more poles the compulsator has, the slower the shaft must spin for the output signal to reach a certain frequency. The rail gun is fired when a switching system closes the circuit between the compulsator output, the rails, and the projectile. The rail gun will continue to fire as long as the switch remains closed, the compulsator is spinning and powered, and the projectile is completing the circuit between the rails.
In practice, the compulsator output signal
frequency will have a much longer period than the time the projectile
is in between the rails. With a 4km/s muzzle velocity and 10m rails, the
projectile is in contact with the rails for 0.005 seconds. If your compulsator
has 8 poles and is spinning at 900 RPM, the output signal frequency will
be 60 Hz and a period of 0.017 seconds. In general a compulsator with
a greater number of poles is desired in order to reduce the required rotational
speed of the rotor assy. This is advantageous because of the size and
mass required of the compulsator to withstand the forces and energy dissipation
required of such a high power device. Thus the higher the required rotational
speed the more critical rotor balancing and bearing forces become.
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How can I switch the Rail Gun current?
Closing the circuit between the compulsator and the railgun itself is a design challenge unto itself. The "switch" must withstand all of the current that goes through the rails and projectile, which as we saw earlier can be very substantial. Basically, the switching mechanism MUST NOT weld itself closed. If it does, the pulses of current will continue to be sent through the circuit, possibly overheating your rails, thus warping, delaminating, melting, or destoying them. If the projectile has cleared the rails, this is of course of no consequence. However, as you will discover, working the bugs out of any system, especially a mega-joule rail gun, is a difficult task and you don't want your entire project to turn to melted slag if your projectile gets stuck in the rails.
This said, you should design each separate component of your rail gun to be self-reseting or self-arresting. This means that the switching mechanism should not rely on the projectile to clear the rails to turn off the gun, the compulsator should not continue to spin or discharge unchecked, etc.
The switching system must also introduce the minimum of electrical resistance to the completed circuit. Since the compulsator is a relatively low-voltage device (<1kV), all effort must be spent on keeping the resistance of the total circuit as close to zero as possible. In addition, the higher the resitance of the switch, the more heat it will generate and thus the faster it will destory itself. As you may imagine this is a tenuous balancing act.
The best sulution is good old solid state electronics. The SCR (silicon controlled rectifiers) is basically a diode that can be turned on and off. Designed for power systems, SCRs are available in ratings that will handle rail gun sized current pulses for short durations. By using a properly sized SCR or array of SCRs the compulsator power signal may be tapped using a a fixed or adjustable voltage sensing circuit that actuates the SCR at the desired point in the output wave. Proper heatsinking of the SCRs will determine their service life and rate of fire. As you may imagine, the SCR will introduce a voltage drop like any semi-conductor device.
Another consideration of the SCR, is the turn off time. Once an SCR is triggered "on" it does not stop conducting until the voltage signal across it drops below a threshold level. In an AC situation like our compulsator, this is where the voltage signal crosses zero. So the longest pulse an SCR can pass is one half waveform, roughly from zero to peak and back to zero. Therefore, if the projectile has not cleared the rails, you must wait a half cycle to pulse again, or use two SCRs in a triac type formation to pass the last half of the wave, zero to negative peak to zero. This will result in a current through the rails of opposite direction as the previous pulse, and you will have to "destroy" the field established by the previous pulse before building the new opposite sign field. This will introduce phase lag, increased inductive impedance, and other effects that may be less than optimal.
So, the point I am trying to make is, your compulsator and switching system should produce a pulse of sufficient duration and power to eject the projectile just as or just after the SCR shuts off. This balancing can be performed by rail length, peak voltage, pulse shaping, output frequency, etc.
Thanks go out to Ben for his contributions
to this section.
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How can I inject or load the projectile?
As mentioned in other areas of the site,
projectile/rail welding is a common problem in rail guns. In order to
avoid this, many designs call for a mechanical injection system to give
the projectile forward motion before it comes in contact with the rails
and currents. As you know, kinetic friction is less than static friction.
(think about four wheel drifting in your favorite automobile...)
One important consideration when designing an injection system is of course timing. Making sure that the projectile has entered the rails while the compulsator is tapped at the desired voltage and so forth is very important, considering that the compulsator may be tapped for a duration of less than 10 msec. It is for this reason that highest power rail guns do not use an injection system, but rather a simple breech loading procedure.
If an injection system is desired, there are a number of ways to "squirt" the projectile into the rail assy. You can use spring-loaded, centrifigual, compressed air, or any other method you can think of. The best method is most likely compressed air, which can provide a large force and thus higher injection speeds. With a projectile cross section of 2 square inches and 50 PSI air, you could use PVC as your material and generate 100 pounds of force on the projectile. With a 1 pound projectile, this would generate an acceleration of roughly 3 g's.
For timing, you could use an optical gate
to switch you SCRs as the projectile crosses the breech of the rails themselves.
This would allow fairly precise timing, as most SCRs have a fairly rapid
response time (on the order of a dozen or so msec). By the time the compulsator
is tapped the projectile would have traveled only a few inches.
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Page last updated 4/24/02
