PowerLabs ElectroThermal Gun!

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Project Introduction:

Electro-Thermal, or Electro-Thermal-Chemical guns (ET and ETC guns, respectively), along with Electro-Thermal Ignition (ETI) guns (these the subject of a later study) are not a novel idea, nor are they complex: ET/ETC guns (from here on termed �ET� guns on this page) operate on the very same basic underlying principles of the simplest and oldest firearms; pressure is produced inside a sealed barrel, and this pressure is used to propel a projectile out of the barrel at high speeds. The modern twist employed here is that, by placing the energy source outside of the barrel, a much finer control of the propellant pressure achieved can be obtained. This opens up new possibilities that were up until now unexplored, such as the ability to �dial in� to a very high degree of precision whatever muzzle velocity or energy is desired. Also, and more interestingly, by utilizing an external power supply to create propellant pressure, ET guns are not bound by the limitations of conventional propellants; energies and velocities can be as high as the structure of the gun will allow; as long as more power is introduced, the projectile will never outrun the propellant burn rate, as occurs with conventional propellant firearms at speeds of around 5000fps.

My particular motivation for starting this research is my ongoing quest for more knowledge in what is currently my favorite research topic; electromagnetic propulsion. As with the railgun research, I am not looking at developing a weapon but rather trying to improve on this technology so that it may become more useful for all of the potential applications it has.

Project Description and Goals:

The objective of this research is to:

1- Produce a compact and reliable .50cal (1/2�, 11.22mm) caliber electro thermal gun also capable of operating in Electro Thermal Chemical regimen.

2- Quantify gun performance in terms of muzzle velocity and kinetic energy for different types of projectiles in both ET and ETC regimen, calculate energy conversion efficiency from electrical power supply, and compare it with a similar conventional propellant gun.

3- Optimize gun design with the ultimate goal of surpassing the conventional propellant gun in muzzle velocity achieved.

Theory (a simplified overview):

Looking down the ETC barrel and seeing the NGK 3 prong spark plug.

Under Construction The basic design of an ET/ETC Gun consists of a high voltage pulse discharge power supply, most often a capacitor bank, that is connected to the gun via a switch capable of handling the very high currents seen during the discharge. The gun itself is made up o a barrel with a central electrode surrounded by insulating material at the breech.
A charge, either water, or a water/aluminum slurry, is turned into a plasma via an electrical discharge between the insulator and the barrel material. The plasma generated produces propellant pressure that pushes the projectile out of the barrel.
Increasing the breech energy will increase the peak temperature of the plasma and thus the pressures generated. In the case of the ElectroThermal Chemical gun, chemical energy is released when the aluminum powder oxidizes in the presence of water. This energy is in the order of 400J/gram.

Barrel Design:

ET/ETC Gun Barrels serve the exact same purpose as conventional gun barrels; they hold back pressure generated at the breech, and allow that pressure to guide a projectile down the length of the barrel. Due to the much higher temperatures generated at the breech, Electrothermal guns are a lot more demanding on barrels, and barrels tend to last a lot fewer shots. The main problem is erosion, followed by the structural limitations of the material, which can easily be exceeded once the breech energy levels become substantial.

Specifications: These barrels are made of Heat Trated Cromium Molybdenum steel (4140alloy). They are heat treated for maximum hardness. The barrels have a 1" outer diameter and are bored out to 50 calibre (1/2" inner diameter). The design operating pressure is 60 000PSI.

The barrels are treated with a special bluing solution (a form of oxide coating to protect the metal) and then varnished on top of the bluing. Corrosion resistance is important since the ETC gun will be constantly exposed to electrolytes.

Here are the finished barrels (blued and varnished). I used the 1/2 dia. bolt below as a test for my bluing method. I read on numerous gunsmithing sites that DIY bluing was not worthwhile; I managed the results pictured here with a $4.00 bottle of bluing solution and about an hour's worth of cleaning and polishing. Not bad...

Setup:

The current setup is embarrassingly primitive, but it works. Since I am not conducting this research at the University or with any external funding, everything is as simple and inexpensive as possible, while still being safe, of course - roulette-game.org.

A 12kV 60MA Neon Sign Transformer is rectified by a 100kV X-Ray transformer full wave oil immersed rectifier and then used to charge a 20kV 10uF Aerovox oil filled pulse capacitor. The capacitor is then discharged into the gun through a solenoid actuated ball gap. 12 gauge 40kV high voltage silicone wiring is used for the charging connections and 8 and 4 gauge oxygen free copper cable is used for the gun connections. A high voltage probe is used to monitor charging.

Electrode Design:

Currently automotive plugs are being employed. This greatly simplifies the design, even though it is non optimal. Automotive spark plugs are designed to withstand combustion temperatures of 1600F maximum at pressures of around 1000PSI. The ETC gun was designed to withstand pressures of up to 60 000PSI and it is estimated from other works that the plasma temperature is in the 30000 - 50000F range.
In the future a proper electrode will be designed.
Specs on the plugs are: Non resistive, copper cored, 14mm thread.

The center plug has been fired and has had its center section (pillar insulator) pulverized as a result. I am currently attempting to fill the gap with epoxy so as to avoid these problems in the future.

Results:

The ceramic at the tip is gone, as is the back.

At low energies the spark plugs stay together and produce a sharp crack and a jet of mist that will split thin wooden boards in half. At higher energies the spark plugs explode in half.
No attempts have been made yet to fire projectiles or to increase energy past 1kilojoule.

Phase 2:

Here are some renderings of my newest design for an ElectroThermal Gun. Due to the constant failures, even at low energies, of the spark plugs, I have decided to design my own breech insulator and center electrode. The center electrode is now 1/4" thick and made of Molybdenum, and the insulator is attached in such a way as to require a calculated 138,000PSI to blow apart.

Energy Source:

This gun will be powered by a proper energy source: initially, a single self-clearing ICAR High Energy Density (HED) Biaxially-oriented Polypropylene (BOPP) Energy Storage Capacitor, rated at 12kV, 267uF. At full charge, this unit stores an impressive 19.2kJ; almost as much as the entire capacitor bank powering all previous PowerLabs railguns. Even more impressive: the capacitor only measures 14 x 9 x 5.5", and weights 36lbs! With an energy density of 1.7J/cc, this is the state-of-the-art for pulse rated energy storage, and it is quite a priviledge to have one of these available for research (just don't ask me how I got it ;)

ICAR Capacitor Internals

Power Switching:

Along with the research-grade High Voltage Pulse Capacitor, this ETC gun will also be utilizing a high quality power switch, designed to safely, quietly, and efficiently switch the energy from the pulse capacitor into the breech of the gun. This particular unit is a General Electric TZ7512. Specs as follows:
Minimum Voltage: 300V
Maximum Voltage: 40kV
Current: 50kA
Charge transfer: 0.7Coulombs/shot
Firing time: <1uS

It is triggered by a 5kV pulse from a 1uF capacitor at a current not to exceed 350A.

I will, of course, be grossly over-driving the tube. In literature it has been tested to 100,000Amps for several dozen shots, so I expect it to last the entire initial phase of testing.

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