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ETAv2.1 High Velocity Launcher

Built a miniature cannon? Then post it here! This section is for completed, finished cannons that you have built. Please include pictures and information.
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Unread postAuthor: Fnord » Wed Sep 15, 2010 2:05 pm

I just have one question: What should I use to drive the wedge into the switch? An ETG, perhaps?


No, use a LGG to drive the wedge. The LGG should use a 200x propane-air mix since we're not allowed to discuss BP :)


I'm not sure If you've considered this, but I think it might be worth a shot.

Make a similar design, with a fixed flat-plate contact and a (heavy)spring-loaded moving contact. Then immerse the entire thing in mineral oil. The oil will stop the arcing until there is almost no space between the contacts. Chances are you can get the switch closed before the pulse ends with this method. The impact+arcing should drive out enough of the oil for a solid connection to be formed.
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Unread postAuthor: DYI » Wed Sep 15, 2010 7:58 pm

The dielectric strength of oil is only about 3-5 times that of air. While the speed required would definitely be more reasonable, it would still be very high, I'm guessing.

The main information of interest here is how fast dielectric breakdown occurs. If you can find some reliable source for that, a discussion on such switching methods would be productive.

As it is, the switch works very well, the arm (which is very inexpensive) will probably last for over a hundred shots, and I'm unlikely to change the basic design in the near future. I would, however, like to improve the containment vessel. It currently absorbs most of the sound produced by the switch through volume alone (the walls are only 1/2" thick), and as such takes up a good deal of space. I'd like to switch to pneumatic actuation and a sturdier, smaller switch body if I get the chance next summer.

EDIT:
This morning, while searching the forums I happened across the first thing loosely callable an "electrothermal gun" to appear on Spudfiles, made by Singularity way back in early 2008. Back then, people (myself included) were wondering if someone would be able to build a supersonic ETG eventually. :lol:
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Unread postAuthor: SpudBlaster15 » Sat Oct 02, 2010 8:56 pm

DYI wrote:This morning, while searching the forums I happened across the first thing loosely callable an "electrothermal gun" to appear on Spudfiles, made by Singularity way back in early 2008. Back then, people (myself included) were wondering if someone would be able to build a supersonic ETG eventually. :lol:


I recall that, it's amazing how things have changed since then. :wink:
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Unread postAuthor: inonickname » Sat Oct 02, 2010 11:06 pm

Here's the drawings for the new ETC. The chamber (well, the bulk at the end) and the threads will probably get longer.
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Unread postAuthor: Technician1002 » Sun Oct 03, 2010 12:51 am

DYI wrote: The main information of interest here is how fast dielectric breakdown occurs. If you can find some reliable source for that, a discussion on such switching methods would be productive.


Have you considered using SF6 in your switch gap? It may do the trick and allow the contacts to be much closer before the arc started. Substations use SF6 breakers to break the arc when opening a high tension breaker under load. They might work well when closing into your low impedance load.

You would want a smaller one than this one.
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Here you can see 2 of them in series to the right of this disconnect switch. The left one worked OK and the right one failed to open. This left the full potentiial on the left one which arc'ed over instead of interupting the current. The disconnect then opened and the current drew a very impressive arc.

Watch the 2 sets of insulators at the begining of the video. They are SF6 switches. The left one arcs over. The SF6 prevented an arc inside the switch. The current is about 100 Amps at about 360 KV. The line is 500 KV phase to phase.
[youtube]http://www.youtube.com/watch?v=6GiIVze2Tac[/youtube]
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Unread postAuthor: DYI » Sun Nov 07, 2010 5:12 pm

Firstly, in response to Tech's comment:
I have no interest in further complicating a switching system which already works well. When time and money permits I'll move to pressure drop switching, but the current setup hasn't given me any trouble so far.

I had some free time today, so I wrote a simple launcher simulator. It assumes the pressure gradient to be zero (i.e., local SOS >> muzzle speed), perfect flow through the "valve" (which is essentially the case here) and that there is no friction with the vessel walls. The expansion is assumed to be adiabatic (also reasonably accurate on a microsecond timescale). With high valve flow and low launch speeds in comparison to the SOS in the propellant gas, it agrees with GGDT to within 10%.

I don't claim this to be an accurate model, but it gave me some useful ballpark figures to work with. Picking a muzzle speed of 2600 m/s and a γ value of 1.14 (based on the scarce literature on pure ET guns with water as a working fluid), we get the following:

-Initial pressure: 43500 psi
-Travel time: 136 μs
-Velocity at joint between "chamber" and barrel: ~1950 m/s

-Barrel length needed to exceed Mach 1 with 6mm steel ball bearing: 1/4"
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Note that the above is for a 3kJ shot, and that ETAv2.2 should be capable, with the necessary modifications, of running at least 6kJ.

At 48.4μF, the discharge current peak occurs at ~16μs, so the approximation based on instantaneous pressure generation produces a somewhat high value for initial chamber pressure. However, I suspect that the muzzle speed is likely over 50% of the SOS in the propellant gas, which will cause the estimated starting pressure to be considerably lower than the actual value. As such, I'll say that the current maximum chamber pressure is probably between 50000 and 80000psi, which is well within the capabilities of the chamber (which will yield at ~92% of its yield stress in internal pressure, or about 120000psi). However, I may need to upgrade to Grade 8 steel next summer.

It's not clear from what I know so far whether or not extending the barrel would be worthwhile, but I can say that I'm currently getting at least 75% of the possible muzzle energy (based only on varying barrel length), and probably quite a bit more when the pressure gradient is taken into account.

Assuming that the allowable maximum chamber pressure will not exceed 140 kpsi in the near future, the upper limit on the speeds achievable with an airsoft round will depend on the chamber volume, the stresses in the capillary tube area which may exceed the chamber pressure, the propellant gas composition and temperature, and the gains caused by the non-instantaneous pressure generation. By the looks of it, I'll probably run into energy input limitations before any of the above prevent further gains: at 1.2mL chamber volume and 965MPa starting pressure, the muzzle speed could exceed 5000 m/s. With a maximum 9kJ input, a more realistic upper bound on the speeds I might achieve next summer is 4000 m/s.

Any suggestions for improving the estimates for any of the quantities discussed above are appreciated. So far, it's clear that higher pressures and temperatures should be pursued as far as is possible without destroying the projectile and/or the chamber. Also clear is that summer 2011 will be a very interesting time of year around here :D
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Unread postAuthor: ramses » Mon Nov 08, 2010 9:01 pm

May I inquire as to the general operation of your model? Do you simply "move" gas from the chamber to the barrel based on density, pressure across the "valve", etc, and then account for the corresponding change in temperature, or...

Thanks, and great job!
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Unread postAuthor: Technician1002 » Mon Nov 08, 2010 9:12 pm

Knowing the mass of oil and the amount of KE it will take from a fast switch (think shock wave of projectile into water) and the problems of gas breakdown, I'm wondering if a high vacuum switch could be made.. I know it is not feasible at this time due to the turbo pump required to get the pressure into the miliorr range low enough to work.
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Unread postAuthor: DYI » Fri Nov 12, 2010 2:36 pm

May I inquire as to the general operation of your model?...


Currently, for a given "step" (about 100ns for ETG modeling) acceleration of the projectile is constant, and based on the system pressure (which is constant throughout, as I mentioned earlier). The model finds the new chamber+barrel volume at the end of the step, based on the projectile's displacement from the start of the step. From this, it calculates the new pressure as P<sub>new</sub> = P<sub>initial</sub> * (V<sub>initial</sub> / V<sub>final</sub>)<sup>γ</sup>

It can also be told to show the temperature throughout the process, although this is of limited usefulness at the moment. On that topic, I have a few questions:

The ideal gas law, on which the derivation of the above relation is dependent, is not particularly accurate at high pressures. Taking a wild guess of steam at 4000K and 300MPa (the actual propellant gas will also contain O<sub>2</sub> and H<sub>2</sub>), we get a density of 163kg/m<sup>3</sup>, which is rather high for a gas. Is the ideal gas law even a reasonable approximation at such high density? If not, can anyone recommend a more accurate equation of state to work with in this case?

For air at 4000K and 300MPa, it looks like the compressibility factor would be around 1.3, leading a simple SOS calculation to be low by a factor of 1.3<sup>0.5</sup>. A good compressibility chart or calculator for steam would be appreciated, although it's still just guesswork - as mentioned earlier, it won't be all steam. Also, the temperature and pressure will be out of the range of most available information on the topic. There's certainly a lot to be learned here...
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Unread postAuthor: SpudBlaster15 » Fri Nov 12, 2010 3:19 pm

The ideal gas equation will be nowhere near accurate when dealing with pressures in the range of 300MPa. Intermolecular forces at such pressures are far too significant to ignore.

There are other equations of state which model the behaviour of specific gases more effectively. The two that come to mind are the van der Waals equation:

(P + a / V<sub>molar</sub><sup>2</sup>)(V<sub>molar</sub> - b) = RT

And the Redlich-Kwong equation:

(P + a)(V<sub>molar</sub> - b)/(V<sub>molar</sub>(V<sub>molar</sub> + b)T<sup>0.5</sup>) = RT

The variables a and b are empirically determined values specific to each compound, with a relating to attraction between molecules in the gas (Dipole-dipole forces), and b relating to repulsion between molecules (London dispersion forces). For water, the constants are approximately:

a = 5.536 L<sup>2</sup>bar/mol<sup>2</sup>

b = 0.0305 L/mol

Of course, the propellant gas is not entirely steam (As you noted), but assuming it to be is probably a relatively close approximation.
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Unread postAuthor: DYI » Wed Feb 09, 2011 12:25 pm

I've started into two very good texts, (compressible flow and CFD, by J.D. Anderson if anyone's interested) which should allow me to come up with a significantly better model for the ETG in a few months. It will definitely incorporate the effects of the very high pressure encountered in the firing process, as well as non- calorically perfect gas effects. Whether or not I'll get to the stage of modeling the chemical interactions (non- thermally perfect effects) occurring in the barrel is questionable (as I'm not all that clear on the starting conditions just yet), but I think I'll achieve a high enough degree of accuracy to start making optimizations. Me and my roommate are also working on the design of a wire-break chrony system which should be able to measure average projectile speed over a distance of 10cm to an accuracy of +/-100m/s at up to 4000m/s. There may yet be solid numbers for this thing on their way :D

I'll be doing the repair mentioned on the first page of this thread over the "reading week" break, and perhaps even a test firing or two if I happen to get time. If I do, and the repair works, I'll probably just launch a 6mm steel ball through 10mm plate or something, and leave the hypervelocity tests for the summer when I have more time to tinker. If it does work out, I'll be sure to post a picture or two in here. Sometime near the end of the summer there'll be a new thread for ETAv2.2, which should contain much more in-depth information on performance than this one did.

I hope to attain the knowledge base necessary to start modeling the electrical part of the system (from capacitor discharge through to the generation of the initial firing conditions in the chamber) in the next year or so. Hopefully, I can learn enough to provide basic criteria for efficient operation of the design, much the same as we know effective C:B ratios, pressures generated by combustion, and the effective limiting projectile speeds for various gas mixes in pneumatics and hybrids. With such a baseline in place, ETGs may seem somewhat less intimidating to the hobbyist. Who knows - in a few years we might just have an ETGDT :wink:
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