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High Velocity Launch Systems

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Unread postAuthor: DYI » Sun May 16, 2010 5:17 pm

Although your reasoning is basically sound, my approach is not without precedent; I invite you to take a look here. Now, I'll grant you that they were NOT attempting hypervelocity launch, and simply copying their design "because they're professionals" would be laughable.

There is, however, a purpose behind my current design choice - firstly, what I have now allows me to test widely varying amounts of chamber volume and different propellant combinations. Second, I simply cannot build what you suggest at this point. The equipment I have access to does not allow for designs with capillary tubes (let's call them CTs from this point on) over 1/4" in diameter.

In some propellant configurations, unifying the chamber and CT could be detrimental or impossible. If pressure around the arc becomes too high, as can happen if the ID is too low or liquid has filled the tube, it WILL quench. Ridiculous as this seems, considering the current and voltage involved, it does happen, and has happened to me. In one extreme case, I discharged at 450J and was left with 360.

When I wrote about efficient conversion to kinetic energy, I was referring to KE of a gas/plasma, not direct generation of KE in a macroscopic projectile. Obviously something like a railgun would be more suitable in that regard. There is, however, a real lack of development in the field of pure ETGs, and it may well be that there's a better method than CTs out there.

I'm unclear as to the origin of CT plasma generators. If you do find out why they were originally developed, I'd be happy to know.

Larda's ETG did use aluminum gas/plasma as a working fluid. So far as I know, it never exceeded 5% efficiency. There are clearly better propellant gases than something which boils at 2800K :roll:
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Unread postAuthor: axi0m » Sun May 16, 2010 7:17 pm

Do you believe that increased pressure makes the arc more likely to quench, or the composition of the ablation material? (i.e. water) Or, perhaps both. What was the case when your's quenched with 350 joules remaining?

Also, you mentioned testing varying chamber volumes, is that to optimize the arc? Otherwise, would we not agree that if everything else remains constant, a smaller chamber would result in increased pressure -> increased performance?

In my design, I am using 450V electrolytic capacitors. Rather than placing them in series for higher voltages, I am keeping them all in parallel for lower resistance. As we know, once a plasma is formed, it is an excellent conductor with hardly any resistance. As such, a higher voltage isn't needed, only higher current, which can come from less internal resistance. With all of my caps in parallel, there is an ESR of 2.5 milliohms. As such, if they were shorted, there would be a 180KA pulse.

Initially, the arcing material will present many times that resistance, but once it arcs, the resistance will drop and all remaining energy will be pulsed through the plasma at a very high current.

My worry, however, is that 450V might not sustain the arc, especially given how violently dynamic the environment will be once the discharge begins.

This leads me to another design that I have encountered on the internet. It consists of the arcing material being incorporated with the ablation material. For example, aluminum powder mixed with water. This design will not have the problem of the arc quenching, because the arcing material will be ubiquitous.

My initial tests, however, will still involve an arcing material and a separate ablation material, like your design. Correct me if I'm wrong, but the ideal arcing material would have a very high boiling temperature, while the ablation material would have a very low boiling point. Once the arc is formed, the surrounding plasma and gas would be hot, with a tungsten wire the gas would start at 10,000 degrees F, thus exposing the ablation material to a temperature that would vaporize it as rapidly as possible, being that it ideally has a very lower boiling point.

I have a length of 0.005" (also .010 and .015) pure tungsten wire that I intend to use to arc and ablate a surrounding volume of water inflated with a super-absorbent polymer to increase its surface area.

Hopefully I haven't typed too much. Thanks.
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Unread postAuthor: DYI » Mon May 17, 2010 7:20 pm

My guess would be that quenching occurred due to excessive pressure. Energy increases as the square of voltage, leading me to believe that pressure (and thus resistance) in the arc may do the same, being as it is confined. Obviously more energetic arcs will be hotter, so this isn't a simple relationship (nothing is in plasma physics :roll: ). I had been using a very low ID (1mm, I believe) capillary tube, 25mm long - an extreme case tested simply to see what would happen (discharge was 7.5kV, 16uF). Achieving this effect was MUCH easier when using electrolytic capacitors in the 1.5kV range. I suspect that in your design it will create a significant problem.

A smaller chamber may or may not result in increased pressure; we're trading heat for pressure here by ablating a liquid/solid material, and if that is removed completely the launcher is nearly useless. We obviously need higher temperatures (meaning higher energy:working fluid mass ratio) for higher speeds, but that's a separate issue. These systems are far too complex to be easily predicted by generalizations such as lower volume = higher pressure.

As I noted earlier, very low voltages will cause you trouble. You'll need an inefficiently large ID on the capillary tube to compensate.
There's another issue here: while resistance outside the ETG should be minimized, you WANT high resistance in the plasma, as much as possible without quenching the arc. Once upon a time, I found a paper about measurements of resistance in capillary tube discharges. The figure was in the tens of milliohms.

If the plasma has very low resistance (stemming from excessive temperature or insufficient pressure, as can be caused by poor capillary design) it will be an efficient conductor, the exact opposite of what you should be looking for. You want a resistance spike after vaporization of the fuse wire, not a drop.

Your worry is well-founded. While adequate for the ETGs seen in common hobbyist use, 450V is likely not sufficient for a serious build using a capillary tube plasma generator system.

I've seen what you describe as well, though only in the context of improved optical properties of the plasma for ignition of propellants. Could you link me to the paper you found on the matter for pure ETGs? I doubt its usefulness, considering that such material will result in more efficient conduction (causing a drop in ballistic efficiency), and the fact that the quenching problem can be solved by optimizing capillary tube dimensions without sacrificing efficiency. I am, however, by no means an expert, and this intuition may turn out to be entirely wrong. Aluminum powder mixed with water is a poor example, as it results in a reasonably energetic chemical reaction which produces hydrogen gas, going far beyond simply modifying electrical properties of the plasma.

I know very little about what precisely goes on between switch closing and beginning of capillary tube ablation, and can offer you no useful advice on that matter. Links to papers dealing with that topic would be greatly appreciated.

Super-absorbent polymer? Tell me more. A water-based foam would be ideal as a working fluid, and I imagine that this polymer, whether or not filled with water (apart from obvious shape and sealing issues which I will assume to be countered by the particulars of your design) would make an excellent capillary tube material.

On that note, I'd really like to see your design.
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Unread postAuthor: 245Tommy » Mon May 17, 2010 8:28 pm

In my gun, the arc would extinguish even if there was a little water in the capillary tube at 660 and 1000v.
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Unread postAuthor: axi0m » Mon May 17, 2010 10:19 pm

DYI,

"A smaller chamber may or may not result in increased pressure; we're trading heat for pressure here by ablating a liquid/solid material, and if that is removed completely the launcher is nearly useless."

No variable is being removed; I am proposing that with a given amount of ablation material and a given input energy, assuming the same percentage of the energy is absorbed by the material, a smaller chamber, not CT, will yield higher pressure. I will go further than propose; I'm stating that it is the case.

"As I noted earlier, very low voltages will cause you trouble. You'll need an inefficiently large ID on the capillary tube to compensate. "

I think that voltage and CT length would be inversely proportional with a given efficiency, not voltage and CT diameter.

"Energy increases as the square of voltage, leading me to believe that pressure (and thus resistance) in the arc may do the same, being as it is confined."

Is this statement suggesting that pressure increases with the square of voltage? Grammatically it does, but I'm not sure it was meant to conceptually. This is perhaps the crux of my equivocation on my design. Maybe you are saying that resistance increases with the square of pressure? This would bear semblance of your other statements indicating higher pressure will cause quenching. Do you have any theoretical or empirical evidence that suggests this is the case?

I have purchased the super-absorbent polymer (SAP) and am going to try and make a test batch of it tonight. I will compare the density of water with the overall density of saturated SAP. Being that the composition is highly porous, a lower density will indicate that there is a high level of surface area exposed due to channels and cavities.

245Tommy,

"In my gun, the arc would extinguish even if there was a little water in the capillary tube at 660 and 1000v."

What was the length of the arc material? Was the water in contact with the arc material before discharge?
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Unread postAuthor: axi0m » Tue May 18, 2010 12:03 am

I performed a preliminary test using a ratio of 5 cubic centimeters of SAP and 2 ounces of water.

The water had a weight of 60 grams for a 1/4-cup volume.

The saturated SAP had a weight of 19 grams for the same volume.

It is looking like the S-SAP could be an ideal ablation material for an ETG.
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Unread postAuthor: 245Tommy » Tue May 18, 2010 1:42 am

245Tommy,

"In my gun, the arc would extinguish even if there was a little water in the capillary tube at 660 and 1000v."

What was the length of the arc material? Was the water in contact with the arc material before discharge?
The strips of aluminum foil were 4x20-25mm and yeah sometimes the water would get pushed into the capillary tube when I loaded the bullet and contact the foil.
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Unread postAuthor: DYI » Thu May 20, 2010 8:00 pm

This is going to be another long one, so bear with me.

1) I stated nothing regarding a smaller CT. We're not dealing with a simple fixed volume of gas at fixed temperature here. If there was no working fluid in the chamber, what you state would be entirely correct. As it is, ablating a liquid/solid increases the pressure while decreasing the temperature (as compared to the case with no working fluid in the chamber). The optimal amount of propellant will likely depend on the desired muzzle speed, with higher speeds requiring higher temperatures and less working fluid.

2) Experiments will show you that both have an effect. I'm not qualified to tell you what the proportionality is, but it's there. Increasing capillary length may not alter the internal pressure much, but it will affect the length of the conductor, increasing the resistance. Decreasing diameter WILL increase internal pressure, meaning more resistance for a given length.

3) I was suggesting quite simply that, if one takes a capillary tube of fixed dimensions and a capacitor back of fixed capacitance, he will, through increasing the voltage, eventually reach a voltage such that the arc will quench. I do not claim a k<sub>1</sub>P = V<sup>2</sup> = k<sub>2</sub>R proportionality*, I only offer that approximation as the simplest explanation I can muster of my empirical results.

4) The higher surface area is highly beneficial to performance in the role of a working fluid, so I would expect good results. What's the composition of you SAP?

I'm interested to know how exactly you plan to adapt the SAP to a capillary tube application.

It's nice to see someone really doing his research into an ETG project. I expect you'll do very well if you persist.

*where k<sub>1</sub> and k<sub>2</sub> are constants.
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Unread postAuthor: axi0m » Fri May 21, 2010 12:11 pm

DYI,

I'm sorry for being so persistent on the size vs. pressure debate, perhaps I have falsely recalled one of your designs. It seems that I remember seeing you having a design that had a CT, a chamber larger than either the CT and the barrel, and then the barrel. Regardless, the debate was still valid I suppose. From what I see, your design has a CT and then a chamber and barrel which are the same diameter. This is how I believe my design will be.

The SAP can be read about here: http://en.wikipedia.org/wiki/Sodium_polyacrylate

I have two variations: one turns into a gel, the other into a very lightweight snow-like substance that is extremely porous.

For the time being, I'm planning on using a 0.125" bore to obtain a high velocity. I'm also planning to use a cartridge-based system so I can quickly and easily reload the system. The cartridge for the SAP will probably consist of a nylon 6/6 tube, 0.25" OD 0.125" ID, with the ID simply being filled with the SAP. I will test different arcing materials including aluminum foil (0.002" thick) and tungsten wire (0.005, 0.010, 0.015" diameter). The arcing material will be placed through the length of the ablation material, completely centered.

I'm not entirely sure how long the CT will be. I need to do more research into the resistance of these materials when they are turned into a gas and then plasma. Ideally, the parasitic resistance of the system (including ESR of the capacitors) should be equal to the resistance of the load, according to http://en.wikipedia.org/wiki/Maximum_power_theorem.
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Unread postAuthor: DYI » Fri May 21, 2010 7:39 pm

You are correct - none of my ETG designs have had a chamber diameter greater than the barrel diameter. The chamber diameter has always been the same as the barrel diameter to allow for adjustment of chamber volume.

A word of warning on 0.125" bore:
Beneficial as high pressures are, using them on a first attempt may lead to a lot of grief as a result of deformed parts. I'm not trying to discourage you, just warning you to be cautious with the design.

I've always wanted to do a cartridge system, but the machining required has remained too daunting. There is no margin for error on the tolerances, especially around the capillary tube. Also, I strongly suspect you'll have quenching issues at 400V (or 4000, for that matter :roll: ) with such a low capillary tube ID. Done in the tens of kV range, it would make for a highly interesting experiment. However, I'm sure you'll be able to work something out with a bit of testing.

Mathematically, ablation in a capillary tube is quite the nasty problem, far beyond my current capabilities. Comprehensive understanding of most plasma processes also eludes me for the moment. I am, however, reasonably certain that your application of the maximum power theorem to the problem is incorrect. What you want is high energy transfer into the plasma, which calls for high efficiency, not maximum power. Ideally, you want much less parasitic resistance in the system than load resistance. Essentially, as much resistance in the plasma as can be managed without quenching, and as little resistance elsewhere as achievable.
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Unread postAuthor: 245Tommy » Fri May 21, 2010 8:24 pm

I'm also planning to use a cartridge-based system so I can quickly and easily reload the system.
You think like me. :) http://www.spudfiles.com/forums/4mm-etg ... rt,15.html
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Unread postAuthor: axi0m » Sat May 22, 2010 12:03 am

To prevent structural deformation, I'm going to use a very thick chamber made from hardened 440C, which is extremely hard and strong.

To prevent barrel erosion, I'm hoping to install a barrel liner made of a high-alumina ceramic.

Also, for any given CT ID, however small, I can shorten the CT such that it reliably arcs. Would you not agree?

I have ten capacitors, so I can arrange up to 4,500 volts with all of them in series. With all of then in series, however, there is 250 milliohms of ESR. So, I have to find a balance between all of these variables.
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Unread postAuthor: ramses » Sun May 23, 2010 8:10 pm

keep in mind that the caps in series will not share the voltage equally. You will need fairly hefty balancing resistors to keep them in line, and that will dissipate a fair amount of power. There is a formula for the resistance, but I forget it at the moment.
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Unread postAuthor: DYI » Mon May 24, 2010 2:38 pm

To prevent structural deformation, I'm going to use a very thick chamber made from hardened 440C, which is extremely hard and strong.


If you can get that manufactured, it should be quite effective. Just be careful with the pressure and watch for signs of cracks developing.

To prevent barrel erosion, I'm hoping to install a barrel liner made of a high-alumina ceramic.


Do you have unlimited equipment, or are you just wealthy enough to get all this stuff done for you? :P

Also, for any given CT ID, however small, I can shorten the CT such that it reliably arcs. Would you not agree?


Yes.
However, in my designs it has been far easier to adjust ID than to adjust length.

I have ten capacitors, so I can arrange up to 4,500 volts with all of them in series. With all of then in series, however, there is 250 milliohms of ESR. So, I have to find a balance between all of these variables.


Did you decide to use electrolytics for cost/availability issues, or did you buy them before knowing precisely what was required? They really are far from optimal here.
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Unread postAuthor: axi0m » Mon May 24, 2010 3:48 pm

You should wander around McMaster.com and you'd find a plethora of exotic materials and parts that can find great use in such an undertaking as this.

"Very High Temperature Nonporous High-Alumina Ceramics"
Tube shape - 1/8" ID 1/4" OD 12" Length
for $13.48, and the tolerances are relatively tight.

I purchased the electrolytics for a railgun that I never built. I decided on an ETG because the rail erosion in a railgun would ruin an expensive set of rails in potentially one firing.

I would love to obtain a true pulse capacitor, but don't really know where to start.

Also, the 440C, while extremely strong, is indeed brittle. Perhaps it isn't the best material for a high-pressure application, but I haven't found a similar metal, especially stainless steel, that is comparable in strength.

Edit:

I just received a quote on some 0.001"-thick tungsten foil on which I inquired. They are asking $38.50 per square inch. Being that I would only need strips about 1/8" wide (guessing) by perhaps 3/4" long, that wouldn't be a budget breaker.

I might as well mention that I have sheets of 0.0005"-thick aluminum foil for a price several orders of magnitude lower. The reason I'm interested in the tungsten foil is because it boils at 10,600 F.

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