SpudFiles

Im just speaking from personal experience. If your using a aluminum wire, that gives you no chance of finding any fragments. you would be lucky to find a aluminum splatter. When ever I use aluminum, the left over aluminum always has a nasty habit of looking exactly like soot or any burned material...

Has anyone tried to build a gun that safely harnesses the power of DDT?

I've seen some research papers on PDE's that claim achieving DDT in 11 inches or less with a 1x mix of propane/oxygen. They use a schelkin spiral to increase turbulence for such a short distance.

Any thoughts on whether this is feasible and could be made safely or what kinds of velocity one could achieve with such a gun?

daberno123 wrote:Has anyone tried to build a gun that safely harnesses the power of DDT?

I've seen some research papers on PDE's that claim achieving DDT in 11 inches or less with a 1x mix of propane/oxygen. They use a schelkin spiral to increase turbulence for such a short distance.

Any thoughts on whether this is feasible and could be made safely or what kinds of velocity one could achieve with such a gun?

I thought about that but the shock wave might just pass through bullet.

I had a discussion topic posted on DDT guns a while ago, although I believe most of the discussion was focused on how to build one, as opposed to how it would perform.

What we really need to ask is how DDT affects chemical P.E. release, combustion products, and temperature. We all know that it produces a short high pressure spike, suggesting that efficiency in propelling heavy projectiles would not be changed for the better, and would probably decrease (as Larda's accidental DDT with HyGaC seems to indicate). Efficiency with very light projectiles, which tends to be adversely affected by use of lower pressure, may well be improved, and higher speeds may be attainable. Considering the SOS limitations in a normal propane/air hybrid, it seems that a mix of 40X or higher would probably be in order for dedicated testing.

DYI, I've seen two different images of your ETG design, one on here and one on 4HV, but I'm still having a hard time understanding how it works. I have a couple of questions, if you don't mind.

What exactly is the capillary tube for?

Why is there a chamber in addition to the capillary tube?

Why is there a second chamber-like feature half-way down the barrel?

What exactly produces pressure behind the projectile in your design? It seems that there is a very small amount of water and a piece of aluminum inside of a much larger chamber, does that alone generate all of your pressure?

I've been interested in creating my own ETG, but have just been thinking of having the combustion chamber simply be part of the rear of the barrel.

Thanks for your time.

This looks like such a clean and educated topic, I don't wish to dirty it with any newbie comments, but I do have one question/suggestion.

Has anyone ever tried building (and of course collecting data on) a small-bore piston hybrid? It seems to me that would be possible, but so many of the hybrids on this site have massive barrel IDs, I wonder what we could do with a much smaller bore barrel on a piston hybrid?

@saefroch
lol no it isn't noobish.. in fact, that's a pretty good question
AFAIK captnlaw has been experimenting with a smallbore one but IDK whether it was a piston or a burst disk hybrid

It's a burst disc one. And, HGDT said 720 m/s for the highest velocity shot... which i think is low compared to what DYI is talking about...

... isn't 343 m/s the generally accepted speed of sound at room temperature? 331 + .7T, where T is the temperature in Centigrade?

EDIT: And seeing as I'm a newbie here still and at building spudguns, I'll see what I can do to post some pics and explanation about a tee piston hybrid I'm thinking of constructing.

Something like this yes...

@axi0m: In response to your queries;
1. At the essence of its function, the capillary tube efficiently converts electric current to kinetic energy of a plasma through rapid Joule heating. It is the most efficient readily achievable such system that I'm currently aware of.

2. Because the created plasma is very hot, prone to causing erosion issues, and not a particularly efficient propellant due to its chemical composition, along with the other problems. Using a chamber allows ablation of a suitable propellant (water, here) to create a high temperature, low molecular weight gas (NOT plasma) which is both easier on the launcher components and more efficient for propulsion.

3. There is no such thing on either design, so far as I am aware. You may have mistaken the barrel-chamber coupling as something which it is not.

4. Pressure results from the ablation of the water in the chamber by means of the capillary tube plasma. The aluminum foil weighs less than 10mg, and is simply a connection between the two electrodes, the distance between being more than the voltage used could bridge if it were simply left as an air gap. This foil is known in the literature as a "fuse wire", and is melted, boiled, and subsequently ionized VERY quickly at the beginning of the discharge. I would conservatively estimate the pressure balance here as stemming at least 98% from Joule heating, the overwhelming majority of the remainder being caused by a reaction between the aluminum plasma and the water in the chamber, yielding Al₂O₃ and H₂.

5. I wouldn't advise building an ETG where the barrel and chamber are one piece. Any design involving a reusable chamber (most or all "serious" ETG builds uses cartridges, as far as I know) requires frequent cleaning and other maintenance (re-facing seals and such), and high modularity to be successful. My reload cycle is more than 20 steps, and requires some sort of tool work on every part exposed to the capillary tube plasma (the anode face, for example, needs to be reground after every shot).

The speed of sound is indeed around 340m/s at SATP in air, but this topic is for discussion of high speed launch methods, in this case preferably in excess of 1200m/s.

Speaking of hybrids, for some reason I only recently discovered that the SHARP light gas gun (the most energetic ever constructed, capable of launching kilogram-plus payloads in excess of 5km/s) was powered not by solid propellants, but 50X air/methane. Only around a tenth the energy density of conventional solid propellants, but considerably safer and cheaper. Unfortunately, I doubt many people in the amateur community would be willing to take on the trouble of building a light gas gun of any size. Shot-to-shot maintenance on the ETG seems pretty minor in comparison...

On the ETG front: I've discovered yet another design problem, this one originating not from the firing process but the reloading, which causes mechanical failure in the insulator after a few loading cycles. It's a minor design fix that will take a lot of machining to remedy. Don't expect it to be running again until the summer.

I read about light gas guns a few weeks ago, and was amazed. Somehow, I don't think many people on spudfiles are capable of a build of that scale... but maybe one or two.

Oh, I thought you said 1,200 fps, not m/s. Gotcha.

I think you had some sort of way past post about oxygen/propane mixes being much more effective than air/propane mixes because the inert nitrogen slows down combustion and the pressure wave in the chamber... do you mind posting a link? I can't find it.

If I did post such a thing, it wasn't recently. Considering the CO₂ produced in oxy/propane combustion, some nitrogen in the initial mix is almost certainly favourable. It also acts as a convenient buffer gas against detonation. Helium would obviously be preferable, although it's considerably less abundant.

The best mix I can come up with in terms of achievable muzzle speeds would be fuel-rich oxy/hydrogen, something that Larda was considering at one point for an upgrade to HyGaC20.

Two stage LGGs need not be huge, and in their simplest form would not be a huge challenge to build, especially if using helium instead of hydrogen. The real issue appears in the reload cycle, which, at the best, requires the removal of a piston which has been extruded into the barrel. Shot-to-shot, LGGs are an expensive and time consuming operation. And let's not forget the obvious hazards posed by their energy sources...

Would it be possible to construct a LGG with a hybrid chamber powering the piston, something high-powered like what Larda recently built, but designed (I have a few ideas in mind) to fail? What I'm thinking here is that the piston isn't extruded into the barrel, but rather that it tears off the adapter and barrel assembly , leaving the piston rather undamaged, and just a few sheared off steel rods to replace. With such an accurately calibrated system like Larda built, I would assume the strengths of the materials could be balanced fairly closely so that the front assembly detaches at fairly low velocity. Would something like that work at all? I know there are elements of the design I haven't considered, but I'm targeting what seems to be one of the biggest.

DYI,

Thank you for your reply. I have been looking forward to it.

I read several papers from the 13th EML symposium on the topic of electrothermal-chemical systems. Most, if not all, of the work done in those papers reflected an interest in the improved ignition of conventional propellants (chemical) in large caliber guns via a plasma injector (electrothermal). It seems that you are basing your design, which is almost entirely an electrothermal system (i.e. converts electrical energy to kinetic energy via joule heating and consequent ablation, instead of using chemically stored energy) on the designs of the electrothermal-chemical systems.

Please correct me, but I believe the plasma injector and/or capillary tube concept was conceived in the pursuit of improved ignition of conventional propellants, not to efficiently convert electrical energy to kinetic energy.

Essentially, what I'm getting at seems to be that for a design that is strictly electrothermal, the capillary tube/plasma injector and combustion chamber should be the same thing.

Mathematically, the larger the combustion chamber, for a constant amount of material to be ablated, the lower the pressure will be. Hence why I'm suggesting the water, in your case, should be incorporated into the initial region of heat generation, rather than spread out around a much larger secondary chamber.

It is relatively late here and I have been paying more attention to the circuit design of my upcoming ETG project recently rather than the aforementioned physics involved, so hopefully this all makes sense.

axi0m

p.s.

I read a thread about another ETG that was developed by, I believe, Larda. It was interesting that, if I remember correctly, all of this propulsive force was generated solely by the vaporization of the aluminum.