Question for those who've played with high mixes...

[D_Hall]

What are the reasonable limits?

Put it this way: I may be building a hybrid out of 6" sch 160 pipe. Oxygen may very well be involved. The point is: Anybody have experience that may save me some time/energy?

I know there have been some very high mix guns, but IIRC, they were also very small guns.

Blah, blah, blah. This just landed in my lap about 20 minutes ago so I freely admit my ducks are not in a row at the moment.

Thoughts?

[saefroch]

Oxygen may very well be involved.

I rather hope so... I'd be concerned if you were planning a chloro/propane hybrid.

Being inexperienced with hybrids, I can't say much of use, but I'd be concerned if you started pushing 20X with that diameter.

[D_Hall]

Oxygen may very well be involved.

I rather hope so... I'd be concerned if you were planning a chloro/propane hybrid.

LOL... As in, using oxygen+propane, not air+propane. Possibly even pulling a vacuum on the chamber before fueling to get rid of ambient nitrogen.

Being inexperienced with hybrids, I can't say much of use, but I'd be concerned if you started pushing 20X with that diameter.

Well, lessee... Yield of sch 160 at 6" should be right around 10,000 psi. As such 5,000 psi should be doable in quasi safety. Figure 40-45X?

edit: D'oh! That's with air, not Ox.

[saefroch]

Do you have yield figures for our usual steel alloy sch 40?

[ramses]

the theoretical maximum is determined, as you probably know, by the partial pressure on the fuel gas. With propane, assuming a vapor pressure of 150 PSI, this is around 255x.

The biggest problem will be ignition; if you can use a glow wire, you have pretty much infinite capability within the bounds of your insulator. DYI knows all about extruding insulators at high pressures.

I'd say, as long as your parts can take the pressure and you have a sufficiently large barrel and strong burst disk... you will see more power with more fuel and air. Again, as you know, you will get more power with a lower mix with air (inerts present) than with pure O2.

[DYI]

The largest hybrid I ever played with had a 3.8" ID chamber, 30" long, running at 30X. The mix was roughly 50%(oxy/propane) and 50% nitrogen, and there were never any problems with detonation or the like. To the best of my knowledge, it was the largest, in terms of (mix number)*(chamber volume), hybrid built by anyone here until Vera was first fired. The gun performed well up until it was stolen, which regrettably happened before I got the chance to do much experimentation with it.

I've started reading "Combustion", by Yetter and Glassman, and it yields some excellent insights on the topic. Essentially, with a low energy thermal ignition in a non-turbulent chamber free of major obstructions (like Schelkin spirals), the DDT runup distance at atmospheric pressure for very reactive fuels like hydrogen or acetylene is about a meter. Propane's as much as ten times that, long enough that it's difficult to measure. Considering the mechanism by which DDT occurs, which I'm sure you're familiar with, it's apparent that while this runup distance decreases with increasing pre-ignition pressure, it doesn't do so very drastically. It does so more dramatically with oxygen, dropping to a similar value to air/acetylene or air/hydrogen.

Oxygen/propane isn't really conducive to performance (or fuel efficiency, but fuel's dirt cheap). The only real advantage gained there is lower pre-ignition pressures and very slightly higher SOS in the propellant gas (but if you were going for SOS, you'd be using helium or hydrogen as a buffer gas, almost doubling what you'd get out of oxy/propane). Also, I assume that sealing the chamber pre-shot shouldn't give you guys much trouble, considering the use of flanges. As such, I'm confused as to the desire to use oxy/propane.

In my opinion, with low energy ignition points spaced at one chamber diameter apart for the length of the chamber, using any sane fuel mix (and likely even some blatantly insane ones), detonation should not be a concern. In terms of fueling procedures, it would of course be wise to add as much of the buffer gas as possible before the oxygen goes in to avoid any of those lovely spontaneous combustion incidents. With those aspects in mind and a healthy safety factor in place, there's not really a practical limiting mix (as SCH 160 pipe should never be taken anywhere near 200X).

It's those "lovely spontaneous combustion incidents" that worry me, if you persist in using oxy/propane rather than buffering with helium or hydrogen. I have gone up to 30X oxy/propane in small launchers, but if I were you I'd pressurize a small tube with the internals slathered in grease, soaked in oil and full of hydrogen or acetylene to about twice the oxygen fill pressure with pure O₂, just to be certain before doing so in a large hybrid. Maybe subject it to some shock loading while you're at it.

In response to saefroch: the diameter of the tube actually works in his favour, slowing down the formation of a roughly planar wavefront in the mix. 20X is near the safe pressure limits of standard hardware store pipes and fittings, but certainly not a concern as detonations and the like go, assuming the use of safe ignition practices.

@ramses: my failing insulator have been in the range of 50-100KSI, possibly exceeding what's found in modern MBT main guns. I'll have you know that my high mix hybrid insulators running mixes similar to what Mr. Hall proposes never did anything of the sort

[D_Hall]

The largest hybrid I ever played with had a 3.8" ID chamber, 30" long, running at 30X. The mix was roughly 50%(oxy/propane) and 50% nitrogen, and there were never any problems with detonation or the like.

Thanks. That's precisely the sort of information I was looking for when I posted.

Essentially, with a low energy thermal ignition in a non-turbulent chamber free of major obstructions (like Schelkin spirals), the DDT runup distance at atmospheric pressure for very reactive fuels like hydrogen or acetylene is about a meter.

Of course, even with some minor obstructions and turbulence that changes radically. I've personally induced acetylene to detonate with slightly over 1 inch run up.

Oxygen/propane isn't really conducive to performance (or fuel efficiency, but fuel's dirt cheap). The only real advantage gained there is lower pre-ignition pressures and very slightly higher SOS in the propellant gas

Lower pre-ignition was/is precisely the thought behind it. I'd like to stick to a modified automotive ignition system. Spark plugs make for nicely done through-bulkhead igniters.

[D_Hall]

D'oh! It just dawned on me that I've got some 8" pipe at work that has like 1.5" wall thickness.... Hmmmm......

[DYI]

Of course, even with some minor obstructions and turbulence that changes radically. I've personally induced acetylene to detonate with slightly over 1 inch run up.

Definitely. With a fast, sufficiently energetic ignition, DDT isn't even necessary for a detonation. I'd still advocate for some percentage of buffer gas in there, even if it's only 20% or so. That seems to be sufficient to prevent detonations in the case of both my testing and Larda's (and he did get a detonation even in his tiny chamber when he removed the remaining nitrogen).

Lower pre-ignition was/is precisely the thought behind it. I'd like to stick to a modified automotive ignition system. Spark plugs make for nicely done through-bulkhead igniters.

In that case, one more piece of personal experience: the hybrid I referred to earlier would break the plugs and fire them out the back with significant speed (consistently), hence my switch to a new ignition setup. Granted, the plugs I was using were 1/2" NPT thread, made for older engines with lower compression ratios, but I'd still be concerned about their failure in your system. Also, if the firing pressure's staying the same, what influence does the starting pressure have on the ability to use sparkplugs?

[D_Hall]

In that case, one more piece of personal experience: the hybrid I referred to earlier would break the plugs and fire them out the back with significant speed (consistently),

Define "significant speed". And did you notice any performance losses? JUST losing a spark plug isn't a big deal. Provided it's the spark plug that fails I can easily just call them a consumable item and use a new plug for every shot.

Also, if the firing pressure's staying the same, what influence does the starting pressure have on the ability to use sparkplugs?

Well, if I want to get to (say) 5,000 psi, I could start with a 45X mix. That's a pretty dense mix and I'd have to push for a spark much harder than I would if I had a 10X mix (what the plug would have been designed for). Alternatively, if I double the O2, I could get away with a 25X mix (if that makes sense). That's a bit easier to push a spark through. Numbers are pulled out of my ass, but I'm sure you get what I'm saying.

[DYI]

During the tests where the hybrid was backed by a gravel berm, the plug ends would embed themselves 6-12" into it. Due to the low framerate of my camera at the time, all I can say is that they were going faster than 150ft/s. Keep in mind that your plugs are modern and more solid, and if they do fail probably won't do so as violently (and it's easy to test for yourself either way).

The sparkplugs do become an issue in high mix hybrids. I developed a quick change burning-wire ignition a while ago to combat that. Convenient on two fronts - it allows the use of higher pre-ignition pressures, and can be run straight from a car battery. If you would still prefer to stick with sparkplugs though, I'd suggest at least a small inclusion of buffer gas (helium would be ideal). I'll play around on GasEq later to see what effect that will have on the propellant gas conditions, but I suspect it'll be an insignificant loss, or even somewhat beneficial.

[ilovefire]

[ I could get away with a 25X mix (if that makes sense). That's a bit easier to push a spark through.

i just want to add that i have successfully ignited a 27x mix in my 4ml hybrid with just a high energy stove ignitor and the end plugs were just epoxy and it was fine, im sure you'll be able to easily go way past 27x with what you would have on had, plus im only 17 so if it was easy for my it should be hell easy for you

[Fnord]

Can't you just machine a custom spark plug that won't fail? It would also let you keep the spark toward the center of the chamber if you choose to.

My "unprofessional" setup would be a hollow threaded rod with an electroded running through it, encased in high-strength epoxy. It would work much like a normal spark plug, though the spark position could be adjusted. You could even add a small ceramic washer to the end of it to keep the epoxy from being carbonized during ignition.

I'd at least experiment with nickel-chromium wire plugs if you're concerned with problems from arcing through a 45x mix (Now personally I think your EE's would enjoy creating a 50Kv ignitor, but still...).


Larda and SB15 had problems with ball valves being destroyed with high mixes.

Larda switched to needle valves which seemed to work well. I've had a 600 psi, w/o/g rated ball valves leak after being exposed to 10x

D'oh! It just dawned on me that I've got some 8" pipe at work that has like 1.5" wall thickness.... Hmmmm......

I've had trains of though that ran something like this. They all eventually de-rail and crash in a heap somewhere around vera.

[D_Hall]

Can't you just machine a custom spark plug that won't fail? It would also let you keep the spark toward the center of the chamber if you choose to.

I'm sure I could. However....

COTS spark plug: $30 at the absolute worst.

Custom plug: $300 minimum, possibly as much as $1200 (machinists don't come cheap around here).

With numbers like that, it makes sense to try the COTS solution first. If it works, great. If it doesn't work, I haven't lost much.


In any event, I ended up spending about 30 minutes talking to an engineer at Autolite. He recommended a plug that they make for the top fuel dragster crowd. I'll try it and keep my fingers crossed.

[Fnord]

I'm not really sure how things actually work at your jobsite, D. Hall, but such a plug could probably be made with some off-the-shelf parts in a matter of minutes. I know you probably don't do any machining yourself but a quick run to the shop during a free hour would take care of it.
Just sayin'

If you prefer to use some ready-made F1 plugs, that sounds like it should work fine though.