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The Pipe Dream lives...

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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The Pipe Dream lives...

Unread postAuthor: D_Hall » Mon Feb 11, 2008 8:45 pm

...well, not yet. But the idea is alive and well and gaining some support at the office (these things take time!).


For those of you who've no idea what I'm talking about, allow me to introduce myself. My name is Dave Hall and I was a member of the old Spudtech Forum. On that forum I discussed a design I playfully referred to as the "Pipe Dream." 'Twere so named because it was to be built using a bunch of spare pipe I have lying around and also because I knew from the get-go that the odds of having it actually built were small.

In any event, the Pipe Dream is (if it ever gets built) a 19" bore 2X hybrid designed to shoot a 100 pound projectile at 1000 fps.

A pic...
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For scale, the barrel is 40' long and it's hard-mounted to a semi trailer (flatbed).


Progress? I have the trailer in my possession! Beyond that, I know my "money guy" has been pitching the gun at a high level and so far they aren't laughing (although nobody has given him any money either). Also, my welding staff keeps bugging me for details; they think it sounds like a hoot. In other words, momentum is building.


So why am I posting?


Because I've a couple questions.....

1) I'm very concerned about the possibility of a DDT in the full scale gun. I think I've outlined a pretty useful test series that I could run to characterize the gun. Basically, it would be a 20' section of 29" pipe. One end blank flanged. The other end with a 19" and 13" diameter holes. The idea would be to treat it as one "leg" of the gun. The 19" hole would be used to characterize burst disk designs. The 13" hole would be used to characterize combustion. The good news is that the test apparatus would (in theory) tell me if I've got any DDT problems and it would be virtually 100% recyclable into the final design (ie, would BECOME one of the legs). Does anybody here have any thoughts/concerns regarding this approach?

2) I've also been asked to design a gun that can push a 100 mm projectile to 800 m/s. This is normally a powder-gun's domain, obviously, but I'm wondering if I can't do it with a 10X hybrid or something as a propane powered gun has certain administrative advantages (read: cheap fuel that doesn't have to be tracked!). I have some 5" ID pipe with 1.5" wall (HIGH pressure stuff) that could be used, but I'm wondering what the practical limits are before propane starts getting tough to predict. In any event, I'll probably run a test series similar to the above mentioned one but at higher pressures. Again, anybody have any thoughts/concerns?

Am I re-inventing the wheel?

Discuss!
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Unread postAuthor: D_Hall » Mon Feb 11, 2008 8:52 pm

Hmmm.... Hate to follow up my own post but I posted before I'd even done back of the envelope numbers for the 100 mm gun as I'd already planned on posting the Pipe Dream question this evening but found out about the 100 mm question this afternoon.

Doesn't look like anything short of a powder gun will even come close so I'll close disucssion on that front early.
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Last edited by D_Hall on Mon Feb 11, 2008 9:04 pm, edited 1 time in total.
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Unread postAuthor: potatoflinger » Mon Feb 11, 2008 9:00 pm

Wow, that will be one hell of a gun (if you ever build it). I don't really know much about hybrids, but I have heard that the longer the chamber, the greater the risk of DDT is, so it would definitely be a good idea to perform multiple tests.

Also, would you mind telling me what program you used to make that design?
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Unread postAuthor: Carlman » Mon Feb 11, 2008 9:01 pm

If this cannon is built than it WILL be the biggest on this site :shock:

I cannot offer any help as i know very little when it comes to hybrids but that thing is massive. Before reading anything i thought it was a 3 way ball valve pneumatic and like 5' long :D

19" bore :shock:

flatbed :shock:
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Unread postAuthor: D_Hall » Mon Feb 11, 2008 9:07 pm

potatoflinger wrote:Wow, that will be one hell of a gun (if you ever build it). I don't really know much about hybrids, but I have heard that the longer the chamber, the greater the risk of DDT is, so it would definitely be a good idea to perform multiple tests.

Yeah, the longer the flame propogates the better the odds of DDT, but there are an insane number of variables involved.

Oh, one other question along those fronts... Does anybody know what the auto-ignition temperature for a stoichiometric mix is? I'm concerned about pre-compression causing the remaining fuel to diesel.... although if that happens it MAY not be bad. I just don't know!

Also, would you mind telling me what program you used to make that design?

I use Alibre
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Unread postAuthor: bigbob12345 » Mon Feb 11, 2008 9:08 pm

Wow that is huge Im just trying to imagine the energy and noise produced from something like that.
19in bore that may be the biggest spudgun ever made if it gets made.
Are there any bigger than this i dont think there is.
good luck with building it you probably already realize but this is going to cost "The Bomb".

Edit: What will you use to fuel it.
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Unread postAuthor: potatoflinger » Mon Feb 11, 2008 9:09 pm

D_Hall wrote:
Also, would you mind telling me what program you used to make that design?

I use Alibre


Thanks!
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Unread postAuthor: Fnord » Mon Feb 11, 2008 9:14 pm

Bah. The title got my hopes up too much :)

If you get this thing finished, are you going to be able to get pictures of it? From what I've gathered the job you have probably wouldn't be the kind that would approve of on-site stuff being posted on the internet.


You may be able to avoid DDT by placing a spark gap every X number of feet. The only other way I can think of is to either create a significant chamber blockage, or dilute the gas further. Perhaps butane would be a better option because of its slow flame front speed?

I think the 100 mm hybrid sounds like a real possibility. With pipe like that there really isn't much of an upper mix limit, just as long as you don't wait for the propane to liquefy. You may consider artificially raising the chamber temperature to combat this.

Edit: oh. Well, anyway, how heavy would the projectile be?
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Unread postAuthor: D_Hall » Mon Feb 11, 2008 9:21 pm

bigbob12345 wrote:good luck with building it you probably already realize but this is going to cost "The Bomb".

It's hard to say. I did a rough order of magnitude analysis and decided that it would cost approximately $250k if I were starting from scratch.

But I'm not starting from scratch. I have the trailer in my possession (got it surplus from another division in my organization that was getting ready to scrap it). Most of the pipe? Ditto. Really, all I've got to fund is labor, instrumentation/electronics, and misc. hardware.

The labor will be "free" as this is a spare time endeavor. If there's "real" work to be done, the guys will do the real work. 'Tis only in dead time that they'd be working on the gun.

Instrumentation and electronics? The firing circuit is trivial. Many moons ago I got a patent for an igniter design; I figure I'll just recycle it. Cost for the firing circuit? Meh... $500. Instrumentation? Oooo... That stuff's expensive. I can piggy-back on some other projects we've got going... Call it $10k. Misc hardware? Meh... Probably $2k.

Of course, things always cost more than you expect them to so... Hopefully I can do the hole thing for 1/10 the nominal cost: $25k.

Edit: What will you use to fuel it.

Sorry if I was obtuse. The fuel will be propane and air. Although I'm contemplating a mix of propane, oxygen, and helium for "hot" shots but that bridge is a long ways from now (not that I ever expect to cross that bridge, but if anybody ever wants to push the capabilities of the gun I've already got an idea of how to do it).
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Unread postAuthor: D_Hall » Mon Feb 11, 2008 9:31 pm

_Fnord wrote:If you get this thing finished, are you going to be able to get pictures of it?

I'll find a way. And while I'm at it, we have video cameras that operate at 10,000 frames/second. I'll get some good video. ;)

Seriously, it shouldn't be difficult to get something like this through public relations. The technology is decidedly low. That's the point! I could very easily see this thing ending up on Mythbusters or something.

***IF*** it gets built.

You may be able to avoid DDT by placing a spark gap every X number of feet.

Currently I'm planning on a 7 point ignition system. Three ignition points in each leg of the chamber and 1 point at the "Tee."

The only other way I can think of is to either create a significant chamber blockage

As I understand it, a chamber blockage would actually increase the chances of a detonation by creating turbulence which in turn increases flame propogation rate.

or dilute the gas further. Perhaps butane would be a better option because of its slow flame front speed?

Interesting thought. I've been pursueing propane due to the ease and cost of procurement. I mean, I already have the propane guys on contract to fill some propane tanks we have. 'Twould be no big deal to have him fill an 50 gallon tank or something. But butane... That's a whole 'nuther logistics train. Still, if it avoids DDT it'd be worth looking into!

Edit: oh. Well, anyway, how heavy would the projectile be?

I'm not sure. I know what the application is and I expect a pretty robust projectile (several pounds at least). I'd figured on saboting... But I'm gonna have to go back to the drawing board (or get more data on the projectile).
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Unread postAuthor: Fnord » Mon Feb 11, 2008 9:39 pm

As I understand it, a chamber blockage would actually increase the chances of a detonation by creating turbulence which in turn increases flame propogation rate.


A certain level of blockage will, but a "forest" of closely placed vertical steel rods would slow it down.

I tried to GGDT a 100mm hybrid and got a DBZ error. I laughed as it's the first time I've had the program crash and it was your gun that did it. :)
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Unread postAuthor: Ragnarok » Mon Feb 11, 2008 9:47 pm

Wow - a visit from the great D_Hall.

That is one hell of a set of cannon plan. I think DYI, Schmanman, Killjoy and their ilk will have to give up any hope they have of holding energy records.

I don't have that much to say about point 1, DDT is outside my area of knowledge - I know what it is, and what might cause it, but I'm hardly informed on exact numbers, mainly because of my strong preference for pneumatic cannons (or electromagnetic ones) over the combustion based variants.
You'll have much better knowledge of that than I will. I'm only in my first year of my engineering degree (Mechanical), which hardly compares to your qualifications and experience.
Not to mention I'm out of my skull with exhaustion.
Nonetheless, I'm always one to help, so I've gone and found your old thread on Spudtech - trying to find a little more information on the concept.

The general consensus these days is that DDT is not a major concern in short and small chambers - there has even been some proposals that simply due to flame front speeds, it's nearly impossible. I'm not entirely sure about that, as there have been many recorded instances of such things in real situations, which as the famous saying goes, puts the flaw in the theory rather than the data.

However, what you're proposing is not a short chamber, certainly not typical in any respect - and how drastically chamber bore affects your dilemma, I don't know. I can foresee problems with flame fronts getting long run-ups in a chamber like that. I don't know how much using heavier hydrocarbons would help - it would have slightly reduced risk of GGDT, but also reduced velocity - still, both only to an extent, a small change like that isn't likely to have that much effect compared to some other things.

As for auto ignition, I'm not sure, I would work out the activation energy and convert that to the required temperature or pressure to create the particle energy necessary, but I'm going to be lazy and use Google.
Seems propane auto ignites at between 750 to 800 K depending on exact pressure and humidity of the air involved.

But I think people like boilingleadbath and jimmy101 will have a better knowledge than what my tired mind will let me remember.

My other thoughts on said cannon are, having read the old thread, have you any thoughts on creating a consistent mix? This is beyond the league of conventional chamber fans, but equally, such a method (scaled up hugely, of course) would create turbulent flow effects, generally accepted as a large factor in DDT occurring.

But even though you've "closed" it, point #2 sounds a possible, if difficult achievement. Killjoy's FEAR has reportedly been chronographed at - I think - 2700 fps (822 m/s) with what IIRC was a 4x mix. Assuming the figure is actually correct, there is no reason I can see why the feat could not be repeated in a higher calibre.
Increasing mix strength would allow heavier projectiles to be used.

Anyway, my bed is looking increasingly appealing now (I'm on GMT), so I'll have to go sleep. However, I'll keep this in mind, and if I get any brainwaves, you'll be the first to know (unless someone else reads the thread post first, but I can't help that.)

@_Fnord: Really? If I'm entirely honest, GGDT getting a bit picky with me isn't unknown.
I am trying to run it on Vista, and it's usually only when I try modelling certain things which are getting "extreme" in either direction, but it's not flawless.

But don't get me wrong, by no means am I implying the program is poor - you should see the messes I've made with my programming.
I once wrote a program to find prime numbers, but the first draft added in all the odd square numbers as well before it crashed, which was something of a shambles.
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Unread postAuthor: MrCrowley » Mon Feb 11, 2008 10:10 pm

D_Hall wrote:
potatoflinger wrote:Wow, that will be one hell of a gun (if you ever build it). I don't really know much about hybrids, but I have heard that the longer the chamber, the greater the risk of DDT is, so it would definitely be a good idea to perform multiple tests.

Yeah, the longer the flame propogates the better the odds of DDT, but there are an insane number of variables involved.


I did find a paper somewhere online that discussed DDT quite in-depth and they carried out tests with propane. I think they said a chamber beyond a few meters is very prone to DDT, I don't remember for sure, i'll try and track it down for you.
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Unread postAuthor: DYI » Mon Feb 11, 2008 10:28 pm

I remembered reading about this in the archive, and assumed that it had disappeared like so many ideas do. I was pretty excited when I saw the title of this thread. Someone actually started a thread here a few months ago asking if the Pipe Dream had ever got built.

As far as energy records, in a year or so I should have something that can get roughly 1/30 the muzzle energy that this will... Which puts a lot of things in perspective, and makes me wish I had military funding :roll:

Apparently the DDT runup distance in propane/air is ~35', although the larger diameter chamber and slightly increased pressure will affect this. As far as I know, multiple spark gaps should significantly lower the chances of DDT ocurring (which would be absolutely epic in a pipe this size) by decreasing the runup distance, but you certainly know a lot more on the subject than I do.

As for your 100mm gun, propane won't start to liquify until ~204x, which would need to be pressurised to ~130psig with propane before air injection. Obviously, that isn't possible, but the general consensus is that even 20 and 30x mixes can be done without risking DDT as long as the chamber is relatively short and fat, and free of obstructions. The speed of sound in the chamber post-combustion should be high enough to consistently allow 800m/s muzzle velocities.

It's great to see you on Spudfiles.
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Unread postAuthor: MrCrowley » Mon Feb 11, 2008 10:31 pm

I think this is what I was talking about:
http://www.galcit.caltech.edu/EDL/publi ... _paper.pdf

Or view it <A HREF="http://209.85.173.104/search?q=cache:ZlgVykp4Q_4J:www.galcit.caltech.edu/EDL/publications/reprints/jannaf99_paper.pdf+deflagration+detonation+transition&hl=en&ct=clnk&cd=1&gl=nz">Online</a>




Yeah it is great to see you on spudfiles, do you have any thoughts on upgrading GGDT, or maybe making a patch to fix any bugs or add anything?
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