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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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Unread postAuthor: Velocity » Thu Feb 14, 2008 12:52 am

You should get an old car, and shoot it point blank with this monster... but aim the cannon at the hood of the car, so the projectile has to pass through the engine, interior, trunk, etc... that would be sweet 8)
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Unread postAuthor: Carlman » Thu Feb 14, 2008 12:53 am

bloody hell you have got the best job, my 5 acres is sufficient for my spudding needs lol

Velocity wrote:You should get an old car, and shoot it point blank with this monster... but aim the cannon at the hood of the car, so the projectile has to pass through the engine, interior, trunk, etc... that would be sweet 8)


just get random stuff like cars, caravans, statues...
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Unread postAuthor: DYI » Thu Feb 14, 2008 6:16 pm

Firing saboted, 80 pound darts made from 3" steel round bar this could easily reach out over 5 miles. But loads of big guns can do that, and more, with much more impressive effects.

This, in my opinion, should be more about what random, improvised projectiles can do to tough targets, while knowing that you're using only propane for a propellant.

And I agree with Carlman and countless others, you really do have the best job a spudder could hope for.
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Unread postAuthor: D_Hall » Thu Feb 14, 2008 11:36 pm

And today I started a complete redesign of the gun.

The design ya'll have seen was a preliminary design. Better than back of the envelope, but not something to send to the welders, ya know?

So today I started on the redesign.

So far.... I'm going to incorporate a multiple barrel capability. Since the cost of a shot is likely to be dominated by the cost of a sabot, I figure it'll be a good idea to have multiple barrels around to simplify sabot design. Why try to build a 19" diameter sabot if the projectile would do well in a 12" barrel? So, based on my pipe inventory and such I guess I'm looking at.... 29"x30' barrel. 19"x40' barrel. 12"x40' barrel. I've got smaller pipe, but I don't anticipate projectiles that would ever be significantly smaller.

I'm also re-examining the barrel carriage. Rather than have that huge contraption that holds the barrel and moves around, I'm thinking it would be better to have a permanent framework with a trolley that rides on a single rail. Besides making the whole assembly much sturdier, this will allow me to move the pick point of the barrel further forward. This, in turn allows me to balance the barrel slightly to the "ass heavy." Result? While it would never happen this way, it could be operated by a single person. Manufacturing tolerances would likely be looser as well.
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Unread postAuthor: D_Hall » Sun Feb 17, 2008 2:15 pm

MrCrowley wrote: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>

MrCrowley,

Today I went back and reread the thread to see if there was anything I'd missed.... I definately missed the above.

Thank you. That's some good reading. In fact, if I read it correctly (page 7, figure 8 ), they were unable to get a detonation using propane, oxygen, and 80% nitrogen (ie, essentially propane+air) until they used spirals and such (or am I reading it wrong?). Granted, their system is much different than mine, but it's a great data point.
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Unread postAuthor: jimmy101 » Sun Feb 17, 2008 2:23 pm

D_Hall

I'm still jealous.

Any thoughts that the T fitting that carries the barrel might get crushed by the recoil?

DDT... might not be a problem. First of all, when it occurs is critical. If it takes the burning of 98% of the fuel to get to DDT then the rapid burning of the last 2% is no big deal.

Multiple sparks should greatly reduce the chances of DDT. I would say that you want a spark every pipe diameter. In other words, lots and lots of sparks. You can probably reduce the number of sparks by a factor of 2 or 3 without affecting performance, or DDT probability, too much.

The above is based on a very simplistic model of combustion.

Real combustion spud guns are much more complex. (well duh) I've always wondered if the flame front speed is coupled to the projectiles' motion. Obviously, if the shell starts moving before combustion is complete then the expanding gas volume (and the resulting drop in temperature) would be expected to slow down combustion. But...

Combustion is proceeding. At some point the shell starts to move. Since the shell is moving the gases (both burned and unburned) start to move in the chamber. At some point the velocity of the gases exceed the Reynolds' number and the movement of the gases through the system become turbulent. The turbulent movement of the gases would be expected to give a turbulent flame front. A turbulent flame front burns much faster than a laminar one. (This applies to a gun without a fan running during firing.) How fast would the combustion gases have to be moving in a typical spud gun chamber to get the movement to be turbulent? How fast for a jumbo gun?

The velocity of the gases in the gun depends on where in the gun you are looking. The small volume of gas immediately behind the shell is moving at the speed of the shell. The small volume all the way back at the breech isn't moving at all. For a gun with the same diameter for chamber and barrel the velocity (ignoring friction and turbulence) is just a linear function of the distance of the volume element to the butt of the shell. For a more typical spudgun configuration, you have to take into account the large ID of the chamber versus the barrel, so the gases in the chamber are moving slower. Can a gun be fine-tuned by changing the barrel to chamber diameters, lengths, number and position of sparks etc. to maximize the amount of fuel burned with a turbulent flame front? How does this affect the number of sparks that should be used? With lots of sparks, combustion is done before the gases have time to move much. Turbulent flow causing a turbulent flame front is not significant with multiple sparks. Which method, multiple sparks versus turbulent flow/front, is the best way to get the most oomph out of a gun?

Wild-ass idea. Multiple spark gaps but fire just the breech end spark gap first. Wait for combustion to proceed a bit so the unburned gases start to move, fast enough to have turbulent flow. Now spark the remaining spark gaps and get the benefits of both multiple spark gaps and a turbulent flame front.

The autoignition temperature for most organics is in the vicinity of 500C. The actual value for propane in air is 480C.
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Unread postAuthor: D_Hall » Sun Feb 17, 2008 11:57 pm

jimmy101 wrote:Any thoughts that the T fitting that carries the barrel might get crushed by the recoil?

Yes and no.

Yes, the thought has occured to me. No, I don't think it'll be a problem. Mind you, I've not yet done that structural analysis, but when I do... Well, if it turns out that there is a potential problem I can just brace the back of the T fitting appropriately. It's crude, but it's easy and it should work.

DDT... might not be a problem. First of all, when it occurs is critical. If it takes the burning of 98% of the fuel to get to DDT then the rapid burning of the last 2% is no big deal.

Disagree. Even if it's only the last 2% you're still going to see one hell of a localized pressure spike. That spike could be enough to crack the gun. Even if you don't have a catostrophic failure on THAT shot, you've still just signed yourself up for some major maintenance at the very least.

The velocity of the gases in the gun depends on where in the gun you are looking. The small volume of gas immediately behind the shell is moving at the speed of the shell. The small volume all the way back at the breech isn't moving at all. For a gun with the same diameter for chamber and barrel the velocity (ignoring friction and turbulence) is just a linear function of the distance of the volume element to the butt of the shell.

This would be true if you're dealing with a gas of homogeneous density, but you're not. The thermal gradients are going to give rise to density gradients. Your stated end conditions will hold true, but between those two points all bets are off.

Turbulent flow causing a turbulent flame front is not significant with multiple sparks. Which method, multiple sparks versus turbulent flow/front, is the best way to get the most oomph out of a gun?

I imagine it doesn't make much difference provided that (a) your chamber can hold up to any abuse and (b) your burst diaphram doesn't go until combustion is very near completion.

The autoignition temperature for most organics is in the vicinity of 500C. The actual value for propane in air is 480C.

Hmmm....

Looks like we may very well have some dieseling (even if I am too lazy to run the numbers right now). In a way that's good, however. It implies that most hybrids experience such (*assuming* there is dieseling) but DDT isn't a problem associated with it.
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Unread postAuthor: MrCrowley » Mon Feb 18, 2008 12:06 am

D_Hall wrote:
MrCrowley wrote: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>

MrCrowley,

Today I went back and reread the thread to see if there was anything I'd missed.... I definately missed the above.

Thank you. That's some good reading. In fact, if I read it correctly (page 7, figure 8 ), they were unable to get a detonation using propane, oxygen, and 80% nitrogen (ie, essentially propane+air) until they used spirals and such (or am I reading it wrong?). Granted, their system is much different than mine, but it's a great data point.


Glad I could help. And Yeah, I think last time I read it, I came out with a similar thought.
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Unread postAuthor: jimmy101 » Mon Feb 18, 2008 4:43 pm

Disagree. Even if it's only the last 2% you're still going to see one hell of a localized pressure spike. That spike could be enough to crack the gun. Even if you don't have a catostrophic failure on THAT shot, you've still just signed yourself up for some major maintenance at the very least.

The pressure spike is an intensive, and not extensive property. (I believe I am recalling the correct terminology. "Extensive" depends on the extent of the object or system, "intensive" does not. Temperature is intensive, pressure is intensive, heat is extensive, energy is extensive.)
You may well get a "one hell of a pressure spike" but if the mass of the gases is small then the chamber won't even notice it. With a high pressure, low mass, high speed pressure excursion the effective strength of the pipe is much greater than it's static rating because the pipe has inertia. A localized DDT involving a small amount of fuel is not going to excessively stress the chamber, the chamber won't even notice it.

I imagine it doesn't make much difference provided that (a) your chamber can hold up to any abuse and (b) your burst diaphram doesn't go until combustion is very near completion.

I think it will make a difference. Given the dimensions of this gun (a very high surface to volume ratio) heat loss is going to be massive. Even if you could come up with a perfect burst disk I suspect you want the disk to rupture well before combustion is complete. That will be an interesting study, "muzzle velocity vs. rupture pressure". I suspect that the optimal rupture pressure will be significantly less than the peak pressure.

Besides, the longer combustion takes the more heat is lost. The laminar flame front speed is so slow that if you ignited a chamber with the proposed dimensions at one end you could out walk the flame front for the first couple of feet.


BTW, how many spark gaps are you planning on using?
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Unread postAuthor: D_Hall » Mon Feb 18, 2008 5:20 pm

jimmy101 wrote:I think it will make a difference. Given the dimensions of this gun (a very high surface to volume ratio) heat loss is going to be massive.

Huh?

The larger the system, the lower the surface to volume ratio given similar aspect ratios. Admittedly, this one has a high aspect ratio but... Just for fun I'll run some numbers.

6" diameter sphere (the sphere having the lowest surface area for a given volume and the volume being "typical" for around here):

Volume = 4/3 * pi * R^3 = 113 in^3
Area = 4 * pi * R^2 = 113 in^2

Surface/Volume = 1 (honest, just a coincidence that I picked the "perfect" sized sphere)

For my system, roughly a 688" long x 29" diameter cylinder:

Volume = pi * R^2 * Len = 454437 in^3
Surface = 2 * pi * R^2 + 2 * pi * R * Len = 64002 in^2

Surface/Volume = 0.141

I'll put that ratio up against ANY system being built around here. Big pipes just inherently have a low surface/volume ratio as a consequence of the square/cube law.

Even if you could come up with a perfect burst disk I suspect you want the disk to rupture well before combustion is complete. That will be an interesting study, "muzzle velocity vs. rupture pressure". I suspect that the optimal rupture pressure will be significantly less than the peak pressure.

Not argueing the point per se, just curious as to your rational.

BTW, how many spark gaps are you planning on using?

Current drawings show 9.
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Unread postAuthor: jimmy101 » Tue Feb 19, 2008 1:51 pm

Huh?
The larger the system, the lower the surface to volume ratio given similar aspect ratios. Admittedly, this one has a high aspect ratio but... Just for fun I'll run some numbers.

Oops, I was just looking at the aspect ratio and I thought it was 12" pipe, which would give a ~50:1 aspect ratio.

For your 454,437ci chamber the comparable spherical chamber would have a radius of 48". The Surface area/volume ratio for a sphere is just 3/r (a handy thing to remember). So a 48" radius sphere has a area/volume ratio of 3/48 = 0.063. Your proposed gun has an a/v of 0.14, amazingly close to the sphere.

I was thinking of more typical sized guns. My closed chamber (3"x15") has an a/v of 1.47. Which is more or less typical for "reasonable sized" spudguns. That a/v is ~10x higher than what your gun will have.

Edjumacated guess would put the thermal half-life of your gun at 1~1.5 seconds. So you have a "lot" of time to extract useful work from the hot gases before the energy is lost as heat transfer to the gun.


As to why you might want the burst disk to burst before peak pressure....

As long as the gases in the gun are more or less stationary (i.e., the projectile hasn't started to move) the gases themselves are a pretty good insulator. (Air, if stagnant, is an excellent insulator.) Once the projectile starts to move the gases do as well and heat loss is going to increase significantly. It might be better if some of the movement of the gases occurs before the fuel has been completely burned. The cooler the gases the less heat they will loose.

Of course, you still want all the gases burned before the ammo exits the barrel but there might be a "sweat spot" in the system that balances all the conflicting design charactersistics.

Any thoughts on how your are going to purge the combustion gases from a gun that is ~260ft<sup>3</sup> (~2000 gallons)?
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Unread postAuthor: D_Hall » Tue Feb 19, 2008 7:28 pm

jimmy101 wrote:As long as the gases in the gun are more or less stationary (i.e., the projectile hasn't started to move) the gases themselves are a pretty good insulator. (Air, if stagnant, is an excellent insulator.) Once the projectile starts to move the gases do as well and heat loss is going to increase significantly. It might be better if some of the movement of the gases occurs before the fuel has been completely burned. The cooler the gases the less heat they will loose.

The gases WILL move even before the disc bursts.

The freshly burned (ie, very hot) gases want to expand, and they do! This creates higher pressures in the chamber as a whole which in turn compresses the cold gases up against the "far" wall of the chamber (which reminds me, I need to start a thread on that....).

Eventually, everything burns and there's a "backlash" of sorts as everything returns to it's original position (or would if there was no such thing as friction/turbulence), but the point is that there's significant movement going on in there. As much as there would be with the projectile moving? No, but non-negligable all the same.

Don't get me wrong, your point is well taken. I'm just arguing that I believe that the sweet spot you spoke of will be closer to the "max pressure side" than you (apparently) think it be.

Any thoughts on how your are going to purge the combustion gases from a gun that is ~260ft<sup>3</sup> (~2000 gallons)?

I've gotta do a crush test on some spare pipe before I finalize, but right now I'm leaning towards a vacuum pump. Just pull a vacuum on it to get everything out, and then open it up again. Voila! A fresh batch of air!

If that doesn't work... Well, I've got a 150 HP air compressor that I'd like to use but there are some political issues with that (using expensive toys in ways that the people who paid for them don't necessarily approve of) so I'd like to avoid it if I can. Beyond that? I could come in with a 12-pack of 3000 psi K-bottles and just dump 'em.

I really like the vacuum idea though as it eliminates mixing. I mean, fill a 5 gallon space full of combustion gasses with 5 gallons of new air and you get 2.5 gallons of air and 2.5 gallons of combustion gasses in the end due to mixing; so you've got to replace your volume MANY times to get a good purge. But with a vacuum pump? Hey, the bad stuff is gone on the first try!
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Unread postAuthor: jimmy101 » Wed Feb 20, 2008 2:03 pm

D_Hall wrote:The gases WILL move even before the disc bursts.

The freshly burned (ie, very hot) gases want to expand, and they do! This creates higher pressures in the chamber as a whole which in turn compresses the cold gases up against the "far" wall of the chamber (which reminds me, I need to start a thread on that....).

Well there is "movement" and then there is "movement". The "sloshing" in the closed chamber combustion phase is going to be pretty minor compared to the gas moving at ~1/8 the projectile's velocity (based on a 1' ID barrel, and two 2'ID chambers, at a location near the T).

Vacuum sounds like an excellent way to purge the chamber, very clever. It will also help to dry out the inside of the gun. (It would be a pita to have to put a drain valve in and then have to lift the gun to get it to drain.)

You could inject the fuel in the inlet air stream as the fresh air is sucked into the chamber. Perhaps totally avoid the fuel mixing problem? (I still think it'll take days for propane injected at the middle of a 30' long pipe to diffuse to the ends.)
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Unread postAuthor: D_Hall » Thu Feb 21, 2008 1:39 am

jimmy101 wrote:You could inject the fuel in the inlet air stream as the fresh air is sucked into the chamber. Perhaps totally avoid the fuel mixing problem?

An interesting thought....

(I still think it'll take days for propane injected at the middle of a 30' long pipe to diffuse to the ends.)

Well, keep in mind that you'd never be starting with a "puddle" of propane and waiting for it to spread around. That'd be the whole point of high energy injection. You'd have a "mostly" mixed chamber to begin with and would just be waiting for it to "finish."
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Unread postAuthor: squarles2007 » Tue Feb 26, 2008 11:52 pm

quick question what kinda job do you have where you make 100 mm spud(thats one hell of a spud) guns, because i want that job!!
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