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Alright, I'll tip my hand a bit...

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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Alright, I'll tip my hand a bit...

Unread postAuthor: D_Hall » Fri Mar 28, 2008 12:19 am

Obviously I've been asking lots of weird questions. I've flat out stated that I'm working on a modeling package for combustions/hybrids. Right now I'm sleep deprived and going to do something very stupid...

...I'm going to release an EARLY EARLY beta version.

Now, keep in mind....

1) No user's manual yet (ie, webpage). It should be obvious how to use it for anybody who's used GGDT and is familiar with it.

2) It's not finished. Not by a long shot. The tools directory (pulldown) is mostly non-functional. Also, right now combustion within the barrel is not modeled. I had it doing that but there were some bugs that I seemed to be making worse the more I tried to fix them so tonight I wiped the slate clean. Result? Currently there's no combustion in the barrel but I've revamped the underlying program architecture in such a way that it should be MUCH easier for me to keep things straight and do the job right.

3) Mostly I'm just looking for "look and feel" feedback. As I said, the model isn't done yet. The lack of combustion within the barrel means that the simulated muzzle velocities are going to be lower than one would normally expect.

4) For those who want to model a straight combustion gun (not hybrid), just set your burst diaphram to rupture at 0 psi.

5) Right now it only models propane even though MAPP is a selectable fuel. Oh, and no, I've no intention of providing support for "spray" propellants.

In any event, constructive feedback appreciated. I don't promise to impliment every request or such, mind you. This gig doesn't pay a dime so you get what you pay for ;) . Still, somebody might point out an obvious need for the program or something.

In any event, here's a screen capture.
Image

And here's a link to the executable
Link
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Unread postAuthor: Carlman » Fri Mar 28, 2008 12:27 am

Nice work David!

Ill just point out some things i have found:

when you change the configuration to metric the inner diameter of the chamber stays measured in inches

EDIT: and when you configure settings for projectile mass it doesn't show anything other than ounces although the number changes
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Unread postAuthor: Hubb » Fri Mar 28, 2008 1:17 am

Is this going to be backed up with experimentation? Jimmy101 may be able to assist with this. He's been doing a lot of experimenting with combustion chambers lately.

Also, there are maybe some things you would like to add, such as other types of fuels (butane), more spark inputs (spark location, number of sparks, etc).

Also, I notice there is a spot for turbulence. Does this mean the fuel receives a mix, the fan is on during combustion, or both?

Those are just some suggestions. I'm looking forward to the finished product. Nice work.
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Unread postAuthor: SpudFarm » Fri Mar 28, 2008 6:23 am

NICE this is going to beat EVBEC down in the boots :)
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Unread postAuthor: Brian the brain » Fri Mar 28, 2008 6:59 am

Although I´m not into combustion I can see this will help out a lot of guys out here.
Your work is greatly appreciated by many.
Good job, keep at it!
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Gun Freak wrote:
Oh my friggin god stop being so awesome, that thing is pure kick ass. Most innovative and creative pneumatic that the files have ever come by!

Can't ask for a better compliment!!
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Unread postAuthor: Jared Haehnel » Fri Mar 28, 2008 7:10 am

Looks good I look forward to crunching some numbers with it I'll let you know if I come up with anything....Thanks for all you do8)

Edit: After running a few numbers I realized the range calculator seems a bit optimistic...
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Unread postAuthor: psycix » Fri Mar 28, 2008 7:36 am

AWESOME!
Ive always wanted this :)
It looks very good, cant wait till its fully developed.
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Unread postAuthor: sniperjosh » Fri Mar 28, 2008 7:42 am

Well done, Ill have to do some testing with this thing now :P. only bug I've found so far is when you change burst disc pressure box to blank (backspace old value) the mix(nx) value resets to 1.
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Unread postAuthor: SpudFarm » Fri Mar 28, 2008 7:44 am

jared is right.. i got 100m/s (333fps) more than on evbec..
can be a good thing to :P

mabe that was because of the burst disk stuff.
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Unread postAuthor: D_Hall » Fri Mar 28, 2008 8:24 am

hubb017 wrote:Is this going to be backed up with experimentation? Jimmy101 may be able to assist with this. He's been doing a lot of experimenting with combustion chambers lately.

I've already tested it against some of his closed chamber work. Agreement isn't perfect, but I wouldn't expect it to be. The model is in the right ballpark, however.

I'm also willing to look at any data anybody else has got for me, but it needs to be good data. I mean, chrony data does me no good if you can't tell me your load and projectile information.

Beyond that, if (when!) I get some real work done towards the realization of the Pipe Dream I'll be running a series of combustion experiments at the office. That data will (obviously) be incorporated.

Also, there are maybe some things you would like to add, such as other types of fuels (butane), more spark inputs (spark location, number of sparks, etc).

More fuels are easy to add, but they'll be the last thing I add as they're just tweaking numbers. I want to make sure I've got the theory down first.

As for the spark stuff... There's already the allowance for multiple sparks but the locations are assumed to be evenly spaced. This is about the best I'll ever be able to do, unfortunately. To get into user defined (read: uneven) spacings and such would require a LOT more knowledge than I've got. My combustion model is very simplified. Throwing that other stuff in there would take it to full blown hydra codes. I'm not smart enough to do that and you don't have a computer good enough to run it. :)

Also, I notice there is a spot for turbulence. Does this mean the fuel receives a mix, the fan is on during combustion, or both?

Yes and no all at the same time. Again, my combustion model is very simple. To it, turbulence is just a velocity delta (burn faster, damnit!). Until such time as folks around here are capable of measuring the turbulence in their chambers, it's about all I can do.

In case it isn't clear, HGDT is never going to be as accurate as GGDT. This is just a much tougher nut to crack with the tools and knowledge I have available. Still, I should be able to get it in the right ballpark.
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Last edited by D_Hall on Fri Mar 28, 2008 9:21 am, edited 2 times in total.
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Unread postAuthor: D_Hall » Fri Mar 28, 2008 8:27 am

Jared Haehnel wrote:Edit: After running a few numbers I realized the range calculator seems a bit optimistic...

The range calculator is only as optimistic as your Cd. ;)

Seriously, that's the one aspect of all this that I could (literally) write a book about. In a former life I wrote flight simulations for a living. And I'm not talking about video games, I'm talking about engineering evaluation tools for the military. I've written stuff that's being used by the military 10 years later(*). I've written stuff that's been reviewed and mostly approved by the FAA. I *know* how to write a range calculator.

The one in GGDT/HGDT (yes, I just cut/pasted that code) is simple; no doubt about it. But within it's input constraints I guarentee it's a LOT more accurate than your knowledge of Cd is going to be.



(*) Personal anecdote I found amusing: We just picked up a new hire at the office. She came from another area of expertise. Somehow flight sims and issues with them came up. I'm talking about some stuff and she's all of the sudden like, "Wait a minute... You're not THAT Dave Hall, are you?" ....Turns out that she was using my software at her previous employment. Gives an engineer a warm fuzzy! :)
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Unread postAuthor: D_Hall » Fri Mar 28, 2008 8:33 am

Carlman wrote:Ill just point out some things i have found:

Thanks for the heads up.
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Unread postAuthor: D_Hall » Fri Mar 28, 2008 8:34 am

sniperjosh wrote:Well done, Ill have to do some testing with this thing now :P. only bug I've found so far is when you change burst disc pressure box to blank (backspace old value) the mix(nx) value resets to 1.

Hmmm.... That might be a tough one to fix. An empty string has a value of zero. GGDT looks for burst disc values of zero to drop it into simple combustion.

Can ya just type over the old value rather than deleting it? :)
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Unread postAuthor: Jared Haehnel » Fri Mar 28, 2008 8:37 am

I wasn't criticizing and I realize you know a heck of a lot more about this then I do :oops:

With a muzzle velocity of 467fps and object weight of 10oz, cd of 0.5, 3834ft max range seemed a bit far to me I didn't know if it was something you might wanted to look into.

Its not a performance I was able to easily get with GGDT using a large pneumatic...
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Unread postAuthor: D_Hall » Fri Mar 28, 2008 8:40 am

Yeah, but think about it a sec... a 10 oz 1" diameter projectile? That's lead ball territory. Why is the default projectile mass 10 oz? I have no idea. That number got put in there and I haven't had a reason to change it since it still does a fine job of helping me evaluate the physics package. In some ways, it's actually tougher to model than a lightweight projectile (reasons are obscure and not worth going into here).
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