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Where did the 100 psi assumption come from?

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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Unread postAuthor: jimmy101 » Fri Nov 21, 2008 2:39 pm

john bunsenburner wrote:Some where we should have a huge pdf with ALL the formulas so that any moron like me can go and put the numbers into their calculator and tada have everything in black and white, or maybe some one could even make a program with ALL the formulas we will ever need...just a suggestion...

1. GasEq for a closed combustion chamber.

2. HGDT for an operating combustion spudgun.

3. You can also search the old Spudtech forum for the various combustion modelling posts.

4. Go to the "Ideal Gas Equation" wiki page for the general model for expanding gases.

5. Go to the Spud Wiki page on fuels for the thermodynamic values for various fuels.
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Unread postAuthor: john bunsenburner » Fri Nov 21, 2008 3:04 pm

ok, now howa bout some one who knows the makers of the programs mentioned get them to sit down and make one big program wiht all spud formulas needed, we can call it the spudipedia or spud formulas or somthing, im just giving suggestsions here
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Unread postAuthor: Ragnarok » Fri Nov 21, 2008 3:07 pm

I don't think you've grasped how much work that would be. You can't ask other people to do something like that for you, just because you're too lazy to switch between programs when working.

If you want to see something like that, the data is all out there... nothing to stop you doing it.
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Unread postAuthor: john bunsenburner » Fri Nov 21, 2008 3:14 pm

I would do it but i guess im too dumb, but by all means making a pdf is not that hard, i would do it and its not really for me its more for people like the ones who started this post(no offence). With a program or PDF like that lots of questions would be answered easyly but who cares really.
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Unread postAuthor: ramses » Fri Nov 21, 2008 4:00 pm

on a somewhat off topic note, in the recent topics box this topic is displayed in a slightly amusing way:
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Unread postAuthor: Ragnarok » Fri Nov 21, 2008 4:14 pm

ramses wrote:Image

Blimey, that would be a lot of curry and beer to achieve that. :lol:

I wouldn't want to be around when the inevitable happened... :shock:
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Unread postAuthor: MrCrowley » Fri Nov 21, 2008 5:20 pm

john bunsenburner wrote:I would do it but i guess im too dumb, but by all means making a pdf is not that hard, i would do it and its not really for me its more for people like the ones who started this post(no offence). With a program or PDF like that lots of questions would be answered easyly but who cares really.

There would be loads of formulas that relate to the spudding world. You could probably cover most formulas used in physics. Gas and projectile density probably could come under nuclear, electronics is obvious, that'd cover DC circuits, maybe even electric fields, generators, magnetism and induced voltage. Linear mechanics, energy, springs, circles (centripetal and centrifugal forces), momentum and projectiles is also obvious, maybe even torque, vectors is another obvious one. You probably could even cover waves, such as sound and light waves and different frequency radio signals if your cannon is remote control...


Anyway, i'm sure you could cover every topic in physics with some sort of spudding application, i've got a small pamphlet here with formulas, and it has 3 pages just for Mechanics, Electromagnetism and Waves.

Then you still need to learn the formulas so you know what variable goes where, and you need to learn how to rearrange them incase you don't have two of the variables as opposed to one. If you can't do that, it'd require someone making a spreadsheet that automatically does it for you etc...

In short, you may as well just stop being lazy and learn physics yourself.
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Unread postAuthor: john bunsenburner » Sat Nov 22, 2008 9:19 am

As stated before i am not the one in need of those formulas, i build a gun see if i like it and if it is incredible THEN i will test it. I only build pneumatics(excuse my spelling i got into a fist fisht two days ago and my head is till somewhat messed up [i shall kill any one who says my head is always messed up]) and therefore i frankly do not care how many psi a propane gun at this and this temperature can achive(though i think 120psi is somewhat pathetic if you compare it to pneumatics which are easyer to build...) I was acctually only tring to help, asking if it would be possible to make a pdf or program with all the formulas, i would do it for pneumatics but im not sure about hybrids or combustions. I just wanted it to be like a little really helpful program that anyone could use for any gun and that would give them all the formulas ever needed, but it was just an idea and the reaction to it shows that it was not the best one, so never mind about it.
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Unread postAuthor: D_Hall » Sat Nov 22, 2008 10:02 am

Two thoughts....

1) Jimmy has already covered the thermodynamic theory of why you can't get over 122 psi so I won't do it again. I will say, however, that HGDT's model is pretty simple: I ignore chemistry changes (they're a second order effect when you're talking about air... 80% of your mixture is inert!). I then just magically assume that the air increases to XXXX degrees (I'd have to look up the exact number... suffice to say: stoichiometric flame temp). What does this do to your pressure? That's a high school level physics problem. The only problem is that it assumes that the combustion is instantaneous. It's not, of course. Thus, when I say that my combustion model is crude, what I am discussing is my flame propogation model. There are simply too many variables in the real world to do it right with anythen less than a hydracode. So I make some other assumptions (there are threads discussing what I did if you use the search functions).

2) You state/ask.....
jitup wrote:what type of a meter would you use to find this, I am thinking that you would have to have an expensive electronic pressure sensor hooked up to a computer to monitor the whole event. Do these sensors even exist?

As a matter of fact, yes, such gages exist and I've got a couple on order as we speak. They aren't cheap, of course. I'll be honest and say that I'm not 100% sure what gages I have on order as I've a guy who works for me who specializes in such knowledge and I simply told him what I was trying to do and he took it from there (I do know they're kulites (http://www.kulite.com), however. Cost? IIRC, he said he would be spending about $3k on pressure gages.).

My point: Give me a year and I'll be able to definatively answer the question you ask as I will be doing some tests very similar to what Jimmy has done but on a larger scale and with a few million dollars worth of instrumentation at my disposal.

Note: The above is NOT meant to poo-poo Jimmy in any way shape or form. What he did is flat out amazing (especially given his budget!). I just happen to be in a position to throw some significant resources at the problem.



edit: I've no idea if this is what I'm getting, but here is a gage that would do the job.... Design application is measureing the pressure inside an internal combustion engine's combustion chamber ( http://www.kulite.com/pdfs/pdf_Data_She ... CS-190.pdf ). edit again: Well... almost. I see that it's range is a bit high, but it proves my point about such gages existing (the hybrid folks would love that gage though!).
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Unread postAuthor: jimmy101 » Mon Nov 24, 2008 12:48 pm

How's it going D_Hall? I know you said that you can't really comment until everything gets published but any hints of where you are?

BTW, another useful source of info on the chamber max pressure, burn rate, techniques for measuring etc. is all the work that has been done on gasoline engines. You can indeed measure the pressure versus time of an operating ICE, you can also photograph it, measure power output and a ton of other things. Since an ICE is basically a ~10X hybrid spud gun, the info is pretty nearly directly useable. Need to do a bit'o fudging for the difference between gasoline and propane but there is surprising little difference between the various hydrocarbon fuels.
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Unread postAuthor: D_Hall » Mon Nov 24, 2008 7:21 pm

Your hint: It already weighs in excess of 3000 pounds and I've got approximately $3,000 worth of nuts and bolts on order. You read that right, $3,000 worth of nothing but nuts and bolts.
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Unread postAuthor: Ragnarok » Tue Nov 25, 2008 7:14 am

D_Hall wrote:You read that right, $3,000 worth of nothing but nuts and bolts.

That's either a lot of bolts, or some very high quality bolts - or somewhere in the middle.
...Maybe it's a load of spares as well?
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Unread postAuthor: Moonbogg » Wed Nov 26, 2008 7:43 pm

I bet people with experience just judge by how much power thre gun has. They have shot pneumatics and just judge what the PSI is likely to be from a combustion.
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Unread postAuthor: D_Hall » Wed Nov 26, 2008 8:23 pm

Ragnarok wrote:
D_Hall wrote:You read that right, $3,000 worth of nothing but nuts and bolts.

That's either a lot of bolts, or some very high quality bolts - or somewhere in the middle.
...Maybe it's a load of spares as well?


A few spares, but not many. Size is large (>1" diam). Quality is grade 5.


And I'm up to $4500 worth of nuts/bolts/washers.
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Unread postAuthor: starman » Wed Nov 26, 2008 8:47 pm

D_Hall wrote:And I'm up to $4500 worth of nuts/bolts/washers.


Mere coffee machine money for you guys, right Dave?... :wink:
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