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Predicting the performance of pneumatic guns is a well-established field, dominated by the <a herf="http://www.spudfiles.com/spud_wiki/index.php?title=GGDT">GGDT</a>, but predicting the performance of combustion spudguns is (as of the time of righting) rather speculative.
To the knowledge of the author, there are only two software packages which attempt to model the performance of combustion cannons:
1) <a href="http://www.spudfiles.com/forums/viewtopic.php?t=1602&">sim gun</a>
2) <a href="http://www.spudfiles.com/spud_wiki/index.php?title=EVBEC">EVBEC</a>
The first, sim gun, does not give results which agree with the <a href="http://www.burntlatke.com/launch.html">latke</a> chamber:barrel ratio tests when using reasonable physical constants.
It is possible to get reasonably close by, for instance, setting the pressure Increase to a factor of 2.5:1 - but such a expansion factor is not consistent with observed data.
...and it's dangerous to use garbage inputs in a physics-based modeling technique.
The second, EVBEC, <i>does</i> agree with the burnt latke test data.
This is because it simply extrapolates from the burnt latke test data - which, incidentally, leads directly to it's weakness:
Because it's not physics based, the accuracy of EVBEC falls off when the launcher deviates from the configuration of the launchers used in the tests.
The author of this article believes EVBEC to be the superior application; at least for the moment.
Jimmy (of the spudtech forums) is working on a physics-based model of combustion launcher, which will hopefully be more accurate than the last one.
<b>A bit on the workings of EVBEC</b>
EVBEC 1.4, as I said, simply extrapolates from the performance of the latke launchers.
When preforming this extrapolation, it assumes that the energy produced by the chamber is proportional to it's volume and it's pressure, that the fraction of this energy given to the projectile is constant for any specific chamber:barrel ratio, and that one can account for atmospheric pressure if one knows the amount of energy it takes to push that air out of the barrel.
This means that it has to take the following into account to get muzzle energy:
1) Chamber volume
2) Chamber pressure
3) Atmospheric pressure
4) Barrel volume
And, finally, it has to know the mass of the projectile to turn the muzzle energy into a muzzle velocity.
<b>Use of EVBEC</b>
Step one, of course, is to <a href="http://www.spudfiles.com/forums/viewtopic.php?t=1212&">get EVBEC</a>. It is an excel spreadsheet, so you'll be needing microsoft <i>excel</i> (although I believe it also runs in <i>open office</i>)
Second, is to enter the some data pertaining to your launcher and projectile; there are 5 of them:
This variable is entered in cell C4
If your combustion chamber is a simple shape, you can calculate it using the mini-tool labeled "volume calculator". This tool assumes that the air in your combustion chamber is in the shape of a cylinder, and you'll need to know the inside diameter and length of your chamber.
Remember that the inside diameter of the pipe is <i>not</i> it's Actually inside diameter - the Actually diameters for various nominal pipe sizes are listed <a href="http://www.harvel.com/pipepvc-sch40-80-dim.asp">here</a>.
If, on the other hand, your chamber is not a simple cylinder, you have two options: you can measure the volume of your chamber directly (by filling it with water and measuring the volume of that water), or by calculating through geometry.
Remember than EVBEC wants this value in cubic inches, so if you calculate the volume of the chamber in some other unit, make sure that you convert the values.
enter in cell C5
This is the average pressure of the gas in the bore in PSIA - which, most of the time, is equal to the atmospheric pressure.
If you don't know your atmospheric pressure, it'd be best to leave this at 14.7 psi
This variable allows you to model the launching of combustion cannons at altitude, or with evacuated bores.
Note that, if you have an evacuated bore - unless you have a perfect vacuum - your gasses will be compressed, so the actually average pressure will be higher than the initial pressure... I haven't incorporated a tool to do this with, but such a tool will appear in the public release of V1.5.
This is the pressure inside the launcher, in PSI relative to the pressure in the bore. Although you'd generally set it to 0, you wouldn't in the following situations:
1) When running a hybrid mix. (in this case, you'd set the pressure to the pressure you gauge reads before ignition)
2) When operating with an evacuated bore. (in this case, you'd use the pressure differential between your "atmospheric pressure" value and the actual pressure in your chamber. This means that the sum of the "atmospheric pressure" and the "interior pressure" is equal to the actually "interior pressure" in PSIA)
This is the volume of the barrel in cubic inches.
If your barrel is round (as is all pipe), you can use the mini-tool directly below (labeled "volume calculator").
Remember to <a href="http://www.harvel.com/pipepvc-sch40-80-dim.asp">look up</a> the inside diameter of the pipe, because it's not equal to the nominal diameter.
If your barrel is square or something, you'll have to calculate it's volume manually.
Remember to input your answer in cubic inches.
This is the mass of the projectile in grams. Simple enough.
There are roughly 450 grams in an pound, or 28 in a ounce.
There is a mini-tool entitled "mass calculator" directly below for calculating the mass of a projectile (assuming that it is cylindrical).
Use is fairly simple; just enter the length, diameter, and density of the projectile.
For reference, a potato is roughly 1.1 g/cm^3.
You'll want to at least enter your barrel diameter in this mini-tool, as it is critical for the operation of cell I6.
Once these values have been entered, look at cell K6; it will tell you which dataset to use (which depends on the sectional density of the projectile - the result in cell I6).
There will be 3 lines going diagonally, and one going vertically.
If K6 said "use 3/4 or 2.5 inch data", find the intercept of either the blue or the green lines (these are the lower two) and the vertical line. Read off the velocity value on the y-axis of the graph.
If K6 said "use 1.5 inch data", find the intercept of the magenta (top) line and the vertical line. Read off the velocity value on the y-axis of the graph.
If you wish, you may attempt to manually extrapolate between these values... your guess on how to do it is as good as mine.
The 3/4" data was done at a sectional density of 10g/cm^2
The 2.5" data was done at a sectional density of 12g/cm^2
The 1.5" data was done at a sectional density of 26g/cm^2
Your sectional density can be found in cell I6
<b>Limitations of EVBEC, accounting for them</b>
As I said, EVBEC is limited. Here's my guess on how to adjust for differences between your launcher and theirs:
<i>Very small (under 150 ml) chamber</i>:
Increase velocity slightly
<i>Very large (over 5 liters) chamber</i>:
Decrease velocity slightly/somewhat
Decrease velocity somewhat
<i>Huge numbers of ignition points</i>:
Increase velocity slightly
<i>Using MAPP gas</i>:
Increase velocity 10%
<i>NOT using a fan</i>:
Decrease velocity 25%
Ya know what?
I really shouldn't have made it a poll.
Anyways, one person said that the "use" section needs improvement.
I don't imagain I'll be so lucky as to get that specific voter to answer - but that's not important - just so long as <i>someone</i> tells me what I need to improve.
Voted "limitations" since your proposed adjustments are educated guesses at most.
There are so many variables (some random) involved that I don't think accurate modeling is even a realistic goal: projectile friction along the barrel, the heterogeneous nature of the fuel/oxidizer mix and the influence of heat on the aforementioned, plus spark position, temperature and surface, altitude, humidity etc.
Obviously some have a more dramatic effect than others, but the sheer number of options make even observational methods biased.
However I do feel that the derivative method is the most practical; it just needs to be extended to include as much and as diverse data as is possible.
Perhaps more data can be collected at meetings and/or gathered online.
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