Part V: Adiabatic Gun Model

Post questions and info about combustion (flammable vapor) powered launchers here. This includes discussion about fuels, ratios, ignition systems and anything else relevant to launchers powered by igniting things like hairspray or propane.

Postby Freefall » Tue Feb 06, 2007 12:05 pm

<blockquote id="quote"><font size="1" face="tahoma,verdana,arial" id="quote">quote:<hr height="1" noshade id="quote">Of course, this approach limits the placement of the sparks. Symmetrically placed sparks would be easy. Asymmetric sparks, for example, two sparks in a 10" long chamber with the sparks located at 2.5" and 5", might be tricky.<hr height="1" noshade id="quote"></font id="quote"></blockquote id="quote">
Not really. Just model it as individual chambers with an imaginary barrier halfway between each spark pair.
Example: If you have a 10-inch chamber with three sparks placed at 3, 5, 7 inches, model it as one chamber four inches long with the spark at 3 inches, one chamber 2 inches long with the spark at 1 inch, and one chamber 4 inches long with the spark at 1 inch.

Using your own example, you'd model it as one virtual chamber of length 3.75", with the spark at 2.5" and a second virtual chamber of length 6.25" with the spark at 1.25".

Of course, the flamefront center and virtual chamber ends will have to move with each timestep. That shouldn't be too difficult, since you're already moving the flamefront center to accomondate the gas expansion due to the breechside flamefront.
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Postby jimmy » Tue Feb 06, 2007 12:30 pm

Yes, but that gets back into the problem of actually modeling the individual flame fronts. Three sparks with non-uniform spacing means I have to model 6 different flame fronts (and "burn out" events). That would be fairly easy if I rewrote the model in a real programming language. I think it'll be a real arbeit in a spreadsheet.

The beauty of modeling symmetrical sparks is that only one of the sparks is <u>actually</u> modeled and it makes no difference if there is two or ten sparks.

I'll need to cogitate on this more.
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Postby Ric » Tue Feb 06, 2007 10:37 pm

OUCH.

I'm glad to have models like these, but this all reminds me of why I sucked at calculus.
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Postby FLONE » Wed Feb 07, 2007 10:26 am

<blockquote id="quote"><font size="1" face="tahoma,verdana,arial" id="quote">quote:<hr height="1" noshade id="quote">Originally posted by jimmy
Flone: Yes, that should work, but you would need a combustion gun with a schrader valve and a pressure gauge. (Come to think of it, my standard gun has a schrader valve as the fuel port.) If I wasn't so lazy, I would just take a spud and shove it halfway into a barrel then invert the barrel and ram rod and push it down onto a bathroom scale and try to measure the static and dynamic friction.

<i>"Jimmy, are you going to try a longer barrel on your spudder to verify the calcs?"</i>
I can't change the barrel on the "standard gun", I would have to build a new gun.

<i>"And I am curious about the short fat chamber concept, if anyone has built a short 6" instead of a longer 4". Yes it is a lot more expensive."</i>
That would be a great experiment wouldn't it? The model suggests that the difference in performance should be pretty significant. It would be cheaper if you went the other way. Instead of building 6"D and a 4"D guns build 4"D and 2"D guns (with the same chamber volume).
<hr>
<hr height="1" noshade id="quote"></font id="quote"></blockquote id="quote">
As for your formula's, would using a bathroom scale would result in a different unit of measure than the psi idea?

What better motivation to build a new spudgun than proving a point? Is the $30 cost an issue?

My 6" chamber thought would use a 2" barrel. As was mentioned before the length of the fittings available locally would create a chamber about 9" long. Think that would be "short fat" enough?

Thanks.
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Postby jimmy » Wed Feb 07, 2007 2:00 pm

FLONE said: <i>"As for your formula's, would using a bathroom scale would result in a different unit of measure than the psi idea?"</i>

Actually, the current model uses pounds force as the input value for the friction terms so the reading from a bathroom scale would be correct. (The model converts from pounds force to Newtons for the calc's.) Even if it wasn't, it would be easy enough to convert to the proper units.

<i>"My 6" chamber thought would use a 2" barrel. As was mentioned before the length of the fittings available locally would create a chamber about 9" long. Think that would be "short fat" enough?"</i>

I think so. The following is what the current adiabatic model predicts.

A 6x9 chamber has a volume of 255in<sup>3</sup>. A 2" barrel would need to be 101" (2.56m) long for an 0.8 C:B.

<table border=1><tr><td align=center> Chamber<br>Dimensions,<br>DxL (inch) </td><td align=center>  FPS  </td><td>    FPS<sup>2</sup>  </td></tr><tr><td align=center>3x36</td><td align=center>307</td><td align=right>94,000</td></tr><tr><td align=center>4x20-1/4</td><td align=center>490</td><td align=right>240,000</td></tr><tr><td align=center>6x9 </td><td align=center>690</td><td align=right>476,000</td></tr><tr><td align=center>6.9x6.9 </td><td align=center>699</td><td align=right>489,000</td><td> <--"perfect" short-fat</td></tr></table>
So it looks like the 6x9 chamber should significantly out perform the same sized chamber made from 3" or 4" pipe.
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Postby Numbuh 16 » Thu Feb 08, 2007 12:46 pm

These threads make my concept models look like total crap. >.<
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Postby FLONE » Thu Feb 08, 2007 12:54 pm

One observation regarding spud friction. When I partially start the spud in the muzzle, do my spray and pray, cap it, then seat the spud, it rarely will stay put. If the barrel has an inside bevel, it usually will stay still. No bevel, no stay. This is on my 1.5 and 1.7 c:b combustions.
Thanks
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Postby FLONE » Thu Feb 15, 2007 9:45 am

Let me see if I can make sense of my question.

Looking at the Latke site, especially the 1.5" tests I wonder what if: at each phase of the tests, prior to shortening the barrel to increase the C:B ratio, the 2600 cc chamber would be replaced with the 3000 cc chamber. That would effectively increase the C:B ratio by about 15%, which would approximate the next ratio to be tested.

So, would the spud speed from the 3000 cc chamber be higher than the speed from the 2600 cc chamber with the same barrel length? Hope I make some sense and not replicate a previous post.

Interesting to note that a lot of the spudgun chambers are larger than a lot of todays car engines!
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Postby jimmy » Thu Feb 15, 2007 2:59 pm

Flone:

I think the answer to your question is a definite Yes/No/Maybe.

I would think that if the current barrel on the 2600cc chamber was too big (C:B too small) then a larger chamber would raise the C:B, getting it closer to the optimal C:B, and the muzzle velocity would increase.

If the current barrel on the 2600cc chamber was too small (C:B too big) then using a larger chamber would make the C:B even worse and lower the velocity.

So, yes, a bigger chamber is better if the barrel was too long to start with, but a bigger chamber is worse if the barrel was already the correct length or if the barrel was already too small.


<i>Interesting to note that a lot of the spudgun chambers are larger than a lot of todays car engines!</i>

Yep, but you have to remember that a typical car engine is essentially a "10:1 hyrbrid spud gun with a 0.1 C:B ratio". A "10:1 hybrid" puts out a heck of a lot more power than a "1x" spud gun.
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Postby boilingleadbath » Thu Feb 15, 2007 7:13 pm

...arn't engeins measured by their maxium displacement - so that 10x (sub-optimum) mixture is only 1/10th of the volume listed?

Of course, most decent-sized spudguns can't do a few thousand spuds-per-second, so I think it's a rather silly comparison.

An interesting thing I noticed is that, when one takes my new EVBEC 1.5 and model a launcher with an fat chamber/skinny barrel operating in a vacume, and a projectile of mass 0 (which, no, does not cause it to fail)... the projectile velocity is rather constant across the ratios.

This (along with a host of other things) sugests to me that the combustion of the FA mixture is alot quicker than your model gives credit to.

Which confuses the heck out of me (because of your pressure trace)... but that's the way it appears.
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