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Postby fullmetaljacket » Wed Nov 01, 2006 11:16 pm

It's been a while since I played with some proper physics, is there some way you can calculate the pressure in the barrel required to accelerate the golf ball of given weight and diameter to the velocities you've measured within the length of the barrel you used?

edit: I did some vague calculations, it's at least 2,000 psi by my reckoning...
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Postby joannaardway » Thu Nov 02, 2006 7:35 am

I can totally believe the velocity figures. I haven't watched the videos (yet - not working from school).

Plugging the known figures into my range calculator and seeing what velocity would be required to get the estimated range, then I get figures of about Mach 1.5, or higher.

The range calculator isn't entirely accurate into and above the transonic region, but if anything it has underestimated the velocity required.

It would be interesting to see if the results from the chrony are massively effected by moving it back slightly. I wouldn't want to hit the chrony though - it would be totally wrecked.

Although the pressures required would be incredible, they are achievable.

The average acceleration needed to reach Mach 1.5 in 5 feet of pipe would be about 8500 Gs - not impossible, and actually fairly common with smallish projectiles.

The force needed would be an average of 3750 newtons. Dividing by the barrel area of 14.52 cm^2, that gives a required pressure of 25.8 bar - 374 psi on average.

374 psi would be totally new and unforeseen, but achievable.

So, in saving you some of the maths - *sits down to wait for responses*
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Postby fullmetaljacket » Thu Nov 02, 2006 9:24 am

To be fully convinced I´d need to see some actual penetration figures, say how many equally spaced 3/4 inch sheets of ply (like will the golf ball penetrate, compared to other launchers firing similar ammunition that have been independently chronied.
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Postby SpudStuff » Thu Nov 02, 2006 12:49 pm

Hm. That seems like and effictive way. Although I shot a golf ball at 1/2" ply and it bounced back. It missed me by a couple feet. I'm not doin that again.
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Postby sgort87 » Thu Nov 02, 2006 1:03 pm

This should go through one 3/4" ply easily. The Blue Flame goes through one just fine. I haven't tried two or more.
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Postby jimmy » Thu Nov 02, 2006 1:14 pm

BigBang... cool someone who actually wants to figure it out, and has the background to do so.

Your analysis of the energy lost to recoil appears to be correct though I'll need time to digest it.

The calculated energy in the amount of fuel used needs to take into account the final temperature of the gases. The heat of combustion is generally expressed relative to absolute zero (IIRC). In addition, the calc needs to take into account the pressure in the chamber when the projectile exits the barrel since that energy is also lost. In order to get most of the work out of the hot gases the chamber pressure would have to be at atmospheric when the projectile exits the barrel and the gases would have to be near ambient temperature. I suspect that if you fire your gun at night you will see a muzzle flash, indicating the gases are still 1000C or hotter when the GB leaves the barrel.

In order to get a supersonic launch then you need to get combustion gases moving through the barrel at supersonic speeds. I don't believe that is generally possible. At least not with the amount of energy we have available in a combustion gun. In compressed gas guns you have to use helium as the gas since the speed of sound is faster in helium. The helium is moving sub-sonic (for helium) but supersonic for air.

The "generally accepted" maximum chamber pressure in a typical propane combustion gun is 50 or so PSI. The calculated maximum pressure is only about 100psi. These numbers are for conflagration. Detonation could reach much higher pressures though people get a little worried about safety when a gun is working in the detonation mode. Hence the general aversion to the use of acetylene in spud guns. One advantage MAPP has over acetylene as a fuel for torches is its much lower tendency to explode (detonate) compared to acetylene.

Looking at the numbers joannaardway posted, "374 PSI on average" is not realistic in a combustion gun. It isn't even realistic in a compressed gas gun. If the average pressure is 374 then the <i>peak</i> pressure would be even higher since the chamber is about twice as big when the GB exits the barrel then it was when the GB started to move.

It would be interesting to dig up (or calculate) the energy in say a 50 caliber military round. The weight of a 0.50 is probably somewhat less than a GB. IIRC, the muzzle velocities for a 0.50 is ~2000fps (roughly mach 2). So we have roughly similar velocities and roughly similar projectile masses. How does the amount of energy in XXcc of MAPP compare to the energy in YYgrains of modern powder used in the 0.50? Are the two amounts of energy even within an order of magnitude of each other? Heck, are they even within two orders of magnitude?

One last thought. A supersonic spud gun has been one of the "stretch goals" of spudding for a long time. Very few guns have ever been able to achieve it. To get to Mach 1 takes a concerted effort and generally a fair amount of $$$$. If PVC, MAPP and a GB in a moderately large gun easily breaks the sound barrier people would know about it. I am not aware of anyone ever reaching Mach 1 with a standard combustion gun. The only examples of Mach 1 I know of were compressed gas guns using helium for the gas.

Does anyone know of a hybrid gun that has believably broken Mach 1?
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Postby SpudMonster » Thu Nov 02, 2006 1:47 pm

Both Jspellman and Spudinator's hybrids broke mach 1, and I bet other people's hybrids did too.

Don;t forget, in a combustion gun, the temperature of the air is highly increased, and as the temperatureof a gas increases, so does the speed of sound in that gas. Propane burns at 3600 degrees fahrenheit. MAPP burns slightly hotter. At 3000 degrees fahrenheit, the speed of sound in air is over 4,000 FPS according to ... ouspe.html
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Postby sgort87 » Thu Nov 02, 2006 1:57 pm

We are comparing it to the speed of sound in the atmosphere into which the golf ball is launched.
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Postby aturner » Thu Nov 02, 2006 5:09 pm

<blockquote id="quote"><font size="1" face="tahoma,verdana,arial" id="quote">quote:<hr height="1" noshade id="quote">Originally posted by aturner
[br]jimmy, are you aware of any other data on the velocity of GB's fired from a combustion launcher such as this? 3800ml chamber and a 5' barrel of 1.5" SDR21? I don't recall every seeing any similar data prior to this.
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">

<blockquote id="quote"><font size="1" face="tahoma,verdana,arial" id="quote">quote:<hr height="1" noshade id="quote">Originally posted by jimmy
[br]Is it really all that different than a hundred other guns? The projectile doesn't make a lot of difference, it has mass, it has friction..... <hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote"> Yes, but a GB in 1.5" SDR21 is nearly a float fit, depending on the specific GB and pipe. Quite different than a potato or a gasketed wooden slug. It's Apples to Oranges.

Personally, I don't recall anyone trying what BigBang has tried. Because of this, I contend there is no reference point from which to judge BigBang's claimed data as being correct or not. However, I am intrigued by the fact that he achieved relatively stable velocities over a string of shots.

So, I think it is much more productive for a couple of folks to attempt to independently replicate the test, rather than argue. I can tell you that as soon as I have a free evening, I'm planning to run a few GB's though my chrony. It's got to be one of the simplest tests that could be run, b/c the ammo is relatively standard.
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Postby BigBang » Thu Nov 02, 2006 6:19 pm

Wow, lots to consider here! Regrettably, I'm pretty much out of ideas to further the discussion. I'm a laser jock by trade so these topics are wandering well away from my technical comfort zone. I just wish the weather were a little warmer here so I could take the Crusader out and collect more data. I will, of course, when I get the chance.

Till then, I will leave it to those of you better prepared than I to wrestle with the issue of whether or not supersonic speeds are possible with this type of cannon. Of course, I'd be happy to discuss the gun with any who are interested.

Thanks to all for your kind comments, and for the illuminating debate. Our silly little hobbie will be better for it in the end, no matter what the result.

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