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Hydrotesting Vs. Shock Pressure

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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Hydrotesting Vs. Shock Pressure

Unread postAuthor: randompkguy » Fri Jan 29, 2010 12:39 am

Is hydrotesting an accurate assumption of safety for hybrids since they produce a pressure shock, as opposed to releasing stored pressure like a pneumatic does?

I'm building a 20x hybrid, and I plan to hydrotest it, but I'm worried that since the explosion creates an instant pressure load, the acceleration of the gases will create more force than expected.

The barrel for the hybrid is .065" wall .745" id DOM tubing. Playing it safe i calculated the hoop stress from 80% of the lowest known yield strength of 1020 steel i could find (40,640psi) with a 3x safety factor for a safe working pressure of 2364 psi. The Chamber is .188" wall 1.625" id DOM with .75" thick end caps TIG welded in place. I calculated the working pressure to be 3134 psi.

I think I'm going to hydro at around 4000 psi to be safe.

Here is a sneak peak
Image
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Unread postAuthor: inonickname » Fri Jan 29, 2010 12:42 am

With the dimensions and information supplied, you'll be safe. Hydrotesting it at 4000 psi will be plenty. Hydrotesting will place more stress on the components than the temporary shock as the pressure drops rapidly.

Looking forward to seeing the rest.
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Unread postAuthor: randompkguy » Fri Jan 29, 2010 12:44 am

wow that was fast....
Thanks for the assurance. If all goes well i plan to make an equally nice stock so i can shoulder fire it :lol:
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Unread postAuthor: Jeeperforlife » Fri Jan 29, 2010 12:46 am

Very nice, I think it is time I get a TIG. What length is the barrel and what kind of ammo is planned?
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Unread postAuthor: randompkguy » Fri Jan 29, 2010 1:21 am

34 inches or so. One more thing....is it necessary to stress relieve the welds? I believe I have the stuff to do so if it is.
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Unread postAuthor: EvilFish » Fri Jan 29, 2010 1:31 am

Sorry Randompkguy, but I don't think that a 34 inch long hybrid is going to be able to be shoulder fired...
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Unread postAuthor: randompkguy » Fri Jan 29, 2010 1:55 am

err.. i meant hand held, and why not? the barret 50 caliber rifle generates at least 3 times the muzzle energy of a 20x hybrid

EDIT: can you tell this is my first hybrid?
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Unread postAuthor: Killjoy » Fri Jan 29, 2010 1:59 am

34" inches is actually a reasonable length for many firearms so I don't see why a 34" hybrid would be any different, it would just need a good stock. Hell, I have a 60" 20x hybrid I can shoulder fire.

I don't know about stress relieving welds, you might want to shoot larda an email about that one, I know he would know for sure. Else, just wait till a member who knows rolls by.

Looking forward to seeing more.
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Unread postAuthor: randompkguy » Fri Jan 29, 2010 2:01 am

sorry to add to the questions, but any idea how to calculate burst disk failure pressures?
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Unread postAuthor: inonickname » Fri Jan 29, 2010 2:27 am

randompkguy wrote:err.. i meant hand held, and why not? the barret 50 caliber rifle generates at least 3 times the muzzle energy of a 20x hybrid

EDIT: can you tell this is my first hybrid?

Yeah :wink:

The .50 bmg casing will produce substantially higher pressure than any gun that has been posted here, over a much longer time. However, guns like this are scalable, and a 20x hybrid with a 2" bore could easily be more powerful than an m107.

On that note, the M107 is not intended to be shoulder fired. This is some rubbish they come up with in games like call of duty (another example, a PTRS-41, which produces around 25,000 foot pounds of muzzle energy and can shoot through 4+ cm of hardened armor plating won't penetrate completely through a body..yay for cod-and without "stopping power" they will survive 80% of the time- frankly, bullshit.). The gun is extremely heavy. Yes, 1 or 2 people can carry it around. One person can shoulder it and fire it. Practical? no. The recoil from guns like this is massively over represented. An M107 can be fired (with a bipod) by a 10 year old. I'm sure I've even seen a video of it.

http://www.youtube.com/watch?v=Gyjo5BOb ... re=related

It's not meant to be fired from the shoulder or hip by any means. It's meant to be firmly on the ground when you pull the trigger.

But yes, you could probably put a stock on this and fire it without support.
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Unread postAuthor: jackssmirkingrevenge » Fri Jan 29, 2010 2:39 am

inonickname wrote:The .50 bmg casing will produce substantially higher pressure than any gun that has been posted here, over a much longer time. However, guns like this are scalable, and a 20x hybrid with a 2" bore could easily be more powerful than an M107.


This is what many people fail to appreciate when it comes to scaling projects up or down. A 2" hybrid has 4 times the projectile diameter of the 50 BMG, meaning it has 16 times the projectile base area. This means that what the '50 can do with say 50,000 psi, the hybrid can do with "just" 3,125 psi.

Make a 4" cannon and the base area is now 64 times bigger than the '50, meaning with just 780 psi from say a paintball tank and a burst disk valve, you could conceivably get even more energy from a pneumatic ;)

This works in reverse too, which is why a 0.1" calibre pen gun needs high pressure for good performance as the base area is decreasing exponentially.
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Unread postAuthor: mechtician » Wed Apr 07, 2010 7:34 pm

I've been lurking for several months, reading about hybrids and calculating and scheming before building one, but after seeing this topic I couldn't help but speak up :P Very nice welding, by the way! IMHO, TIG welds on CrMo don't need to be stress relieved, as long as you used the correct type & size of filler rod, which (judging by the good looking weld) it looks like you did. Same goes for stainless. Just out of curiosity, did you backpurge the chamber during welding?

As far as hydro testing and working pressure calcs, in my experience CrMo is similar enough in strength to SS tubing that SS pressure rating charts are a good benchmark. For example; http://www.unifiedalloys.com/products/pdf_directory/main_collection/stainless_pressure_tube.pdf Shows the theoretical burst pressure of 1.625x0.148Wall 316L toob to be 13000ish psi. I agree with inonickname, hydrotesting will place more stress on components than shock loading with gas. In short, you have nothing to worry about :)
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Unread postAuthor: randompkguy » Wed Apr 07, 2010 11:45 pm

yeah, I'm no longer worried. Also where are you getting CrMo from? DOM tubing is usually 1026 mild steel. No backpurge, as the end plugs were well machined to close tolerances, and about .75 inches thick, so the gas wouldn't help too much.

thanks for teh comment on mah welds. I love my miller :D
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Unread postAuthor: Gippeto » Thu Apr 08, 2010 12:07 am

randompkguy wrote:sorry to add to the questions, but any idea how to calculate burst disk failure pressures?


Might help you...pull your thinking cap on tight and hold on. :lol:


http://www.ansys.com/events/proceedings ... RS/227.pdf
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Unread postAuthor: SpudBlaster15 » Thu Apr 08, 2010 2:22 am

randompkguy wrote:sorry to add to the questions, but any idea how to calculate burst disk failure pressures?


If you assume the disk bursts uniformly around its circumference and doesn't deform significantly under pressure, this formula I derived a few years ago should be fairly accurate:

P<sub>B</sub> = 2*S<sub>T</sub>*t/r

Where P<sub>B</sub> is burst pressure, S<sub>T</sub> is the tensile strength of the material, t is the thickness of the disk, and r is the radius of the union port.

That formula is simplified, but it's based on taking the lateral surface area of the disk, multiplying it by the tensile strength of the material to obtain the force required to rupture it around the circumference, then dividing that figure by the cross sectional area of the disk to determine the failure pressure.
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