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Hybrid Fuel Meter 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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Hybrid Fuel Meter Pressure?

Unread postAuthor: kjjohn » Thu Nov 05, 2009 6:09 pm

I was calculating the fuel meter size/pressure for a 6x hybrid on HGDT, when I realized that the number start to get very big. With a 127ci chamber volume and a .5" diameter by 12" length fuel meter, I need to set the MAPP regulator at 96 psi!!! MAPP/Propane tanks can't even put out that much pressure without a regulator! Is this correct, or is there something wrong?
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Unread postAuthor: c11man » Thu Nov 05, 2009 6:13 pm

are you sure you have the 127 cubic inch chamber correct?

in the summer you can get a easy 150psi out of mapp. winter is a different story. and if you cant get that much pressure two shots of 48psi out of the same meter will give you the same amount of fuel
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Unread postAuthor: kjjohn » Thu Nov 05, 2009 6:21 pm

The chamber in question is 3" dia. by 18". I tried making the fuel meter with a larger volume, increasing the diameter of the pipe to 1" on HGDT, and that worked out better. How long would a MAPP tank last with using such high pressures?
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Unread postAuthor: c11man » Thu Nov 05, 2009 6:36 pm

it will last for a decent amount of shots. but its not like your gonna be doing rapid firing with a hybrid this big
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Unread postAuthor: kjjohn » Thu Nov 05, 2009 7:06 pm

you mentioning rapid-fire made me think of something something: how hot will the chamber become after multiple shots? I know it will heat up significantly, but I am actually planning on making a cooling system surrounding the main chamber.

edit: Also, I have a question about the accuracy of HGDT. Will a larger barrel diameter actually achieve a higher velocity? I'd think the opposite, but HGDT seems to think that a 3" chamber with a 3" barrel will get the highest velocity!
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Last edited by kjjohn on Thu Nov 05, 2009 7:19 pm, edited 1 time in total.
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Unread postAuthor: c11man » Thu Nov 05, 2009 7:11 pm

um it wont heat up to much, this just just too big of a cannon to do rapid fire easily, i was being sarcastic
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Unread postAuthor: D_Hall » Thu Nov 05, 2009 7:27 pm

kjjohn wrote:edit: Also, I have a question about the accuracy of HGDT. Will a larger barrel diameter actually achieve a higher velocity? I'd think the opposite, but HGDT seems to think that a 3" chamber with a 3" barrel will get the highest velocity!

Are you using the same mass projectile? If so... Yeah, that makes sense.
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Unread postAuthor: kjjohn » Thu Nov 05, 2009 7:47 pm

I always enter the projectile diameter as the same diameter as the barrel. I always set the mass at about 4.5 oz, which is around the same mass as Sureshot's 2" UHMW round. But no matter what the mass, it always thinks that a same diameter barrel/chamber provides the most power. I suppose making the barrel too small would certainly choke the flow and make the velocity lower, but this confuses me a little.
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Unread postAuthor: daberno123 » Thu Nov 05, 2009 9:18 pm

If you don't scale the weight of your projectile as you increase the diameter of the barrel your velocity will always be higher.

Think of it this way. If you increase the diameter of the projectile there is more area for the pressure from your gun to act upon. Remember- the force on the projectile is equal to pressure times area. Therefore, by changing the area while keeping the same pressure the same you increase the force on the projectile, thus increasing the velocity.

Hopefully that made sense, if not I'm sure D_Hall could clarify as I'm sure he has a much clearer idea on how this all works.
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Unread postAuthor: kjjohn » Thu Nov 05, 2009 10:37 pm

Oh, now I understand. Thanks for the help! :)
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Unread postAuthor: MrCrowley » Thu Nov 05, 2009 11:29 pm

Just make the volume of your meter bigger to drop the required metered fuel pressure. I hope the chamber isn't made from PVC :)
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Unread postAuthor: D_Hall » Thu Nov 05, 2009 11:33 pm

Rule of thumb is the square/cube law.

Basically, if you scale the diameter of the projectile, you should scale the mass of the projectile by the cube of the linear scale.

Example: If you have a projectile that is 0.5" in diameter and weighs 4 oz and you want to get an idea of what would happen in a 1.5" barrel...

Scaler = 3. (1.5 in / 0.5 in = 3)

Scaler^3 = 3*3*3 = 27

New mass? 27 * 4 = 108 oz.
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Unread postAuthor: kjjohn » Thu Nov 05, 2009 11:41 pm

@MrCrowley - No, my chamber certainly isn't PVC! I'm going to be using SCH 40 galvanized steel.

@D_Hall - That explains a lot, thanks for the help.
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Unread postAuthor: starman » Fri Nov 06, 2009 12:21 am

kjjohn wrote:How long would a MAPP tank last with using such high pressures?


A tank of MAPP is likely to last the useful life of your cannon....mostly meaning until you get tired of shooting it.
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