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HDGT and Burst Disc 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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HDGT and Burst Disc pressure

Unread postAuthor: sharpshooter » Wed Oct 30, 2013 7:57 pm

I have been playing the numbers game for my newest design, and am getting in my target muzzle energy range by having a high burst pressure despite a small chamber and short(ish) barrel.

Specs are something like this (Dont have the data in front of me, as my home computer runs Linux)
2" x 10" chamber
2" x 36" barrel
2" burst disc, 400 psi rupture
5x mix
.5oz projectile
results in 1800 ft/lbs or so.

Does this translate into real world results? or are my calculations going to get skewed as I get to high and higher burst pressures?

Also, while on the subject of HGDT, how close will this calculation be to a piston hybrid? (I had something similar to SpudBlaster's TBMA-230 V2.0 and Crowley's Mjollnir). I remember reading a 6ms opening time.
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Re: HDGT and Burst Disc pressure

Unread postAuthor: jackssmirkingrevenge » Thu Oct 31, 2013 5:18 am

The higher the burst pressure of the disk, the higher pressure is generated in the chamber before it does burst, simple as that, and since pressure is directly related to the force on the projectile, this will obviously result in greater muzzle energy.
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Re: HDGT and Burst Disc pressure

Unread postAuthor: sharpshooter » Thu Oct 31, 2013 8:48 am

That makes sense to me, I just don't understand the math going on in the background. Is there any known limits to the software where the results start getting unrealistic?
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Re: HDGT and Burst Disc pressure

Unread postAuthor: jackssmirkingrevenge » Thu Oct 31, 2013 9:52 am

I think you'll have to ask D_Hall for the best answer to that, though in my experience it has been fairly accurate and I have modelled and tested up to 28x.
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Re: HDGT and Burst Disc pressure

Unread postAuthor: sharpshooter » Thu Oct 31, 2013 1:47 pm

Well if its not common knowledge, I think its safe to say I would have to go pretty extreme to break it. Perfect, as always, D Hall
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Re: HDGT and Burst Disc pressure

Unread postAuthor: DYI » Tue Nov 26, 2013 10:16 pm

HGDT is typically less accurate than GGDT for the "common" performance envelopes considered around here, but is probably still within 20% (GGDT can be scary accurate at low speeds, say less than 200m/s). HGDT and GGDT are both 0D codes (meaning that the gas is considered to have negligible mass and very high sound speed relative to the problem being modeled), and this is where their major limitations arise. These limitations are not, however, important in "most" spudding applications. HGDT has a further limitation in that combustion is an inherently 3D process, so Dave ended up starting with an approximate 1D theoretical model based on the chemical kinetics of the problem and tweaking it to produce output more closely fitting the (admittedly sparse) experimental data. That's my understanding of the situation, at least. Unfortunately, the code is closed-source, so no one but Mr. Hall knows for sure.
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