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Thoughts on 10X large hybrids and DDT?

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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Thoughts on 10X large hybrids and DDT?

Unread postAuthor: D_Hall » Fri Nov 03, 2017 7:46 pm

OK, so I'm looking into building a large 10X hybrid and I'm aware that I'm not a DDT expert. Don't know that there are any around here but sometimes this place surprises me. In any event, I'm thinking that the standard multi-point ignition to minimize runup distances should work but I concede I've not gone to 10X before. Nor am I aware of it being done on a large scale. Thoughts are that it shouldn't be a problem but.... anyone disagree?

edit: D'oh! Propane and air. Nothing fancy.
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Simulation geek (GGDT / HGDT) and designer of Vera.
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Re: Thoughts on 10X large hybrids and DDT?

Unread postAuthor: Zeus » Sat Nov 04, 2017 4:00 am

For one, can I express my excitement.

Secondly, I daresay the risk of DDT under sane conditions is quite overplayed. Provided you avoid excess turbulence and stay away from silly geometries you shouldn't have problems,. But I gather you need to prove it safe to the higher ups/bean counters before you get the ok.

How large scale are we talking?
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/sarcasm, /hyperbole
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Re: Thoughts on 10X large hybrids and DDT?

Unread postAuthor: jackssmirkingrevenge » Sat Nov 04, 2017 6:09 am

I don't have much experience with large scale hybrids but in my many forays into relatively high mixes (up to 28x) with relatively small length to diameter ratio cylindrical chambers I've never experienced this phenomenon.

For one, can I express my excitement.


+1, I presume this is work related? Something along these lines?
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Re: Thoughts on 10X large hybrids and DDT?

Unread postAuthor: D_Hall » Sat Nov 04, 2017 11:40 am

Zeus wrote:How large scale are we talking?


Basically a rehash of Vera but 10X.
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Re: Thoughts on 10X large hybrids and DDT?

Unread postAuthor: jackssmirkingrevenge » Sun Nov 05, 2017 6:02 am

Basically a rehash of Vera but 10X.


I'm guessing the burst disk alone is going to put most EFPs to shame...
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Re: Thoughts on 10X large hybrids and DDT?

Unread postAuthor: jimmy101 » Sun Nov 05, 2017 2:07 pm

I'm obviously not an expert but ...

I kind of doubt anyone in the world knows much about DDT in a gas fueled gun that is this large. Everything I've ever read about DDT suggests it is difficult to predict and even difficult to reproducibly create.

Still, as a WAG, multiple sparks might reduce the chances of DDT since there wouldn't be sufficient run-up time nor sufficient distance.
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Re: Thoughts on 10X large hybrids and DDT?

Unread postAuthor: wdr0 » Wed Nov 15, 2017 6:56 am

I haven't a large sample of tests to go off but my hybrid at 10x has spark gap roughly every chamber diameter and to my knowledge have not experienced ddt.
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Re: Thoughts on 10X large hybrids and DDT?

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Re: Thoughts on 10X large hybrids and DDT?

Unread postAuthor: GalFisk » Mon Mar 19, 2018 11:14 am

The gas explosion handbook may have some interesting info in the chapter about explosions in vessels, pipes, channels and tunnels: http://www.gexcon.com/article/handbook-chapter-9
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Re: Thoughts on 10X large hybrids and DDT?

Unread postAuthor: matti » Mon Mar 19, 2018 1:22 pm

You probably know this, but Utrons CLGG might give some ideas on burst disc and combustion control, there is some good information in the chapter "Combustion and pressure control"

here is the document:
http://www.dtic.mil/get-tr-doc/pdf?AD=ADA462130

example,

"Pressure waves in the propellant gases are mitigated by careful ignition design.
Experiments in the 45-mm CLGG have shown that the speed of ignition and the
subsequent production of undesirable pressure oscillations can be controlled to a large
degree by the amount of energy used in the ignition process and the location of the
ignition sites.

The amount of diluent also plays an important role in combustion control and in
preventing the formation of pressure waves. A diluent can be either an inert gas such as
helium or excess fuel such as methane or hydrogen. It has been shown that if the diluent
level is high relative to the amount of free oxygen available, undesirable combustion
events (such as pressure waves or gaseous detonations) are simple to avoid.

The combination of obturator design, ignition stimulus, and “proper” propellant
chemistry allows the CLGG to operate without pressure anomalies even at very high fill
pressures."
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