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Possible large burst disk hybrid build(Update with video)

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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Re: Possible large burst disk hybrid build

Unread postAuthor: mark.f » Thu Sep 18, 2014 5:29 pm

Arduino's the easiest thing in the world to learn. Interfacing with a sensor like this is a simple few lines of code, and there's libraries for sending the output to an LCD like mobile chernobyl said, or you can simply do something like light an LED within a certain window containing your target pressure, or a bar of LED's (red -> orange -> yellow -> green or something like that).

I could also show you how to use a cheap LM339 comparator (one IC would be able to give you a two LED bar display) if you have a model of transducer you plan on buying, but like mc said as well, with the Arduino you can do other stuff to further automate the fueling/firing as well, and the 339 involves calibration and trimpots which could be a deterrent.
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Re: Possible large burst disk hybrid build

Unread postAuthor: wdr0 » Thu Sep 18, 2014 9:33 pm

I will defiantly look into it. I think I am going to start with building the cannon itself, fueling and ignition will come after. I do already have a plan for ignition though. I am going to continue looking for transducers, I may just start with the two gauges. I am really interested in going the automated electronic route though.
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Re: Possible large burst disk hybrid build

Unread postAuthor: D_Hall » Sat Sep 20, 2014 10:40 am

Shouldn't even need something like an arduino. VERA simply uses a cheap three digit gauge that takes a 0-5 V input. Since my transducers (yes, VERA uses a 2 transducer system) have 0-5 V outputs.... Well, it's butt simple. Plug the goesouttas into the goesinnas and voila... a 3 digit read out of current pressure.
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Re: Possible large burst disk hybrid build

Unread postAuthor: wdr0 » Mon Sep 22, 2014 10:54 pm

I have been working on the welding, I have one flange left to weld on. Thanks for all the suggestions on pressure sensing, I have decided to go with the transducer route as I was able to get readouts for cheep on ebay. I should have the welding done sometime this week and possibly a first test firing by the end of next week.

I as of tonight have all of the remaining components ordered. I am going to try an interesting idea for the spark gap(s). I am having a freind build a Marx generator. It's a capacitor arrangement that will generate a high voltage pulse. I am going to attempt to discharge it through 20 spark gaps(one every diamiter so for ten feet of 6" pipe 20). I expect I will end up having to use fewer gaps to get a reliable spark though.
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Re: Possible large burst disk hybrid build

Unread postAuthor: wdr0 » Sun Sep 28, 2014 11:06 pm

So the marx generator plan did not work out. It worked, just did not make a long enough spark, and it discharged inconsistently. I really didn't quite like the idea of waiting for 30 seconds with no exact idea when the cannon would fire. However I did get a working ignition system. Two ignition coils connected with to 120 with capacitors and a dimmer switch produce a almost continuous 3" spark. Should be plenty to overcome the 20 gaps and pre ignition pressure.
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Re: Possible large burst disk hybrid build

Unread postAuthor: matti » Mon Sep 29, 2014 4:17 am

Not sure if 20 spark gaps makes a much different then one or two in a hybrid. I would think that the burning time does not matter very much in hybrid where bust disk or valve keeps the chamber closed as long as the pressure is risen to the opening level ? But.. it wont propably do any harm, so if you can make 20 spark gaps, I dont see no problem in that :)
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Re: Possible large burst disk hybrid build

Unread postAuthor: wdr0 » Mon Sep 29, 2014 8:02 am

That was about what I was thinking. My main concern was DDT. The current advice on here seems to indicate multiple spark gaps will prevent it, and it is more likely in long skinny chambers. So with such a long skinny chamber I decided having a large number of sparkgaps couldn't hurt so that's what I am doing. The newest issue that has arisen is I am not sure if my transducers are compatible with my readouts. But I have not had much time to play with them yet so I may just not have hooked them up correctly.
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Re: Possible large burst disk hybrid build

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Re: Possible large burst disk hybrid build

Unread postAuthor: matti » Mon Sep 29, 2014 1:15 pm

wdr0 wrote:That was about what I was thinking. My main concern was DDT. The current advice on here seems to indicate multiple spark gaps will prevent it, and it is more likely in long skinny chambers. So with such a long skinny chamber I decided having a large number of sparkgaps couldn't hurt so that's what I am doing. The newest issue that has arisen is I am not sure if my transducers are compatible with my readouts. But I have not had much time to play with them yet so I may just not have hooked them up correctly.


Well.. You propably wont have problem with DDT if the chamber is shorter than 50 times the inner diameter of the chamberpipe.. or to be safe, lets say 30 x chamber diameter or so... At least if the chamber is relatively smooth on the inside with no obstacles that could shorten the distance where the transition occurs..
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Re: Possible large burst disk hybrid build

Unread postAuthor: wdr0 » Mon Sep 29, 2014 1:25 pm

I ended up just using all the pipe that was available with the initial thought that I would have to make less cuts(that theory didn't work out). So the barrel is somewhere around 20 ft, the chamber was in theory going to be ten, but turns out is like 12, so 6/144=1/24, which is actually pretty close to 1/30. And it is just a strait steel pipe. So it seems based on what you said there may be a risk as I thought. I spent awhile searching for info, but it seems ddt alone is too short to search and everyone uses the abreviation so searching for the whole words brings up few results.
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Re: Possible large burst disk hybrid build

Unread postAuthor: matti » Mon Sep 29, 2014 2:31 pm

wdr0 wrote:I ended up just using all the pipe that was available with the initial thought that I would have to make less cuts(that theory didn't work out). So the barrel is somewhere around 20 ft, the chamber was in theory going to be ten, but turns out is like 12, so 6/144=1/24, which is actually pretty close to 1/30. And it is just a strait steel pipe. So it seems based on what you said there may be a risk as I thought. I spent awhile searching for info, but it seems ddt alone is too short to search and everyone uses the abreviation so searching for the whole words brings up few results.


I still would not worry about it, i think the normal rule is the 50times the pipe diameter and its from the ignition source.. So unless you have 50times the diameter long pipe with one spark gap ignition at the END of the pipe you should be Ok.. BUT.. Some safety factor is something to consider about of course :D

You can find some information of DDT in pipes by searching about detonation arrestors.
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Re: Possible large burst disk hybrid build

Unread postAuthor: wdr0 » Mon Sep 29, 2014 8:06 pm

Yeah, like you mentioned and i was thinking it can't hurt anything, just might be harder to build. I have designed the system to only penetrate the chamber at one point.
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image.jpg
Here is the fuel system. The hose on the left will have an air qc fitting installed. Hose on the right is propane. 0-250 1% transducer on air, 0-30 .25% on propane. The large tee connects to the chamber, the black rod serving as the high voltage feed through for ignition. The two compression fittings will make the seal. The qc fitting on the tee is rated to 10k psi so should serve well with only 1000psi combustion pressure.

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Re: Possible large burst disk hybrid build

Unread postAuthor: matti » Mon Sep 29, 2014 10:40 pm

Looks good ! Btw. With this large bore hybrid you will need some good burst disk material, you could try to use brass shim stock that they sell in different thicknes. In shim metal materials the thicknes is very accurate so burst pressure should be too.
You could find some with good price if you sarch ebay with "brass shim stock"
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Re: Possible large burst disk hybrid build

Unread postAuthor: wdr0 » Mon Sep 29, 2014 10:51 pm

I plan to use aluminum for the disk. Specifically I think it is 3/64" thick, which should burst somewhere around 800 psi with a 20k pound tensile strength.
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This is the ignition box.

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Re: Possible large burst disk hybrid build

Unread postAuthor: mark.f » Tue Sep 30, 2014 6:41 am

Is that a light dimmer -> capacitor -> ignition coil setup? :D I like it.
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Re: Possible large burst disk hybrid build

Unread postAuthor: wdr0 » Tue Sep 30, 2014 7:15 am

Indeed it is, capacitor and ignition coils x 2. It seems to work very well.
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