large burst disk hybrid build(Update September 2017)

[mark.f]

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.

[wdr0]

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.

[D_Hall]

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.

[wdr0]

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.

[wdr0]

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.

[matti]

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

[wdr0]

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.

[matti]

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..

[wdr0]

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.

[matti]

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

You can find some information of DDT in pipes by searching about detonation arrestors.

[wdr0]

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.

[matti]

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"

[wdr0]

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.

[mark.f]

Is that a light dimmer -> capacitor -> ignition coil setup? I like it.

[wdr0]

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