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Problems with my very first combustion spudgun.

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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Unread postAuthor: hi » Wed Apr 20, 2011 11:56 am

Generally you need about one second or so worth of spray. 10 seconds is WAY too much.
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"physics, gravity, and law enforcement are the only things that prevent me from operating at my full potential" - not sure, but i like the quote

you know you are not an engineer if you have to remind yourself "left loosy righty tighty"
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Unread postAuthor: warhead052 » Wed Apr 20, 2011 12:01 pm

@hi, we were messing around at that point, 1 - 2 seconds of spray wasent enough for his chamber size (about 3 inch by 1 foot), so yeah...
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Unread postAuthor: 50bbx » Wed Apr 20, 2011 3:17 pm

saefroch wrote: That usually indicates a fueling problem.


Ok, I will try to spray the right quantity of fuel next time.

saefroch wrote: I've attempted to used a very large spray 'n' pray before, and I can tell you that fueling correctly is incredibly difficult to get the hang of. So long as your ignition source is producing a spark at all under atmospheric conditions, it should function perfectly well for this build.


Ok, well, I've tried to burn the hairsprai on the floor with the sparks and it worked.

saefroch wrote: I suggest syringe fueling, JSR's favourite method. It really works, and is quite simple. You may also be having an issue with mixing. What warhead052 talks about with allowing air to get into the chamber isn't really that, it's allowing the gases to diffuse together and become a uniform mix. If the mix isn't uniform, you may end up with a region of gas around the spark that isn't combustible.


Ok, definitely a fuel problem.


saefroch wrote: If there are holes in the chamber, it will reduce performance, but you will still be able to ignite a combustible mix. However, be aware that if there are any holes a jet of very hot gas will shoot out during ignition, and can burn through duct tape, hair, and clothes.


Distance ignition is just for that. I don't want to blow up with the potatoes :-/

Technician1002 wrote:Very carefully check if the hairspray is flammable. I don't recommend holding the lighter in your hand. If you manage to spray your hand it could be very bad.
http://www.youtube.com/watch?v=H8BfP6zrUBY
Some hairspray is more water and less butane or propane.


I've double checked, and the spray is extremely flammable

Technician1002 wrote:In re-examining the photos, does the chamber have a clean out cap? The breech looks too short to contain a threaded and glued ends. Lack of fresh air between shots will prevent igniton.

You need a way to open the chamber wide open for full venting of spent gas and replacing it with all fresh air.


Ok, understood... but, yes, at the bottom od the chamber there's a glued and threaded cap. I cannot open it, but just because when it wasn't glued it allowed air to escape, so I have to glue it. The initial idea was to spray the fuel through the cap. So, the problem is always air :(

warhead052 wrote:Oh I just realized this. Dont use bell reducers because they are weak. They cant handle pressure the way bushings do, just a tip! I dont know if they work better in combustions, or if they are safer, but I know I wont use them unless its for decorations.


well, I do not know exactly aht bell reducers are, but looking through the internet doesn't sound like something I used... :oops:
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Unread postAuthor: warhead052 » Wed Apr 20, 2011 3:20 pm

Its the cone shaped reducer you have on there. I dont know if its for the mechanics or for looks, but they can be dangerous.
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Unread postAuthor: 50bbx » Wed Apr 20, 2011 3:41 pm

warhead052 wrote:Its the cone shaped reducer you have on there. I dont know if its for the mechanics or for looks, but they can be dangerous.

so it's a plastic piece... no, I went to the shop, I ask for reducer and they gave me this. sorry, I'm totally inexpert. which is the best one?
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Unread postAuthor: warhead052 » Wed Apr 20, 2011 3:43 pm

A bushing, but thats ok. What store did you goto? I havent seen those types in forever.
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Unread postAuthor: 50bbx » Wed Apr 20, 2011 3:54 pm

I live in italy, maybe there was even a bushing, but I didn't see it and they didn't offer me to buy one. or maybe it's an unusual kind of joint.
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Unread postAuthor: saefroch » Wed Apr 20, 2011 4:57 pm

If it's pressure rated, it's okay. It probably is, from the looks of it. I've used very similar fittings on spray 'n' prays myself, and I haven't had any issues.
Please don't make sweeping generalizations.
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Unread postAuthor: mark.f » Wed Apr 20, 2011 10:27 pm

warhead052 wrote:Oh I just realized this. Dont use bell reducers because they are weak. They cant handle pressure the way bushings do, just a tip! I dont know if they work better in combustions, or if they are safer, but I know I wont use them unless its for decorations.


Plenty of people have used DWV bell reducers in combustion launchers. I agree it's nice to err on the side of caution and use pressure rated fittings, but DWV fittings are usually an accepted construction material for even advanced combustions.
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Unread postAuthor: 50bbx » Sun Apr 24, 2011 9:55 am

Ok, tried few days ago and I've perfectly understood why it's called spray and pray... First shot: chamber not even perfectly sealed, range of 30 meters, more or less 90 feets. I was very happy, BUT, not even another single trial worked :( I guess I figured out why it didn't work: I need a bigger hole to spray the fuel, from behind it would be perfect, but i've glued the cap. The fuel doesn't spread well in the chamber spraying it from the bottle neck.
I have an extra tube cap, I will try replacing the old one and hoping it doesn't allow air to escape. :)
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