Burn night (fake) Machine Gun

[CDS] topher · Post by **[CDS] topher** » Sat Nov 04, 2006 11:04 am

I encountered, and spoke with, someone with a fake machine gun setup on the way out on burn night, around 3:30 and esplanade or so.

I attempted to elicit details from him about the details of thye design, but he was rather...reluctant.

His fake machine gun, which fired at roughly 10 'rounds' per second and featured a flash spreader and two gas bottles (O and H2, I think).

I am a competent DC electrician, have worked with solenoids, programmed microcontrollers, expansion chambers, various gasses, etc., and have begun work on similar design - found a machine shop to help me to the milling work, gas supplier, etc., but still have some issues I ould use some help with, including choice of solenoid, gas pressures, volume of expansion chamber, barrel length, etc.

If you are, or know the man with that gun, I'd appreciate some assistance to help me along to save some misery with failed designs and wasted machine shop cash.

- topher,
Camp Cul-de-sac

MikeVDS · Post by **MikeVDS** » Sat Nov 04, 2006 8:07 pm

I have a little experience in the area. The main problem with designing something like this is fluid dynamics doesn't work when your output is defined by "a cool looking flame".

Are you looking to make sound as well as flame? I'm not certain that just igniting H2 out of a barrel will give you the sound of a machine gun. To produce a machine gun type flash I'd think a liter accumulator and 50psi would work well. You could probably even get away with 20 psi (if you design to 50 psi it should be easy to manage and you can always run lower pressures). As far as a bang, I've never done anything along those lines with a gas. Barrel length should not matter much. The longer the barrel the faster the gas will leave, but the flame won't change much anyway. Design it to be changed so you can tweak it after it is complete. No matter how well you design it you'll always want to change something.

MikeVDS · Post by **MikeVDS** » Sat Nov 04, 2006 8:10 pm

You probably don't even need an accumulator. Just open the line out of the H2 tank and ignite it at whatever pressure it runs. If needed you can regulate it down to a better pressure. Find out the pressure range of the tank you'll run. Some propane goes up to 300 psi at 120 degrees F so that may alter the types of fitting you'll use.

[CDS] topher · Post by **[CDS] topher** » Sat Nov 04, 2006 8:54 pm

One of the few bits of info I got from him was that there were TWO expansion chambers, with differntial solenoid timing (which is why I would use a micrcontroller to fine-tune the timing). In the first chamber the gasses are ignited after trigger pull. After X period of time - experimentally arrived at, I suspect - when the pressure reaches its maximum (or just before this), a second solenoid is opened, allowing the high-pressure ignited gas into the barrel, thus creating both a) a flame visible in the playa night environment coming out of the spreader on the end of the barrel, and b) a significant and loud sound. Having seen a Browning .50cal in action, this was't significantly different.

[quote="MikeVDS"]I have a little experience in the area. The main problem with designing something like this is fluid dynamics doesn't work when your output is defined by "a cool looking flame".

Are you looking to make sound as well as flame? I'm not certain that just igniting H2 out of a barrel will give you the sound of a machine gun. To produce a machine gun type flash I'd think a liter accumulator and 50psi would work well. You could probably even get away with 20 psi (if you design to 50 psi it should be easy to manage and you can always run lower pressures). As far as a bang, I've never done anything along those lines with a gas. Barrel length should not matter much. The longer the barrel the faster the gas will leave, but the flame won't change much anyway. Design it to be changed so you can tweak it after it is complete. No matter how well you design it you'll always want to change something.[/quote]

MrMullen · Post by **MrMullen** » Sun Nov 05, 2006 7:43 pm

Fake guns don't kill people, fake people kill people.

K-mom · Post by **K-mom** » Thu Nov 09, 2006 8:26 pm

Are you trying to tell me Tony Montana was a fake!?!?

spectabillis · Post by **spectabillis** » Fri Nov 10, 2006 10:18 am

[CDS] topher wrote:... a second solenoid is opened, allowing the high-pressure ignited gas into the barrel, thus creating both a) a flame visible in the playa night environment coming out of the spreader on the end of the barrel, and b) a significant and loud sound.

sounds tricky. how do you put an ignited flame through a solenoid? i would think solenoids couldnt take that kind of heat, but i dont know.

Isotopia · Post by **Isotopia** » Fri Nov 10, 2006 10:25 am

I don't think that's exactly how the thing is set up. The plans I have are a bit different.

spectabillis · Post by **spectabillis** » Fri Nov 10, 2006 11:01 am

my solenoid is a 1/4" normally closed 12v with 30psi max, only rated to 160F. never worried about the high end of the temp rating, just the low since pushing propane through smaller openings sometimes has the tendency to freeze up.

[CDS] topher · Post by **[CDS] topher** » Fri Nov 10, 2006 11:58 am

[quote="Isotopia"]I don't think that's [i]exactly[/i] how the thing is set up. The plans I have are a bit different.[/quote]

I'm going from burn night memory, which is naturally a bit unreliable. The man I talked to was reluictant to share info.

Care to share what you have?

spectabillis · Post by **spectabillis** » Fri Nov 10, 2006 12:39 pm

i was thinking the popular two tank setup where one holds the fuel and the second is the expansion chamber, but if he had it at 10 times per sec that sounds difficult. maybe his fuel choice makes that big of a difference? i know the fx industry sometimes used a hybrid gas choice with spark plugs mounted to a flared opening, but they add special sound effects over the shots.

tanks: its easy to find spare 20lb bbq tanks where you can remove the opd valve for a good 3/4" npt connection. i am still looking for a lighter and smaller tank source myself, kidde and amerex never answered my q's about their 10lb fire extinguishers - but they are rated to 300psi.

StevenGoodman · Post by **StevenGoodman** » Fri Nov 10, 2006 2:24 pm

Sometimes you can find old 5lb and 10lb propane tanks; they should also have a standard NPT connection.

Fire extinguisher connections are completely non-standard. The type of connection could be most anything. I have a 5lb extinguisher being use as an accumulator, but I had a custom 1"-12 UNF to NPT adapter made for it. (by the machinist at work).

The extinguishers Flaming Lotus Girls used for the Serpent Mother had garden house threads!

If you can find older steel extinguishers they are often rated to 500+lbs.

spectabillis · Post by **spectabillis** » Sun Nov 12, 2006 10:17 am

well thanks, i know to avoid that route then.

anyone know a better sealant besides pipe dope to use on brass/galvanized fittings for high psi propane (low temp)?

Bob · Post by **Bob** » Sun Nov 12, 2006 11:34 am

http://www.rectorseal.com/slnts.htm

spectabillis · Post by **spectabillis** » Sun Nov 12, 2006 3:42 pm

dayum, that was quick.

t'nks...

StevenGoodman · Post by **StevenGoodman** » Sun Nov 12, 2006 11:06 pm

[quote="spectabillis"]well thanks, i know to avoid that route then.

anyone know a better sealant besides pipe dope to use on brass/galvanized fittings for high psi propane (low temp)?[/quote]

Teflon Tape. The best and easiest for anything with NPT connections.

Lassen Forge · Post by **Lassen Forge** » Wed Nov 15, 2006 3:42 am

I could see if you had both a fuel gas and compressed air set up against timed solenoids to punch a fuel charge out, then followed by a air blast, maybe cycling at 5-10 "burps" per second. Igniion would have to be by some constant source (spark plugs fed by a "buzzbox" ignitor coil in the ignition chamber?).

Effect could be something like the output of a pulsejet engine of a V-1 rocket... with the right size/dimension of an expansion chamber you'd have both flame and sound effect.

[CDS] topher · Post by **[CDS] topher** » Wed Nov 15, 2006 5:06 pm

[quote="Bay Bridge Sue"][color=darkred][b]I could see if you had both a fuel gas and compressed air set up against timed solenoids to punch a fuel charge out, then followed by a air blast, maybe cycling at 5-10 "burps" per second. Igniion would have to be by some constant source (spark plugs fed by a "buzzbox" ignitor coil in the ignition chamber?).

Effect could be something like the output of a pulsejet engine of a V-1 rocket... with the right size/dimension of an expansion chamber you'd have both flame and sound effect.[/b][/color][/quote]

Sounding pretty close to what I saw and had (briefly) described to me that night. I believe that the fuel was NOT propane, but O2 and H2 or something else. The setup I saw also looked pretty analog - not microcontroller rigged (which is DEFINITELY how I'd go about it, if only to ease tweaking of solenoid timings and the like.)

Frankly, for all I know, the barrell outlet solenoid was simply pressure-switched, rather than controlled by specific timing. I dunno. Will be spending time in the shop come january seeing what I can come up with.

Lassen Forge · Post by **Lassen Forge** » Thu Nov 16, 2006 4:39 am

H2 burns almost too clear to see. Either need to inject another material or gas, or use a dif fuel source.

Acetylene is flashy, noisy, and can look nice, tho acetylene is explosive as well. Maybe a fuel gas with an injected liquid (either a fuel oil or somethig, or maybe a different gas combo?) would work?

You can pulse liquid fuels, tho not as easily as vaporous ones.

Hugz...

spectabillis · Post by **spectabillis** » Fri Nov 17, 2006 6:00 am

StevenGoodman wrote:Teflon Tape. The best and easiest for anything with NPT connections.

tried that, at connections around the pull valve it developed leaks. my guess was the low temp.

Post by **mdmf007** » Fri Nov 17, 2006 9:28 am

when we need ignition at variable speeds, we use a cheap strobe light from Walmart, the kind with the adjustable timing. Wire it into a narrow gapped spark plug, and you can adjust the timing by turning the control knob. Youalso have to short the safety lead to the bulb. (it wont fire without a bulb in the holder. ) Ill get you a model number. There 39 bucks or so.

later all.

fishfool · Post by **fishfool** » Thu Jun 26, 2008 2:12 am

wowowow um I would really be itneresting in getting more specs on this setup or seeing some photos if anyone is so inclined, this thing sounds AWESOME please let me know, thanks,
-Fishfool @ [url=http://www.thereeftank.com]The Reef Tank[/url]

Toolmaker · Post by **Toolmaker** » Sat Jun 28, 2008 12:54 am

I wonder if there is a pressure relief valve method to avoid the solenoid method. Something low pressure with a needle valve to control "speed/rate of fire", more pressure = faster "reload/cycle" times on the relief valve that opens.

I have had good luck with cheap stun guns for low pressure propane effects so far, and the spark plug trick works great too. Pencil torches and full size torch nozzles are good for the larger stuff. I have very little experience so I haven't gotten into using solenoids and whatnot yet.

gyre · Post by **gyre** » Sat Jun 28, 2008 5:02 am

That is exactly the sort of arrangement a friend suggested about a different type application.
I wonder if this thing was a theatrical prop?
I recently found fully automatic blank pistols made for this.
They were 9mm however.

Post by **trilobyte** » Sat Jun 28, 2008 6:09 pm

This may be completely irrelevant, but did anybody see the episode from the 2008 Doctor Who entitled "Sins Of The Fathers?" There's a 'Doctor Who Confidential' episode that goes with it called "The Doctor's Daughter" - in the episode they had some seriously badass futuristic weapons that looked like they were some kind of propane gun. In the 'confidential' (behind the scenes) episode they kept going on about how it was so much better to work with than a regular gun with blanks, etc etc. I'd love to know how those babies were made.

Trilo