Propane rifle

This is a nice effect:
http://technabob.com/blog/2015/05/18/diy-propane-rifle/

Wrapping the tubing gives the flame front the appearance of a longer transit time.  You could imagine doing a helical wrap around a clear tube and getting a nice mostly-linear effect.  And using the propane torch as the mixing element is also nice, because it's designed to mix air-propane at the inlet.  No calculations needed!  Might even be able to modulate it by modding the inlet a bit...

Money, money, money...

I bought some round telescoping tubing from McMaster-Carr, hoping that it would give me tight tolerance fittings, but it doesn't.  The tolerances are crappy, like 1/8" difference in ID and OD.  I'm hoping to retry the linear model with 4 ft long tubing, and also possibly a new rotational valving model with the following changes:

• Tighter tolerance tubing
• Have the outer tube holes all facing upwards, and the inner tube with the helical holes
• Use 1/16" holes instead of the 1/8" holes I used in the original prototype

I'm just going with my gut on these changes, that they might improve the performance and appearance of the effect.  The problem with swapping the helical holes into the inner tube is that it requires the inner tube to rotate, and the inner tube is connected to the gas supply.  Rotating it by 90 or 180 degrees should be possible as long as the hose is long enough.  I looked at gas swivel joints, but they're really expensive.

On the other hand, I found a really cheap source of solenoid valves on Alibaba, like less than $10 a valve.  The shipping is expensive (from China) but if I decide to go with these in bulk, it's pretty tractable.  I think testing a system with about 10 valves along a 4 foot line might be actually quite doable from a cost and complexity standpoint.  I was thinking this would be prohibitive, but really, I think $3000 in valves isn't actually that much in the grand scheme of things.   I had also originally thought this would require me to to build several independent lengths with holes and their own inputs but I want to try simply attaching them under a square U-Channel with holes drilled at intervals.  It would depixelate the effect but still be pretty simple to build.  The control logic becomes much more complicated, but control logic is in some sense the easy part: one controller per edge, and a central controller.  Not that big a deal.

Linear mechanical valve prototype: qualified success!

!!! Big Red Warning Header !!!

I have zero training in flame art apart from what I've been reading on the internet, and having a doctorate in physics.  Don't attempt any of this.  Don't blame me if you get hurt.  Don't build dangerous things in your basement.  It's a bad life choice for 99.9% of the population.

So, this is based on the same mechanical valve principle, but instead of the rotary motion, it uses a linear motion, with distances between the inner and outer holes progressively offset, so that moving the outer shield uncovers different sets of holes.  I set it up with two holes per offset, and half-hole offsets per stop, so that there's always a progression: as one set of two holes is fully opened, the adjacent holes are half-opened.  This helps give the flame a more continuous effect, and ensures that it gets translated as the new holes valve open.

Some observations:

• I bought hollow square stock from McMaster-Carr, and specifically found stock with very tight tolerances, to prevent leakage from one hole to the next.  I also used a layer of high temperature automotive grease between them, both for the lubrication and to help prevent gas seepage.
• Working with square stock is hard.  I am a very bad welder, and I tried to weld the ends shut (one end I welded a cap, so that I could disassemble it if needed.)  It's not easy to weld a gas-tight seal, and I wound up using silicone sealant and rescue tape (which I spotted at my local hardware store when buying the silicone sealant), and the seal still wasn't fantastic.  Silver solder would have given me a better seal, but on parts this big I worried that it would take a shitload of propane to get them to the right temperature.  And, I can't use silver solder in production, because of the melting risk (FAST prohibits it, and unlike my other, ahem, projects, I care about getting this one approved.)
• Also, I should be working with brass.  I didn't really check if there was brass stock available with the right dimensions, but there are fewer options in general, so I might not have been able to.  On the other hand, it's so much easier to work.  I broke a few drill bits trying to drill screw holes into the ends of this thing (don't ask), and gave up and welded the ends on instead.
• While this looks like a good option, it's going to be very hard to scale up.  Precision machining on a 12" scale is time consuming.  I literally have no idea how I would do this with a 4 foot part, the mill won't accommodate it.  And then multiply that by 30 edges.  I'd like to see if I can get a similar effect with pipe stock, which is cheaper.  Also possibly make the holes much further apart; in that case, each valve would have to have its own pilot (via a continuous copper line, probably), but the machining wouldn't need to be precision, you could simply measure and drill.  The trick again I think is getting parts with tight enough tolerance.  But if the holes are spaced far enough, that may not even be an issue.  Will have to experiment more...