Sunday, September 27, 2015

Build details on shuttle prototype v5.1

After the relative success of the shuttle prototype, I've been trying to scale this up into a design that is more similar to what I imagine the eventual final design will be like, which is to say, I've been working on building something that doesn't suck, and that I can use as a blueprint for a final design. I've stopped keeping track, but I think given the gas mixing prototype, rotational valving, translational valving, multiplexing, and shuttle prototypes, this is major version 5, making this Flaming Icosahedron v5.1.  I've done some basic experiments with it, but I wanted to take the time to write up some of my build notes here.

The sheath is made from 6"x3"x5 foot rectangular tube, 0.120 wall, which I got from Bayshore Metals.

I drilled 1/4" holes at 1/2" intervals along the top.  I managed to basically destroy my 1/4" bit along the way.  I actually took it over to the bench grinder and (after watching a YouTube video) re-ground the tip by hand, badly, but well enough to get me through the end of the drilling.  But I bought a $20 bit grinding jig for next time (and also a new 1/4" cobalt tip.)

I've posted photos of the new shuttle build before,in my last post.  I moved the rail holders to the inside to reduce the width.  The rail holders are just cut up pieces of 3/8" black pipe (which is weldable, unlike galvanized pipe.  Welding galvanized metal is a great way to inhale zinc byproducts, which will kill you slowly and painfully.)  
I also made some little clips for holding the 16ga cable in place (in v5.0, I was using picture wire twisted in place, because it's what I had lying around.  None of that bush league crap this time.)

Here you can see the shuttle in place on the rails (1/4" rod from the hardware store), with the gas supply line in place on the shuttle.

The rails are held in place on the ends with 16ga sheet metal that I bent using a handbrake and drilled and tapped holes into the sheath.  I welded 3/8" pipe to the ends, and tapped holes for set screws.
Initially, I hand measured and cut the sheet metal, but for the second set I made, for the fan intakes (below) I ran these on the water jet cutter, and it saved a lot of time.  Those needed a hole in the center for the fan intake, which would have been much harder to do by hand anyway.

Fully assembled (without the fan intakes), you can see the shuttle running along the rail very smoothly, dragging the looped-back gas line with it:

This is another one of those things where, after a week of construction, I was so happy with the results that I sat there playing with it for at least 10 minutes, just pulling the shuttle back and forth, because it worked so smoothly.  It's not a complex machine.  It's just great to imagine a thing, sketch it on the back of a donation insert at high holiday services, and then make it.

I ran a preliminary test without the intake fans, to see how it would work without any additional air, and the results were pretty predictable; at about 30 PSI, I could get okay functioning near the ends, but towards the center the flame snuffs out due to too little air:

  At 50 PSI (closer to where I'd like to run it), it doesn't really work at all.  I attached the intake fans and tried again, and the results were at first better, but there was a flame-out inside the sheath.  I see this pretty often: if not enough air gets into the middle, the fire can go out.  When propane reaches the open end, it flares up and comes out the end in a jet.  This is what you see at the beginning of this demo from v5.0.  There's no video of this run, but in this case, the flame jet came out through the intake (which was providing the air that the fire was seeking), destroying the intake fan:

So, from a build perspective, it's solid, and it works well.  I'm going to disassemble it today to check the hose, and make sure the flame-out didn't damage it.  But getting enough air in is going to require a different design.  Luckily, there are several design I'm considering: adding forced air through a compressor, adding more ventilation to the sheath, and also, carburation.  I found this design online, and reached out to the author:

I have an SMC vacuum ejector that I plan to use as my next test after I check for hose damage.  Progress!

Wednesday, September 16, 2015

Building industrial grade fire art

I was able to make the shuttle prototype functional, at least marginally, by simply slapping a fan on one end to push more air into the chamber.  Here are the results:

There's some wind, and I'm not running the gas at very high volumes, but you can see it clearly translate from one end to the other as I pull the shuttle through.  If you look at the second video closely, you'll also notice that the tygon tube has burned off the end by the time I get it back through: stress + heat = failure.  I'm very happy with this design overall, though, and am now trying to scale it up to fix the problems.  First, I came up with a new, much larger design spec, using a 3" x 6" x 5 foot sheath.  The 6" height should give me enough space to use a low-pressure braided LPG hose, which is much less flexible, and keep it well out of the way of the shuttle.  I've also redesigned the shuttle, which will run along rails

It came out a little too wide, I'm going to have to grind off the tubes which run along the rails (actually 3/8" black pipe) and move them closer to the center.  But the concept is there.

The rail will run along the top half of the 6" height, leaving the bottom half for things like hose clearance (mostly), but also having a fan on either end to push air in, some room for gearing and a motor to move the shuttle back and forth, etc.  I also started machining the ends of the sheath, which will hold the rails in place (and the fans):
These are 16 ga steel sheet which I cut and bent, then tapped holes in the sheath to hold them in place.  Fun stuff.  I screwed up the bending the first couple of times, took some practice to get it right.  But this one fits nicely.

So, this is not a "proof-of-concept" prototype.  This is intended to be an actual model for what I might eventually build to bring to the Playa, and is industrial-grade.  No joke.

Friday, September 11, 2015

Carburetion vs forced air

Some really interesting reading this morning on strategies for mixing air and gas to the right ratio.  Based on last night's experiments, this will be key to getting this to work, and I've seen gas/air mixing as critical to almost every prototype I've worked on.

Basically, you need about 96% air and 4% propane to get a good burn.  In a confined space, like the sheath tube I'm using, getting enough air in there requires an active strategy.  One strategy is to just pump air into the sheath.  This is the simplest thing, and probably what I'll try first: drill some holes in the side of the sheath, and hook up an air pump.  It's inexact, but it should work.  If I go to a pressurized air tank, I can get higher flows, and use a needle valve to tune the ratio.

But an interesting alternative is to build a carburetor that pre-mixes the propane and air.  A carburetor basically uses the Venturi effect to draw air into the stream actively before the gas reaches the outlet.  This is a good example of such a design, for a propane burner that can burn much hotter than just burning raw propane:

The writing is unfortunately pretty bad, but the gist is that the teeny hole in the middle pipe lets out propane, and it draws air in through the giant hole in the reducer.  There's not much in the way of tuning, but it works pretty well apparently, possibly just based on the specific hole sizes used.  Using a design like this in the shuttle would be complicated; it's not clear how I would adapt this.  But it's a starting point for some thinking on how to make this work.

Thursday, September 10, 2015

Quick update on the shuttle prototype

Got back from the burn and have been working on the shuttle prototype; had some good time to think about it while I was at TTITD.  Here's was the first version of the shuttle:

It used a pair of circular brackets, made from pieces from previous prototypes (I always get a bit teary-eyed when I cut up an old prototype to make a new one, but the cycle goes round-and-round.)  This was elegant, but it tended to rotate while passing through the sheath, so I had to scrap it.  The current version looks like this:

It's strapped to a couple of pieces of shelf bracket, and I've laid an aluminum rail down on the bottom of the sheath (not shown.)  I didn't film my first experiment, but the result was interesting: it worked well, once.  But it's unreliable because of poor circulation of air to the interior of the sheath.  This is an issue I've seen a lot.  I'm starting to think it might be a major issue with a number of my prototypes.  I can either pump air in (I bought a Shraeder-to1/4" NPT adapter so I can use a car tire as a compressed air source) or open up the sheath a bit.  Haven't exactly decided which.  But a quick test with just using a fan to blow air into the sheath improved it a lot.  Unfortunately the viton came out of the compression fitting after that.  Making sure the compression fitting is secure will be tricky.  In the original shuttle design above, I used a barbed hose fitting with some copper tubing soldered on to make it fit right, and ziptied it in place.  While I think this is actually more secure than the compression fitting, it's not as "legal" according to BMOrg rules.  Getting this approved will require working with them on the design, but for the prototype either is fine.