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!

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