Intermolecular Forces Strike Again

When it came to pouring the asphalt fuel grains, I did not anticipate how long it would take.  Starting with approximately ten pounds in two large chunks, I pulled/sliced the bitumen (asphalt without the gravel-just the tar) in small pieces.  This was more difficult than I thought... the long, high mass hydrocarbons in the asphalt would not let go of each other.  With a teflon pan I started to melt the chunks of asphalt over a hot plate, trying to evenly heat the black pitch into a uniform liquid.  This proved daunting, as the asphalt much preferred the solid state, and refused to melt.  After probably half an hour of coaxing, I filled half of one phenolic tube with liquid asphalt, leaving another tube and a half to go.  It became apparent that it would just be easier to pick up the asphalt on an aluminum stick and melt it with a blowtorch than to use a pan.  The whole time, between the spatters of hot pitch (the same material they used to tar and feather people in the days of old; I can only imagine the horrific burns they suffered across their bodies in addition to the public shaming), I was continuously taunted by the pesky properties of this hydrocarbon amalgamation. Next time I'll just drive to the La Brea tar pits and dip the grains in-it'd almost be faster that way.

Melting Asphalt

Another major objective was to drill/tap a hole in the bulkhead where a pressure transducer could fit.  This was necessary because without the pressure transducer, I would not be able to quantify force/pressure vs time in each rocket testing.  The end result was not pretty, but works.  However, I made a terrible mistake and partially destroyed the threads that allow the bulkhead to screw into the combustion chamber.  The damage was mostly repairable and I avoided having to start all over in making/modifying a new bulkhead.

Bulkhead with Transducer Fitting

Later in the week, I began to experiment with the HTPB fuel grains.  HTPB (hydroxyl-terminated polybutadiene) is a liquid rubber base, and in the presence of MDI (Methylene Diphenyl Diisocyanate), cures rapidly into a solid rubber.  I used other additives in making the fuel grains.  I planned to make two HTPB basic grains, and one with powdered magnesium as a fuel additive.  Each fuel grain followed a basic mixture as follows:

40g HTPB
8g MDI
2g CuO (cupric oxide), a stabilizer and fuel supplement
~20mg Triphenyl Bismuth, a contaminant that allows for quicker ignition

The Mg+HTPB grain also included 5g Mg.  This was all relatively easy, with the exception of extracting MDI from its storage container.  The barrel is very tightly sealed, as MDI reacts very quickly with water, tearing apart the isocyanate and rendering the MDI useless.  Additionally, MDI should be kept in a controlled climate, meaning the barrel is housed in a foam igloo.  MDI also has very low viscosity, making extraction via vacuum somewhat difficult, as the vacuum seal was not perfect and allowed MDI to drip very quickly back into the barrel and out of our reach.  Oh, and it's harmful to skin and lungs, pretty harshly so.  Anyways, after mixing all the components, I cast the rubber into the phenolic tubes, each capped with aluminum tape to prevent leakage.  They look like huge shotgun shells.

The Fuel Grains Curing (well, not really curing....)

After this, Robert and I constructed the control box, a task that we did incorrectly.  I have to go move some wires and resolder after I post this.  Simple fix.

Fast forward to today, after a long weekend.  The nitrous valve is mounted, the tubing is in, the bottle is oxygen-cleaned to ensure safety.  All this would be great, but there was one large complication. 

The HTPB grains didn't cure.... Critical Mission Failure.

Left: Failed Fuel after Extraction. Right: Empty Phenolic

The two pure rubber mixes were still incredibly liquid, which by a strange twist of fate, was much better than the magnesium-HTPB grain.  How do I describe it?  The Mg grain was sticky like glue, stretchy like putty, and is like bread dough from hell. And toxic.  But oh it's a very nice color... I think I ought to sell it as a premium automobile paint color... Anyways... since I didn't want to waste the phenolic tubing by throwing away the whole grain, I had to remove the fuel.  What a task that was.  After half an hour, it was all out.  The intermolecular forces really taunted me there.  I can imagine all those electrons, each participating in a subatomic tug-of-war against me, completely determined to make me waste my time. 

Tomorrow I will revisit HTPB.  I am increasing the proportion of MDI to be 20% by weight, hopefully enough to fully cure each grain.  I want to be a chemical engineer so I better get used to finicky reagents. 

I am on schedule to fire by Thursday, or Monday at the latest.  Thanks for viewing.

Graphite Dust, White Shirt

All in all, it was a good day.  But not for my white shirt.  My electrode-grade graphite rod came in, and that was all I needed to start fabricating the nozzle.  What I thought was going to be incredibly laborious ended up actually being very simple.  Rather than follow my plan as explained to my BASIS advisors (sorry guys), I decided to ditch using the lathe altogether.  The lathe at Mishaal can only grab items larger than 2in diameter; the graphite rod is 3/4in.  Not wanting to putter around trying to make a collar to hold the graphite rod in the lathe and wait and wait and wait until it finished, I decided to use the drill press instead.  My on-site advisor Mr. Loehr and his son Robert keenly brought out an array of drill bits that could be improvised for reaming the graphite piece. 

12.5in by .75in diameter graphite rod, Dollar bill for scale

 Using a hacksaw, I lopped off roughly 3/4in of the graphite rod, held in a vice.  I wasn't careful, and gravity pulled the piece off before I could finish it with a clean cut.  This resulted in a chip in the graphite and a bit left to file down.  After this, I drilled a 1/8in pilot hole through the small graphite cylinder, shown below.  I thought it looked like a cinder cone volcano-the red laser beam looking like flowing lava. As much as I thought it was an industrial piece of art, I had to blow the dust off and stare at the pilot hole, just a tiny bit off-center (the drill press sucks, it wobbles, and the belts come loose.  Every guy in the shop is waiting anxiously for a new one).
 

 Left: Cinder Cone volcano in Utah. (wiki images)      Right: Graphite Nozzle after drilling pilot hole. Click to enlarge.

With a 15/64in drill bit, I bored through the nozzle.  My throat has to be 1/4in, so I accounted for the wobble and sized one bit down, yielding a nice 1/4in diameter throat through the graphite nozzle.  Only it wasn't centered.  I repeated the process,  more careful with the hacksaw and centering. The second attempt worked beautifully.  To finish the nozzle, I had to taper both ends.  Here is the diagram:

L to R: Boring Bit, 1/8in bit, 15/64 bit, stepped drill bit, finished graphite nozzle

The convergent end of the nozzle (noted as 'CONV' on the diagram, left side)  is the piece that is on the interior of the rocket. It is where the gasses produced during reaction seek to escape, and it is important that the interior wall of the nozzle be smooth here.  Using a boring drill bit (gray, far left on image), we achieved roughly a 30° half-angle taper.  Success.  For the other side, the divergent end (DIVR), where the gas escapes and creates thrust as it expands, we used a step drill, a drill bit that has wider and wider sections as it goes in (bronze, rightmost on image).  This achieved roughly a 15° half-angle, stepped of course.  A small section of 1/4in throat lies in between the convergent and divergent holes.  This is a successful, well made nozzle, if I may say so. 

The nozzle is a tad small for mt rocket chamber, but some O-rings will wedge it in just fine.

After this, Robert and I tried to mount the nitrous valve to the aluminum sheet holding up the rocket, but our quick method of using zip ties just didn't hold.  We moved on to putting a small section of 1/8in nylon tubing into the rocket's bulkhead, and it will connect to the nitrous valve in the future. 

The nylon tubing is wonderful because it has a burst pressure of approximately 1500psi.  That means that if there was a backfire with the rocket, the nylon tubing will rupture, rather than letting the reaction travel into the nitrous valve and blow up my $42-with-free-shipping gadget. 

After this, I thought I would take off.  However, I decided to watch as some experiments were going on.  Friction Stir Welding.  Never heard of it? Neither had I until today.  But it's about the coolest damn thing ever (if you're into this stuff at least).  The premise: weld two pieces of metal together without melting either.  "What? How?  That doesn't make sense," I said, as you might be saying yourself as you read this.  I can't say I fully understand the specifics, but I'll try to explain best I can:

1.  A high pressure, high rotation speed (1000-2000 rpm) specially made drill bit presses into the crease between two separate aluminum coupons.
2. It bores a hole, and starts slowly moving along the border between the two sheets of aluminum.
3.  The heat and pressure and movement from the friction of the drill bit increase the plasticity of the metal, and literally mix the still-solid aluminum from both pieces together, like play-doh.
4. The weld cools, and both pieces are fused to each other, and a hollow trough under the weld is formed.

The weld is clean, strong, and consistent.

It's vague from what I said, sorry.  I have a hard enough time understanding it myself. It was discovered in 1991, and very little data is available on it.  However, these welds are convenient because they do not melt the metal ever, meaning that the metal does NOT have to be heat treated to regain its original, pre-weld strength.  This cuts cost and allows aluminum alloys like 70-71 to be used more easily in the fabrication of complex, high strength components.  SpaceX is currently experimenting with the same method, but on a larger scale.  What does this all mean?  A more cost effective, practical, and safer way to weld pieces together that yields a high tensile strength.  The primary use, at least in the aerospace field: build high pressure tanks out of sensitive lightweight aluminum alloys.



I honestly hope to take a little detour from my project to play with this tomorrow.  Seeing metal manipulated like play-doh is not something I'm used to.  I'm still fascinated.  And a little in shock.  I think you can tell that this excites me.

However, fun metal molding aside, I have things to do.  Finish the tubing, wire up my control box, and cast some fuel grains: that's the agenda for the next few days.  Firing next week, too! (I hope)

April is my testing month, and I'm ready to get rolling.

Until next time,
Will