Sustainer final assembly

All that's left to do on the sustainer assembly is the two aft centering rings, the motor retainer, and application of a glob of epoxy to solidify this high-impact zone.

       I had to dig out/chisel the foam that arose above the bottom of the fin tabs to ensure the next centering ring would seat well.  That took a surprisingly long time as this polyurethane foam is resilient and bonds well!  I also had to plug the bottoms of the fin slots (epoxy plus tons 'o' silica) to prevent resin from running out.  I then poured in epoxy on the fin bottoms, slid the centering ring into place, and tapped it down with a dowel and a hammer to ensure good contact.  I flipped the sustainer over while the epoxy cured.    As I mentioned in my documentation package I needed to pre-assemble the retrofit-ready Allen bolts into the collar of the very cool motor retainer to match my design intent:

Then I loaded up a batch of epoxy with silica to form gel and globbed this into the bottom inside edge of the sustainer.  I pushed the final centering ring into this epoxy and leveled it with the L-angle.  I then pushed this ring in the rest of the way using the threaded nuts on the motor retainer (~0.375"), centered the motor retainer on the motor tube, and tacked it into place with CA adhesive.  

You can also sort of see that I applied electrical tape around the inner interface between the retainer and motor tube so epoxy would leak through and into the motor tube (this could be disastrous!). Finally I mixed up a big batch of epoxy to fill this end volume:

The masking tape forms a dam to prevent epoxy from leaking over.  Ok so why is it so goopy and bubbly you might ask?  Well that first huge batch of epoxy wasn't sufficient to fill up all the way to the edge so I mixed another half that size and poured it in on top.  Even that was still just shy of the top so I mixed/poured a third smaller and final batch in and that did the trick.  As I began stirring/pulling the bubbles out of this pool I noticed some microbubbles creeping up the sides in a few points.  Then I noticed some light  smoke wafting out above the pool....

     Now I'm a chemical engineer but I haven't built a rocket of this diameter for years now.  Just like the foam mentioned below this epoxy resin is highly exothermic (energetically favorable) and liberates a bunch 'o' heat.  The three batches were mixed only minutes apart but when the bottom-most batch began curing it heated up the layers above and basically boiled them as they began to cure.  Duh.  It's no biggie but I'll need to sand fill that end a bit to achieve my customary aft finish.