Friday, July 3, 2009

Continuing composite saga

OK so I've worked in chemical engineering-related R&D for almost 15 years now. As such I can attest to the fact that, when you're pushing the limits of your personal or collective knowledge, things can fail.
My most recent rocket science failure stemmed from a) the hasty use of b) a handy resin ratio spreadsheet I cooked up for some odd materials. For those of you who have used two part systems like epoxy you'll know that most employ simple ratios like 4 parts resin to 1 part hardener or the even more common 1:1 ratio.
Well Dave Triano teaches that "UV Smooth Prime" is the best possible filler and primer for rocketry use and I agree. I calculated its "resin":hardner ratio to be ~79:1. Gee that's easy. Similarly Dave recommends Aeropoxy as the resin system of choice for rocketry composite material lay-ups. It's ratio is exactly 100:27. So, as I prefer to tailor each material batch size to fit the job, I assembled a table to get those odd ratios right:

This table has proven invaluable to me this year. However when you're rushed and throw together a batch of Aeropoxy to test a new part design and refer to the first block of columns, UV Smooth Prime, you're ratio will be WAY off!! And how the hell did I not notice the ridiculous imbalance of otherwise familiar Aeropoxy components?!
The result of this error was that my vacuum-cured, carbon fiber-faced fin never cured and all the gelled monomer selectively sucked into the breather/bleeder fabric. Sadly I'd also used the excess resin to bond the reinforced coupler into my M750 rocket's payload section. All this did was wet both parts with monomer. Good thing this only affected about 7 previous hours of work on the constituent parts!! :(
Yesterday I stopped moping, manned up, scrapped the fin, and cleaned the payload/coupler up with acetone. I got the latter clean enough that I was able to properly bond the other ends of each part and all is well again. Lesson learned: think before you pour!!

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