Sunday, July 12, 2009

EM-SEM-FITY fin attach

For months now I'd planned to reinforce the 1/4" fins with at least one fascia layer of carbon fiber/epoxy prior to attaching them to the booster. I mentally devised several layup scenarios ranging from vacuum bagging individual fins through stacking all four, stratified, and curing simultaneously under a 10 lb weight. In the end I decided the risk was too high for pre-reinforcement and reverted to what I know -- bonding plywood fins to the tube then post-reinforcing. This results in a less optimal resin:reinforcement ratio but oh well... this thing is already extremely lightweight!! I started by laying out the fin positioning guides in Illustrator:
I then printed this on a 13x19" sheet of HP Advanced Photo Paper using my HP B9180 printer. I cut the two guide patterns out using a sharp X-Acto knife and set the freed inner areas aside leaving inset guide patterns in the larger sheet. I laid these guides on a piece of 3/16" foam core, traced them, and again cut the inner pattern leaving these two 12x12" fin alignment guides:

I then test fit the four pre-sanded fins with these two guides on the booster tube:
All parts fit perfectly so I then marked a single, silver line for the first datum fin. The purpose of this first fin is to prevent unintended rotation of the two guides relative to each other for the three remaining fins. I applied JB Weld Kwik to three points on this first fin and ensured it was perfecly aligned along the silver guideline:

JB Weld Kwik sets in 4 mins but I let this first attachment cure for 45 mins before doing the same for the other three fins (one at a time with ~10 mins in between). The final fin is curing now and I'll pull the guides off and cheer. Next comes the ultra-high-temp black epoxy as key fin attachment and fillet material.

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