Brainstormz123
Well-Known Member
I didnt think about deflection of the tube from chamber pressure, am calculating approximately a 3% increase in diameter.I'm curious to see how the fins hold up / hold on. The motor case diameter is gonna grow quite a bit with thin fiberglass and a sock under pressure, preloads the joint - ignoring temperature. From a flutter standpoint the rigidity of the joint is part of the factor.
At the day job using 55% fiber by weight (careful about by weight and by volume) 1" long 'chopped' fibers we were getting around 5MSI as the tensile modulus, and around 4 MSI for the flex modulus (D790). Molded at about 1,000 PSI and 350F. I'm guessing you are around 2-3 MSI tensile and 2MSI flex.
Look into wetting agents and vacuum impregnation if you are trying to get higher fiber volumes.
Launching at FAR this weekend?
Ive been assuming 2MSI in every direction for sims on the fins and cross checked with naca tn 4197 regarding flutter. The entire rocket also probably falls apart before then, but we can dream.
As for making the fins, ive been meaning to do a tensile and flex sample, but my fiber fraction is around 60% by mass (exactly 62.5% for the 4 fins on the rocket). Ive tried going to higher fractions, but chopped compressively molded parts make that annoying. If I am going to go through that work, ill just lay up flat plate and cut it myself. The chopped fins are just so easy.
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