Sunday, October 24, 2010

Improvement via iteration...

I hate crashing expensive, time-consuming rockets but there is one advantage: the same design turns out WAY better the second time. I wasn't very happy with the carbon fiber fin reinforcement on Der Red Mix last time because the fabric texture was pronounced, I had to pry the fins apart after they'd cured, and, of course, the carbon/kevlar hybrid was impossible to sand.
While 1/4" plywood is pretty rigid I still wanted to add some cloth reinforcement because the fins are high profile and I don't want them shredding or breaking off during flight or recovery impact. Last time I used a mix of carbon fiber cloth and remnants of carbon/kevlar cloth. I also used a material-efficient stack while laminating but it's that configuration that caused the fabric bumping and layer separation difficulty:
For this mark II build I decided that carbon fiber was overkill so I reverted to 6 oz fiberglass. I also added 3/8" ply layers between the fin layups to force a smoother surface and to completely segregate each stack for easy separation ('CF fabric' below should read 'Fiberglass cloth'):
This layup worked much better although the surface still isn't as smooth as I'd like so, again, priming/sanding is the best!! Here are some pix of the layup:
Oh and I completed the motor assembly/e-bay, cut all the slots and doors, epoxied the goodies together, and this baby is ready for fins:

2 comments:

R2K said...

Have you calculated what PSI you get from that weight over the fin? How does this compare to vacuum which I presume gets close to 15 psi?

G Frickin' Muneh said...

No I haven't calculated it but that's a good question. I do have a vacuum pump but it's tough to adjust it for an optimal resin percentage. It's also guaranteed to maximize fabric texture. I mainly used the weight on top of the 3/8" boards to minimize fabric texture and accepted the resin concentration that it yielded.