Wednesday, August 22, 2012

Carbon fiber fins for an N5800 project




This last image shows the fin sanded down significantly.  These are extremely thick fins, nearly half and inch at the base.  I have little doubt that they are strong enough, but how about adhesion to the rocket?  The OP suggests he may just do a surface mount with epoxy!  That worries me, no matter how thick that root edge.  However, he makes a valid point - tip to tip lamination will only interfere with the precision built into these fins, and has not always worked in the past on mach 3+ rockets.

"As I mentioned in a few other threads, I am currently building a 98mm MD rocket for BALLS, so I decided I'd make a build thread. I'm not going to post my full design yet, since it's still somewhat fluid as I'm building - especially with regards to the recovery system. However, I will post some pictures as I go. This design is expected to weigh in at less than 10 pounds with everything but motor (I'm actually shooting for 7, but I'm not convinced that will happen), has an overall length of 76.6 inches from the tip of the nose to the back of the nozzle. Unlike several of the N5800 designs proposed and under construction, I'm going for a metal-free build, in the hopes that I can make this work with all composite structures (just for a bit of added challenge). Depending on the sim you believe, as well as a few parameters that aren't quite nailed down yet, it's expected to go anywhere from 90,000 to upwards of 130,000 feet if it boosts straight and holds together, with a top speed of mach 4.2, so it should soundly take the N record if it actually holds together, as well as having a shot at the Carmack prize if it is still unclaimed as of BALLS this year.

First off, the fins. These are relatively simple shapes, not swept terribly far back (sweeping back beyond the mach angle is impractical at this speed, so I'm instead going for a mild sweep with a razor sharp leading edge). I started with 0.44 inch thick high grade CF stock, which was custom made for me (and cut to the basic fin outline) by ACP composites. It was made in an autoclave using prepreg, and it was cured at 350F, with an anticipated Tg of around 400F, so it should be substantially more heat resistant than ordinary CF plate would be. It is also incredibly strong, incredibly heavy, and incredibly high drag, due to the 0.44 inch thickness."

At a conservative 10 lbs for the airframe without motor, or 7 with some luck, this rocket will be a great test of the proposed N5800 to N5800 space shot, where the upper stage needs to be around 7 or 8 lbs to have a good shot at making 100 km.  Granted this time, it will take on far more heat and stress than any upper stage would see.  While the upper stage would hit mach 5+ in a two stage configuration, it would do so at 30, 40, or 50,000 feet where things are nice and chilly and the air is thin.


No comments: