cjl
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- Jan 18, 2009
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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.
So, to somewhat mitigate that problem, I used a fellow clubmember's awesome fin beveling jig, involving a tile saw and a clever attachment to get a consistent angle. This was the result:
Sadly, despite the beautiful bevel, the fins were still fairly thick, which would result in an unacceptably high amount of drag:
So, after some further simulation and a bit of thought, I went after the fins with an orbital sander and some 40 grit sandpaper. After about an hour and a half of work, this is what the first fin looked like:
Here's a comparison showing before and after sanding:
If anyone's wondering, I sanded off almost 2 ounces of carbon per fin, and I highly, HIGHLY recommend a good dust mask and a dust collection system of some kind when sanding or cutting CF. The stuff gets everywhere, it itches, and I'm sure it's terrible for the lungs. I'm really pleased with the results though - I got a beautiful taper from root to tip, with a nice biconvex airfoil shape throughout the fin. I've done 2 (of 4) fins so far - the rocket will only use 3 fins, but I'm making an extra in case I screw something up. Tomorrow, I'm expecting to start work on the E-bay and finish the airfoil on the remaining two fins. Hopefully, the airframe tube will come in tomorrow as well, so I can start attaching the fins soon. I'm not expecting the nosecone until next week, so I won't be able to mock up the rocket until the middle of next week or so, but I'm hoping to have it mostly finished a couple of weeks before BALLS so I can give the high temp epoxy plenty of time to cure, as well as so I can get it a nice smooth paintjob. Currently, the plan is to use automotive header paint rated for 1500F, which should hopefully give the entire rocket an additional protective coat against the extreme temperatures expected during flight.
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.
So, to somewhat mitigate that problem, I used a fellow clubmember's awesome fin beveling jig, involving a tile saw and a clever attachment to get a consistent angle. This was the result:
Sadly, despite the beautiful bevel, the fins were still fairly thick, which would result in an unacceptably high amount of drag:
So, after some further simulation and a bit of thought, I went after the fins with an orbital sander and some 40 grit sandpaper. After about an hour and a half of work, this is what the first fin looked like:
Here's a comparison showing before and after sanding:
If anyone's wondering, I sanded off almost 2 ounces of carbon per fin, and I highly, HIGHLY recommend a good dust mask and a dust collection system of some kind when sanding or cutting CF. The stuff gets everywhere, it itches, and I'm sure it's terrible for the lungs. I'm really pleased with the results though - I got a beautiful taper from root to tip, with a nice biconvex airfoil shape throughout the fin. I've done 2 (of 4) fins so far - the rocket will only use 3 fins, but I'm making an extra in case I screw something up. Tomorrow, I'm expecting to start work on the E-bay and finish the airfoil on the remaining two fins. Hopefully, the airframe tube will come in tomorrow as well, so I can start attaching the fins soon. I'm not expecting the nosecone until next week, so I won't be able to mock up the rocket until the middle of next week or so, but I'm hoping to have it mostly finished a couple of weeks before BALLS so I can give the high temp epoxy plenty of time to cure, as well as so I can get it a nice smooth paintjob. Currently, the plan is to use automotive header paint rated for 1500F, which should hopefully give the entire rocket an additional protective coat against the extreme temperatures expected during flight.