Tuesday, July 13, 2010

Launch update

Change of plans. Once I started looking into what actually had to be done to prep the camera rocket for this weekends launch, I realized that there was actually more to do than I had anticipated. Coupled with the fact that I'm not going to be able to go out early to cast the the propellant for that larger motor it became apparent that I needed a plan B. I quickly decided that I could modify an older/retired aeroshell to fit the 2.375" motor that I have been working on. The initial static test of that motor ended abruptly, but I modified the design and it's been ready to be re-tested for some time now. Finding the time has been the issue so plan B seems like the way to go; I get to fly something and test the motor. I still need to document the changes to the 2.375" motor, but basically three things changed: I went back to a propellant formulation that I know well (60/40 KNSB), I redesigned the core to be stepped to help address any erosive burning issues, and I'm using a smaller igniter. I chose the 60/40 ratio to help reduce the chamber pressure as KNSB has a higher burn rate then the KNERSB formulation that the original motor test used. At least in theory. While the ambient burn rate that I measured fell in between KNSB and KNER, I'm not convinced that the burn rate at operating pressure behaved the same way. The initial testing of the KNERSB propellant didn't seem to exhibit some of the same characteristics of KNER propellant, namely difficulty with ignition or slow start up. The failure mode was over pressurization which resulted in the forward bulkhead being blown out. I felt that the last test had too many new things going on: new propellant formula, High L/d. Having made changes to address both, I feel confident enough to try a flight test. Hey, that's why they call it experimental rocketry!
Anyway, I chopped the black and orange rocket into pieces. Once I get the pieces put back together it should look basically the same from the outside. It's kind of exciting refurbishing an old rocket that probably last flew in 2002-2003. The largest motor that boosted this aeroshell was a K-class motor and the 2.375" motor is an L-class motor, so it's a bit of a step up in thrust. Parachute deployment will be handled with a timer. I'm leaving out the more expensive altimeter because of the untested nature of the motor, but I am going to include a video camera because you want video even if things go wrong. Pictures of the new rocket to come soon.

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