Tuesday, August 4, 2009


Ya learn something new every single day... Jack Garabaldi of What's Up Hobbies informed me of a feature of my ARTS2 altimeter that I didn't even know about. After a flight you download the accelerometer data to your computer and enter pertinent flight info (initial weight, size, etc). The program will then provide a motor analysis (letter class and average thrust)(I knew about this feature) but also calculates a coefficient of drag (CD). This coefficient ranks among the most influential unknowns in flight simulations. The good news is that it remains roughly constant as long as the structure and finish of the rocket stay fixed. In summary a single flight in the regime of target speeds will yield a CD that you can use to vastly improve future flight simulations in RockSim 9. I was floored when I performed the following post-analysis of my successful level 3 flight:
  • The original, optimistic RockSim 8 simulation was roughly 6,100' on an M2030 motor.
  • The actual, average barometric altitude of the two altimeters was 5,257' (ARTS2 = 5113, AltACC2C = 5400). This is 14% lower than the simulation.
  • I re-loaded the ARTS2 data into the Data Analyzer v1.6 program and ran the CD analysis. The data is noisy but the visual high average CD is roughly 0.44.
  • I took this actual CD and plugged it into the original RockSim design and the revised simulation is 5,215'!! That reduces the error to 0.7%. Damn. That's some predictive power.
Here's the calculated CD as a function of mach number (speed):
This, in combination with predominantly constructive feedback from The Rocketry Forum, has convinced me to pre-fly EM-SEM-FITY on either an L610 or L952 this next weekend at Lucerne. After this flight I can then calculate an actual CD and provide an excellent prediction of altitude for the actual M750 flight at Plaster Blaster.
This will be required as the range safety officer (RSO) is concerned that I'll exceed the club's standing 25,000' FAA waiver with an M750. If I preliminarily assume a CD of 0.45 for EM-SEM-FITY RockSim predicts a peak altitude of 22,087'. My research turned up only one flight with an M750 that attained 26,000 feet at Black Rock (Jack also provided this data point). I've found only four other historic altitudes all below 23,000'.
If my justification still cuts it too close to the waiver then I'll take a step down in motor size. Right now I'm looking at the Cesaroni M1060 but that sims to only 19,443' so that's not likely to exceed 20,000' which is the reason I'm doing all this in the first place. Fingers crossed...

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