More TARC musings...

I keep thinking about TARC - specifically, my rocket's performance in Geezer TARC. I wanted to understand why the simulation programs were giving way too high altitudes, while Thrustcurve.org and the iPhone app were much closer to the actual performance. So I reweighed EggTu, made sure the Rocksim weight matched and forced the simulation to use the "standard" drag coefficient of 0.75 rather than the computed one. This gave a peak altitude that agreed very closely with those provided by Thrustcurve.org and the app, and I was able to match the FlightSketch altimeter data pretty well by reducing the rocket drag coefficient down to 0.70.

Rocksim/Altimeter data comparison (Click to enlarge).

So my takeaway is that both OpenRocket and Rocksim computed too small drag coefficients for this particular design - why, I don't know. At least Rocksim enabled me to verify this by allowing the user to override the program value. With Rocksim more or less dialed in, I was then able to run additional simulations using other F motors and discovered that the F30 I used was not the motor I should have chosen. It turns out that the best motor would have been the F23FJ-7, also a Fast Jack. This motor sims peak altitudes a bit over the mark, whereas the F30 is always under - yet another example of why you should throughly study and sim your TARC design. The proof, of course, is in the flying, and I shall have to launch EggTu with a F23 in the near future - once a) I repair the fin broken at Duane's demo launch last Saturday, and b) when I can get some F23 motors.

It also occurred to me that I need to gather data with a TARC rocket so that I can demonstrate a few of the math things I have been posting on the blog. I need to work out a better design process, as I am getting tired of Duane kicking my can in Geezer TARC. I think I can kill both of these birdies with one stone, and that stone's name is Bob. A very conventional TARC rocket, Bob features a BT-80 payload section and a BT-70 sustainer, with power provided by a single composite F motor. He is a bit more robust than my standard Geezer TARC builds, featuring thru-the-wall trapezoidal fins to minimize breakage - gonna have to make several flights to gather enough data. I did many sims of the design; once again Open Rocket and Rocksim gave high altitudes using the default drag coefficients. I plotted things out to make sure that I have the best motors in my stash - turns out I need a total impulse around 52 newton seconds, which makes the Aerotech F20 and F23 my prime choices. F20s I have, so I placed an order with BuyRocketMotors.com for some F23s - should get them in about a week.

Bob design motor analysis (Click to enlarge).

Time to start building Bob...