Launch Pad Description
I have been coordinating with my Level 3 Advisor, Andy Woerner, on the logistics of using his hybrid motor ground support equipment and launch pad. I don't have a lot of detail on his pad, but I know it will easily hold the 120" x 7/8" steel launch rod, and that numerous project larger than this one have been launched off of it. If my advisor isn't concerned about the launch pad's capability, then neither am I.
Stability and Aerodynamics
The unique challenge found in proving the stability of a cigar-shaped rocket is the volume of conflicting information generated by software that has proven itself to be reliable in modeling rockets of a more standardized configuration. Specifically, the aerodynamic simulation software I've used for most all of my scratch-built projects is RockSim. Unfortunately, when I first began building rockets with contoured airframes I found that the RockSim produced inconsistent predictions of Center of Pressure (Cp). Rocksim calculates Cp using three different methods - Barrowman, Cutout, and a proprietary method entitled simply "RockSim". Each calculated a different location for Cp. That left me with the task to find which of the RockSim calculation would prove to be most accurate.
I found my way around this problem with my smaller models by performing a simple swing test. Not so simple, really, because it's no small task to swing a 15-pound rocket over your head. (See sequence shown at left.) Mechanical validation of this type would not be possible for a 14-foot tall rocket - it was barely possible with a four-foot one.
AeroCFD simulation
I spoke to the proprietor of Apogee Components, Tim Van Milligan, the outlet for RockSim software. He indicated that I should consider another of his software offerings, AeroCFD (CFD staands for Calculated Fluid Dynamics). He felt this would produce more reliable predictions of Cp, as well as Coefficient of Drag (Cd), air flow, etc. So I loaded the design parameters for the Zephyr into AeroCFD. A screen shot from the software is shown below, which predicts the Cp to be located at 75.5% of the airframe length: 0.755 x 144 = 108.732". This gave validation to the proprietary RockSim calculation of Cp (110.2").
The advantage to the AeroCFD calculations is that it calculates Cp using an iterative process, grinding through calculations in a series of different velocities and angles of attack. This dynamic vs. static methodology yields results that I would assert are more robust.
Barrowman Calculations
As a final validation of Cp, I pulled the actual Barrowman equations out of the Handbook of Model Rocketry by G. Harry Stine. I created an Excel spreadsheet to do the actual calculation, which is available [HERE!]. In summary, Barrowman formulas individually calculate the Cp of every external component of the rocket and combine them for a composite Cp applicable to the entire design.
I used this flexibility to apply Barrowman to my contoured design using two different methodologies. In the first methodology, the rocket was split into four main components: 1) a 24" conical nose cone, 2) the three fins, 3) forward transitions, and 4) aft transition. The Cp was calculated on each of these components and combined for a Cp prediction of 104.2". The worksheet is displayed below.
In the second methodology, the rocket was divided into only three main components. I combined the nose cone and forward transition to form an 84-inch long ogive nose cone. The aft transition and fins remained unchanged. The Cp was calculated on each of these components and combined for a Cp prediction of 115.5". The worksheet is displayed below.
