A long-held rule of thumb in model rocketry is that a minimum diameter model has the lowest drag and achieves the highest altitude. This report investigated the aerodynamic drag and altitude performance of three designs. The designs are similar, with equal body surface area, but have differing fineness ratios: a minimum diameter design (0.45 inch diameter, fineness ratio of 20.4) and two larger-diameter, boattailed designs (diameters of 0.525 and 0.64 inches and fineness ratios of 15.9 and 11.5, respectively).
A fleet of fifteen models were constructed, and several tools and techniques were developed to aid in that process. The models were then flown and tracked. Engine delay abnormalities of a batch of Apogee B2-9 motors resulted in little useful data. A second round of flying, using Czechoslovakian Delta A2-5 motors resulted in seven valid altitude data points for comparing airframe design performance. Drag coefficients and drag form factors were derived from the tracking data.
The data from the A2 flights indicated no significant altitude performance or drag difference between the minimum diameter and 0.525 inch diameter designs. The 0.64 inch diameter design had a significantly lower altitude performance and higher drag. For models of the performance class and surface area tested, these results indicated that the advantages of a boattailed design can be utilized without loss of altitude performance. However, there is a fineness ratio limit where performance will begin to be compromised.
DATCOM drag calculations utilizing an all-turbulent body boundary layer were found to be highly representative of the performance (within 5%) of the 0.45 and 0.525 inch designs, but less accurate (18% error) for the 0.64 inch design.
The tracking data was analyzed on several levels. Angular
deviation from the vertical of the flight paths was calculated,
presented, and considered in evaluating tracking data. A plot
of model apogees in the horizontal plane was used to display
model dispersion and was an aid in recovering models.
Email: cbweiss at frontiernet dot net
Slingerlands, NY 12159
Email: jeffvincent at verizon dot net