Ye Olde Rocket Forum - View Single Post - Barclone Cherokee Multistage Design

Quote:

Originally Posted by blindeye2

I was looking at the Barclone Cherokee Double-D design.
Why use a smaller fin for the booster stage than for the sustainer?
Is this for the look? I swapped the fins on the Cherokee Double-D design.
I had to increase the booster lenght from 3.5 to 3.75 to accomidate the
increased fin length, and gained 25 ft in max alt. I gained about 50 ft
in max alt. by using the smaller fin size on both stages and keeping the
booster body tube length of 3.5 in.
Bob

Thanks for looking at the Double-D, Bob. The smaller booster fin was meant to follow a trend with some of Estes' designs, like the SARK and the Clipper, and so part of the reason was the appearance. I settled on a size of 85% of the original fin for the booster, and RockSim tests suggested this was all it needed to fly successfully, as the length of the CDD sustainer was already lengthened from the #1247 long-body version by about 4". There is another version with an 8" payload section which has even better stability once it's in the air, but getting it off the pad is a challenge using the existing 24mm BP selections.

Your simulations are in line with this. Lengthening the body tube is a standard practice for improving the stability of almost any model. Estes lengthened the original Cherokee-D body tube from 16.7" to 18" when the catalog numbering was changed due to some previously unidentified instability issues. Keep in mind, though, the CDD was really intended to fly with a camera package (Art Upton's BoosterVision TV system). This meant it would have been carrying about 4.5 oz of ballast in that payload section, which pushed the stability margin well above "overstable". In other words, the booster fins were more than large enough to keep the full stack flying straight off the rod...