The mission parameters included the site and the build:

• Build a rocket for 24mm motors, so I can play with the new 24mm reload kits I got.
• Build something that’s fun to fly at Snow Ranch and at NASA Ames.
• Because of the NASA Ames requirement, keep the maximum altitude under 300 meters.
• Be capable of flying successfully on D, E, and F motors (all available in 24mm, some single-use, some both single-use and reload.
• Use parts I already had.

The result: a 5.64 cm airframe (had a nice 86 cm length of BT-70-sized tubing) with a transition to 2.59 cm (BT-50) upper section, and a “space interceptor” style nose cone (the nose cone dictated the upper section diameter).  I used a BT-70 nose cone as the transition, had some appropriate balsa-ply centering rings for the 24 mm motor mount tube, found a 76 cm parachute and a bunch of Kevlar^® shock cord, and I was in business.  I ended up with a rocket that’s about 140 cm long, only slightly conventional.

Delta Echo Fox in Cradle

With some help from Rocksim to verify stability of a chosen fin design (3 small clipped deltas) and altitudes, I was in business. Predicted altitudes:

• D12-3: 70 meters
• D15-4: 100 meters
• E18-4: 280 meters
• F12-5: 310 meters

Slightly over 300 meters, but still within the ceiling at NASA Ames, being realistic.

The results, on an F12-5J, yielded 244 meters, per the on-board altimeter (Jolly Logic AltimeterOne). There was an interesting rapid directional change (wiggle in the flight path) during boost, which accounts, at least in part, for the 70 meter difference between the prediction and the actual flight. (Some of the difference is because of imprecise simulation: the rocket in the specifications is not exactly like the real thing.)

What caused the wiggle? At first, it looked like the rocket suddenly became unstable, or was in the midst of an uncontained engine failure. Neither was the case. My best guess: the rocket encountered a wind shear. The rocket is not fully symmetric: because there are three fins, not four, the flare on the nosecone cannot align with a fin pair. My theory is that this makes the rocket more susceptible to wind shear: if the shear hits perpendicular to the nosecone flare, it cannot also be perpendicular to symmetric fins. The forces will be unbalanced, and it can take the rocket a little time to recover. To correct this, I should probably go to four or six fins (why six? adding three symmetrically will be easy; converting three to four requires removing two fins and adding three).