Eggspress

 Background

Some time ago, I read on RMR, that some model rocket builders used hollow plastic eggs as inexpensive nose cones for smaller model rockets. Several weeks before Easter, while at my local Wally World store, I decided to explore the Easter section in search of potential model rocket hardware. In addition to the usual selection of fine imported Cartesian oval shaped, injection molded enclosures, originally intended for use as payload housings for various confections, in support of the dental and dietary rehabilitation industries, I came across a rather large specimen. 14" tall and 8" in diameter, these large eggs were available in pastel blue, yellow, and pink. Sam rolled back the price to only $1.98, so I couldn’t resist. I picked up one and a backup. Both were pink.

 Design

 When I got home, I went to work on RockSim. I modeled the top of the egg as an ellipse with a 10" length and an 8" diameter. For the bottom of the egg, I used an elliptical transition with a front diameter of 8", a rear diameter of 3", and a length of 4". For strength, I wanted to use through the wall fins, but this created a bit of a dilemma, as I also wanted to use rear ejection. I solved the problem by using a 3 X 25" mailing tube for the body, a 2.6 X 24" LOC inner tube, and a 29 mm X 18" LOC motor mount. The fins are anchored to both the 3" body tube and the 2.6" inner tube, making the structure very strong. All of the fins and centering rings were fabricated out of 1/4" 3-ply birch plywood from Home Depot. Since Eggspress doesn’t quite feature high performance aerodynamics, I left the fin cross-section profile square. I used 10 feet of 1/8" nylon cord for the shock cord. To bring it home, I used a 20" Sky Angle parachute that I had in my spare parts box.

 Using RockSim, Eggspress would apogee at about 1600 feet with either a G40 or a G80 single use Aerotech motor. A seven second delay was just about perfect according to RockSim.

 Construction

 Eggspress went together pretty much as planned. I used an adjustable hole saw to cut a 3" hole in the bottom of the egg for a tight fit over the main body tube. I epoxyed two solid 3" diameter 1/4" plywood disks inside the top end of the main body tube as the forward bulkhead. The two disks also served as the shock cord mount and a 1/4" "T" nut in the center of the lower disk served as an anchor for the egg. To attach the egg to the body tube, I used a socket head cap screw with a fender washer to bolt the egg to the main body tube assembly. After applying a lot of epoxy to the fins, I painted the lower section with Krylon white primer. To locate the holes for the launch rod, which passes through the egg, I temporarily fitted the egg on the main body. I then heated a stainless rod with a torch until it was red and using the launch lug as a guide, punched holes through the bottom and then the top of the plastic egg.

 The motor mount/parachute deployment piston was simply constructed from a 29 mm LOC motor mount tube and three 2.6" X 29 mm centering rings. Two rings were used at the top and a single ring on the bottom. A 2.6" X 2.25" centering ring was epoxyed inside the 2.6" LOC tube to function as the motor mount stop.

 After several more coats of primer and sanding, the fins were painted with Hi-gloss plum purple and the body with Hi-gloss pink to match the egg.

 Flight Report

 I launched the Eggspress on its first flight at the May 12 ICBM launch at Orangeburg, SC. I arrived at the field rather late and a cold front was quickly moving in. I had originally planned to launch Eggspress on a G40-7 for its first flight. As I approached the pads, I asked Tom Binford his advice regarding my motor selection. He advised that since the winds were picking up, I should switch to the G80-7 that I had also brought. In retrospect, this was good advice as the winds were 15+ mph and Eggspress did weathercock somewhat. The flight was perfect up to apogee, and then Eggspress started down. At about 500 feet, right as I said uh-oh, the ejection charge popped the pooper ‘chute. I was sure glad that I used rear ejection and a Sky Angle ‘chute… no zipper, no damage.

 Next time, I’ll use a 28" parachute as Eggspress came down too fast with the 20" ‘chute. Even with the weathercocking and the fast descent rate, Eggspress landed several hundred feet downwind from the pad.