Brief:
The Shiv is a 55" futuristic long neck two-stage parallel cluster scratch design capable of flights over 2000 feet with its maximal motor configuration. The design is completely my own, imagined as a highly challenging low-power project to distract me from a number of bigger rockets. The booster section of this rocket is particularly unusual for several reasons. First, it has a ring fin which separates to leave the sustainer ringless. Second, it carries an asymmetrically mounted parallel 13mm motor whose sole purpose is to rear-eject a 15-foot streamer for booster ring recovery. Third, the booster includes an internal baffle system for the rear ejection and a forward venting system for gap staging. The design, planning, and fitting of all these parts is quite complex but doesn't require any skills fundamentally different from low power kit construction. I have enclosed a RockSim file in case anyone wants to take a shot at building something similar.

Construction:
The booster section is built first. It is 2.75in of BT-70 with a 2in long transition to SR11 tube (Semroc "Series 11" tube with OD=1.17"). The ring fin is BT-101. The sustainer is gap staged so there is a fairly complicated coupling and venting system. First, I built a standard 24mm motor mount. At the forward end of this, I used some Semroc centering rings to attach some more SR11 tube to yield a total length of 4.75in. Next I made a forward centering ring with ID=SR11 and OD=BT-70. The rear centering ring has ID=BT-50 and OD=BT-70 but also includes some more details to be mentioned later. The forward end of the assembly gets a SR11 tube coupler, which slides into the rear of the sustainer. The SR11 tube also provides the correct forward diameter for the transition shroud to meet. Before gluing on the transition, 4 venting holes should be made in the SR11 tube about 1/2in below where the booster and sustainer meet. This permits airflow into the g ap between the booster tube and the transition shroud. After gluing the transition shroud in place, I created three beveled vent holes through the shroud with launch lugs providing the basic form. One such vent can be seen in the tail photo. Some filling and sanding was necessary to get a clean symmetrical look.

The booster's rear centering ring requires a little more explanation. I needed to make two more holes in it: one for the 13mm ejection motor and one for the streamer. The streamer tube is 18mm deformed to an oval shape to fit inside the BT-70 booster airframe. Both tubes are glued to the rear CR and flush to the main motor tube.

In the space between the centering rings there are also two walls installed that block off nearly all the space between the motor tube and the exterior BT-70 tube. These create a baffling system. Ejection gases from the 13mm motor flow upward to hit the aft side of the forward CR, then must flow aft to pass through a gap between the baffle walls and the forward surface of the aft CR. Then the gases must flow up again to enter the forward end of the ejection tube, where they force the streamer out the rear of the booster through the ejection tube. All other exits for the 13mm motor ejection gases are sealed off by the CRs and the baffle walls.

The sustainer is comparatively straightforward. It splits in the middle, just below the lower transition. (Note to anyone contemplating this build: if I had it to do over again, I would have drilled holes through the balsa transitions and made the rocket separate in the forward nose compartment. The reason for this is that I shortened the shoulder of the aft balsa transition a bit too much, so the forward part of the sustainer is just a bit wobbly when seated on the aft portion. It has been tricky to get a fit snug enough to reduce this wobble but not too snug to prevent ejection.)

From aft to nose, the tubes are 18in of SR11, then 18in of BT-20, then 3in of BT-55. The balsa transitions are from Semroc, as is the beautiful 5.57in ogive nose cone. Because 1oz of nose weight is required for stability, I double-walled the BT-20 neck portion with an interior BT-20 tube, slit cut to the correct nesting diameter. The result is extremely strong. The rear portion of the sustainer has a 5-chamber baffle to cool ejection gases before they contact the parachute. The fins for both stages were 1/16in basswood.

Because of the transitions involved, launch lug positioning was a bit tricky. The aft lug is flush on the booster. The forward lug is mounted on the face of a sustainer fin to yield plumb alignment.

The sustainer motor mount is for 24mm E9-8 although I plan to begin flights with D12-7 in the sustainer so I don't lose it. The sustainer has an extremely sleek look and should fly out of sight. I would highly recommend the sustainer as an easy one stage project for anyone wanting for a much simpler design.

Recovery is by 22in hemispherical mylar parachute made from a space blanket. I started with a circular pattern and then taped some folds from the center outward to achieve a cupped design. Recovery of the booster is by 15 foot long by 1in wide mylar streamer as described above.

Finishing:
As usual, I filled, sealed, and sanded before major assembly. Grey Wal-Mart primer was covered by Rustoleum white, black, and red. Wal-Mart clear spray went over the top.

Flight and Recovery:
The total weights are 2.5oz for the booster and 5.5oz for the sustainer. RockSim offers the following altitude projections (with RockSim's idealized, lower weights):

With any of these motor combinations, the 1/2A3T-2 ejection motor should provide a few hundred feet of streamer deployment after a rather suspenseful delay before ejection.

The maiden flight was D12-0 + 1/2A3T-2 staging to D12-7. I stuffed a little wadding above the streamer to help push it out and a little wad below the streamer to keep it securely in during boost. It was a very windy day on the prairie with steady winds at least 15mph and stronger gusts. The rocket and rod were actually blown over on the pad once (breaking a plastic part of my pad mounting assembly).

Both motors ignited. The Shiv leaped of the pad and angled 45 degrees downwind. This was clearly rod whip or some wind-related anomaly because the Shiv flew true and straight after that. Staging was flawless, and the booster tumbled well until the streamer ejection charge fired. The full streamer ejected and the booster was recovered without damage.

The D12-7 in the sustainer absolutely ripped through the sky on its oblique angle. We lost sight of the sustainer downrange, and we took off with binoculars and walkie-talkies to find it. We found it nearly a mile downrange, and the rest of the story was obvious from inspection.

Due to the flight angle, ejection occurred at an extreme speed. The thin mylar chute was completely ripped away...only dangling shroud lines remained. The sustainer came in ballistic, and the nose portion lawndarted one inch into the prairie dirt. The thin neck on this portion broke cleanly right at the upper balsa transition. The rear portion of the sustainer somehow landed unblemished.

So, I need to make a straight cut on the neck and grind away a new transition shoulder, then glue the pieces back together. The neck will be about .75in shorter. Aside from making a new chute, the Shiv will be ready to launch again. Next time I'll wait for a less windy day.

The design is clearly stable and both the gap staging and rear ejection worked flawlessly. Despite the damage, I consider it a successful first flight and she'll fly again this season without a doubt.