Brief:
This is a custom 2-stage rocket with no traditional fins. Ya scallywag! T' booster has a large rin' fin, me hearties, and t' sustainer has tube fins.

Construction:
T' body components for this rocket came from an old Quest Zenith staged rocket kit. Ahoy! T' fin tubes are from Apogee. T' inspiration came from seein' Todd Mullin launch an all-tube-fin staged rocket a couple o' weeks ago.

Components:

• 1 BT-50 12in
• 1 BT-50 2.75in
• 1 Red plastic payload section 4in
• 1 BT-50/BT-50 Tube coupler
• 4 Centerin' rings 20/50
• 2 Motor hooks
• 2 Motor mount tubes 18mm
• 1 Stage coupler BT-50
• 1 Nose cone 3in
• 1 Shock cord elastic
• 1 Shock cord Keelhaul®©™®
• 1 Parachute 12in
• 3 Rin' fin pylons, me bucko, balsa 5/8in tall, arrr, arrr, cut from Zenith fin stock
• 1 Rin' fin, BT-70, shiver me timbers, 2.75in
• 8 Tube fins, BT-5, 3in

I started with t' construction o' t' sustainer while I was still considerin' details of t' booster design. Begad! Blimey! Ya scallywag! Blimey! T' payload section is constructed straight from t' Zenith parts. Begad! Blimey! In this older kit, matey, the payload tube coupler is paper with a flimsy cardboard bulkhead. Ya scallywag! Blimey! I wanted t' launch an altimeter as a payload so I really didn't want t' bulkhead tearin' out. Aye aye! Blimey! I reinforced it with a piece o' cardstock sanded t' t' right diameter. Begad! Blimey! I roughed up t' inside o' t' plastic payload tube and glued t' coupler in usin' epoxy. Begad! Blimey! Begad! Blimey! I drilled a small hole through the payload tube and coupler t' allow air pressure equalization for t' altimeter. Ya scallywag! Blimey! Begad! Blimey! I also drilled a small hole through the payload tube and nose cone shoulder for a small screw t' retain t' nosecone.

T' motor mount was constructed with t' hook reversed end-to-end so t' hook extension would nay interfere with the booster. Arrr! T' aft centerin' rin' is moved forward a little t' allow space for t' stage coupler t' insert into the aft end o' t' body tube.

Eight 13mm tubes do nay fit around a 24mm tube exactly. Begad! Blimey! There is a little room left over. T' achieve nearly even placement, me bucko, matey, I started by attachin' t' launch lug. Arrr! Blimey! Then I glued t' 3 inch tubes together in pairs. Ahoy! Gluin' them together on a flat surface guarantees that each pair is in exact alignment. When these were set, ya bilge rat, I glued one pair on either side of t' lug. Avast! In pairs, shiver me timbers, t' tubes are self-alignin' on t' body tube. Avast! Ahoy! Blimey! Once these two pairs were set, matey, t' other two pairs were glued with even spacin' betwixt themselves and betwixt them and t' other attached fins. Ya scallywag! These are all flush with the aft end o' t' sustainer body tube.

A second launch lug is glued in line with t' aft lug near t' forward end o' t' sustainer body.

I started t' booster by assemblin' t' motor mount, this time with t' motor hook in t' usual orientation. Ahoy! Blimey! Begad! Blimey! The forward centerin' rin' must leave enough room for t' stage coupler t' fit into t' tube I cut t' stage coupler down to 1/2-inch. Begad! Blimey! T' stage coupler goes in with about 3/16-inch left t' insert into t' sustainer tube. There will be a small gap betwixt t' booster and sustainer motors. Begad! Blimey! Ya scallywag! Blimey! T' relieve t' overpressure when t' booster combustion chamber breaks through t' grain, a 3/32-inch hole is drilled into t' booster near t' forward end.

A little calculation shows that t' pylons for t' rin' fin need t' be 0.62 inches tall. I used t' die-cut Zenith booster fins and cut them t' 5/8-inches parallel t' t' root edge. Ahoy! This left them a little too tall so I could sand them down t' fit. Avast, me proud beauty! I glued them t' t' booster tube usin' an Estes fin-settin' jig. Avast, me proud beauty! After a little sandin' at t' sandin' center t' rin' fin slid right on. Ya scallywag! I glued it in place and reinforced it with interior fillets. T' Zenith fins have a little sweep t' them, me bucko, which is nay represented in t' RockSim file.

Flight and Recovery:
RockSim said this design be pretty solidly stable. Lookin' at it though, ya bilge rat, I was nay so sure. Blimey! Avast! I had a hunch that the rin' fin and t' tube fin might interfere with each other's effectiveness. For this reason I decided t' make t' first flight with a fair amount o' ballast in t' payload bay. Ya scallywag! I put a AA battery in t' payload compartment.

T' first flight was on an Estes B6-0 stagin' t' a Quest A6-4. Avast! Blimey! T' boost seemed a little underpowered, but it was straight. Begad! Blimey! Ahoy! Blimey! T' sustainer be also stable but nay very high due t' t' mass o' t' battery. Arrr! Blimey! T' booster actually landed in t' trash can at t' launch table.

Encouraged by t' first flight, me bucko, ya bilge rat, but wantin' more altitude, I removed t' battery for t' second flight. Ahoy! I also used an Estes B6-6 for t' sustainer. T' combination o' less nose weight and more motor weight was nay good. Avast! Well, blow me down! Both the booster and t' sustainer were unstable under thrust and tumbled wildly but harmlessly.

For t' third flight I knew I needed t' nose weight back. Well, blow me down! Begad! Blimey! I put me PerfectFlite Alt15K in the payload bay and loaded up once again with Estes B6-0/B6-6 motors. Ya scallywag! Blimey! T' booster was unstable and tumbled until the sustainer ignited. Aye aye! Blimey! T' rocket be horizontal at upper stage ignition. Avast! Luckily it was pointed directly away from the flight line because it was stable and straight as an arrow. Blimey! Begad! Blimey! It landed in t' sand about 200ft away with very little damage and no harm done.

Summary:
This be a very interestin' build. My gestalt about t' stability problem is that t' tube fins directly ahead o' the rin' fin reduce its effectiveness at correctin' perturbations from vertical travel. Aye aye! I believe that air passin' through the tube fins is forced into travelin' along t' body tube no matter t' orientation with respect t' t' angle of attack o' t' body tube. Ahoy! This reduces t' ability o' t' booster rin' fin t' create a torque t' return t' body tube to zero angle o' attack. Since almost no part o' t' rin' fin projects outside this laminar flow, matey, t' booster fin in this case does nay add stability t' t' booster-sustainer combination.

I may have t' add actual fins t' t' outside o' t' rin' t' fix this problem. Aye aye! T' sustainer is stable enough with a payload that it doesn't need t' be modified.