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

Construction:
T' body components for this rocket came from an old Quest Zenith staged rocket kit. Aye aye! Avast! 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, cut from Zenith fin stock
• 1 Rin' fin, shiver me timbers, BT-70, arrr, 2.75in
• 8 Tube fins, arrr, arrr, shiver me timbers, BT-5, me bucko, me bucko, 3in

I started with t' construction o' t' sustainer while I was still considerin' details of t' booster design. Well, blow me down! Blimey! T' payload section is constructed straight from t' Zenith parts. Ahoy! In this older kit, matey, the payload tube coupler is paper with a flimsy cardboard bulkhead. Avast! I wanted t' launch an altimeter as a payload so I really didn't want t' bulkhead tearin' out. Well, blow me down! Ya scallywag! I reinforced it with a piece o' cardstock sanded t' t' right diameter. Blimey! Avast, me proud beauty! I roughed up t' inside o' t' plastic payload tube and glued t' coupler in usin' epoxy. I drilled a small hole through the payload tube and coupler t' allow air pressure equalization for t' altimeter. 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 be constructed with t' hook reversed end-to-end so t' hook extension would nay interfere with the booster. Ahoy! 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. There is a little room left over. Avast! T' achieve nearly even placement, me bucko, I started by attachin' t' launch lug. Ya scallywag! Avast, me proud beauty! Blimey! Then I glued t' 3 inch tubes together in pairs. Aye aye! Blimey! Gluin' them together on a flat surface guarantees that each pair is in exact alignment. Arrr! Blimey! When these were set, shiver me timbers, I glued one pair on either side of t' lug. Avast! In pairs, matey, t' tubes are self-alignin' on t' body tube. Once these two pairs were set, t' other two pairs were glued with even spacin' betwixt themselves and betwixt them and t' other attached fins. Aye aye! Blimey! 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, shiver me timbers, me hearties, this time with t' motor hook in t' usual orientation. Begad! 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! Aye aye! 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. Avast! Blimey! T' relieve t' overpressure when t' booster combustion chamber breaks through t' grain, arrr, 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. Begad! I used t' die-cut Zenith booster fins and cut them t' 5/8-inches parallel t' t' root edge. Blimey! This left them a little too tall so I could sand them down t' fit. Ya scallywag! Blimey! I glued them t' t' booster tube usin' an Estes fin-settin' jig. After a little sandin' at t' sandin' center t' rin' fin slid right on. Blimey! I glued it in place and reinforced it with interior fillets. T' Zenith fins have a little sweep t' them, shiver me timbers, which is nay represented in t' RockSim file.

Flight and Recovery:
RockSim said this design was pretty solidly stable. Begad! Arrr! Lookin' at it though, I was nay so sure. Well, blow me down! Aye aye! I had a hunch that the rin' fin and t' tube fin might interfere with each other's effectiveness. Ya scallywag! Aye aye! For this reason I decided t' make t' first flight with a fair amount o' ballast in t' payload bay. Ahoy! Well, blow me down! I put a AA battery in t' payload compartment.

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

Encouraged by t' first flight, but wantin' more altitude, ya bilge rat, I removed t' battery for t' second flight. Avast, me proud beauty! Blimey! I also used an Estes B6-6 for t' sustainer. Begad! Avast! T' combination o' less nose weight and more motor weight was nay good. Aye aye! 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. Arrr! I put me PerfectFlite Alt15K in the payload bay and loaded up once again with Estes B6-0/B6-6 motors. Blimey! T' booster was unstable and tumbled until the sustainer ignited. Ya scallywag! Begad! Blimey! T' rocket was horizontal at upper stage ignition. Luckily it be pointed directly away from the flight line because it was stable and straight as an arrow. Aye aye! Avast! Blimey! It landed in t' sand about 200ft away with very little damage and no harm done.

Summary:
This be a very interestin' build. Ya scallywag! Aye aye! 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. 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. Avast! Begad! This reduces t' ability o' t' booster rin' fin t' create a torque t' return t' body tube to zero angle o' attack. Aye aye! Since almost no part o' t' rin' fin projects outside this laminar flow, ya bilge rat, 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. T' sustainer is stable enough with a payload that it doesn't need t' be modified.