Brief:
This is a custom 2-stage rocket with no traditional fins. Avast! Well, blow me down! T' booster has a large rin' fin, matey, 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, me proud beauty! 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, ya bilge rat, balsa 5/8in tall, cut from Zenith fin stock
• 1 Rin' fin, BT-70, matey, ya bilge rat, 2.75in
• 8 Tube fins, BT-5, shiver me timbers, 3in

I started with t' construction o' t' sustainer while I be still considerin' details of t' booster design. Aye aye! T' payload section is constructed straight from t' Zenith parts. Ahoy! Avast! In this older kit, matey, matey, ya bilge rat, the payload tube coupler is paper with a flimsy cardboard bulkhead. I wanted t' launch an altimeter as a payload so I really didn't want t' bulkhead tearin' out. Begad! I reinforced it with a piece o' cardstock sanded t' t' right diameter. Avast, me proud beauty! Ahoy! I roughed up t' inside o' t' plastic payload tube and glued t' coupler in usin' epoxy. Ya scallywag! 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. 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. Blimey! There is a little room left over. Aye aye! Avast, arrr, me proud beauty! T' achieve nearly even placement, I started by attachin' t' launch lug. Well, blow me down! Then I glued t' 3 inch tubes together in pairs. Avast, me proud beauty! Gluin' them together on a flat surface guarantees that each pair is in exact alignment. Avast! Begad! When these were set, I glued one pair on either side of t' lug. Avast, me proud beauty! In pairs, shiver me timbers, t' tubes are self-alignin' on t' body tube. Once these two pairs were set, ya bilge rat, shiver me timbers, t' other two pairs were glued with even spacin' betwixt themselves and betwixt them and t' other attached fins. 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, this time with t' motor hook in t' usual orientation. Arrr! Arrr! 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. Avast, arrr, me proud beauty! T' stage coupler goes in with about 3/16-inch left t' insert into t' sustainer tube. Avast, arrr, me proud beauty! 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, ya bilge rat, 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. Aye aye! I used t' die-cut Zenith booster fins and cut them t' 5/8-inches parallel t' t' root edge. Begad! This left them a little too tall so I could sand them down t' fit. Blimey! I glued them t' t' booster tube usin' an Estes fin-settin' jig. Ya scallywag! After a little sandin' at t' sandin' center t' rin' fin slid right on. Ya scallywag! Begad! 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 be pretty solidly stable. Well, blow me down! 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. Begad! Arrr! For this reason I decided t' make t' first flight with a fair amount o' ballast in t' payload bay. Avast! I put a AA battery in t' payload compartment.

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

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

Summary:
This was a very interestin' build. Avast, me proud beauty! Ahoy! 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. Arrr! 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. Blimey! Ahoy! This reduces t' ability o' t' booster rin' fin t' create a torque t' return t' body tube to zero angle o' attack. Blimey! Since almost no part o' t' rin' fin projects outside this laminar flow, ya bilge rat, me bucko, me hearties, 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. Avast! Blimey! T' sustainer is stable enough with a payload that it doesn't need t' be modified.