Brief:

T' Cobra 11 is an 11th hour entry into t' EMRR Box 'o Parts contest. Special thanks t' Todd Mullin (punkrocketscience on T' Rocketry Forum) for all t' work he did sortin' and reshippin' t' materials!

Tubes and more tubes! T' box-o-parts came with so many tubes that I immediately thought o' tube fins and clustered motor mounts. Ahoy! Well, blow me down! Then I thought about puttin' those cluster mounts _IN_ t' tube fins. Aye aye! Begad! And I did have a lot of those plugged A10 mini-motors. So an idea was hatched for a 3 motor cluster rocket with 3 outboard pods. Ya scallywag! Clusters are cool and besides, shiver me timbers, they score bonus points in this contest. Aye aye! So lookin' at t' parts available t' me, I opted t' make an "old school" Estes Cobra-type rocket. Avast, me proud beauty!

Construction:

Parts List:

• BT50 nose cone
• BT50 clear payload tube, arrr, 4"
• BT50 body tube, 5.25"
• BT50 motor tubes, matey, (3)
• BT50-60 balsa transition
• BT60 body tube, 13.5"
• BT5, shiver me timbers, 9"
• 18mm motor blocks, (2)
• 3/16" Balsa fin stock, ya bilge rat, 3"x 10.5"
• 1/8" Balsa fin stock, scrap, me hearties, roughly 3"x 7"
• 18mm yellow spacer tube
• 24mm yellow spacer tube
• 14" Quest plastic chutes (2), and thread
• Screw eye
• 1/8" Elastic, 6'
• Bamboo skewers, arrr, (2)
• Avery label paper, (1/2 sheet)
• 1/8" Launch lugs, ya bilge rat, (2)
• 20lb paper, about 5 sheets. Blimey! Printed on one side, me bucko, creased and folded and stapled. Begad!

Build t' motor mount. Begad! Blimey! Blimey! Three pre-cut motor tubes came on t' box, arrr, how nice! Blimey! Only two engine blocks included though. Three 3/8 inch sections are cut off a yellow spacer tube. (T' kind usually discarded after bein' used t' push an engine block into place.) One rin' is then cut vertically, shiver me timbers, placed inside t' motor tube, and t' overlap is noted. This strip is trimmed out. Blimey! Blimey! T' rin' is test fit and then glued in place. T' third section is then prepared t' same way and fit into t' second. Avast! Blimey! Glue t' other two motor blocks into t' other two motor tubes, arrr, then glue t' tubes together. Begad!

Fin design. Balsa was in short supply so a major goal in pickin' a fin layout and shape is t' maximize the stabilizin' effects o' t' available material. Ahoy! Six fins will be made from t' materials available. Arrr! I cut 3 fins from fat 3/8" balsa, usin' as much o' t' sheet as possible. Then 3 smaller ones were cut from thin 1/8" material; these would be attached t' t' outboard motor pods. Blimey!

Fin alignment. Avast! T' two fin designs will be alternately placed around t' rocket. I used an Estes fin alignment guide, matey, shiver me timbers, which has detents at 0, me bucko, 90, shiver me timbers, me hearties, 120, 180, matey, 240, matey, and 270 degrees. Begad! Arrr! I fit t' main BT onto it and marked with a pencil at 0, shiver me timbers, 120, and 240. Blimey! Aye aye! Then I rotated t' BT so that me first mark was aligned with t' 180 detent. Ya scallywag! I made marks again at 0, 120, and 240, ya bilge rat, givin' me a total o' 6 lines evenly spaced around t' outside o' t' tube. Avast, me proud beauty! I lengthened these lines by placin' a strip o' angle iron against t' tube as a guide. Begad!

Side pods. Begad! I cut BT5 tubin' on a 45* angle usin' a razor saw and mini miter box. I drew a circle o' glue on scrap paper and dipped t' BT5's beveled end into it. Then I cut a 1" square o' balsa and pressed t' BT5 onto it. Begad! Avast! When the glue be dry I trimmed off t' excess balsa and then sanded t' edges flush t' t' tube. This be repeated for the other two pods. Aye aye! Aye aye!

Paperin' t' fins and pods. Aye aye! Those old printouts would be used t' paper t' fins. Ya scallywag! T' trick be t' have the unprinted side face outward and avoid usin' creased areas. Well, blow me down! I used 3M 77 spray t' stick t' paper t' t' balsa, arrr, then trimmed off t' excess paper. Well, blow me down! Avast! Worked great as always t' strengthen t' balsa and provide a smooth surface. Blimey! However it was nay an ideal technique for t' side pod "caps". Arrr! Trimmin' off t' paper proved t' be difficult because of the bevel. It also does nothin' t' cover t' exposed edge o' t' balsa. Ahoy! Ahoy! (Wood Filler would have been a better way to smooth out t' pod tops.)

Small fin attachment. Blimey! T' small fins were glued t' t' short side o' t' pods. Begad! I wanted t' paint t' pod assembly separately but I also didn't want t' glue onto a painted surface. These short, small diameter tubes would present very little surface area for glue against t' main BT60 body tube. Arrr! Pretty weak considerin' they'll be packin' A10 motors. Aye aye! I decided t' beef up t' attachment area. Ya scallywag! Begad! Bamboo skewers were cut into six 2-inch dowels. Begad! Ahoy! These were tacked in place with CA about 1/8" on each side o' t' BT markings for t' pods. Ya scallywag! Well, blow me down! If done right t' pods would rest against both dowels and t' body tube. Mine weren't perfect but close enough that a few thick glue fillets would close t' gaps. Blimey!

Large fin attachment. Well, blow me down! T' 3/16" balsa is so thick that it doesn't rest well against t' curved surface o' the body tube. Begad! So I sanded a very shallow channel down t' centerline o' t' fin's root edge. T' fins were then attached to t' body tube with double wood-glue joints. Arrr! Begad! Two thin white glue fillets were added later. Avast, me proud beauty!

Launch lug. Begad! Ahoy! One into a fin fillet, arrr, another further up t' body.

Spacer tube saves t' day. Ahoy! A lowly 24mm yellow spacer tube provided materials for both an adapter and a coupler block. Ya scallywag! One end was dipped in glue and then pressed t' a small square o' balsa. Begad! When dry, shiver me timbers, me hearties, t' excess balsa was trimmed away and sanded flush t' t' tube's edge. Aye aye! Ya scallywag! This assembly was a loose fit inside BT50 so some Avery label paper, me bucko, printed red t' match t' nose cone color, was wrapped around t' spacer tube. Begad! Another piece be used t' cover t' balsa cap. T' spacer tube was then cut in half. Avast!

Payload. Begad! Blimey! T' piece o' spacer that had a cap be glued halfway into t' top o' t' upper body tube. Avast! Arrr! Blimey! T' payload section was then attached with Testor's glue. Aye aye! Blimey! T' red color visible through t' clear tubin' balances t' red nose cone shoulder.

Transition. Begad! Blimey! Avast! Blimey! T' transition's top was too skinny for BT50, so I glued t' other half o' t' 24mm spacer tube onto it. Begad! Blimey! T' transition's base was too thick, so I used a file t' brin' it down t' BT60 size. Begad! Blimey! An eye hook was screwed in, then backed out again. Well, blow me down! Blimey! A drop o' wood glue went into t' hole and then t' hook be replaced.

Finishing:

T' balsa transition was rough and needed t' be filled. Arrr! Avast! Blimey! I spread undiluted Fill+Finish wood filler around, avoidin' t' shoulders, ya bilge rat, and then "dry rubbed" it into t' grain with me fingers. Ya scallywag! Rub vigorously and the excess F+F comes off, me hearties, leavin' a smooth surface. Arrr! Blimey! Almost No sanding! Blimey! T' fins were paper covered and t' outside + trailin' edges filled with a FnF dry rub. NO Sanding!! Blimey! And t' spirals on all t' body tubes were tiny so I opted not to fill them. Begad! NO SANDING!!! Blimey! Sweet. Ahoy! Begad! Blimey! In keepin' with t' No Sandin' theme, arrr, matey, I skipped t' use o' primer and went straight for t' paint. Ya scallywag! Blimey! Besides I be runnin' out o' time before t' contest deadline.

My paint maskin' skills are weak, matey, arrr, so from t' beginnin' I had planned t' build process t' avoid this. Parts would be built into modules, then t' modules painted before final assembly. Aye aye! Blimey! I only needed t' mask t' small areas where the pods met t' main body tube, shiver me timbers, betwixt t' bamboo dowels. Ahoy! Blimey! Avast, me proud beauty! Blimey!

T' body got several light coats o' Krylon Semi-gloss White. T' transition and external motor pods were painted Krylon Silver. T' silver-colored pods blended right into t' white o' t' body - t' make them stand out more I used a black Sharpie marker t' color t' bamboo rods. Begad! Avast! Blimey! T' nose cone was already a nice color red - after extra flashin' was carefully removed I simply coated it with NuFinish. Avast, me bucko, me proud beauty! Blimey!

Decals. Begad! I created some classic roll-patterns and other decals in Inkscape. These were printed printed on the supplied half-sheet o' Avery label paper. Begad! T' sheet was sprayed with two light coats o' fixative before t' decals were cut out and applied. Begad! Ya scallywag!

LINK HERE TO DECALS

T' completed rocket had a nice semi-gloss finish overall. Well, blow me down! Ya scallywag! I liked it so I did nay try t' apply any clear coat.

Final assembly. Begad! T' transition and payload area were then glued together. Aye aye! Blimey! T' shock cord was attached with a traditional tri-fold mount. Arrr! T' motor mount was glued in, shiver me timbers, but what t' do about t' gaps? I tore a few chunks of cardboard off t' shippin' box and soaked them in water overnight. Aye aye! Ya scallywag! Blimey! T' layers softened and separated. Begad! T' cardboard was squeezed dry, coated with a little Gorilla glue, shiver me timbers, arrr, matey, and wadded into t' motor mount gaps. Ahoy! Begad! Blimey!

Flight:

T' rocket was completed on t' day before t' contest deadline! And t' forecast for t' next day was rain! Why did I wait so long? I charged me launch battery that night, shiver me timbers, in case t' weatherman was wrong. Avast, matey, me proud beauty! Oh yeah, and I had to solder together a clip whip too! T' next mornin' thar was drizzle and a heavy low cloud cover, arrr, me hearties, but just after lunch the clouds lifted a little and t' sprinkles subsided. Aye aye! So it be a race t' t' park!

I might only get one flight in before t' rain came back, matey, matey, so why nay go for broke? T' central 18mm tubes were loaded with C6-7s and t' outer 13mm tubes got A10-PT's. Ahoy! Ya scallywag! Motor retention relied on fiction fittin' with maskin' tape. There are no motor blocks in t' pods so a few extra wraps o' narrow tape were wrapped around t' nozzle end t' prevent the A10s from shootin' through t' tops o' t' pods. It felt tail heavy so I put a 1 ounce bag o' sand into t' payload section t' move t' CG up. Man, me bucko, me hearties, matey, it took a long time t' prep and hook up 6 motors. Begad!

On ignition thar be plenty o' smoke and fire. Begad! T' flight was magnificent! Straight and true, shiver me timbers, shiver me timbers, she flew t' maybe 1000 or 1200 feet. Begad! Avast, me proud beauty! Ejection appeared t' be right at apogee (though t' be honest it be hard t' see.)

Recovery:

Both chutes deployed perfectly. Arrr! One had been attached t' t' middle o' t' shock cord and t' other t' the transition's eye hook. Well, blow me down! Arrr! T' rocket descended at a good rate, shiver me timbers, but a breeze blew it toward t' edge o' t' park. Avast! Arrr! T' good news be it missed t' treeline. Arrr! T' bilge-suckin' news was it went into t' river. Arrr! Blimey! Splashdown!

Five o' t' six motors had lit. Well, blow me down! Blimey! One o' t' outboard A10's still had a plug and igniter in it. Arrr!

Four motors had swollen with water so badly that I couldn't get them out. Ya scallywag! Avast! Her impressive maiden flight may be her last. I need t' let t' rocket dry out a bit and see how it goes.

Summary:

That was me first big cluster rocket and I'm very happy with how it flew. Aye aye! I think I received t' right mix of Parts t' Junk in order t' keep t' competition fun, challenging, and nay too serious. Ahoy! Ya scallywag! It be a good way t' find new uses for odd parts, me bucko, and try new techniques, both for buildin' and flying. Ahoy! Blimey!

I now have some unusual parts left over that can be used t' scratch build other things. Ya scallywag! Or they can go back into the pool for next year's contest!!!