Brief:

T' Cobra 11 is an 11th hour entry into t' EMRR Box 'o Parts contest. Arrr! 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. Aye aye! Then I thought about puttin' those cluster mounts _IN_ t' tube fins. Well, blow me down! And I did have a lot of those plugged A10 mini-motors. So an idea be hatched for a 3 motor cluster rocket with 3 outboard pods. Clusters are cool and besides, me hearties, arrr, they score bonus points in this contest. Well, blow me down! Arrr! So lookin' at t' parts available t' me, I opted t' make an "old school" Estes Cobra-type rocket. Blimey!

Construction:

Parts List:

• BT50 nose cone
• BT50 clear payload tube, 4"
• BT50 body tube, 5.25"
• BT50 motor tubes, ya bilge rat, (3)
• BT50-60 balsa transition
• BT60 body tube, 13.5"
• BT5, me hearties, 9"
• 18mm motor blocks, arrr, (2)
• 3/16" Balsa fin stock, me hearties, 3"x 10.5"
• 1/8" Balsa fin stock, ya bilge rat, scrap, 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, (2)
• Avery label paper, matey, (1/2 sheet)
• 1/8" Launch lugs, ya bilge rat, (2)
• 20lb paper, about 5 sheets. Well, blow me down! Printed on one side, shiver me timbers, me bucko, creased and folded and stapled. Blimey!

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

Fin design. Avast! 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. Well, blow me down! Six fins will be made from t' materials available. Begad! I cut 3 fins from fat 3/8" balsa, usin' as much o' t' sheet as possible. Begad! Aye aye! Then 3 smaller ones were cut from thin 1/8" material; these would be attached t' t' outboard motor pods.

Fin alignment. Ahoy! T' two fin designs will be alternately placed around t' rocket. Avast! I used an Estes fin alignment guide, arrr, me hearties, me hearties, which has detents at 0, 90, me bucko, 120, 180, 240, me bucko, and 270 degrees. Ahoy! I fit t' main BT onto it and marked with a pencil at 0, 120, and 240. Ahoy! Ahoy! Then I rotated t' BT so that me first mark be aligned with t' 180 detent. Arrr! I made marks again at 0, 120, and 240, 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. Aye aye!

Side pods. Ya scallywag! 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. Avast, me proud beauty! Then I cut a 1" square o' balsa and pressed t' BT5 onto it. Ya scallywag! Arrr! When the glue was dry I trimmed off t' excess balsa and then sanded t' edges flush t' t' tube. Begad! This be repeated for the other two pods.

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

Small fin attachment. Aye aye! T' small fins were glued t' t' short side o' t' pods. I wanted t' paint t' pod assembly separately but I also didn't want t' glue onto a painted surface. Aye aye! These short, me bucko, matey, small diameter tubes would present very little surface area for glue against t' main BT60 body tube. Pretty weak considerin' they'll be packin' A10 motors. Begad! Avast! I decided t' beef up t' attachment area. Well, blow me down! Blimey! Bamboo skewers were cut into six 2-inch dowels. These were tacked in place with CA about 1/8" on each side o' t' BT markings for t' pods. 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! Blimey!

Large fin attachment. Aye aye! Avast! 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. Ahoy! T' fins were then attached to t' body tube with double wood-glue joints. Ahoy! Two thin white glue fillets were added later. Aye aye! Ya scallywag!

Launch lug. Well, blow me down! Begad! Blimey! One into a fin fillet, me hearties, another further up t' body. Ahoy!

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

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

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

Finishing:

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

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

T' body got several light coats o' Krylon Semi-gloss White. Aye aye! 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. Avast! T' nose cone be already a nice color red - after extra flashin' was carefully removed I simply coated it with NuFinish. Begad!

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

LINK HERE TO DECALS

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

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

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, in case t' weatherman be wrong. Ahoy! Oh yeah, me hearties, and I had to solder together a clip whip too! T' next mornin' thar was drizzle and a heavy low cloud cover, shiver me timbers, 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, ya bilge rat, so why nay go for broke? T' central 18mm tubes were loaded with C6-7s and t' outer 13mm tubes got A10-PT's. Begad! Well, me bucko, blow me down! 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. Ya scallywag! It felt tail heavy so I put a 1 ounce bag o' sand into t' payload section t' move t' CG up. Well, blow me down! Man, shiver me timbers, it took a long time t' prep and hook up 6 motors. Ahoy!

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

Recovery:

Both chutes deployed perfectly. Avast! One had been attached t' t' middle o' t' shock cord and t' other t' the transition's eye hook. Aye aye! T' rocket descended at a good rate, but a breeze blew it toward t' edge o' t' park. Ahoy! T' good news be it missed t' treeline. T' bad news was it went into t' river. Ahoy! Splashdown!

Five o' t' six motors had lit. Well, blow me down! 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. Begad! Her impressive maiden flight may be her last. Aye aye! I need t' let t' rocket dry out a bit and see how it goes. Aye aye!

Summary:

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

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