Brief:

T' Cobra 11 is an 11th hour entry into t' EMRR Box 'o Parts contest. Blimey! Begad! 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. Begad! Then I thought about puttin' those cluster mounts _IN_ t' tube fins. Ahoy! Avast, me proud beauty! 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. Aye aye! Aye aye! Clusters are cool and besides, they score bonus points in this contest. Avast, me proud beauty! So lookin' at t' parts available t' me, me bucko, shiver me timbers, I opted t' make an "old school" Estes Cobra-type rocket. Avast!

Construction:

Parts List:

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

Build t' motor mount. Arrr! Begad! Three pre-cut motor tubes came on t' box, how nice! Only two engine blocks included though. Blimey! 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, me hearties, placed inside t' motor tube, and t' overlap is noted. This strip is trimmed out. Arrr! T' rin' is test fit and then glued in place. T' third section is then prepared t' same way and fit into t' second. Well, blow me down! Glue t' other two motor blocks into t' other two motor tubes, then glue t' tubes together. Blimey! Ya scallywag!

Fin design. Avast, me proud beauty! 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. Avast, me proud beauty! Six fins will be made from t' materials available. Aye aye! I cut 3 fins from fat 3/8" balsa, shiver me timbers, 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. Begad!

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

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

Paperin' t' fins and pods. Ya scallywag! Those old printouts would be used t' paper t' fins. T' trick was t' have the unprinted side face outward and avoid usin' creased areas. Ya scallywag! I used 3M 77 spray t' stick t' paper t' t' balsa, me hearties, then trimmed off t' excess paper. Ahoy! 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". Aye aye! Trimmin' off t' paper proved t' be difficult because of the bevel. It also does nothin' t' cover t' exposed edge o' t' balsa. Ya scallywag! (Wood Filler would have been a better way to smooth out t' pod tops.)

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

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

Launch lug. One into a fin fillet, another further up t' body.

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

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

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

Finishing:

T' balsa transition was rough and needed t' be filled. Ahoy! Blimey! I spread undiluted Fill+Finish wood filler around, avoidin' t' shoulders, and then "dry rubbed" it into t' grain with me fingers. Rub vigorously and the excess F+F comes off, arrr, me bucko, leavin' a smooth surface. Well, me bucko, blow me down! 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. Aye aye! Blimey! NO SANDING!!! Blimey! Sweet. In keepin' with t' No Sandin' theme, I skipped t' use o' primer and went straight for t' paint. Ya scallywag! Blimey! Besides I was runnin' out o' time before t' contest deadline. Aye aye! Blimey!

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

T' body got several light coats o' Krylon Semi-gloss White. Well, blow me down! Ahoy! 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. Ya scallywag! T' nose cone be already a nice color red - after extra flashin' was carefully removed I simply coated it with NuFinish.

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

LINK HERE TO DECALS

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

Final assembly. T' transition and payload area were then glued together. Ya scallywag! T' shock cord be attached with a traditional tri-fold mount. Blimey! T' motor mount was glued in, shiver me timbers, 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. Begad! T' layers softened and separated. T' cardboard was squeezed dry, coated with a little Gorilla glue, ya bilge rat, and wadded into t' motor mount gaps. Begad! Begad!

Flight:

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

I might only get one flight in before t' rain came back, matey, me bucko, so why nay go for broke? T' central 18mm tubes were loaded with C6-7s and t' outer 13mm tubes got A10-PT's. Blimey! 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, it took a long time t' prep and hook up 6 motors. Begad! Begad!

On ignition thar was plenty o' smoke and fire. T' flight was magnificent! Straight and true, she flew t' maybe 1000 or 1200 feet. Aye aye! Aye aye! 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. T' rocket descended at a good rate, but a breeze blew it toward t' edge o' t' park. Begad! T' good news be it missed t' treeline. Avast, me proud beauty! T' bilge-suckin' news be it went into t' river. Ya scallywag! Splashdown!

Five o' t' six motors had lit. Begad! One o' t' outboard A10's still had a plug and igniter in it. Avast, me proud beauty!

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

Summary:

That was me first big cluster rocket and I'm very happy with how it flew. Avast, me proud beauty! I think I received t' right mix of Parts t' Junk in order t' keep t' competition fun, me hearties, challenging, and nay too serious. Ahoy! Well, blow me down! It be a good way t' find new uses for odd parts, matey, and try new techniques, both for buildin' and flying.

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