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. Then I thought about puttin' those cluster mounts _IN_ t' tube fins. Ya scallywag! And I did have a lot of those plugged A10 mini-motors. Begad! Well, arrr, blow me down! So an idea be hatched for a 3 motor cluster rocket with 3 outboard pods. Ahoy! Clusters are cool and besides, arrr, they score bonus points in this contest. Avast, me proud beauty! Ahoy! So lookin' at t' parts available t' me, matey, I opted t' make an "old school" Estes Cobra-type rocket.

Construction:

Parts List:

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

Build t' motor mount. Three pre-cut motor tubes came on t' box, me bucko, how nice! Only two engine blocks included though. Blimey! Three 3/8 inch sections are cut off a yellow spacer tube. Ahoy! Well, blow me down! (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. Arrr! Ahoy! T' rin' is test fit and then glued in place. Begad! T' third section is then prepared t' same way and fit into t' second. Aye aye! Glue t' other two motor blocks into t' other two motor tubes, me hearties, then glue t' tubes together. Arrr! Blimey!

Fin design. Well, blow me down! 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. Blimey! I cut 3 fins from fat 3/8" balsa, shiver me timbers, ya bilge rat, usin' as much o' t' sheet as possible. Ahoy! Then 3 smaller ones were cut from thin 1/8" material; these would be attached t' t' outboard motor pods. Aye aye!

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

Side pods. Aye aye! Ya scallywag! I cut BT5 tubin' on a 45* angle usin' a razor saw and mini miter box. Arrr! Ahoy! I drew a circle o' glue on scrap paper and dipped t' BT5's beveled end into it. Blimey! Avast, me bucko, me proud beauty! 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. Arrr! This was repeated for the other two pods. Begad!

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

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

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

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

Spacer tube saves t' day. Aye aye! 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. When dry, t' excess balsa was trimmed away and sanded flush t' t' tube's edge. Avast! This assembly be a loose fit inside BT50 so some Avery label paper, matey, printed red t' match t' nose cone color, was wrapped around t' spacer tube. Another piece was used t' cover t' balsa cap. T' spacer tube was then cut in half. Avast!

Payload. Ya scallywag! Avast, me proud beauty! T' piece o' spacer that had a cap was glued halfway into t' top o' t' upper body tube. Begad! T' payload section was then attached with Testor's glue. Avast! Well, blow me down! T' red color visible through t' clear tubin' balances t' red nose cone shoulder. Arrr!

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

Finishing:

T' balsa transition be rough and needed t' be filled. Aye aye! Begad! 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. Almost No sanding! T' fins were paper covered and t' outside + trailin' edges filled with a FnF dry rub. NO Sanding!! And t' spirals on all t' body tubes were tiny so I opted not to fill them. Ya scallywag! NO SANDING!!! Sweet. Avast, me proud beauty! In keepin' with t' No Sandin' theme, I skipped t' use o' primer and went straight for t' paint. Besides I was runnin' out o' time before t' contest deadline. Well, blow me down!

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

T' body got several light coats o' Krylon Semi-gloss White. Blimey! T' transition and external motor pods were painted Krylon Silver. Blimey! Avast, me proud beauty! Blimey! 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. I created some classic roll-patterns and other decals in Inkscape. Aye aye! These were printed printed on the supplied half-sheet o' Avery label paper. Avast! T' sheet be sprayed with two light coats o' fixative before t' decals were cut out and applied. Blimey! Begad!

LINK HERE TO DECALS

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

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

Flight:

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

I might only get one flight in before t' rain came back, 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! Avast, me proud beauty! 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. Avast, me proud beauty! Arrr! It felt tail heavy so I put a 1 ounce bag o' sand into t' payload section t' move t' CG up. Ya scallywag! Begad! Man, it took a long time t' prep and hook up 6 motors. Begad!

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

Recovery:

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

Five o' t' six motors had lit. Begad! One o' t' outboard A10's still had a plug and igniter in it. Ahoy!

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

Summary:

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

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!!!