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. Aye aye! And I did have a lot of those plugged A10 mini-motors. Begad! Aye aye! So an idea be hatched for a 3 motor cluster rocket with 3 outboard pods. Clusters are cool and besides, they score bonus points in this contest. Blimey! So lookin' at t' parts available t' me, ya bilge rat, arrr, I opted t' make an "old school" Estes Cobra-type rocket. Well, blow me down!

Construction:

Parts List:

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

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

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. Six fins will be made from t' materials available. I cut 3 fins from fat 3/8" balsa, matey, ya bilge rat, arrr, usin' as much o' t' sheet as possible. Begad! Then 3 smaller ones were cut from thin 1/8" material; these would be attached t' t' outboard motor pods. Arrr!

Fin alignment. T' two fin designs will be alternately placed around t' rocket. Arrr! I used an Estes fin alignment guide, which has detents at 0, me bucko, 90, me bucko, me hearties, matey, 120, 180, 240, matey, and 270 degrees. I fit t' main BT onto it and marked with a pencil at 0, 120, and 240. Begad! Aye aye! Then I rotated t' BT so that me first mark was aligned with t' 180 detent. I made marks again at 0, 120, and 240, matey, givin' me a total o' 6 lines evenly spaced around t' outside o' t' tube. Avast, me proud beauty! Aye aye! 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. I drew a circle o' glue on scrap paper and dipped t' BT5's beveled end into it. Ya scallywag! Then I cut a 1" square o' balsa and pressed t' BT5 onto it. Ya scallywag! When the glue be dry I trimmed off t' excess balsa and then sanded t' edges flush t' t' tube. Ahoy! Arrr! This be repeated for the other two pods. Arrr!

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

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

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

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

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

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

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

Finishing:

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

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

T' body got several light coats o' Krylon Semi-gloss White. Avast, me proud beauty! T' transition and external motor pods were painted Krylon Silver. Ahoy! Begad! 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 was already a nice color red - after extra flashin' was carefully removed I simply coated it with NuFinish. Ya scallywag!

Decals. Avast! I created some classic roll-patterns and other decals in Inkscape. These were printed printed on the supplied half-sheet o' Avery label paper. Well, blow me down! T' sheet was sprayed with two light coats o' fixative before t' decals were cut out and applied. Blimey! Ahoy!

LINK HERE TO DECALS

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

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

Flight:

T' rocket was completed on t' day before t' contest deadline! Blimey! And t' forecast for t' next day was rain! Blimey! Why did I wait so long? I charged me launch battery that night, me hearties, matey, in case t' weatherman was wrong. Begad! Blimey! Oh yeah, matey, shiver me timbers, 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, 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. 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. Avast, me proud beauty! Begad! It felt tail heavy so I put a 1 ounce bag o' sand into t' payload section t' move t' CG up. Blimey! Begad! Man, it took a long time t' prep and hook up 6 motors. Aye aye!

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

Recovery:

Both chutes deployed perfectly. Begad! 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, ya bilge rat, but a breeze blew it toward t' edge o' t' park. Aye aye! Begad! T' good news be it missed t' treeline. Arrr! T' bad news was it went into t' river. Well, blow me down! Splashdown!

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

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

Summary:

That was me first big cluster rocket and I'm very happy with how it flew. 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. Ya scallywag! It be a good way t' find new uses for odd parts, and try new techniques, both for buildin' and flying. Begad!

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!!!