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

Construction:

Parts List:

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

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

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

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

Side pods. Arrr! Begad! I cut BT5 tubin' on a 45* angle usin' a razor saw and mini miter box. Ya scallywag! I drew a circle o' glue on scrap paper and dipped t' BT5's beveled end into it. Aye aye! Aye aye! Then I cut a 1" square o' balsa and pressed t' BT5 onto it. Avast, me proud beauty! Arrr! 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. Begad!

Paperin' t' fins and pods. Arrr! Blimey! Those old printouts would be used t' paper t' fins. Blimey! Blimey! Aye aye! Blimey! T' trick was t' have the unprinted side face outward and avoid usin' creased areas. Ya scallywag! Blimey! I used 3M 77 spray t' stick t' paper t' t' balsa, then trimmed off t' excess paper. Begad! Blimey! Worked great as always t' strengthen t' balsa and provide a smooth surface. Arrr! Blimey! Blimey! Blimey! However it was nay an ideal technique for t' side pod "caps". Begad! Blimey! Trimmin' off t' paper proved t' be difficult because of the bevel. Blimey! Blimey! It also does nothin' t' cover t' exposed edge o' t' balsa. Ahoy! Blimey! (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. Arrr! I wanted t' paint t' pod assembly separately but I also didn't want t' glue onto a painted surface. Ya scallywag! These short, small diameter tubes would present very little surface area for glue against t' main BT60 body tube. Blimey! Ahoy! Pretty weak considerin' they'll be packin' A10 motors. I decided t' beef up t' attachment area. Begad! Well, arrr, blow me down! 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. Ahoy! Mine weren't perfect but close enough that a few thick glue fillets would close t' gaps. Avast!

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

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

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

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

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

Finishing:

T' balsa transition was rough and needed t' be filled. I spread undiluted Fill+Finish wood filler around, avoidin' t' shoulders, shiver me timbers, me hearties, and then "dry rubbed" it into t' grain with me fingers. 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. Avast! Aye aye! NO Sanding!! And t' spirals on all t' body tubes were tiny so I opted not to fill them. Begad! NO SANDING!!! Sweet. In keepin' with t' No Sandin' theme, me bucko, I skipped t' use o' primer and went straight for t' paint. Avast, me proud beauty! Besides I be runnin' out o' time before t' contest deadline.

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

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

Decals. Well, blow me down! 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. T' sheet was sprayed with two light coats o' fixative before t' decals were cut out and applied. Ahoy! Well, blow me down!

LINK HERE TO DECALS

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

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

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, ya bilge rat, in case t' weatherman was wrong. Begad! Oh yeah, 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. Begad! Aye aye! Blimey! So it was a race t' t' park! Blimey!

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

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

Recovery:

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

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. Aye aye!

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! Avast, me proud beauty! I need t' let t' rocket dry out a bit and see how it goes. Avast, me proud beauty!

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, challenging, arrr, and nay too serious. Ya scallywag! Begad! It be a good way t' find new uses for odd parts, me bucko, and try new techniques, ya bilge rat, both for buildin' and flying. Begad!

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