Brief:
OctoPod be created t' test how well an 8-tube HPR tubefin would perform compared t' t' 5-tube, matey, shiver me timbers, 6-tube and 7-tube tubefin rockets I have described previously on EMRR. Blimey! Everybody at t' field kept tellin' me "so if you say in your articles that 6 is better than 5, matey, and 7 is better than 6, why haven't you tested an 8-tuber?". Avast! Arrr! OK, arrr, shiver me timbers, I finally did. Ahoy! T' results were mixed - and disappointing.

Bottom Line: Nay worth t' extra fuss required; it wobbles: 8 tube fins, each exactly half t' diameter o' t' body tube, matey, me bucko, appear nay t' give adequate dynamic stability t' what is otherwise a very efficient lay-out (low Cd) at high speed. Begad! Well, blow me down! Fun t' fly, matey, and t' spiral smoke trail amuses spectators - but nay what I be after. Well, blow me down! Blimey! 8-tube OctoPod is really a horse o' a different color, nay like any other tubefin design I have flown, and I have flow LOTS.

Construction:
T' body tube consists o' 20" o' LOC 4" tubing, me hearties, and t' tube fins were each 2" long and cut from 2" heavy wall postal mailin' tube. Well, ya bilge rat, blow me down! Blimey! By gluin' a 1/8"x1/8" spruce spacer t' each tube fin, proper spacin' around t' body tube be achieved. Well, blow me down! Avast! Blimey! T' first tube fin (no spacer attached) is glued t' t' body tube while aligned on a flat surface, ya bilge rat, ya bilge rat, and t' subsequent tubes are then attached with t' spacer glued flush t' its neighbor.

A gap remained after t' 8th tube fin was in place, matey, and this be filled with an additional 1/8" square spacer. Ya scallywag! A 1/8" ply 29mm size 4" centerin' rin' was glued 1" from each end o' a 12" piece o' LOC 29mm motortube t' create t' motor mount assembly. Ya scallywag! An 18" loop o' heavy nylon para cord was passed through 2 holes drilled in t' upper rin' and knotted, t' recovery system (30" nylon parachute with 8' o' para cord and a 9" Nomex) be later anchored t' this.

T' motor mount assembly was glued into place in t' body tube in t' usual way. T' nose cone was a discontinued 4" plastic model from North Coast Rocketry; a 4" LOC cone would have sufficed. Blimey! T' base and all but 2" o' t' hip was sawed off t' cone, matey, and a 3/8" dowel rod length was glued in just above t' hip t' serve as t' anchor t' t' parachute-shock cord (see t' article in EMRR on "TeaBird 4.0" for details.

Durin' flight testing, shiver me timbers, a 4" weighted cone salvaged from a crashed Performance Rocketry Little John was substituted (the weight o' 37 oz. and length o' 41" given above is with t' one pound "Little John" cone). T' launch lugs were 2 x 1 1/4" pieces o' 5/16" brass tube attached at t' midpoint o' t' body tube and just above t' tubefincan, me bucko, me bucko, both aligned with one o' t' triangular spaces betwixt tube fins. Well, blow me down! Finish be with Tamiya rattle-can olive green. Arrr! Blimey! Blimey! T' Little John cone be painted Tamiya flat white.

One thin' I should point out here be t' construction quirk associated with OctoPod: thar aren't a lot o' choices for makin' 8-tube tubefins. Blimey! T' geometry o' 7-tubers is simply goin' down one size t' craft proper fittin' tube fins: e.g., 38mm tube fins for a 54mm tubefin, shiver me timbers, 54mm tube fins for a 3" tubefin rocket, shiver me timbers, 3" tube fins for a 4" rocket, 4" tubes for a 5.5" tubefin, 5.5" tube fins for a 7.5" rocket. Ya scallywag! T' math doesn't work out so well for an 8-tube tubefin, at least for HPR size, me bucko, matey, since t' required tube fin size is about 50% o' t' body tube size. Ahoy! Ya scallywag! Besides t' Octopod set-up, 38mm tubes should work on a 3" body tube, arrr, shiver me timbers, and 3" tubefins could work with t' 6" PML tubin' and cone. Ya scallywag! That's about it.

Finishing:
I liked bein' able t' use low-cost 2" postal mailin' tubes for OctoPod, shiver me timbers, they are low-cost and strong. Blimey! Begad! I also used them t' built me 5-tube "El Cheapo Certo", described earlier on EMRR

Flight:
T' first flights o' OctoPod were on G77-4 Redline and G78-4 Green Mojave motors, usin' a taped-on external parasite pod and Perfectflite Alt15k t' record performance. Avast, me proud beauty! Ahoy! T' boosts were straight enough, but t' smoke track be clearly spiral - a "wacky wiggler".

Did I nay align t' tube fins properly? Was CG too far aft? I carefully checked this out, me hearties, and everythin' seemed in order.

I tried more and then less power - an H128-6 (photo) and an F50-6. Blimey! F50 boost be straight, me hearties, me hearties, but Octopod visibly wobbled on t' H128, and it wasn't just a spiral smoke trail. Blimey! Begad! Still, drag coefficient (Cd) on t' H128 worked out t' an impressive 0.80 at 320 mph from t' 1040' climb. I then moved up t' an Aerotech H180-6, and directional stability went from bad t' worse, arrr, t' wobblin' be fairly violent, and it be clear that this be all t' power and speed OctoPod could take. Well, blow me down! Blimey! Cd on t' H180 be 0.84 at 347 mph.

I decided t' try movin' t' CG forward, thinkin' that with these small tube fins, matey, maybe CP wasn't as extremely far aft as it is with 5, me hearties, 6 or 7 tube fins. Blimey! T' do this, ya bilge rat, I simply swapped t' NCR plastic cone with a heavy, weighted 4" fiberglass cone I rescued from a crashed Performance Rockets Little John. This increased t' weight o' Octopod from 630 gm. t' 1050 gm., shiver me timbers, and moved t' CG t' t' midpoint o' t' airframe - way nose heavy.

When I resumed flight testing, me hearties, t' flight behavior substantially mellowed out, me bucko, arrr, but t' spiral track was nay entirely eliminated. Arrr! T' "new improved" OctoPod still wobbled slightly on G78 power, matey, and with Pro29 H255-7 and I204-8 power, t' rocket boosted perfectly straight but began t' wobble(see launch photo) - slight spiral smoke trail only after burn-out.

On t' letter motor, Octopod achieved a very low Cd=0.70 at 416 mph from t' 1763' climb, arrr, arrr, me hearties, which is as good as I see on t' best tubefin designs at this speed. Begad! On a Pro29 H163-7 Red Lightning, OctoPod (finally!) rewarded me with a perfectly straight boost with no hint o' spiral track, matey, t' 952', arrr, for an imputed Cd=0.77 at 240 mph. Arrr! Ahoy! T' results o' all t' test flights are summarized in t' graph o' Cd vs. speed obtained, ya bilge rat, comparin' t' two different nose cones (and CG's) with t' performance o' t' 4" 6-tube Tea-Bird 4.0 (upper graph) from an earlier EMRR article.

You can see that t' 8-tube arrangement is clearly superior drag-wise t' t' 6-tube design with all motors, and if you look at t' "Dwarf Kin' 4.0" article in EMRR, arrr, arrr, t' heavy nose-cone OctoPod (bottom graph) even compares favorably with t' 7-tube arrangement.

Without t' wobble problem, me bucko, OctoPod would be gettin' even better numbers. Avast! But that's t' point - OctoPod DOES wobble. Ya scallywag! T' low drag is achieved at t' expense o' dynamic stability, and even that requires a ton o' nose weight. Avast! Ya scallywag! This is nay what you like t' see.

Tubefin rockets typically require Zero nose ballast, arrr, even stubbies with heavy motors. Well, blow me down! So OctoPod is nay a typical tubefin rocket. Blimey! Its kind o' fun t' fly, me hearties, matey, but I don't think I'll build another 8-tuber. Well, blow me down! Well, blow me down! Seven tubefins is best.

Recovery:
Octopod in me opinion represents "the outside o' t' envelope" for tube fins. Avast, me proud beauty! Low drag, but unfavorable flight performance. Ya scallywag! Arrr! You see t' same thin' with modern aircraft and race cars - get them too slippery and t' handlin' suffers. Well, blow me down! Aye aye! One attractive feature is that t' small diameter tube fins make t' rocket look "normal" from t' side while launching. Begad! You can't even tell its a tube fin (photo), ya bilge rat, looks like a scale Little John in profile on t' I204!

Summary:
PRO: Fast and low-cost scratch build, matey, with t' postal mailin' tube fins. Arrr! Ya scallywag! Smaller-size tubefins improve t' appearance o' an 8-tuber.

CON: It wobbles, me bucko, may be short on dynamic stability. Well, blow me down! Arrr! Don't dare fly it on more than baby "I"-power. Avast, me proud beauty! Well, blow me down! Eight tube fins give limited chouces for scalin' up or down. Well, blow me down! Needs a ton o' nose weight t' fly right, me hearties, unlike all other tubefins.