Brief:
OctoPod be created t' test how well an 8-tube HPR tubefin would perform compared t' t' 5-tube, 6-tube and 7-tube tubefin rockets I have described previously on EMRR. Ahoy! 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, arrr, why haven't you tested an 8-tuber?". OK, me hearties, I finally did. Ya scallywag! T' results were mixed - and disappointing.

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

Construction:
T' body tube consists o' 20" o' LOC 4" tubing, me bucko, and t' tube fins were each 2" long and cut from 2" heavy wall postal mailin' tube. Avast! By gluin' a 1/8"x1/8" spruce spacer t' each tube fin, me hearties, proper spacin' around t' body tube was achieved. Avast! T' first tube fin (no spacer attached) is glued t' t' body tube while aligned on a flat surface, me hearties, 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, and this be filled with an additional 1/8" square spacer. Well, blow me down! Blimey! A 1/8" ply 29mm size 4" centerin' rin' be glued 1" from each end o' a 12" piece o' LOC 29mm motortube t' create t' motor mount assembly. Avast, me proud beauty! 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) was later anchored t' this.

T' motor mount assembly was glued into place in t' body tube in t' usual way. Begad! Aye aye! T' nose cone was a discontinued 4" plastic model from North Coast Rocketry; a 4" LOC cone would have sufficed. Begad! Ahoy! T' base and all but 2" o' t' hip be sawed off t' cone, arrr, and a 3/8" dowel rod length be 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, ya bilge rat, a 4" weighted cone salvaged from a crashed Performance Rocketry Little John was substituted (the weight o' 37 oz. Aye aye! and length o' 41" given above is with t' one pound "Little John" cone). Avast, me proud beauty! Avast, arrr, me proud beauty! 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, both aligned with one o' t' triangular spaces betwixt tube fins. Finish was with Tamiya rattle-can olive green. Arrr! 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. Well, blow me down! 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, 54mm tube fins for a 3" tubefin rocket, 3" tube fins for a 4" rocket, arrr, 4" tubes for a 5.5" tubefin, arrr, shiver me timbers, 5.5" tube fins for a 7.5" rocket. T' math doesn't work out so well for an 8-tube tubefin, ya bilge rat, at least for HPR size, since t' required tube fin size is about 50% o' t' body tube size. Ya scallywag! Besides t' Octopod set-up, 38mm tubes should work on a 3" body tube, ya bilge rat, and 3" tubefins could work with t' 6" PML tubin' and cone. Aye aye! That's about it.

Finishing:
I liked bein' able t' use low-cost 2" postal mailin' tubes for OctoPod, they are low-cost and strong. Begad! Blimey! 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, shiver me timbers, usin' a taped-on external parasite pod and Perfectflite Alt15k t' record performance. Avast! Begad! T' boosts were straight enough, but t' smoke track was clearly spiral - a "wacky wiggler".

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

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

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

When I resumed flight testing, arrr, t' flight behavior substantially mellowed out, me hearties, ya bilge rat, but t' spiral track be nay entirely eliminated. T' "new improved" OctoPod still wobbled slightly on G78 power, and with Pro29 H255-7 and I204-8 power, ya bilge rat, 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, which is as good as I see on t' best tubefin designs at this speed. On a Pro29 H163-7 Red Lightning, shiver me timbers, OctoPod (finally!) rewarded me with a perfectly straight boost with no hint o' spiral track, me bucko, shiver me timbers, matey, t' 952', me hearties, ya bilge rat, for an imputed Cd=0.77 at 240 mph. Begad! Blimey! T' results o' all t' test flights are summarized in t' graph o' Cd vs. speed obtained, me bucko, 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, me hearties, me bucko, 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. Blimey! 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. This is nay what you like t' see.

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

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

Summary:
PRO: Fast and low-cost scratch build, arrr, shiver me timbers, with t' postal mailin' tube fins. Smaller-size tubefins improve t' appearance o' an 8-tuber.

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