Brief:
OctoPod was 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. Ya scallywag! 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, me hearties, me hearties, why haven't you tested an 8-tuber?". Arrr! OK, I finally did. Aye aye! Begad! T' results were mixed - and disappointing.

Bottom Line: Nay worth t' extra fuss required; it wobbles: 8 tube fins, matey, each exactly half t' diameter o' t' body tube, ya bilge rat, me bucko, appear nay t' give adequate dynamic stability t' what is otherwise a very efficient lay-out (low Cd) at high speed. Ahoy! Fun t' fly, and t' spiral smoke trail amuses spectators - but nay what I be after. 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, ya bilge rat, and t' tube fins were each 2" long and cut from 2" heavy wall postal mailin' tube. Avast! Arrr! By gluin' a 1/8"x1/8" spruce spacer t' each tube fin, proper spacin' around t' body tube was achieved. T' first tube fin (no spacer attached) is glued t' t' body tube while aligned on a flat surface, 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, arrr, shiver me timbers, and this was filled with an additional 1/8" square spacer. Avast! Blimey! 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. Arrr! Blimey! An 18" loop o' heavy nylon para cord be 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. Begad! T' nose cone was a discontinued 4" plastic model from North Coast Rocketry; a 4" LOC cone would have sufficed. Avast! T' base and all but 2" o' t' hip be sawed off t' cone, 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, a 4" weighted cone salvaged from a crashed Performance Rocketry Little John was substituted (the weight o' 37 oz. Ahoy! Blimey! Blimey! 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 hearties, ya bilge rat, both aligned with one o' t' triangular spaces betwixt tube fins. Well, blow me down! Begad! Blimey! Finish was with Tamiya rattle-can olive green. Ahoy! 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. Avast, me proud beauty! T' geometry o' 7-tubers is simply goin' down one size t' craft proper fittin' tube fins: e.g., ya bilge rat, 38mm tube fins for a 54mm tubefin, me bucko, 54mm tube fins for a 3" tubefin rocket, ya bilge rat, me hearties, 3" tube fins for a 4" rocket, arrr, 4" tubes for a 5.5" tubefin, matey, ya bilge rat, 5.5" tube fins for a 7.5" rocket. Well, blow me down! T' math doesn't work out so well for an 8-tube tubefin, shiver me timbers, at least for HPR size, since t' required tube fin size is about 50% o' t' body tube size. Avast! Besides t' Octopod set-up, matey, me hearties, 38mm tubes should work on a 3" body tube, arrr, and 3" tubefins could work with t' 6" PML tubin' and cone. Avast! Begad! 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. I also used them t' built me 5-tube "El Cheapo Certo", arrr, described earlier on EMRR

Flight:
T' first flights o' OctoPod were on G77-4 Redline and G78-4 Green Mojave motors, me hearties, usin' a taped-on external parasite pod and Perfectflite Alt15k t' record performance. Well, blow me down! T' boosts were straight enough, matey, 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. F50 boost was straight, ya bilge rat, arrr, but Octopod visibly wobbled on t' H128, 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. I then moved up t' an Aerotech H180-6, me hearties, and directional stability went from bad t' worse, me hearties, arrr, t' wobblin' was fairly violent, arrr, and it was clear that this was all t' power and speed OctoPod could take. Arrr! Cd on t' H180 be 0.84 at 347 mph.

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

When I resumed flight testing, shiver me timbers, arrr, me bucko, t' flight behavior substantially mellowed out, but t' spiral track be nay entirely eliminated. Begad! T' "new improved" OctoPod still wobbled slightly on G78 power, 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, me hearties, 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. Avast, me proud beauty! On a Pro29 H163-7 Red Lightning, arrr, OctoPod (finally!) rewarded me with a perfectly straight boost with no hint o' spiral track, t' 952', for an imputed Cd=0.77 at 240 mph. T' results o' all t' test flights are summarized in t' graph o' Cd vs. speed obtained, shiver me timbers, 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, t' heavy nose-cone OctoPod (bottom graph) even compares favorably with t' 7-tube arrangement.

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

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

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

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

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