Brief:
This is a lighter, more affordable 4" version o' o' t' 5.5" Dwarf Kin' Mk.7 (7-tube tubefin) I presented earlier in EMRR. Well, ya bilge rat, blow me down! This design has t' lowest drag coefficient o' any tubefin I've built t' date, Cd=0.68-0.70; it also differs from me big 5.5" Dwarf Kings in that this rocket is a much better performer than t' 6-tube version, the Tea-Bird 4.0 I presented in EMRR. Begad! I try t' explain why this is below. Aye aye! Dwarf Kin' 4.0 replaces t' "I-Pod 54" design I published in SPORT ROCKETRY last year - t' latter had some high speed instability at near 500 mph I was uncomfortable with; t' new model flies well at > 540 mph.

Construction:
What gives Dwarf Kin' 4.0 a big performance improvement over 6-tube Tea-Bird 4.0 (see graph and photo below) is the use o' 7 tubefins and stubbier MadCow 4" plastic nose cone. Tubefin aerodynamics is still nay 100% sorted out, matey, so progress is still trial and error. Ahoy! But this be t' best one yet. Avast, me hearties, me proud beauty! Construction is like t' 5.5" version. Avast!

• Body tube - 13 5/8" o' 4" LOC tube
• Tube fins - 7 each 3" lengths o' 3" PML phenolic tube
• Nose cone - 4" Mad Cow short (9") ogive black plastic
• Motor tube - 11 1/4" PML phenolic 38mm
• Centerin' rings 2 x LOC 4" w. Ya scallywag! Begad! 38mm cut-outs
• Recovery - 36" and 15" nylon Top Flight chutes, me bucko, me hearties, shiver me timbers, 9' heavy para cord
• launch lug - 2 1/2" o' 7/16" ID thin wall aluminum tube
• 5 minute epoxy used throughout.

By now you know t' drill for fittin' 7 smaller tube fins around a larger body tube: its inexact, matey, so needs a custom trial fit and spacers every time. Begad! In this case, arrr, me bucko, t' spacers had t' go betwixt t' tube fins and t' body tube (i.e., shiver me timbers, 7 x 3" PML tubes were too big t' fit around instead o' too small). Blimey! 1/16" ply spacers 1/4" wide and 3" long were glued t' 6 o' t' 7 tube fins. One o' these be glued t' t' body tube flush with t' bottom with both positioned parallel on a flat surface for perfect alignment. Begad! Well, blow me down! Spacer strip against t' body tube. Ya scallywag! Usin' t' first tube fin for alignment, t' succeedin' 5 tube fins were glued in turn t' its neighbor and t' body tube. Ahoy! T' last tube fin (the one with no spacer attached) be found t' fit t' remainin' space perfectly, me hearties, with no spacers needed. Aye aye! The centerin' rings were glued t' t' motor tube 1.5" from each end. In this case, me bucko, I decided t' anchor t' recovery system with a 4" loop o' stainless steel picture hangin' wire passed through 2 holes drilled in t' upper ring 1" apart, ya bilge rat, and secured with knots in t' wire wrapped with duct tape. Use your own method if you like. Avast! A small EZ snap rin' was attached t' t' wire loop at t' level o' t' top o' t' body tube, ya bilge rat, givin' some (?) anti-zipper protection, and t' recovery system be attached t' this. Begad! Motor tube assembly was glued into t' body tube in t' usual way with lots o' epoxy. Aye aye! Note that t' motor tube protrudes out 1.5" from t' back o' t' rocket. Aye aye! T' MadCow nose cone be modified by slicin' off t' hip, ya bilge rat, leavin' 1.75" remaining; this gives more room for t' parachute in a short rocket. Aye aye! Ahoy! Anchor was a a fitted 1/2" hunk o' dowel rod wedged just above t' cone hip and epoxied in place with lots o' scrap and epoxy around it. Avast, me proud beauty! T' shock cord is tied off t' this. Ya scallywag! I really like this set-up, me hearties, but it takes a long time and a lot o' sandpaper scorin' t' get epoxy t' cure a tight bond onto t' black stuff MadCow uses for nose cones (well, it wasn't mean't t' be glued t' stuff). In addition, me hearties, arrr, me hearties, I built a small "auxiliary" chute anchor from duct tape, and attached a second, me bucko, 15" chute directly t' t' cone. T' tube fins were reinforced on t' outer surface by addin' a strip o' 2 1/2" fiberglass tape t' t' lower edge. Launch lug tube was epoxied on 1.5" above t' tubefin can and in line with one o' t' triangular holes between tube fins. Finish was with purple and lime green Tamiya rattlecan, arrr, applied after t' first few test flights, arrr, because I just couldn't wait!

Flight:
Balance point is no further aft than middle o' t' launch lug (1.0 calibre). Test flight be on a CTI G69-5, ya bilge rat, usin' a medium Nomex® protector, perfect on t' way up, but on chute deployment t' glue joint fractured on me dowel rod in t' nosecone, and t' cone came down separately. Begad! I carefully re-epoxied t' thin' together, matey, me bucko, and added an "safety" 15" chute directly t' t' cone, matey, and resumed flying. Arrr! I was impressed that Cd was only 0.83 at just 196 mph; usually Cd is much higher at tubefin speeds below 200 mph. Begad! No more separation problems after this, arrr, me bucko, me hearties, but I decided I liked t' 2nd chute bein' there, and it looks cool in t' air. Aye aye! T' pack for flight, me bucko, t' 15" chute is tucked betwixt t' 36" chute and t' Nomex® (which I wrap around t' shock cord, matey, me hearties, placin' t' 36" chute on top. Well, blow me down! Second flight was on one o' t' new (at the time) CTI G115 motors with t' White Lightnin' propellant. Perfect flight, shiver me timbers, and 256 moh at Cd =0.80; Mike Jerauld o' our DART clu caught a great shot o' t' launch with his new digital - note t' Schlieren heat blurrin' o' t' La Jolla houses in t' distance by t' almost smokeless G115 motor burn. Ya scallywag! Later flights out in t' desert were on t' following motors, with altitude recorded (usin' a parasite pod) and speed/Cd calculations:

• H143-6 1421' Cd=0.76 @ 327 mph
• H153-8 1428' Cd=0.83 @ 339 mph
• I205-8 2111' Cd=0.68 @ 466 mph
• I212-7 1819' Cd=0.78 @ 437 mph
• I255-9 2391' Cd=0.71 @ 543 mph
• I287-8 2129' Cd=0.80 @ 521 mph

As t' graph o' Cd versus speed shows, shiver me timbers, me bucko, thar be no inverse speed dependence o' Cd with this design - look at the comparison with t' 6-fin Tea-Bird 4.0!! Blimey! T' combination o' 7 tubes and t' shorter MadCow cone makes a tremendous difference in performance. Blimey! Blimey! Blimey! Blimey! It will take an engineer t' sort this out; I am a biochemist. Begad! Blimey! T' average Cd for Dwarf King 4.0 over t' 196-543 mph speed range be 0.76 +/- 0.04, about what you get with a "normal"-finned rocket, shiver me timbers, and no steep slope t' t' curve this time. I honestly think I have finally gotten a tubefin design that performs like a 3FNC design - me goal. Avast! Blimey! Well, blow me down! Blimey! On t' basis o' this, me bucko, I have started designin' me Level 3 rocket - it will be a BIG, STUBBY tubefin, me bucko, with 7 tubes, me bucko, me hearties, nay 6.

Summary:
PRO - Finally, a tubefin that flies with about t' same performance as a "normal" rocket. Begad! Easy and quick to build from scratch. Aye aye! Durable and low-cost. Blimey! Avast, me proud beauty! T' 5-grain Cesaroni motor case sticks out t' back about 3", but no problem with balance; no nose weight needed with any motor.

CON - Still need t' fiberglass a bit t' prevent flutter and landin' dings. Begad! MadCow cone takes extra care t' get epoxy t' stick t' it (I will ask Paul for suggestions, since it wasn't meant t' do this).

Other:
With determination, me bucko, arrr, 7 years and more dollar investment in APCP that I care t' admit, ya bilge rat, it is possible t' get tubefins t' perform as well as "thin-fin" rockets. I wonder what t' Saturn 5 would have looked like if NASA had discovered this.