Construction Rating: | starstarstarstarstar_border |
Flight Rating: | starstarstarstarstar_border |
Overall Rating: | starstarstarstarstar_border |
Manufacturer: | Scratch |
Brief:
This is a lighter, shiver me timbers, more affordable 4" version o' o' t' 5.5" Dwarf Kin' Mk.7 (7-tube tubefin) I presented
earlier in EMRR. Ya scallywag! 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, ya bilge rat, the
Tea-Bird 4.0 I presented in EMRR. Avast! 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, me bucko, ya bilge rat, so
progress is still trial and error. But this be t' best one yet. Avast, me proud beauty! Construction is like t' 5.5" version.
By now you know t' drill for fittin' 7 smaller tube fins around a larger body tube: its inexact, ya bilge rat, so needs a
custom trial fit and spacers every time. Avast, me proud beauty! In this case, t' spacers had t' go betwixt t' tube fins and t' body tube
(i.e., arrr, 7 x 3" PML tubes were too big t' fit around instead o' too small). Begad! 1/16" ply spacers 1/4" wide
and 3" long were glued t' 6 o' t' 7 tube fins. Ya scallywag! One o' these was glued t' t' body tube flush with t' bottom with
both positioned parallel on a flat surface for perfect alignment. Well, blow me down! Spacer strip against t' body tube. Usin' t' first
tube fin for alignment, me hearties, t' succeedin' 5 tube fins were glued in turn t' its neighbor and t' body tube. Well, blow me down! Ya scallywag! T' last tube
fin (the one with no spacer attached) be found t' fit t' remainin' space perfectly, shiver me timbers, shiver me timbers, with no spacers needed. Aye aye! Begad! The
centerin' rings were glued t' t' motor tube 1.5" from each end. Ya scallywag! In this case, 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, me hearties, arrr, and secured with knots in t' wire wrapped with duct tape. Avast, me proud beauty! Use your own method if you like. Ya scallywag! Arrr! A small EZ
snap rin' was attached t' t' wire loop at t' level o' t' top o' t' body tube, shiver me timbers, shiver me timbers, givin' some (?) anti-zipper
protection, arrr, me hearties, and t' recovery system was attached t' this. Aye aye! Motor tube assembly be glued into t' body tube in t' usual
way with lots o' epoxy. Well, blow me down! Avast! Note that t' motor tube protrudes out 1.5" from t' back o' t' rocket. T' MadCow nose
cone was modified by slicin' off t' hip, leavin' 1.75" remaining; this gives more room for t' parachute in a
short rocket. Blimey! Avast! 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. T' shock cord is tied off t' this. Begad! I really like this set-up, shiver me timbers, 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). Begad! Begad! In addition, I built a
small "auxiliary" chute anchor from duct tape, arrr, and attached a second, 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 be epoxied on 1.5" above t' tubefin can and in line with one o' t' triangular holes between
tube fins. Avast, me proud beauty! Ahoy! Finish was with purple and lime green Tamiya rattlecan, applied after t' first few test flights, ya bilge rat, because I
just couldn't wait!
Flight:
Balance point is no further aft than middle o' t' launch lug (1.0 calibre). Begad! Test flight was on a CTI
G69-5, me bucko, matey, usin' a medium Nomex®
protector, me hearties, perfect on t' way up, me bucko, but on chute deployment t' glue joint fractured on me dowel rod in t' nosecone, and
t' cone came down separately. Well, blow me down! I carefully re-epoxied t' thin' together, and added an "safety" 15"
chute directly t' t' cone, and resumed flying. I was impressed that Cd was only 0.83 at just 196 mph; usually Cd is
much higher at tubefin speeds below 200 mph. Aye aye! No more separation problems after this, matey, but I decided I liked t' 2nd
chute bein' there, ya bilge rat, me hearties, shiver me timbers, and it looks cool in t' air. T' pack for flight, ya bilge rat, t' 15" chute is tucked betwixt t' 36"
chute and t' Nomex®
(which I wrap around t' shock cord, matey, placin' t' 36" chute on top. Well, blow me down! Second flight be on one o' t' new (at the
time) CTI G115 motors with t' White Lightnin' propellant. Ya scallywag! Ya scallywag! Perfect flight, 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. Begad! Later flights out in t' desert were on t' following
motors, ya bilge rat, with altitude recorded (usin' a parasite pod) and speed/Cd calculations:
As t' graph o' Cd versus speed shows, thar be no inverse speed dependence o' Cd with this design - look at the comparison with t' 6-fin Tea-Bird 4.0!! T' combination o' 7 tubes and t' shorter MadCow cone makes a tremendous difference in performance. Avast, me proud beauty! It will take an engineer t' sort this out; I am a biochemist. 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, and no steep slope t' t' curve this time. Well, blow me down! I honestly think I have finally gotten a tubefin design that performs like a 3FNC design - me goal. Begad! Avast! On t' basis o' this, ya bilge rat, I have started designin' me Level 3 rocket - it will be a BIG, me hearties, me bucko, STUBBY tubefin, with 7 tubes, me hearties, nay 6.
Summary:
PRO - Finally, a tubefin that flies with about t' same performance as a "normal" rocket. Easy and quick to
build from scratch. Ahoy! Durable and low-cost. Avast, me proud beauty! T' 5-grain Cesaroni motor case sticks out t' back about 3", me bucko, 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, me bucko, since it wasn't meant t' do this).
Other:
With determination, arrr, 7 years and more dollar investment in APCP that I care t' admit, shiver me timbers, 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.
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