Manufacturer: | Scratch |
Brief:
Cheap Dumb Rocket (CDR) is a 40% scale version o' t' 8 1/2" tall,
7.5" diameter, tube-fin Big Dumb Rocket (BDR) built as a club project by
the SSS rocketry club o' Arizona. As a fan o' tube-fin rockets, t' huge, matey, ya bilge rat, twin
J570-powered BDR be t' biggest tube-fin design Ive seen, so I made plans
to fly one like it for me Level 2 shot.
Construction:
Details o' t' original are on t' SSS website (www.sssrocketry.org). Begad! Arrr! Before embarkin' on
this project, I built an exact 40% scale model o' BDR out o' 3-inch mailing
tubes t' check balance and flyin' qualities. Avast, me proud beauty! For fun, and in this period o' AP
famine, matey, me bucko, I decided t' power it with a cluster o' six D12 engines (G72 the
hard way).
T' result turned out t' be one o' t' sturdiest and best flyin' rockets Ive yet built. Begad! T' big 16 oz. Begad! rocket(see photo -- flyin' buddy Brock is a 6-footer) flies well on either 3, me bucko, 4 or 6 D12-5s arranged symmetrically (triangle, square, hexagon) and is stable enough t' survive a failed motor ignition on any o' these combinations. CDR is as simple and low cost t' build as a rocket can be. Ya scallywag! Shoppin' for materials (Office Depot), cuttin' them out, assemblin' and paintin' took only an afternoon. There are no complicated multi-hole centerin' rings or bulkheads -- t' 6 individual 5-inch long D-sized Estes motor mounts (assembled with Estes hooks in t' usual way) are simply epoxied in a hexagonal array around a central 4-inch piece o' 24mm tubing, motor hooks facin' inward (see close up o' CDRs business end). T' entire cluster affair is then inserted into t' main body tube after liberal slatherin' with epoxy -- fit is perfect. Well, blow me down! Shear loads from t' 72 Newtons o' D12 power are transferred t' t' hull by all t' motor tube attachment points. Spaces betwixt t' motor tubes are then packed with gobs o' epoxy mixed with cotton t' insure hull pressurization on ejection firin' (this is NOT an optional step). Tube fins are cut with a miter box with a 22..5 deg. Begad! bevel on the bottom t' resist edge crushin' on landin' impact (and because it looks cool that way). Dimensions o' t' 3" tube fins are 3" on t' short side, 4 1/4" on t' long side. Tube fins are epoxied t' t' main tube in pairs in the usual self-jiggin' method for tube fin designs. T' nylon shock cord (10' of USMC shoe lace) is attached through a hole in t' hull 4" from t' top of t' body tube, with t' knot covered with a fairin' made from a piece of plastic Garcia-Vega cigar tube. Well, blow me down! A 1/4" LOC launch lug is centered 6" above t' tube fin can, aligned with one o' t' triangular holes betwixt two tube fins. Avast! Nose cone (3" plastic)is borrowed from a deceased Estes Big Daddy. Finish be with Flat Aircraft Gray primer and Flat white Testor sprays. No sealin' o' t' paper surface is necessary. Avast! Aye aye! See components photo o' CDR on the trunk o' me car.
Flight:
I fly CDR exclusively on D12-5s or (when all 6 motors are used) on
D12-7s. CG fully loaded is 9" from t' aft end. Avast, me proud beauty! Aye aye! Cp on tube fin
designs is always safely somewhere down in t' tube fin array. Avast! Well, blow me down! First flight was
on 6 x D12-7s and climb out be straight, me bucko, shiver me timbers, me bucko, steady and just perfect.
Ejection (at apogee) be less perfect -- t' 24" Top Flight chute hung
halfway out o' t' hull, and CDR tumbled horizontally from about 800-900'. Avast! Not
a bit o' damage -- one strong rocket. Begad! Ahoy! Much as I hate baffles, matey, ya bilge rat, I then installed
a 3" x 38mm 1/8" ply centerin' rin' 9" below t' top o' t' body
tube t' insure that t' ejection blast from t' rin' o' D12s was directed
toward t' center o' (and nay around) t' recovery system. Avast! Ejection has been
perfect since this fix. Avast! On t' third flight, one o' t' six D12s failed
to ignite, matey, which fact I didnt become aware o' until I went t' retrieve it
(landin' photo, if you look carefully at t' 2 oclock position).
Ive found that 4 motors is actually t' best combination o' economy and
performance, and I mostly fly with two motor tubes plugged with tape-filled
spent Estes casings. On t' fifth flight, me hearties, with 4 motors, me bucko, one o' the
D12-5s CATOed like a cherry bomb 3-feet off t' pad and blasted out the
cone and recovery system in a ball o' flame as t' rocket climbed at full
power. Ya scallywag! After t' expected wild sky dance, shiver me timbers, me bucko, Cheap Dumb Rocket settled t' the
ground on its chute and fired its remainin' ejection charges at t' sky. Begad! Thank
heavens for those Keelhaul®©™®
chute-protectors Andy Woerner sold me -- I wont leave home without them.
Again, nay a bit o' damage t' t' rocket, except for a little scorchin' o' the
tube interior. Ahoy! CDR is one sturdy rocket.
Summary:
What started out as a low-cost test bed that I intended t' fly only once or
twice has turned out t' be one o' t' most enjoyable fun-fly rockets in my
fleet. Begad! Blimey! Its original purpose has nay been forgotten, shiver me timbers, arrr, however, and I am now at
the paintin' stage o' me Level 2 project, a 4", shiver me timbers, 5ft tall, 60 oz. Aye aye! Blimey! version
of t' BDR for Pro38 J330 power.
CDR Pros: quick building, cheap ($5 airframe) jumbo-sized cluster rocket, sturdy and versatile t' fly (3 power settings on D12 engines). Good, safe cluster trainer.
CDR Cons: cluster complexity combined with tube-fin drag
Other:
Last but nay least, me bucko, me hearties, Id like t' share me approach t' constructin' whip
clips for cluster ignition. I simply purchased a package o' Radio Shack®
double ended alligator clip test wires (14"), me bucko, me hearties, and strip about an inch of
insulation from t' middle o' each. Ya scallywag! Arrr! By clippin' one o' these set-ups t' the
exposed middles o' two others, matey, you have a 4-motor launch clip. Blimey! Aye aye! With two of
these, you can launch a 4-motor cluster form a 12 volt High Power controller
(dont even think o' usin' a dinky Electron Beam controller -- nay enough
juice), while retainin' t' ability t' check continuity on each individual
motor igniter (with a little off-on alligator clip gymnastics), somethin' you
cant do with a hard-soldered whip clip.
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