Scratch Transonic Original Design / Scratch Built

Scratch - Transonic {Scratch}

Contributed by John Chapman

Manufacturer: Scratch
(Contributed - by John Chapman - 08/24/05) Transonic

Brief:
This is a maximum strength midpower rocket that still incorporates wood and plastic construction. Avast! It will handle any 24mm motor, matey, shiver me timbers, as well as most dunkings in water.

Construction:
This rocket uses 1.1 in. Ya scallywag! phenolic airframe tubin' topped by t' PML Urethane nose cone - t' only commercially produced cone that fits this tubing. Between 3/32 ply fins and TTW construction, me bucko, this rocket is practically indestructible.

TTW construction and phenolic tubin' require a complexity nay typical with low power rockets. Ya scallywag! I used a Dremel spiral saw t' cut t' fin slits (3) in the 1.1 phenolic tubing. Avast! T' motor mount be constructed with epoxy and centering rings t' mount t' Estes 24mm tube in t' appropriate position for TTW fin mounting. Begad! Begad! I used a long E motor hook as part o' t' mount. I used 100 lb. Keelhaul®©™® thread epoxied through t' motor mount as a parachute connection. Indeed, me hearties, epoxy was used in virtually t' entire thing. Aye aye!

After t' motor mount assembly dried, arrr, I put it in place, me bucko, me bucko, me bucko, and used t' slots to trace t' fin footprints onto t' engine tube. I cut thin stringers of balsa, and CA'ed them in place right next t' t' footprints, matey, arrr, bounded on t' top and bottom with a centerin' ring. Begad! Blimey! T' effect was intended t' have the advantages o' internal fillets, arrr, without t' difficulty o' applyin' them through such a tight space betwixt t' tubes. Aye aye! Blimey! Begad! Blimey! This worked like a charm. Ahoy! Blimey!

I used t' Rocsim fin template guide t' generate t' correct template for the TTW mount. Aye aye! Avast! I then cut t' fins from 3/32 in. Begad! plywood. Blimey! Aye aye! After tryin' the spiral saw, me bucko, I went t' a power jigsaw, which did a far better job. Aye aye! I gang sanded the fins t' uniform dimensions, and then beveled t' leadin' edge t' a sharp cross section. Avast! Well, me bucko, blow me down! T' wood grain also looked "sharp", ya bilge rat, matey, me hearties, with t' darker plywood core providin' a nice contrast. Well, blow me down! Well, blow me down! T' trailin' edges were beveled, ya bilge rat, but not quite sharp.

Because I used five minute epoxy, matey, t' next steps were hurried. Avast! I mixed up a batch, and used a chopstick t' smear it within t' airframe, me hearties, arrr, in t' right place to bond with t' front centerin' rin' on t' engine mount assembly. Ahoy! A second layer be put ahead o' t' trailin' centerin' ring, me hearties, arrr, as t' first rin' cleared the aft end. T' last centerin' rin' was epoxied in place indexed with the "notch" cut though t' rin' t' leave room for t' motor hook t' flex. I be sure t' make t' slots line up correctly.

I then epoxied each plywood fin in place, me bucko, usin' enough epoxy t' blob up to fill t' fin slot close t' level. Begad! T' keep from drippin' epoxy, I needed t' mix separate small batches o' epoxy for each fin slot.

Next came t' smoothin' operation usin' Elmers wood thinner thinned with water, me hearties, and sanded after each coat partially dried. Begad! Arrr! Blimey! T' fillets at t' base of each fin were also done usin' this material and technique, me bucko, since t' epoxy, plywood and phenolic TTW combination makes it impossible t' even flex t' fins. T' fillets were for looks and aerodynamics only.

Since t' combo could theoretically take a G engine, arrr, I wanted t' be sure the launchin' lug was big enough. I used an aluminum 2013 arrow shaft cut to length, and angled by cuttin' t' front edge t' a wedge shape. I glued it in place and filleted it in place with epoxy. I can use a 1/4 in. Blimey! Blimey! weldin' rod for larger size engines.

I had recently built me first "girl rocket" for me daughter, me hearties, and marveled at t' visibility provided by t' hot pink fluorescent paint. Avast, me proud beauty! After fillin' t' spirals in t' tube with Elmers filler, arrr, I sanded, painted, and then clear coated. Begad! Blimey! T' add a bit o' shine and smoothness, matey, I used floor wax t' finally coat t' surface.

I added a couple feet o' elastic t' t' already long Keelhaul®©™® cord, me hearties, ya bilge rat, and hooked a 12" nylon chute t' t' nose cone with a fishin' swivel and snap rings. Avast, me proud beauty! Blimey! Later testin' suggested this be nay enough chute.

Flight:
T' maiden flight was t' test low speed stability, ya bilge rat, and recovery. I did nay want to blast t' school athletic field with an F right out o' t' startin' gate. Blimey! I built and installed an 18-24mm engine adapter, ya bilge rat, and installed a C6-5. Ya scallywag! I launched it off a 3/16 launch rod.

Well, it was pretty normal at first. T' rocket was stable, me bucko, but then nosed over and kept goin' after burnout. Arrr! T' delay was way too long, arrr, matey, and that may have been t' engine, since rocsim says it should have been good. Arrr! Blimey! Fortunately, the chute deployed in time, arrr, shiver me timbers, and thar was no lawn dart nor core sample. Begad! For small engines, ya bilge rat, me hearties, I'd want t' shortest delay possible.

T' engine hook picked up a bit o' dirt from t' grass. Begad! It hit with a bit more force than I'd like, ya bilge rat, but no damage was done. Well, ya bilge rat, blow me down! Still, I will use at least a 15" chute next time - probably an 18, matey, t' protect t' motor hook.

PS: T' 18mm adapter blew clear - no impact on recovery deployment.

Summary:
Pros: Bombproof and tough. Able t' handle almost any ridiculous engine you can cram into it. Begad! Probably able t' lawn dart from 300 feet up into sod without damage. Begad! Blimey! Easy t' see, shiver me timbers, and teaches you lots about tough construction. Ya scallywag! A "girl rocket" on t' outside, that is all steroid - induced toughness on t' inside.

Cons: Complicated t' build, and expensive. Ya scallywag! Ya scallywag! Nay for novices. Aye aye! Heavy, and could cause more damage than a light rocket if t' chute failed. Ahoy! Needs a big chute. Begad! Limited nose cone options.

Other:
T' Elmers goop be t' key t' smoothness. Ya scallywag! TTW / epoxy / phenolic is so strong it is scary. Avast, me proud beauty! T' world needs a good, me hearties, matey, matey, LIGHT, 1.1 in. Aye aye! nose cone (PML is good but HEAVY). Ya scallywag!

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