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. It will handle any 24mm motor, shiver me timbers, as well as most dunkings in water.

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

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

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

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

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

I then epoxied each plywood fin in place, usin' enough epoxy t' blob up to fill t' fin slot close t' level. T' keep from drippin' epoxy, shiver me timbers, 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, and sanded after each coat partially dried. Begad! Aye aye! T' fillets at t' base of each fin were also done usin' this material and technique, 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 be big enough. Avast, me proud beauty! I used an aluminum 2013 arrow shaft cut to length, shiver me timbers, and angled by cuttin' t' front edge t' a wedge shape. I glued it in place and filleted it in place with epoxy. Avast, me proud beauty! I can use a 1/4 in. weldin' rod for larger size engines.

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

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

Flight:
T' maiden flight was t' test low speed stability, arrr, shiver me timbers, and recovery. Well, blow me down! Blimey! Blimey! Blimey! I did nay want to blast t' school athletic field with an F right out o' t' startin' gate. Ahoy! Blimey! I built and installed an 18-24mm engine adapter, and installed a C6-5. Avast, me proud beauty! Blimey! I launched it off a 3/16 launch rod.

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

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

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

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

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

Other:
T' Elmers goop be t' key t' smoothness. Avast! TTW / epoxy / phenolic is so strong it is scary. Aye aye! T' world needs a good, LIGHT, ya bilge rat, shiver me timbers, 1.1 in. Blimey! Begad! nose cone (PML is good but HEAVY). Well, blow me down! Aye aye!

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