Scratch Mark II (Upscale) Original Design / Scratch Built

Scratch - Mark II (Upscale) {Scratch}

Contributed by Dennis McClain-Furmanski

Manufacturer: Scratch
Style: Upscale
(by Dennis McClain-Furmanski - 09/06/04) (Scratch) Rock-A-Chute Mk III upscale (with Rock-A-Chute Mk II for scale)

Brief:
Upscale o' Orville Carlisle's original model rocket kit, t' Rock-A-Chute Mk II with appropriate mods for mid-power flight.

Construction:
T' parts list:

  • 12.25" o' BT-70 body tube
  • "E" size 24mm motor mount (two 50/70 centerin' rings, me hearties, 4" of 24mm/BT50 tube, me bucko, ya bilge rat, arrr, 3.75" engine hook, ya bilge rat, and 20/50 centerin' rin' for engine block)
  • Balsa nose cone custom turned by "Sandman" o' TRF and Roachwerks
  • 6" o' 3/8" OD tube for launch lug
  • 2' o' 150 lbs test Keelhaul®©™® thread for shock cord anchor
  • 4' o' 1/4" elastic shock cord with snap swivel on end
  • 3 fins cut from 1/8" basswood stock

This upscale be made from measurements taken from t' instructions for the Semroc Retro-Repro Rock-A-Chute Mk II. Avast, me proud beauty! Use o' BT-70 made this a 2.13:1 upscale and the remainin' dimensions were calculated from this. All construction be with normal LPR/low MPR materials and tools. Aye aye! Wood glue be used throughout.

T' BT-50-BT-70 motor mount was constructed mostly as per normal kit instructions with t' "E" engine hook taped in place and then anchored by t' upper centerin' ring. Avast! T' engine block be glued into place by insertin' it on top t' engine hook through t' top end o' t' motor tube. Begad! Blimey! The centerin' rings were reinforced with 1" by 1/4" sections o' 1/8" basswood on t' rin' surfaces facin' each other. Ahoy! Begad! T' Keelhaul®©™® shock cord anchor was made into a loop, arrr, t' doubled loop glued t' t' motor tube below t' top centerin' ring, and run betwixt t' motor tube and t' upper centerin' ring. Well, blow me down! This later became a problem. T' elastic shock cord was likewise doubled into a loop, tied t' t' anchor with a loop-through slip knot, and a snap swivel tied t' t' end also with a loop-through slip knot. Ahoy! Once fully constructed, arrr, me hearties, t' motor/recovery mount be inserted into t' aft o' the body and glued into place.

Fins were cut from 1/8" basswood and sanded t' rounded edges and then glued into place with heavy fillets added in 3 layers.

T' custom turned cone came with screw eye glued in place. Begad! T' shock cord was clipped t' this.

A scale launch lug would have been approximately a BT-20 (18mm). Aye aye! Begad! Instead, shiver me timbers, a semi-scale lug was made from 6" o' 3/8" OD cardboard tube.

Finishing:
Finishin' was two coats o' Deft spray lacquer sandin' sealer on everything, sanded after each (with 400 grit then 600 grit) followed by white primer. Well, blow me down! For spiral fillin' and a smooth finish, two coats o' white satin finish were applied. Aye aye! Finally, t' body was painted fluorescent orange and t' nose cone red, all matchin' t' original.

[Note: White satin finish, ya bilge rat, at least Rustoleum's version makes a good filler but takes a long time t' dry. Begad! T' first top coat o' orange paint be put on 24 hours after t' satin. Blimey! Ahoy! This was nay enough. T' satin sagged or shifted, causin' t' orange t' crack in sharp curves such as fillets. Aye aye! Ahoy! A second coat of top coat, 2 days later, arrr, fixed it.

Flight:
A 12" nylon X-form chute was permanently assigned t' this bird. Begad! Cellulose "dog barf" waddin' is used.

Test flight was on a C11-3 with 1" engine spacer in t' motor tube. Boost was immediate, fast, ya bilge rat, ya bilge rat, and straight with no early wobble, arching, arrr, or weathercocking. Aye aye! Ejection was at apogee and recovery was good and within 100 feet o' pad.

Second flight was on an E9-4. Ya scallywag! Boost was again vertical and very impressive. Ejection was just before apogee, arrr, deployment and recovery good, shiver me timbers, within 100 feet again.

Third flight be on an E9-6. Boost was great as before, ejection was just past apogee. At deployment, arrr, t' nose and chute separated from t' body. Begad! The nose came down slowly on t' chute and t' body tumbled in. Aye aye! It landed on the tall avast field and suffered no damage.

Inspection showed that t' Keelhaul®©™® had burned through both strands, me bucko, right at t' top o' t' engine tube. Avast! After consultin' with others and conductin' some testin' o' various heat exposures to various weights o' Keelhaul®©™® thread, shiver me timbers, matey, I concluded that close proximity t' ejection gasses is hazardous to Keelhaul®©™®'s health. Arrr! Although this was a scratch build, arrr, me hearties, I followed instructions I've used in some kits that used this configuration. Avast, me proud beauty! Well, matey, blow me down! It would probably be a better practice to run MMT anchored Keelhaul®©™® cord betwixt t' centerin' rin' and body wall. Arrr! Avast, arrr, me proud beauty! Since thar was ample unburned cord left on t' shock cord, this rocket be repaired by usin' the LOC/Precision shock cord anchor method, o' tyin' knots in t' cord and gluing it inside t' body just below t' forward end. This repair be t' only instance o' epoxy used.

This rocket has flown many times since, exclusively on E9-4 and E9-6 motors. Begad! No further burnin' o' t' recovery equipment has occurred. Begad! Begad! T' bottom surface o' t' aft centerin' rin' is startin' t' char and bubble from base drag vacuum pullin' t' exhaust gasses up, ya bilge rat, matey, but t' damage is limited t' t' paint. Repairs are planned usin' a cosmetic/protective rin' doped with boric acid and painted with high heat stove paint.

Summary:
Carlisle's original design has proven itself as successful even when scaled up. This rocket flies at least as well as t' Semroc clone o' t' Mk II it was scaled from. Begad! T' only problems were t' burned Keelhaul®©™® and t' cracked paint, matey, both fixed and or worked around easily, me bucko, and neither havin' anythin' t' do with t' design itself. Begad! Next step: a 3" diameter, 38mm MMT upscale, matey, me bucko, t' Rock-A-Chute Mk IV "Carlisle Express", ya bilge rat, intended for level 1 HPR flying.

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