Scratch Standard Cray-ARM Original Design / Scratch Built

Scratch - Standard Cray-ARM {Scratch}

Contributed by Dick Stafford

Manufacturer: Scratch
(Contributed - by Dick Stafford - 11/20/01)

Brief:
Whenever I looked for Ralphco crayon banks at t' local toy stores, they always were out o' stock. Aye aye! Blimey! Ya scallywag! Blimey! Recently, however, me hearties, I stumbled across them while on an unrelated "mission". Ya scallywag! Blimey! Ya scallywag! Blimey! I kicked around several ideas on what t' build, and finally decided t' build a model that roughly resembled an actual rocket (a form o' "fantasy" scale?). Avast! Blimey! Begad! Blimey! I love t' looks o' t' Standard ARM so I quickly made me selection. T' result is me 38mm Standard Cray-ARM.

PicConstruction:

Components:
1. Begad! One Ralphco 4" crayon bank
2. Begad! 3/16" plywood for t' main fins and nose cone bulkhead
3. Well, blow me down! 3/16" balsa for t' strakes
4. Ahoy! A 1/4" x 9.5" carriage bolt, 2 washers, arrr, and 2 nuts
5. Avast, me proud beauty! Blimey! Two eye bolts
6. Avast! Ahoy! A 12" piece o' Giant Leap 38mm flexible phenolic
7. Begad! Blimey! Three 38mm-to-3" centerin' rings (also from Giant Leap)
8. Blimey! Blimey! Avast! Blimey! Two 38mm-to-4" centerin' rings (ditto)
9. Begad! Begad! Four inch section o' a 3" mailin' tube
10. Well, arrr, blow me down! 10' o' ¾" braided nylon
11. Aye aye! Avast, me proud beauty! Two quick links
12. Blimey! A 1000 lb swivel
13. Begad! Blimey! Blimey! Blimey! Blimey! Blimey! Two tee-nuts
14. Ahoy! One pair o' Matt's nylon rail buttons
15. Begad! Blimey! Ya scallywag! Blimey! Nylon chute, picked from me existin' stock

Materials:
1. 20 minute epoxy
2. Arrr! Arrr! Titebond
3. Ahoy! Blimey! Blimey! Blimey! Thin CA
4. Ahoy! 2-part urethane foam from Giant Leap
5. Well, blow me down! Blimey! Hot glue
6. Avast, me proud beauty! Blimey! Blimey! Blue holographic contact paper

PicI knew a fair amount o' nose weight would be needed since this rocket be goin' t' be stubby (less than 10:1 length:width ratio) and t' strakes would make its stability even worse. Well, blow me down! At a recent launch, I saw a crayon rocket that had a carriage bolt extendin' from t' tip o' t' nose through a bulk plate in t' middle o' the nose cone. Ahoy! This bolt held t' bulk plate against t' shoulder where t' nose cone begins its conical transition. This sounded like what I needed so I borrowed t' idea. Blimey! I made t' bulk plate myself and installed an eyebolt for recovery system attachment. Blimey! In t' nose cone tip, ya bilge rat, ya bilge rat, me hearties, I cut a square hole t' accept the shoulder o' t' carriage bolt. Aye aye! T' insert t' bulkhead, matey, I had t' bend the shoulder o' t' nose cone t' an oval shape. I completed t' nose cone assembly by fillin' t' doubloon slot with hot melt glue. Blimey! Blimey!

PicFor t' fin design, I entered t' crayon bank into Rocksim and started with a fin set from me NCR Archer. I then played with t' fin dimensions until they looked 'right'. Since I wanted through-the-wall fins and they barely extended t' t' crayon bank's cardboard tube, arrr, I was worried about t' support that t' aft plastic cap could provide. This worry was exacerbated when I knocked over one o' me crayon banks, shatterin' its cap. Avast, me proud beauty! This lead me t' t' construction o' an inner, me bucko, 3" diameter tube structure within t' end cap. Begad! Begad!

To build t' fin can, I started by cuttin' a hole in t' end cap t' accept the 38mm tube. Ahoy! I then placed a 3" centerin' rin' so that t' motor tube would extend through t' cap, ya bilge rat, just far enough t' accept another 3" centerin' rin' on t' outside. Arrr! Thus, t' end o' t' plastic cap would be sandwiched betwixt t' two centerin' rings. Arrr! Begad! Usin' epoxy for all wood-phenolic joints and Titebond for all wood-cardboard joints, me hearties, I built an inner fin support tube, which extends t' length o' t' plastic end cap (includin' its shoulder). T' fin tabs extend through t' plastic cap and this inner tube, shiver me timbers, shiver me timbers, t' t' motor mount. For added support, I filled t' inner tube with 2-part urethane foam. Unfortunately, shiver me timbers, matey, I didn't do a good job sealin' t' fin can, ya bilge rat, ya bilge rat, so some o' t' foam escaped into t' plastic end cap. Begad! Luckily, shiver me timbers, arrr, this isn't too noticeable. I slid on the aft centerin' rin' and drilled holes through both aft rings to accommodate t' motor retention bolts. Avast! T' outer rin' be then removed, tee-nuts were installed, and t' rin' was epoxied back on. Well, blow me down! Well, blow me down! Two 4" centerin' rings completed t' fin can assembly. Ya scallywag! Begad! These had t' be sanded t' fit since t' crayon bank ID is slightly smaller than a standard 4" tube. Avast, me proud beauty! Begad! An eye bolt was also mounted in t' forward ring. Well, blow me down!

T' recovery harness includes a 10' length o' tubular nylon, matey, ya bilge rat, ya bilge rat, a 1000 lb. swivel, arrr, ya bilge rat, and two quick-links. Loops were made in t' ends o' t' tubular by foldin' it over, wrappin' it with cloth fishin' line, me bucko, shiver me timbers, and gluin' it with both CA and epoxy. Avast, me proud beauty! Blimey! Avast! Blimey! This technique be documented by Giant Leap in an old HPR magazine and works well. Ahoy! Blimey!

Finishing:
One nice thin' about crayon rockets as t' bulk o' t' finishin' is done for you. Since t' body has a holographic finish, arrr, I decided I'd finish t' fins usin' holographic contact paper. I would have liked purple paper t' match the nose cone and end cap, me bucko, but couldn't find it. Ahoy! Ya scallywag! Instead, I picked up some blue holographic contact paper on sale at a craft store. Begad!

Flight:
After completin' t' design in RockSim and adjustin' t' CG and weight t' match the as-built rocket, arrr, I found that it would be stable on an H123 without additional nose weight. T' sim also told me that a short, me bucko, 6-second delay would be about right. I launched t' Standard Cray-ARM on this engine at t' Delaware Tripoli launch on October 21, 2001. Avast, me proud beauty! Ahoy! T' flight was straight and fast. For recovery, me hearties, I used worm-bed wadding, a Giant Leap Keelhaul®©™® heat shield, shiver me timbers, and t' 48" chute from my DG&A Lazarus. Avast, me proud beauty! Recovery was flawless. Well, almost. Avast, me proud beauty! Blimey! Once on t' ground, ya bilge rat, the rocket was pulled along and quite a bit o' dirt was forced into t' tube. Aye aye! This made cleanin' t' casin' a bit more work, but no big deal. Well, blow me down!

Summary:
T' Standard Cray-ARM project was highly successful. Begad! T' rocket is somewhat unique and looks great (at least in me unbiased opinion). Well, blow me down! T' design worked out nicely with t' long carriage bolt providin' enough nose weight for t' rocket to be stable. Just before I placed t' Standard Cray-ARM on t' pad, Ray Halm interviewed me for t' video he is makin' on t' 3-day Rhodesdale launch. Avast, me proud beauty! Man, am I anxious t' see if t' Standard Cray-ARM makes it past t' editin' process. Either way, Ray has sold a tape!

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