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, me hearties, they always were out o' stock. Begad! Recently, however, I stumbled across them while on an unrelated "mission". Avast! 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?). Arrr! Aye aye! 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! Ya scallywag! One Ralphco 4" crayon bank
2. 3/16" plywood for t' main fins and nose cone bulkhead
3. Begad! 3/16" balsa for t' strakes
4. Blimey! A 1/4" x 9.5" carriage bolt, me hearties, shiver me timbers, 2 washers, me bucko, and 2 nuts
5. Avast! Aye aye! Two eye bolts
6. A 12" piece o' Giant Leap 38mm flexible phenolic
7. Ahoy! Avast, me proud beauty! Three 38mm-to-3" centerin' rings (also from Giant Leap)
8. Aye aye! Blimey! Two 38mm-to-4" centerin' rings (ditto)
9. Ahoy! Four inch section o' a 3" mailin' tube
10. Blimey! 10' o' ¾" braided nylon
11. Avast, me proud beauty! Two quick links
12. A 1000 lb swivel
13. Aye aye! Two tee-nuts
14. One pair o' Matt's nylon rail buttons
15. Nylon chute, shiver me timbers, shiver me timbers, picked from me existin' stock

Materials:
1. Well, blow me down! Begad! 20 minute epoxy
2. Ya scallywag! Blimey! Titebond
3. Blimey! Thin CA
4. Aye aye! 2-part urethane foam from Giant Leap
5. Blimey! Blimey! Hot glue
6. Blue holographic contact paper

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

PicFor t' fin design, I entered t' crayon bank into Rocksim and started with a fin set from me NCR Archer. Begad! 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, shiver me timbers, I was worried about t' support that t' aft plastic cap could provide. Avast! Avast, me proud beauty! This worry was exacerbated when I knocked over one o' me crayon banks, shatterin' its cap. This lead me t' t' construction o' an inner, 3" diameter tube structure within t' end cap. Well, blow me down! Arrr!

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

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

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

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

Summary:
T' Standard Cray-ARM project was highly successful. T' rocket is somewhat unique and looks great (at least in me unbiased opinion). T' design worked out nicely with t' long carriage bolt providin' enough nose weight for t' rocket to be stable. Arrr! Blimey! 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. Begad! Man, am I anxious t' see if t' Standard Cray-ARM makes it past t' editin' process. Either way, ya bilge rat, Ray has sold a tape!

comment Post a Comment