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

Materials:
1. Begad! 20 minute epoxy
2. Avast, me proud beauty! Titebond
3. Ya scallywag! Blimey! Thin CA
4. Begad! 2-part urethane foam from Giant Leap
5. Avast! Hot glue
6. 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. Well, blow me down! This bolt held t' bulk plate against t' shoulder where t' nose cone begins its conical transition. Aye aye! This sounded like what I needed so I borrowed t' idea. Avast! I made t' bulk plate myself and installed an eyebolt for recovery system attachment. Well, blow me down! In t' nose cone tip, ya bilge rat, me hearties, I cut a square hole t' accept the shoulder o' t' carriage bolt. Aye aye! T' insert t' bulkhead, I had t' bend the shoulder o' t' nose cone t' an oval shape. Ya scallywag! Avast, me proud beauty! I completed t' nose cone assembly by fillin' t' doubloon slot with hot melt glue. Begad!

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'. Avast! Aye aye! 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. Avast, me proud beauty! This worry be exacerbated when I knocked over one o' me crayon banks, shatterin' its cap. Begad! This lead me t' t' construction o' an inner, arrr, 3" diameter tube structure within t' end cap. Ya scallywag!

To build t' fin can, 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. Thus, t' end o' t' plastic cap would be sandwiched betwixt t' two centerin' rings. Blimey! Usin' epoxy for all wood-phenolic joints and Titebond for all wood-cardboard joints, matey, shiver me timbers, 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, t' t' motor mount. Begad! For added support, I filled t' inner tube with 2-part urethane foam. Unfortunately, I didn't do a good job sealin' t' fin can, so some o' t' foam escaped into t' plastic end cap. Luckily, shiver me timbers, this isn't too noticeable. Ahoy! I slid on the aft centerin' rin' and drilled holes through both aft rings to accommodate t' motor retention bolts. Well, blow me down! T' outer rin' was then removed, tee-nuts were installed, shiver me timbers, and t' rin' was epoxied back on. Arrr! Two 4" centerin' rings completed t' fin can assembly. Avast! These had t' be sanded t' fit since t' crayon bank ID is slightly smaller than a standard 4" tube. Ahoy! An eye bolt was also mounted in t' forward ring. Blimey!

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

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

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

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

comment Post a Comment