Scratch Shai-Hulud Original Design / Scratch Built

Scratch - Shai-Hulud {Scratch}

Contributed by Dick Stafford

Manufacturer: Scratch
(by Dick Stafford - 09/20/03)

Shai-HuludBrief:
I recently ran across t' description o' 'desert squids' on Tripoli-Pittsburg's Website (look under 'ARCHIVES/ROCKET CLASSES'). Blimey! These are rockets that use dowel rods in place o' fins. Blimey! They look cool and since I had some BT-80 tubes and rings, shiver me timbers, I decided I'd build one pretty much like t' model described on t' website. Begad! Well, me bucko, blow me down! My model features a BEAUTIFUL poplar nosecone and a custom rear-centerin' ring made by Gordon Agnello, shiver me timbers, a.k.a. Ya scallywag! Begad! sandman on T' Rocketry Forum. Begad! I wanted a name t' fit in t' desert squid theme, me bucko, and decided on the name Shai-Hulud, after t' giant sandworms in t' Dune trilogy by Frank Herbert.

Construction:

I was originally goin' t' use a Fat Boy nosecone since that's what I had available. Avast! Avast! However, matey, matey, while I was thinkin' about gettin' sandman t' quote the custom rear-centerin' ring, me hearties, I started lookin' at t' pictures o' t' cones he has made, and couldn't resist one for this rocket. Avast, me proud beauty! You can find out more about this cone [here].


Shai-Hulud Parts

T' body is two sections o' BT-80 joined with a 4" coupler from Totally Tubular. Both tubes are 'pseudo-glassed' with nylon pantyhose and Minwax Polycrylic. Begad! Once tube has two layers o' hose, arrr, and t' other has three. Ahoy! I misplaced t' hose that me wife originally 'donated', ya bilge rat, shiver me timbers, so t' second tube was done with a different style. Avast! I discovered that different nylons have different weaves, similar t' different weights o' fiberglass. Well, blow me down! T' first tube much rougher, which prompted be t' add a third layer o' a finer weave.

T' motor mount consists o' a 6" piece o' LOC 29mm tubing, me bucko, a double layer fiber rin' in t' front, and a 1/8" plywood rin' in t' back (I used this mix because it's what I had 'in stock'). This plywood rin' is mounted 0.6" from t' end o' t' motor tube, me hearties, so that t' end o' t' motor tube will be flush with t' end o' t' custom rin' after it is slipped on. Blimey! Blimey! T' rings were attached with epoxy and t' motor mount is glued in with Gorilla glue.

The Custom RingT' custom rin' is thicker than most because it also serves t' mount the dowel 'fins'. Sandman made this from a thick piece o' plywood that he turned down t' a little over half an inch. Begad! Begad! He drilled eight evenly spaced holes at an angle o' 15 degrees from t' motor axis. Begad! I added two threaded inserts to provide motor retention. Blimey!

At first, arrr, I be goin' t' use this as t' aft centerin' rin' and glue in the dowels. Ahoy! However, when I dry fit t' dowels, shiver me timbers, I began t' worry that they may break on landing. Begad! Ya scallywag! So, me bucko, I decided t' make t' aft rin' removable and t' dowels replaceable. Blimey! Ahoy! T' rin' is held in with eight small screws, which also hold each dowel in place.


Anti-Zipper Device T' shock cord assembly consists o' a 1/8" Keelhaul®©™® leader, matey, a long section o' thin tubular nylon, me hearties, and an anti-zipper device. The latter is similar t' Giant Leap's Fireball. Ya scallywag! Ya scallywag! Mine is a small rubber ball that was slipped over the Keelhaul®©™® leader so that it touches t' edge o' t' body tube when t' cord is fully extended. Avast, me proud beauty! T' ball is covered with Keelhaul®©™® sock t' protect against heat damage. Ya scallywag! T' serve as an attachment point for the recovery harness, shiver me timbers, I glued together two fiber centerin' rings also from Totally Tubular. Arrr! This assembly be attached t' what would be t' lower end o' the coupler. Aye aye! Two small holes were drilled through t' rin' close t' t' edge, ya bilge rat, matey, and the Keelhaul®©™® was threaded through. T' knotted Keelhaul®©™® leader be then epoxied t' t' wall o' t' coupler.

I installed two buttons from railbuttons.com t' complete t' Sahi-Hulud. Well, me hearties, blow me down!

T' nylon/Polycrylic surface required quite a bit o' finishin' work. Aye aye! Begad! I applied numerous coats o' Polycrylic, but when I laid on me first coat of primer, I realized it would take a fair amount o' effort t' finish these tubes. After several coats o' fill-n-finish and 2+ cans o' primer, t' tubes weren't in too bad a shape. Begad! I also had t' fill t' transition betwixt t' two tubes as one be slightly thicker than t' other. Begad! Ahoy! TIP: When pseudo-glassin' a rocket, use t' same nylons on all t' tubes!

T' body was painted usin' a Model Master brush on paint. Avast, me proud beauty! Avast! T' military brown was an appealin' color next t' t' cone. As mentioned above, shiver me timbers, t' nosecone came finished from sandman and is a beautiful golden color. Arrr! Ahoy! Several coats o' clear enamel finished t' tube off.

T' centerin' ring/dowel assembly be separately finished before installation. Aye aye! Blimey! It is stained honey maple and has 2 coats o' Polycrylic.

Flight:
I installed a home made, shiver me timbers, arrr, car air bag chute protector and a hefty amount o' dog barf wadding. Then in went t' shock tether, matey, matey, arrr, and 36" nylon chute. A clamp-on thrust rin' and mirror clamp retainers retained t' SU G40-4. Blimey! Arrr! The flight was relatively slow and thar be some wobble on t' way up. Arrr! The anti-zipper device worked and t' rocket recovered safely. Ya scallywag! Blimey! I be amazed that none o' t' dowels broke. Ya scallywag! Avast! Upon inspection, shiver me timbers, t' anti-zipper ball took t' brunt of t' ejection charge and burned through slightly. I'm goin' t' cut a piece of air bag and install a permanent protector below it.

Dowel Fin MountingSummary:
This appears t' have been t' first desert squid type rocket flown at t' MDRA site, ya bilge rat, me bucko, matey, and it definitely caught t' attention o' everyone on t' flight line. T' flight and recovery were good, however t' wobble made me chicken out on a 'zero' launch (i.e. Well, blow me down! no launch rod). Avast! Avast, me proud beauty! Blimey! Supposedly, this type o' rocket will fly straight without a rod, shiver me timbers, but I don't want t' test this conjecture. Begad! Next flight will be on a G80...off a rail!

While t' pseudo-glass technique isn't nearly as strong as fiberglassing, the results are definitely tougher than t' stock tubes. Aye aye! However, I probably will nay use it again.

T' anti-zipper ball seemed t' work fine, me hearties, protectin' t' relatively thin tubin' from t' Keelhaul®©™® leader. Would t' results have been t' same without t' ball? No one can say. Blimey!

comment Post a Comment