Saturday, January 01, 2011

Review: Shai-Hulud (Desert Squid type rocket)

I found the description of 'desert squids' on Tripoli-Pittsburg's Website. These are rockets that use dowel rods in place of fins. They look cool and since I had some BT-80 tubes and rings, I decided I'd build one pretty much like the model described on the website. My model features a BEAUTIFUL poplar nosecone and a custom rear-centering ring made by Gordon Agnello , a.k.a. sandman on The Rocketry Forum. I wanted a name to fit in the desert squid theme, and decided on the name Shai-Hulud, after the giant sandworms in the Dune trilogy by Frank Herbert.



Construction:
I was originally going to use a Fat Boy nosecone since that's what I had available. However, while I was thinking about getting sandman to quote the custom rear-centering ring, I started looking at the pictures of the cones he has made, and couldn't resist one for this rocket.

The body is two sections of BT-80 joined with a 4" coupler from Totally Tubular. Both tubes are 'pseudo-glassed' with nylon pantyhose and Minwax Polycrylic. Once tube has two layers of hose, and the other has three. I misplaced the hose that my wife originally 'donated', so the second tube was done with a different style. I discovered that different nylons have different weaves, similar to different weights of fiberglass. The first tube much rougher, which prompted be to add a third layer of a finer weave.

The motor mount consists of a 6" piece of LOC 29mm tubing, a double layer fiber ring in the front, and a 1/8" plywood ring in the back (I used this mix because it's what I had 'in stock'). This plywood ring is mounted 0.6" from the end of the motor tube, so that the end of the motor tube will be flush with the end of the custom ring after it is slipped on. The rings were attached with epoxy and the motor mount is glued in with Gorilla glue.



The custom ring is thicker than most because it also serves to mount the dowel 'fins'. Sandman made this from a thick piece of plywood that he turned down to a little over half an inch. He drilled eight evenly spaced holes at an angle of 15 degrees from the motor axis. I added two threaded inserts to provide motor retention.




 
At first, I was going to use this as the rear centering ring and glue in the dowels. However, when I dry fit the dowels, I began to worry that they may break on landing. So, I decided to make the rear ring removable and the dowels replaceable. The ring is held in with eight small screws, which also hold each dowel in place.

The shock cord assembly consists of a 1/8" Kevlar® leader, a long section of thin tubular nylon, and an anti-zipper device. The latter is similar to Giant Leap's Fireball. Mine is a small rubber ball that was slipped over the Kevlar® leader so that it touches the edge of the body tube when the cord is fully extended. The ball is covered with Kevlar® sock to protect against heat damage. To serve as an attachment point for the recovery harness, I glued together two fiber centering rings also from Totally Tubular. This assembly was attached to the lower end of the coupler. Two small holes were drilled through the ring close to the edge, and the Kevlar® was threaded through. The knotted Kevlar® leader was then epoxied to the wall of the coupler.

I installed two buttons from railbuttons.com to complete the Sahi-Hulud.

Finishing:
The Nylon/Polycrylic surface required quite a bit of finishing work. I applied numerous coats of Polycrylic, but when I laid on my first coat of primer, I realized it would take a fair amount of effort to finish these tubes. After 2+ cans of primer, the tubes weren't in too bad a shape. I also had to fill the transition between the two tubes as one was slightly thicker than the other. TIP: When pseudo-glassing a rocket, use the same nylons on all the tubes!

The body was painted using a Model Master brush on paint. The military brown was an appealing color next to the cone. As mentioned above, the nosecone came finished from sandman and is a beautiful golden color. Several coats of clear enamel finished the tube off.

The centering ring/dowel assembly was separately finished before installation. It is stained honey maple and has 2 coats of Polycrylic.

Flight:
I use a home made, car air bag chute protector and a hefty amount of dog barf wadding. Then in goes the shock tether, and 36" nylon chute. A clamp-on thrust ring and mirror clamp retainers retains the motor. The first flight was on a G40-4. The flight was relatively slow and there was some wobble on the way up. The anti-zipper device worked and the rocket recovered safely. I was amazed that none of the dowels broke. Upon inspection, the anti-zipper ball took the brunt of the ejection charge and burned through slightly. For the subsequent flights, I permanently added a piece of air bag below the ball.

The next flight was on a G80-7. The boost was a lot faster and was arrow straight. However, it still wobbled a lot after burnout. I have also used G79-10 and G77-7 reloads. In both cases there was a fair amount of wobble during boost.

At MDRA's Central Sod Farm launch site, the Shai-Hulud landed smack-dab in the middle of the irrigation pond. Luckily, it floated and the prevailing wind pushed it to shore. To my amazement, the pseudo-glassed Estes tubing held up just fine! While the pseudo-glass technique isn't nearly as strong as fiberglassing, the results are definitely tougher than the stock tubes. However, I probably will not use it again.

Poplar Nose Cone Details
 
This is a custom-turned and finished poplar nosecone that was made for me by Gordon (Gordy) Agnello, a.k.a. sandman on The Rocketry Forum. He does great custom work, but doesn’t have much of a web presence as far as vendors go. If you get a chance, pop on over to TRF and poke around to see more of his work. You can also contact him at gagnello@yahoo.com

I decided hat I wanted a hardwood cone for two reasons. First, I had never used hardwood for anything bigger than a MicroMaxx cone and just want to use one. Second, I wanted to keep the raw wood look.

The cone is 2.6-inches in diameter (BT-80), 5.5-inches long, and has a 1.25-inch shoulder. He pointed out that the cone would be heavy, so he also drilled a 2.25-inch hole to lighten it up. Now that is one big honkin’ hole! I had been pondering how to attach the shock cord to this cone until it arrived…he also mounted an eye-bolt in the cone.   Gordy scratch-builds wooden sailboats, so he is as much an expert on finishing as he is turning cones. When he heard that I wanted the raw wood look, he also offered to finish the cone [for a reasonable price, of course]. He gave the cone three coats of finishing epoxy, which turned the cone a beautiful amber color. To protect the clarity, he gave it a similar number of coats of marine-grade, UV protected varnish.