TECH NOTES

(Courtesy of Star Rocketry)

 

"Hyper Active" by Chuck Andrus

 

Written by Chuck Andrus (Pictured below holding his Hyper Active) -  published in SPORT ROCKETRY magazinechhyper.jpg (16323 bytes).

Finally, a break in the clouds. "Lets go Scott" said Oscar, calling his son to the range head, after patiently waiting all morning and half the afternoon. The 3000' ceiling was finally broken. Moments later, the silence is disturbed by a growing hiss, and a plume of vapor from the motor vent. The countdown commences, followed by an Earth shattering groan as the PML Aurora scorches the Earth on it's way to a mile high! A brief flash at apogee, followed by a faint pop deploys a drogue, breaking up the sleek silhouette in the sky. Just when it looks to the crowd that the Aurora is on it's way to six feet under the Alabama clay, the main parachute snakes it's way to the airstream, gently bringing the projectile to rest near a stand of pines at the edge of the range. "How could it get any better than this," I thought to myself. "I've got to try it!"

Such was the scene at the monthly HARA launch (Huntsville Area Rocketry Association), July 1998, when I witnesses my first Hypertek hybrid powered flight. I knew immediately that Nitrous Oxide (known by unfortunate dental patients as laughing gas) was going to play a significant role in my future rocketry activities. I was most impressed by the simplicity of the Hypertek system, since preparation of the motor takes only moments, and clean up is virtually non existent. Cost per flight is way down there too, since a J powered Hypertek flight is around 30 smackeroos, compared to $50 and up for AP motors. Specialized launch equipment is required, but club incentives offered by the manufacturer make this issue vanish quickly.

Though not flawless, the Hypertek system is easily integrated into new as well as existing airframes. With minimal modifications, the "Standard" 440cc Hypertek motor will fit inside any rocket with 54mm motor mount tube, but I've never been known to take the easy way out. I chose to build my first hybrid powered rocket to fly on the J-300 configuration of the 835cc Hypertek "Hammerhead" motor, so some special considerations were in order. First, I had to ensure the 54mm motor mount tube was short enough to accommodate only the length of the Hypertek J fuel grain, since the tank blisters to 3.2" above the bell. I wanted to fly the same rocket on a variety of different motor combinations in the future though, so I decided to implement a modular motor mount system that would allow me to construct several quick change motor adapters. I elected to build the rocket with a 3.9" diameter airframe more by necessity than choice, since I already had sufficient stock of 3.9" filament wound fiberglass airframe tubing, and couplers to match. I scrounged through boxes and bags until I located a 3.9" PML cone once destined to become the nose of a 3.9" Phoenix missile. I hope the "scale" readers won't frown on my abandonment of the Phoenix project, considering the end product.

"How am I going to build this beast to accommodate any motor configuration up to 98mm motors," I asked myself. No through the wall construction of this bad boy, since I had to leave the inside diameter of the airframe undisturbed. I considered design after design, finally settling on a fin canister approach that would attach the fin unit to the airframe as part of the motor adapter attachment process, similar to the system I use with ACME fin canisters on smaller rockets. The motor mount modules, in turn, would be constructed so the aft ring would thrust against the aft of the airframe, and the bottom of the fin canister.

The fin canister was the first hurtle to overcome, since I thought it would be the most difficult and time consuming. It had to be strong, relatively light, and easy to build with standard construction tools and methods. I chose a short section of 4.0" ID filament wound tubing for the base of the canister, since my airframe tubing was 3.995" OD. I had a section of .090" G-10 sheeting left from a few other projects, so selection of material for the core of the fins was automatic. After cutting out each of the four generously sized fins using a pattern printed with VCP, they were butt-glued to the fin canister with West Systems resin and "fast" catalyst.

Next, two laminations of 5 oz. bi-directional carbon fiber fabric were applied to the entire fin canister assembly utilizing West Systems resin and "slow" catalyst, from one fin tip to the root, across the canister, then up the next fin. The second laminate of carbon fiber was applied with the weave at a 45 degree angle to the first, to maximize strength and rigidity of the material. Topped with a "veil" of 2 ounce fiberglass and several coats of clear lacquer, the carbon fiber weave yielded a very distinct and interesting finish.

Next on the agenda was fitting a motor mount to the inside diameter of the rocket. To transfer energy directly to the airframe, the thrust ring of the fuel grain fits flush against the aft bulkhead, which in turn, pushes directly on the airframe and fin canister. With this construction approach, the only way the motor is going through the airframe is if both 3/8" aft plywood bulkheads splinter. The motor mount tube is there for alignment purposes only, and isn't directly subjected to thrust generated by the motor, or the impact with the ground during recovery. In addition, the 12-1/8" long 54mm motor mount tube allowed just enough length for the plastic fuel grain to screw on the bottom of the Hypertek tank, capturing the motor mount between the tank and the fuel grain for extremely positive retention.

I decided that some scrap fiberglass coupler material would provide the best bearing and alignment surface for quick installation and removal of the motor mount module, so I cut two slices of coupler material to 2" lengths. Using my drill press and a circle cutter, I cut 3/8" thick plywood circles to fit inside the couplers, as well as some to match the OD of the fin canister, making several extras of each for use as bulkheads and spares. I then set up the circle cutter to machine holes to capture the motor mount tube, using the pilot hole of the outside diameter cut as the pilot for the motor mount hole. The centering rings were epoxied inside the 2" slices of coupler tube, then epoxied to the motor mount tube, with the aft ring fitted flush with the end of the motor mount tube. A hole was also drilled through the motor mount tube for access to the vent fitting on the motor tank, which was plummed through the aft end of the mount with silicone tubing, and vented away from the nozzle.

Final fitting of the motor mount and fin canister assembly was accomplished by drilling several holes in the aft end of the fin canister, through the airframe, and through the aft coupler tube slice attached to the motor mount. The mount was then removed, and blind nuts epoxied to the holes inside the coupler slice to provide a solid anchor point for the counter sunk hex key fasteners that would bind the assembly together. Since the aft ring pushes directly on the airframe during boost, the only real purpose the screws serve is to retain the motor mount module inside the the airframe after burn out. However, following my tradition of over-engineering, I used eight. Besides, lots of hex key screws look cool!

Of course, this modular approach opens a lot of doors. The Hammerhead motor module can be used with any 54mm motor, or itcan be replaced with a 75mm module, or one comprised of most any combination of clusters, as long as the selected configuration fits inside the airframe and coupler tube. Motor retention is pretty standard, when using most of the commercial reloadable motors. With a little imagination, even a 98mm motor could be tamed to send this beast toward the stars!

With the exception of the "Zipperless" coupler used on the top of the booster, application of 1/2 inch ACME conformal launch lugs, and a Cambridge IA-X96 powered dual stage deployment payload section, the remainder of construction was fairly conventional. I chose charcoal and silver automotive finish colors to compliment the natural beauty of the carbon fiber fin canister, and added silver vinyl lettering cut by a local sign shop to give the finish my personal touch.

"Hyper Active" lived up to it's name, as it leapt from the launch pad at the "Rocket City Blast Off", October 24th, for my first Hypertek Hybrid and most memorable High Power rocket flight to date. From conception to flight was about 6 weeks, and with absolutely no prior experience with hybrid technology, I managed a safe and flawless flight. Much credit for the safety of this project goes to Korey Kline at Hypertek, who has engineered a simple, cost effective, and above all safe Hybrid motor fueling, launching, and propulsion system. Significant thanks also goes to Oscar Valent of STAR ROCKETRY (http://starrocketry.com), for introducing me to a propulsion system that might have otherwise been discounted as too complex or troublesome. Lastly, the biggest pat on the back has to go to my dad Robert, since his genetic influence on my problem solving and engineering abilities impacts most everything I do, one way or another. "It's in the genes!"

If you have the opportunity to watch some Hypertek Hybrid action, by all means.... go watch. But beware, Nitrous Oxide in this form is addicting, and hybrid flights are contagious! "Hyper Active" is only one of my symptoms!

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