Scratch - Extreme Test Vehicle (ETV) {Scratch} Original Design / Scratch Built

Contributed by Jack Caynon

Manufacturer: Scratch
(Contributed - by Jack Caynon - 05/24/08) (Scratch) Extreme Test Vehicle

Brief:
Rocket designed to handle the most powerful 3-inch motors, commercial and research, notwithstanding.

Construction:
The Extreme Test Vehicle (ETV) is a scratch design that won't shred while flying any 3-inch commercial or research motor. To accomplish this mission parameter, this rocket is equipped with the aluminum fin can made by Hawk Mountain for its four inch tubing. In addition, the rocket has a clever 0.5 inch thick aluminum ring for motor retention that is held in place by all-thread and three nuts. The airframe tubing is Hawk Mountain's Extreme II tubing. This tubing is designed for high speed flight using high thrust M or N motors. Finally, the rocket has a Hawk Mountain 4 inch fiberglass ogive nose cone with a 54mm tube inside it to hold my Walston tracking transmitter.

There were no anticipated issues with this particular rocket.

The recovery system is the drogue-to-main style with a design similar to most of my other rockets. The rocket also has an 8 inch altimeter bay between the fore and aft recovery bays. The altimeter coupler is in the middle of the stack with the drogue chute in the recovery bay below and the main in the recovery bay above the altimeter coupler. Both recovery bays are bolted to the altimeter coupler.

With a maximum launch weight of 31 pounds, standard Blacksky rail guides are sufficient. Two rail guides are screwed through the airframe into the plywood centering rings around the motor mount tube in the motor section. A rail length of 8 feet is required on a suitable launch pad.

The ETV was fairly easy to build, particularly due to the aluminum fin can.

The airframe tubes are four sections of 4 inch fiberglass tubing. The altimeter bay is 4 inch fiberglass coupler tubing with a small, 4 inch piece of airframe tubing to join the main chute section and the drogue sections of the airframe.

The centering rings are 1/2 inch aircraft plywood. There are a total of three centering rings along the length of the motor tube. One is at the bottom and seals the airframe and provides a support for the all-threads to bolt on the motor retainer. The second centering ring is at the forward edge of the fin can. The third ring is at the bottom of the anti-zipper coupler tube at the top of the motor section airframe.

The recovery system deployment sequence starts when the PML Co-Pilot detects apogee. When apogee is detected, the Co-Pilot triggers the Drogue Secondary Pyrotechnic Device at the top of the lower recovery bay. This device pressurizes the Recovery Bay, separating the Recovery Bay from the Motor Section and forces the Drogue Parachute and Drogue Recovery Harness out into the air stream. The airframe then descends at approximately 40 fps on a 24 inch drogue parachute. At 800 ft, the Co-Pilot triggers the Main Parachute Pyrotechnic Device located at the bottom of the main chute recovery bay, separating the nose cone from the top of the main chute recovery bay and forcing the main parachute and main recovery harness out into the air stream. The airframe then descends to a landing at approximately 17 fps on the SkyAngle Classic I 60-inch parachute.

There are two recovery bays, the drogue bay (DB) and the main chute bay (MCB). The DB is located between the motor section and the altimeter bay. The bay is bolted to the Altimeter Bay with three #6-32 bolts into blind nuts on the inside of the bay. The MCB is secured to the top of the altimeter bay with three #6-32 bolts into blind nuts on the inside of the bay. The nose cone is secured from drag separation with two shear pins. The shear pins are #2-52 nylon bolts that are inserted through two holes in the MCB and the nose cone. Both recovery bays are vented with one 1/4 inch hole in each to equalize pressure inside the bays during the boost phase of the flight.

The drogue parachute is a b2 Rocketry Company's SkyAngle Cert-3 Drogue parachute. The main parachute is a b2 Rocketry Company's SkyAngle Classic I 60-inch parachute providing a descent rate of approximately 17fps.

The harnesses are made from 1 inch wide tubular nylon from Rocketman. The ends near the pyrotechnic charges are encased in Kevlar® sheathing to protect the nylon. The drogue harness is 20 feet long and the main chute harness is 25 feet long.

The drogue harness mounting hardware is a U-bolt at the anti-zipper coupler at the top of the motor section and a welded eye bolt that is anchored to the altimeter bay aft fiberglass bulkhead. The harness clips to the mounts using a pair of 1/4 inch quicklinks. The main harness mounting hardware is a Kevlar® cable on the bottom of the nose cone and a welded eye bolt anchored to the altimeter bay forward bulkhead. The harness clips to the mounts using a pair of 1/4 inch quicklinks.

Pyrotechnic devices are 4F black powder contained during the flight in 2.5 gram urethane containers and ignited by Daveyfire electric matches.

(Scratch) Extreme Test Vehicle

Flight and Recovery:
For its maiden flight, I chose the largest 3-inch commercial motor I could find: the Animal Motor Works M2500 Green Gorilla in the 75-7600 case. This case is 40.9 inches long and when it's filled with propellant, that motor weighs a ton! In fact, we're talking about 16.5 pounds for both the propellant and the casing, which is amazing given that the rocket's dry weight is 14.312 pounds.

We decided go day would be Sunday, May 18, 2008. At 1pm, the temperature was 80 degrees at Oregon Rocketry's Brothers, Oregon, launch site. My friend, David Holloway and I lifted the ETV onto our shoulders and carried it out to the M motor launch pad we call "Quadzilla." We slid the rocket onto the rail and switched on the altimeter. Then we lifted the rocket into launch position, inserted the igniter, and checked for continuity. When we were satisfied that the circuit was complete, we returned to the flight line.

Moments later, the LCO announced the flight and performed the countdown. Then he flicked the switch and a second later a puff escaped from the rear of the motor, followed by a column of emerald flame and billowing white smoke as ETV catapulted off the rail into the sky! The rocket flew straight and true on a tight column of smoke with an angry roar for a second or so. Then it grew silent, like the quiet before the storm. At first, I thought the motor had burned out too early. Then, we heard a loud boom, followed by the continuation of the motor's roar. I grinned from ear-to-ear when I realized that I had heard my rocket create a sonic boom as it continued to soar high into the heavens.

(Scratch) Extreme Test Vehicle

Finally when it achieved apogee, the rocket separated and ejected the drogue. The ETV returned to Earth in a controlled descent until it reached 800 feet and ejected the 60-inch SkyAngle parachute from the main chute compartment. The rocket drifted slowly toward the ground and landed about a mile away from the flight line. We drove on a fortuitous dirt road up to the ridge where the rocket had landed and used the tracker to locate the rocket. We found the rocket undamaged and looking good after returning from an altitude of 18,313 feet.

The M2500 Green Gorilla motor sent the rocket streaking skyward at better than Mach 1.6 and the rocket returned unscathed. The fin can easily handled the flight and the rocket flew almost like it was guided by a laser beam so the fins are true. What is so impressive about the ETV is that it vastly outperformed the RockSim simulation by 2000 feet or so. The rocket was very close to our waiver so this is a fast rocket that may require a window if you use a really hot research load.

Summary:
The Hawk Mountain fin can and Extreme II tubing are very impressive and apparently can handle anything the mightiest 3-inch or 4-inch motors can throw at them. If you want a great rocket that can fly fast and high with big 3-inch motors and hold its own with rockets that fly at BALLS, the Extreme Test Vehicle most definitely fits the bill.

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