UNITY IV RECOVERY SYSTEM

Brian Williams*, Paul Neilson*, and Casey Hatchr, Utah State University

Logan, Utah

Jeff Gessaman Woodword Governors Longmont , Colorado

Kent Anderson Puget Sound Naval Shipyard

Bremenon, Washington

Abstract

In March 1992, the Utah section of the American Institute of Aeronautics and Astronautics (AIAA) created a consortium of four Utah universities to provide students with education and experience in hybrid rocket propulsion. During the past few years students, faculty, and local AIAA members from various industrial corporations have worked on the design and testing of a hybrid rocket. An area of major importance to the design is the recovery system. With the assistance of Sandia National Laboratory, a recovery system was developed. Several tests were performed in preparation for the flight test on a March 1995 launch of the hybrid rocket. This paper presents a background of the hybrid rocket, recovery system design, and recovery system test results.

1.0 Introduction

The Unity IV hybrid rocket project was created by the Utah Section of the American Institute of Aeronautics and Astronautics (AIAA) in the spring of 1992. The project consists of designing a hybrid sounding rocket and all the required subsystems to launch and fly it and then return it safely to the ground. The Unity IV design team is a consortium of four northern Utah universities, hence the name Unity

IV. The design team consists of students and faculty from Utah State University (USU), University of Utah (UoU), Brigham Young University (BYU), and Weber State University (WSU). By combining the expertise of the members of each of these universities a better design could be produced. The design team also received assistance from local AIAA members as well as industry in refining and improving the design.

The Unity IV project is a two stage project. The first stage is to produce a small scale hybrid rocket using gaseous oxygen as the oxidizer to lift the rocket to an altitude of approximately 4,500 feet above the ground. The second stage of this project will be to develop a larger liquid oxidizer rocket which can deploy small payloads at an altitude of about 130,000 feet. This paper discusses only the recovery system for the first stage (gaseous oxygen) of the project.

The task of developing a recovery system was given to USU and with the assistance of Sandia National Laboratory (SNL), the design was refined and ground testing has been performed. This paper will discuss hybrid rocket, the design of the recovery system, and the associated ground testing.

Graduate Student, Mechanical & Aerospace Engineering, Member AIAA

Student, Physics Department, Member AIAA

Copyrighta 1995 by the American Institute of Aeronautics and Astronautics, Inc. All rights resewed.

4 3

2.0 Rocket Background

The Unity IV rocket stands 13 feet tall and is 8.625 inches in diameter (see Figure 1). Its initial wet mass is estimated to be 120 lb,. The airframe consists of graphite filament-wound composite sections. This airframe accommodates the hybrid motor, recovery system, and the electronics required to acquire, store, and transmit the data obtained during the flight test. The rocket experiences an initial thrust of 700 lb, which decays exponentially over the 18 second bum. After 20 seconds, the rocket reaches its maximum altitude of approximately

ALT 63 COX BOTTLE

OXYGEN OPERATION VALVE-

MOTOR W E

BOAT T A I L TAPER

Figure 1. Schematic of Unity IV hybrid rocket.

4,500 feet above the ground. At 150 ft after apogee, the recovery system is deployed for a safe return of the rocket.

The hybrid motor consists of a solid propellant, hydroxyl terminated polybutadiene (HTPB), which contains no oxidizer, and an oxygen tank which supplies the gaseous oxygen (GOX), or oxidizer, to the solid propellant during the burn. Hybrids have many advantages over conventional rocket propulsion. They are inherently safe due to the fact that the HTPB fuel is inert without a high concentration of an oxidizer and an energy source present. Also, they can be throttled as well as shut off and do not produce environmentally hazardous exhaust products (if pure oxygen is used as the oxidizer).

The electronicslflight instrumentation for the rocket collects data, stores it in a RAM card, and transmits it to a ground receiver. The motor measurements which will be taken during the flight include pressure and temperature before the throat of the injector, and chamber pressure. An on board altimeter will record the altitude data during the flight. There is also a receiver with the electronics which will allow for a command recovery signal to be transmitted from the ground to activate the recovery system deployment in the event that the autonomous control system fails.

3.0 Description of Recoverv Svstem

The Unity IV recovery system consists of a two-stage, three-parachute configuration located in the top of the rocket. During the rocket's ascent, an Adept Rocketry OBC2B on board computer with recording altimeter will be monitoring and recording altitude data at a rate of 10 samples per second. At approximately 150 vertical feet after apogee, the OBC2B will send a signal to a Pacific Scientific cartridge actuated device (pyrotechnic ram) which will separate the nose cone from the rest of the rocket. As the nose cone trails behind the rocket, a 0.125 inch diameter steel cable, connecting the drogue chute (which is stored in the nose cone) to the remainder of the rocket, will be deployed. Once the steel cable is fully deployed, the 3.5 ft diameter drogue chute will be extracted from the nose cone. A 10 lb, nylon line is connected from the vent of the drogue chute to a 24 inch diameter nose cone chute;

the task of this smaller chute will be return the nose cone safely to the ground. The drogue chute will slow the rocket to approximately 100 ftls.

At the same time that the signal is sent from the altimeter to the pyrotechnic ram, a signal will be sent initiating a timer. Ten seconds after the nose cone separation, another signal will be sent to a Holex cable cutter to cut the steel cable connecting the drogue chute to the rocket. As the drogue chute is released from the rocket, it will extract the 20 ft diameter main chute. The main chute will slow the rocket to its final descent rate of approximately 16 ftls.

As a backup, the altimeter can have two altitude settings: the first one was at 150 ft after apogee and the second one will be set at 500 ft AGL (above ground level). This second switch will be used if the rocket falls to less than the 500 ft AGL before the 10 second timer is up. At that point, the altimeter will send the signal to the cable cutter, releasing the drogue chute and extracting the main chute.

As an additional backup system and recommend by the Air Force's Utah Test and Training Range (UTTR) Range Safety Officer, the on board electronics will include a receiver which can accommodate command signals from the ground to deploy the recovery system. In the event that the altimeter computer does not send a signal to separate the nose cone, a signal can be sent manually from the ground to perform the task. Also, another signal can be sent via the receiver to release the drogue chute and deploy the main chute.

At the completion of the flight, altitude data will be transferred from the OBC2B computer to a personal computer for analysis. Tracking data from UTTR radar stations will also be available for comparison.

3.1 Recovery Svstem Design Procedure

Since the USU recovery system team had practically no experience with recovery systems, outside technical assistance was sought. Don Johnson, a retired expert in the field, came to Utah to present lectures on the basics of recovery systems. Members of the USU team then went to SNL to receive additional instructions. On returning to Utah,

the USU team then proceeded to design the recovery system. After the design was completed, members of the USU team again returned to SNL for a critical design review (CDR) and also to assist in the construction of the parachutes and their associated packing bags.

3.2 Recovery Svstem Design

The interface between the recovery system and the rocket consist of two nose conelrocket body interface rings (Figure 2, items A and B), a base plate (Figure 2, item C), a cylindrical sleeve (Figure 2, item D), and four main chute bridal attachments (Figure 2, item E).

The base plate is made of 1 inch thick aluminum with a 1.75 inch flange on its outside edge; this flange is designed to increase the bonding (epoxy) area between the base plate and the rocket body. The steel cable connecting to a swivel (which is attached to the drogue chute) goes through two holes located approximately 112 inch from the outside edge and the cable cutter is placed on the backside of the plate. Four slots, in which the main chute bridals pass through, are located 90" apart from each other. Four dumbbell-shaped attachments slide through the main chute bridals (after they have passed through the base plate) and bolt to the bottom of the plate. This plate is designed to withstand a maximum snatch force of 2400 lb,, either from the drogue or main chute.

A thin, aluminum, cylindrical sleeve attaches to the base plate via a small grove in the plate and is bonded at its other end to the bottom nose cone ring (item A). The purpose of the sleeve is to provide an unobstructed path for the main chute bag while it is being extracted from the rocket.

The bottom nose cone ring is bonded to the top of the rocket body and the pyro ram is secured to this ring 180" from the hinge slot. It should be noted that neither ring is subjected to any parachute forces (for example snatch force) and both rings are made of aluminum. Two holes, which allow passage of the steel drogue chute cable, are aligned with the holes on the base plate.

The top nose cone ring (item B) is bonded to the inside of the nose cone. Both rings are machined so that they mate to each other without any slippage.

Figure 2. Photograph of the recovery systerntrocket interfacing (items A-E) and the main chute packed in its bag (item F).

A detachable hinge is bolted to this ring and slides into the corresponding slot on the bottom ring. A set screw, located 180" from the hinge and aligned with the pyro ram, ensures a tight contact between the two rings. A brass shear pin goes through the nose cone, through the top ring, and into the bottom ring. This shear pin restricts any movement between the two rings. The drogue chutelnose cone chute bag is attached to the top of the top ring and the nose cone chute attaches to an I-bolt located at the tip of the nose cone.

The bag containing the drogue and nose cone parachutes is approximately 5.75 inches in diameter by 5 inches in height. The total mass of the package is approximately 1.2 lb,. This results in a calculated packing density of 16 lb,/ft3.

Table 1 shows a breakdown in mass of the recovery system components.

4.0 Recovew Svstem Testing

With the assistance of SNL, the recovery The recovery system ground testing consists team packed the main parachute inside of its bag. of four tests: a main chute extraction test, a nose Figure 2 shows the completed package (item F). The cone separation test, a drogue chute extraction test, final bag size is 6.25 inches in diameter by 10.875 and a main chute bag extraction test. All tests were inches in height. Contained in this chute bag is the judged on a "pass/failW basis. 20 ft diameter canopy and twenty 20-ft long suspension lines. The total weight of the bag and contents was approximately 7 lb,; this resulted in a calculated packing density of 37 lb,/ft3.

Table 1. Mass breakdown of recovery system components.

Component

Drogue chute, nose cone chute, and bag

Main chute and bag

Top ring wlhinge and set screw

Bottom ring

Aluminum sleeve

Base plate

Four attachments for main chute bridals

Swivel, pyro ram, and cable cutter

Total

The suspension lines were secured at six-inch lengths to the bag with a single #5 nylon chord. This chord has a tensile strength of 50 lb,. As the lines were pulled from the bag, it was observed on the scale that approximately 50 lb, was indeed needed to break the attachments. Even though this was greater than the 40 lb, specified as the maximum tolerated force, it was determined that these attachments would be dynamically loaded and also loaded only one at a time. The canopy was easily extracted from the bag (negligible observed force) and the vent break chord, which was a single #5 nylon chord, also easily broke.

Due to the unexpected 80 lb, required to cut the first nylon chord, the 400 lb, nylon chord was replaced with a double #6 nylon chord. A separate pull test showed that only 35 lb, would be required to cause the first two knives to cut this chord. Overall, with the modifications made to the system, the main chute extraction test was considered successful and deemed a "pass".

4.2 Nose Cone Separation Test

4.1 Main Chute Extraction Test

The goal of this test was to determine the force required to extract the main parachute (suspension lines and canopy) from the chute bag. The suspension lines were securely mounted to a spring scale and the top of the chute bag was horizontally attached to an electric hoist. This hoist supplied a steady pulling rate. Also, a tape measure was hard-mounted to the scale and allowed to extend with the bag as the lines and canopy were pulled out. A video camera was used to record the force and distance as the system was extracted.

Initially, it was stated that if a force greater than 40 lb, was observed, then the test would be a failure. The first two knife cutters, which cut a nylon chord (400 lb, tensile) which hold the suspension lines in the bag, required approximately 80 lb,. The dynamic loading experienced from the spring-action after these cutters cut caused the four knife cutters used to release the bag lacing to also cut. Therefore, within a fraction of a second, all six cutters had cut their corresponding chords.

The purpose of this test was to determine if the pyro ram could shear the shear pin and separate the nose cone from the rocket under representative aerodynamic loading. A nose cone was placed in a 4 ft by 4 ft cross section wind tunnel at USU for the test. The nose cone was attached to a 10 in. long section of airframe. Contained in the nose cone and in the airframe section were flight-ready aluminum nose cone rings which support the recovery and separation systems. The separation device was the Scientific Pacific pyrotechnic ram and was attached to the airframe section (bottom) ring. This airframe section was then attached to a pre-existing sting in the wind tunnel. A safety cable was attached from the sting to the nose cone to ensure that the nose cone would not blow down the wind tunnel and cause damage. The nose cone was oriented so that it would rotate in a horizontal plane; this was to negate as much as possible the effects of gravity.

The wind tunnel was then turned on and set at maximum velocity (approximately 205 ftls or 140 mph). Two video cameras were located at different locations to record the separation event. A single 9 volt battery was used to supply the 3.5 amps needed to activate the pyrotechnic ram.

After the video tapes were reviewed at a slower speed, it was determined that the separation device did successfully deploy the nose cone and the test was deemed a "pass". In fact, the pyrotechnic ram separated the nose cone from the airframe section with such force that the nose cone was ejected approximately 6 inches forward (until the safety cable restricted any further forward movement) and then fell towards the side of the airframe.

4.3 Drogue Chute Extraction Test

The purpose of this test was to determine the force required to extract the drogue and nose cone parachutes from their bag. A single knife cutter cuts a chord which holds the bag closed. One member of the recovery team held securely to the bag while another person attached the knife cutter to a scale. As force was carefully applied to the cutter in an attempt to eliminate dynamic effects, it was observed that approximately 50 lb, was required to cut the chord. Even though this was greater than the desired 40 lb,, this knife cutter would experience dynamic loading and was therefore considered acceptable.

To prove this, a small length of chord used to hold the bag closed was threaded through the knife cutter. One person held onto the chord while another held on to the cutter. Then, the cutter and chord were jerked apart and the chord was easily cut.

The scale was then attached to the end of the drogue chute's suspension lines and steadily pulled on. Due to the low packing density of this bag, a negligible force was observed while the drogue and nose cone parachutes were extracted. Overall, this test was considered a "pass".

4.4 Main Chute Bag Extraction Test

The goal of this test was to determine the force required to extract the main chute bag from the rocket and also determine if there were any obstructions to the process. The main chute bag was placed inside of the assembled interfacing (i.e. bottom nose cone ring, aluminum sleeve, and base plate) without securing the bridals to the base plate. A scale was then attached to the top of the bag and it was slowly pulled from the canister. This test was repeated several times and always the maximum force was less than 30 Ib,. Therefore, this test was deemed a "pass".

5.0 Flight Test

The rocket launch and recovery system flight test was initially scheduled for Juiuary 18, 1995. Due to problems with the ignition system, this launch date was postponed. As of the writing of this paper, the launch date has been rescheduled for March 28, 1995.

6.0 Acknowledgments

The authors of this paper and the members of the USU recovery system team would like to acknowledge the Parachute Technology and Unsteady Aerodynamics section of Sandia National Laboratory for their technical and financial assistance. We would also like to acknowledge the technical assistance of Don Johnson, formerly of SNL; the financial support and leadership of Dr. Gil Moore, who was spear- heading the Unity IV project; the donation of the pyrotechnic separation devices by Pacific Scientific in Chandler, Arizona; and the donation of the cable cutter by Space Dynamics Laboratory in Logan, Utah.