HiSentinel80: Flight of a High Altitude Airship

HiSentinel80: Flight of a High Altitude Airship Mr. Steve Smith 1 and Mr. Michael Fortenberry 2 Southwest Research Institute, San Antonio, Texas, 78228 Mr. Michael Lee 3 and Mr. Ricky Judy 4 U.S Army Space and Missile Defense Command/Army Forces Strategic Command Huntsville, Alabama, 35807 The first flight of the HiSentinel80, an unmanned high altitude airship, completed a successful test Nov. 10, 2010; launching from Page, Arizona, and tracked northeast toward Utah and Colorado. The payload, part of a U.S. Army Space and Missile Defense Command/Army Forces Strategic Command program, was recovered north of Monticello, Utah, on Nov. 11. The purpose of the test flight was to obtain performance data on the high altitude airship, as well as test various payload capabilities. The objective of the test was to demonstrate engineering feasibility and potential military utility of high altitude systems for persistent payload operations. The HiSentinel80 was aloft for eight hours at an altitude of 66,300 feet collecting valuable command and control and payload connectivity data before flight termination. The Prime Contractor for the HiSentinel effort is Southwest Research Institute (SwRI) and the sub-contractor is Aerostar International, Inc. The team of SwRI and Aerostar launched and recovered the airship. SwRI designed the airship and provided the telemetry, flight control, power, and propulsion systems. Aerostar fabricated the hull and supported in the integration and test flight. The SwRI/Aerostar Team developed the launch system, provided facilities, and launched and recovered the airship. The HiSentinel system is capable of lifting small to medium payloads (20 200 pounds) to high altitudes (>60,000 feet) for a duration of 30 days or greater. HiSentinel80 is 207 feet long and 45 feet in diameter and is designed to cruise at an altitude of 65,000 feet, well above commercial airspace. The HiSentinel80 airship is designed to launch similar to a weather balloon, taking the familiar airship shape as the vehicle reaches its mission altitude. At mission completion, the payload is released from the hull and returns to the ground by parachute and can be refurbished. The hull, or vehicle body, is made of low-cost disposable material designed not to be recovered after a mission. For the purpose of this flight demonstration, the airship hull was recovered for inspection south of Grand Junction, Colorado. This paper will include a technical overview of the HiSentinel80 airship development. An overview of results from the HiSentinel80 flight will be presented. 1 Sr. Program Manager, R&D, Space Science and Engineering, P.O. Box 28510, AIAA Member 2 Principal Engineer, Space Science and Engineering, P.O. Box 28510, AIAA Member 3 Space Systems Analyst, Space Technology Division, P.O. Box 1500, AIAA Member 4 Chief, Space Technology Division, P.O. Box 1500, AIAA Member 1

I. Introduction The concept of a stratospheric or high altitude platform has been around almost as long as stratospheric free balloons. Airships are defined as Lighter-Than-Air (LTA) vehicles with propulsion and steering systems. Designing LTA vehicles to operate in the stratosphere is very challenging due to the extreme high altitude environment and is significantly different than low altitude airship designs. This paper will include an overview of the HiSentinel which is an USASMDC/ARSTRAT programs. The HiSentinel program is a spiral development for a family of tactical launch, long-endurance autonomous, stratospheric airships. The low-cost system will be capable of lifting different payloads to high altitudes for durations of 30 days or greater. These kinds of stratospheric airships will benefit the warfighter by providing a rapid response deployable communication relay, netcentric communications, and a persistent intelligence, surveillance, and reconnaissance capability. A variety of communications and sensor payloads were incorporated during the flight demonstration to show military utility. The purpose of the HiSentinel80 flight was to demonstrate the engineering feasibility and potential military utility of an unmanned, un-tethered, gas-filled, solar powered airship that can fly at greater than 60,000 feet. The HiSentinel80 development team consisted of Southwest Research Institute (SwRI) and Aerostar International, Inc. SwRI designed the airship and provided the telemetry, flight control, power, and propulsion systems. Aerostar fabricated the material and hull and supported the integration and test flight. COLSA Corporation and USASMDC/ARSTRAT provided payload integration, payload testing and field test support. ATA and the AFRL balloon program provided downrange hardware and telemetry and tracking support. II. HiSentinel History The HiSentinel program is a family of high altitude airships to provide persistent communications and ISR capability to the warfighter. The HiSentinel program is to develop a complete system to include the airship, ground support system, weather support system, vehicle control ground station, payload, and payload control ground station. An airship without a payload will not demonstrate warfighter capability so a complete system is being developed and demonstrated. The following sections describe the history of the HiSentinel program and focus on the results of the HiSentinel80. Six high altitude airship engineering flights (Figure 1) have been conducted by the SwRI/Aerostar Team over the years with five of those flights achieving greater than 65,000 feet altitudes. Figure 1. Present and precursor HiSentinel history. 2

Three of those flights were of the Sounder vehicle design which was the predecessor of the HiSentinel vehicle. The HiSentinel program, funded through USASMDC/ARSTRAT, has conducted three flights. All three of the HiSentinel flights achieved greater than 60,000 ft. altitudes. A. HiSentinel20 The first HiSentinel class flight was the HiSentinel20 (formerly known as CHHAPP). It was jointly funded between SwRI Internal Research dollars and the USASMDC/ARSTRAT and was demonstrated in November 2005 from Roswell, NM. The airship and system were assembled in a 4 month timeframe. This flight vehicle was to demonstrate technical feasibility of a powered airship at an altitude of 74,000 feet. The 146-foot-long airship carried a 60-pound equipment pod (20lbs payload weight) and propulsion system. With the 5 hour technology demonstration flight at high altitude under propulsion, HiSentinel20 became only the second stratospheric airship in history to have flown under propulsion at high altitude (Figure 2). Figure 2. HiSentinel20 Hangar Integration Test and aft looking camera at 74,000 ft. float altitude. Since this flight was for a short duration, a regenerative power system was not utilized and batteries were used for the propulsion and support systems. More detailed descriptions can be found in Reference 1. B. HiSentinel50 HiSentinel50 (Figure 3) was test flown on June 2008 from the Alamogordo, NM area. The HiSentinel50 had a dedicated payload mass of 50 pounds and payload power of 50 watts continuous. For this flight, a General Dynamics communications relay and an ITT high resolution camera was integrated and flown to demonstrate military utility. COLSA Corporation integrated these payloads along with a payload control system. The physical dimension of HiSentinel50 was 178.7 foot long with a diameter of 39.7 ft. 3

Figure 3. HiSentinel50 during Hangar Integration Test and at float altitude of 66,186 ft. The system was designed for an altitude of >65,000 feet with duration of greater than 24 hours. The power system had a regenerative capability for day operations and batteries for night operations. More detailed descriptions can be found in Reference 2. III. HiSentinel80 System Description A. Airframe The HiSentinel80 hull was made from a light weight Vectran based material. It was a derivative of the HiSentinel50 hull material but with additional fibers added in one direction. The airship hull design maintained the same hull loading and Class C shape but the cylindrical center section was lengthened to increase the volume for the increased payload mass. The HiSentinel80 physical dimensions and mass properties are provided in Figure 4. The HiSentinel80 airframe configuration is shown in Figure 5. The aerodynamic shaped hull, with three inflated fins arranged in a Y orientation, make up the airframe assembly. Figure 4. HiSentinel80 physical properties. 4

Figure 5. HiSentinel80 airframe configuration. B. Support Systems Several packaged subsystems are attached to the interior and exterior of the hull. The main three segment, equipment pod is mounted (Figures 6 and 7) on the bottom of the hull, just forward of the airships center of gravity via an adjustable position mounting bar. Figure 2. Equipment pod configuration and 3 pod Flight Control, Payload and Battery sections. Figure 6. Equipment pod configuration and 3 pod Flight Control, Payload and Battery sections. 5

The equipment pod contains most of the airships electronics, including the following subsystems: Payload Flight Controller with interface circuits Primary and backup telemetry modems and antennas Iridium satellite transceiver backup telemetry and control GPS receiver and antenna Eight (8) video cameras and a video signal selection switch S-Band Video Transmitter and antenna Rechargeable Battery packs Electronic Compass module Flight termination control circuits Strobe light FAA Transponder and antenna Recovery parachute ARGOS recovery transmitter Forward liquid ballast container and pump Liquid ballast Helium pressure sensors Temperature sensors Figure 7. HiS80 Equipment pod mated with the payload suspension bar. The aft liquid ballast system is mounted to the hull bottom toward the aft end. It is used for trimming the airship once it is at float altitude. The aft ballast system houses: Ballast container Temperature sensor Pump Liquid ballast Valves 6

The non-pointed, internal photovoltaic (PV) 1.2 kw array provided electric power for the airship and was mounted internal to the hull near the nose (Figure 8). The cells were provided by USASMDC/ARSTRAT and were experimental prototypes. They were mounted such that they were z-folded internal to the hull and deployed automatically and passively during ascent. At float they were in a near horizontal configuration. They were assembled in panels of 92 cells in series to get correct voltage (~120V). The five independent (diode isolated) panels were mounted in parallel to provide the maximum power for these type cells. Figure 8. Internal thin film flexible PV array on manufacturing table and deployed in hull. The propulsion system contained the sub systems: Propulsion Motor with folding propeller Gimbal pitch and yaw mechanism for steering the thrust Compass module Motor control module Flight control interface Temperature sensors All of these systems are packaged to protect the equipment from the extreme stratospheric environment and to provide passive thermal control. Figure 9 provides a functional block diagram of the system. Figure 9. Functional block diagram of the Power System. 7

C. Payload The HiSentinel80 mission payload was provided by USASMDC/ARSTRAT with payload components and payload ground station integrated by COLSA Corporation. The payload was housed in an insulated, cuboid container with dimensions 23 inches by 23 inches by 30 inches which could be slid into the central equipment pod section and locked in place. Passive heating and active, electrical heating maintained internal temperature when the payload was not powered, or as required. Otherwise, power dissipated by the payload during operation maintained the internal temperature. The payload housed a repeater, an Iridium transceiver, a high-resolution camera system, data storage, a high data rate transceiver, GPS, an environmental monitoring package, a computer stack, and a power management and control subsystem (Figure 10). The repeater, transceivers, and GPS systems were connected to antennas mounted externally to the payload. Airship power was fed by the HiSeentinel80 Equipment Pod through the power management and control system and then distributed to the payload components. The payload container had a 6 x6 opening to allow the high-resolution camera access to an external view. The aperture covering was transparent in the visual range. BLOS Communications and Persistent Surveillance Payload Components C2 via SATCOM (Iridium) Comms Repeater (Thales MMAR) Imagery downlink/c2 (L3 West Mini CDL) Imaging System (ITT EOS) PIL Mission Control Figure 10. Payload components and functions. IV. His80 Testing And Verification A. Hull Material Three HiSentinel80 fabric constructions were required and designed to meet mechanical specification requirements for the hull, tape and tape bias fabrics. The hull fabric was a plain weave construction with a leno top beam to add stability to the construction. The warp consisted of a mix of 200 denier Vectran and 30 denier Nylon. The Nylon yarns are paired with every fourth Vectran end, their function was to add stability to the weave construction. The filling yarns are 100% Vectran of 200 denier. The Tape Fabric was a plain weave construction with a leno top beam to add stability to the construction. The warp consisted of a mix of 200 denier Vectran with 30 denier Nylon. The Nylon yarns were paired with every fourth Vectran end, their function was to add stability to the weave construction. The Bias Tape Fabric was a plain weave construction with a leno top beam to add stability to the construction. The warp was compromised of a mix of 200 denier Vectran and 30 denier Nylon. The Nylon yarns were paired with every fourth Vectran end to add stability to the construction. The filling yarns were 100% Vectran of 200 denier. 8

Testing of the materials included both room and low temperature uniaxial, biaxial and creep tests. Radiative property measurements were also performed. Further testing included multiple size inflated test structures to failure to determine overall as fabricated performance. B. Fabrication Verification (Scaled Tests) Plans were developed to perform scaled structural tests to validate integrated as built fabrication methods, processes and quality assurance. Two phases were planned for the testing. The first phase was an indoor test to take the hull up to 14 H 2 0 in controlled steps. The pressure level was chosen to obtain the hoop stress in the half scale hull up to a level close to the maximum load that is expected for the flight hull. The first test structure was built half-scale based on the hull diameter with a foreshortened cylindrical mid-section. The second phase was an outdoor test to burst the hull to determine the actual fabricated structural burst pressure. The second test would follow the first taking whatever corrective actions would be derived from the results from the first test. The objectives of the tests were: 1) Validate installation of accessories (valves, patches, inflation ports, etc.) 2) Identify safety factor of burst stress above in-flight skin stress limit 3) Validation of airship system production processes 4) Observe configuration of half scale solar array in a deployed half scale airship On August 5 th, 2009 the first pressurization test was performed on the half-scale model of the HiSentinel80. Several observations were made during the first test: 1) Spiral warp observed down length of airship 2) Partial delamination of solar array patches 3) Small leaks observed in hull during test primarily in hard fittings 4) Some partial delamination and curling up of seam edges 5) Burst at lower than expected burst pressure As a result of the first test, an intensive effort was undertaken to optimize the seaming parameters of the composite seams and to correct the other identified anomalies observed in the first test. The second test hull was built (Figure 11) and tested. Results were very good with burst of the as fabricated structure with a safety factor of 2 over the anticipated flight loads. Figure 11. Photo of second scale structural test model prior to burst. 9

C. Airframe and Systems Integration (Alamodome) A full inflation and systems integration test was conducted in the Alamodome (Figure12) in San Antonio, TX in August 2010. The purpose of the test was to perform a hull integrity inspection and test, integrate all support system with the airframe and perform system balance testing with all systems on board. Figure 12. HiSentinel80 inflation and systems integration. Due to a limited access schedule for the facility, inadequate time was available for a detailed inspection. As a result, a full 100% inspection of the hull was scheduled to be conducted as soon as another large facility could be found. Use of one of the large airship hangars was arranged with NASA for our use at Moffett Field. The 100% inspection of the hull was conducted during the last week of October and first week of November 2010. 1. Airship Balance During the systems integration of the hull, verification measurements were made to verify the balance and center of gravity as compared to those predicted by the structural design models. The hull was connected to load cells anchored to the floor along the length of the airship. Data was logged for approximately 3 hours. The airship was stable during a 40 minute inflation period. A set of equations were developed to solve for the center of buoyancy and the resultant moment about the center of buoyancy. This effectively removed the unknown of changing buoyant force, since it does not contribute to the resultant moment about the point on which it acts. Performing a least squares solution for the system of equations yielded a calculated center of buoyancy at zbar = 91.89 feet (structural model value = 92.78 ft). A balance mass of 33.92 lbf attached at zbar = 150.4 ft was required to balance the system. All data in these calculations was at differential pressures exceeding 1.5 hpa (0.6 water) the required minimum to eliminate buckling in the conical section at the propulsion strut ends 2. Gas Integrity Pressure data obtained during the Alamodome test indicated that a leak was present in the airship hull and fins at an indeterminate location. Detailed inspection found several areas where waviness in the weave of the Vectran fabric allowed small holes to open up in the base film when the pressure increased to higher leaves. The pressure data showed marked improvement in the pressure loss rate when the holes were covered with repair tape. However, due to scheduled facility availability limitations, it was decided to perform a 100% inspection at another facility. 10

During the downtime of rescheduling use of another facility, the Team modified the fin design, tested scaled fins for structural and fabrication efficacy, manufactured the full scale flight fins and installed the new fins in the hull. 3. Fins Mass and balance calculations indicated that the mass of the tail fins could increase, allowing the use of more robust materials. A modified fin design for the HiSentinel80 was developed, fabricated and tested. The base had a symmetrical airfoil shape with a maximum root width of 4.5 tapering up to 1.3 at the tip with 12 baffles. The fin was 24.5 ft high with a length root of 30.3 ft by length tip of 17.5 ft made of an aerostat material. Testing of the new fin was based on the worst case loading of the hull expected to occur during the day at a temperature of 0 degrees C. A thermal chamber was not used for the tests since room temperatures would result in more conservative performance testing. The purpose of the testing was to establish consistency of stomping or the sealing and attachment process on unaltered hull material. This included the fabrication of samples of the fin composite featuring the same construction as the full scale manufactured fin. Measurements of the actual fin and the manufacturing drawings guided the creation of test samples (Figure 13). Samples were constructed with the same stomp parameters dwell, pressure, and temperature. Tests of the strength of the resulting seams were then conducted. The fabrication process was Figure 3. Model test fin. modified as test results dictated. In addition to the basic design and fabrication verification, repair methods were developed for situations that could require repair as well as additional testing of potential stress risers in the fin. At the conclusion of the scaled fin testing program, new full scale fins were fabricated and installed on the HiSentinel80. After installation, the airship hull was shipped to Moffett Field, CA for 100% hull integrity inspection. D. Hull Integrity Inspection (Moffett Field) As a result of the inflation/integration test held in the Alamodome in San Antonio, a full 100% inspection of the hull required to be performed. A facility large enough was found to conduct the inspection and repair. It was arranged with NASA to make use of one of the hangars located at Moffett Field. The inspection of the hull was conducted during October 25-November 5, 2010 (Figure 14). The hull was inflated with air and helium and detailed leak checking was performed. The tail was also marked for the relocation of one tail strut sleeve. The hull was 100% inspected and all identified leaks were tape repaired. The internal PV array was re-installed in the hull. During this time, as well as with the Alamodome test, great difficulty was encountered in trying to quantifying the overall leak rate for performance predictions with limited sensor data. Variable atmospheric conditions, hull strain and constantly changing differential pressure made the process very difficult. A method was developed by Noll 3 to perform this task which was later verified during flight. The hull was shipped on the evening of Thursday, November 4 th to the launch site in Page, AZ. 11

Figure 14. HiSentinel80 undergoing hull integrity inspections at Moffett Field. V. Pre-Launch Activities A. Pre-Launch Agencies Planning and Coordination The flight was scheduled for launch from the Page, AZ airport during the second week of November. As such, the flight required coordination with a variety of agencies including the FAA for flight in the national airspace, FCC for frequency coordination, the SMDC Technical Manager, Government Flight Representative (GFR) Army Safety for flight test plans and safety, airport management for use of airport facilities and airport flight coordination and Homeland Security for descent and recovery. These were obtained and approved for flight. B. Launch Practice All Team members arrived and were present by November 5, 2010. All flight and ground hardware were readied while the HiSentinel80 airframe was in transit to the launch site. As called for under the HiSentinel plans, a test launch of a mockup HiSentinel hull was readied. The purpose of the practice launch was to build proficiency among all flight and ground operations personnel, identify any operational problems and test plan issues and provide familiarity with all systems that would be incorporated in the actual flight. The practice launch took place on the evening of Saturday, November 6 th. The helium truck was positioned and the launch equipment was rigged for the practice launch. A complete dry run was conducted. The practice hull was taken through three separate let-up maneuvers which duplicated the launch procedures. On the third let-up, the hull restraint P-nut was released and the system was allowed to ascend for ~ 10 ft before the restraint chain became tight halting its further ascent. The hull was then deflated by command and reboxed in its shipping container for reuse at a later date. VI. Flight Operations A. Launch Launch of the HiSentinel80 (Figure 15) took place on November 10, 2010 from the municipal airport at Page, Az. The launch occurred between two large low pressure systems. One low pressure system had passed and the next storm system was approaching from the NW. A very small area of high pressure between the systems was overhead the morning of the launch. Surface forecast winds were predicted to be from the SW, however, a SE wind dominated through the morning due to local terrain effects (cold air draining through the canyons into Page). The team assembled at 0730 GMT and began layout of the system at 1100 GMT once a layout direction was selected. Because of some delays during the preflight checkout, the surface winds at the rescheduled launch time were predicted to be from a different direction about 90 degrees perpendicular to the initial layout. This necessitated 12

a layout direction change which was accomplished expeditiously by the launch personnel. The main hull inflation took 12 minutes and completed at 1632 GMT. Winds at launch were calm at the surface to nearly 200 ft with a cloud cover about 20 to 30% at time of launch with clouds approaching from the NW. The launch then occurred at 1639 GMT with no noted anomalies. Due to the experience with prior flights and the previous practice launches, the let-up and launch process was very smooth with all launch crew and equipment performing flawlessly. Figure 15. HiSentinel80 launch sequence B. Ascent/Trim The ascent rate averaged 1400 feet per minute and the vehicle reached pressurized at 1728 GMT. The winds aloft during ascent were high with a maximum of over 120 knots experienced at an altitude of around 32,000 feet. Even with high winds and wind shear during ascent, the HiSentinel ascent dynamics were controlled and the platform was extremely stable going into float. Once HiSentinel was stable and pressurized at altitude, the trim operation was performed to fine tune the pitch of the pod to a level configuration. The trim was adjusted 12 degrees to a 2 nose up pod orientation by 1807 GMT. C. Float Pressurization of the HiSentinel80 hull went smoothly and matched the pre-flight calculations. The system pressurized quickly starting at 1722 and remained pressurized for 5.8 hours. The average altitude was 66,200 ft with a standard deviation of 250 ft while the hull was pressurized (Figure 16). Unfortunately, the propulsion system experienced a failure during the initial portion of the flight. This failure was later found to be caused by failure of a component on the motor controller. The payload remained on and operational during the majority of the flight and was able to test all functionality successfully. As the pressure decreased during the flight, the tail buckled at 2313 GMT once the differential pressure lowered to the pre-calculated point where the weight of the propulsion unit could not be supported by the internal pressure of the hull. Overall, the pressurized performance and gas mass loss due to problems described earlier with the hull material matched the pre-flight calculations based on the developed models. Valuable command and control and payload connectivity data was collected before flight termination because of range limitations. 13

Figure 16. HiSentinel80 Float D. Descent The hull and pod were successfully separated at 2346 GMT. The pod descended by parachute and landed approximately 30 minutes later. The actual parachute descent matched the prediction closely and the pod came down close to the predicted site. The chase plane observed the landing site for the pod and the pod was successfully recovered the next day with only minimal cosmetic damage to any of the systems. The hull came down more slowly than originally planned due to a failure of one of the redundant deflation valves and landed approximately 90 minutes after the initial termination command. The hull was tracked down to the surface and recovered several days later by a heavy-lift helicopter. VII. Summary The HiSentinel80 is the third test vehicle of the HiSentinel series and the sixth in stratospheric airships, five of which have flown above 60,000 ft. The HiSentinel vehicle can provide an excellent complementary asset for the warfighter. With its fast deployment, flexibility in theater, and minimal logistical trail capability, the HiSentinel can provide a persistent, quick response communications relay and ISR capability. The approach taken under the HiSentinel program will mitigate risk and prove the technology of stratospheric airships to be successful. The HiSentinel Team is committed to the success of the HiSentinel Program. Acknowledgments The authors would like to thank the following: continued funding, contract and technical support of the USASMDC/ARSTRAT and Southwest Research Institute. We would like to thank Aerostar International, Inc. for hull fabrication and ground and flight operational support. Further appreciation is expressed for the payload provision and support of COLSA Corporation, the AFRL-Kirtland for use of their ground station hardware for downrange flight support. Further appreciation is expressed to ATA Aerospace for both invaluable expertise and advice for systems, operations and airship as well as manning of the downrange ground station. References 1 Smith, I. Steve Jr. and Lee, Michael, The HiSentinel Airship, 7th AIAA Aviation Technology, Integration, and Operations Forum, Belfast, Northern Ireland, September 2007. 2 Lee, Michael and Smith, Steve and Androulakakis, Stavros, The High Altitude Lighter Than Air Airship Efforts at the US Army Space and Missile Defense Command/Army Forces Strategic Command, 18th AIAA Lighter-Than-Air Systems Technology Conference, Seattle, WA, May 2009. 3 Noll, James R., Determination of Lift Gas Leakage Rate for a Stratospheric Airship Hull, 11 th ATIO Conference-19 th LTA Conference, Virginia Beach, VA, September 2011. 14