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Falcon Hypersonic Technology Vehicle HTV-2

At 0745 PST on 11 August 2011, DARPA commenced the second test launch of the HTV-2. A Minotaur IV vehicle successfully inserted the aircraft into the desired trajectory. Separation of the HTV-2 from the Minotaur carrier vehicle was confirmed by rocket cam and the aircraft transitioned to Mach 20 aerodynamic flight. More than 9 minutes of data was collected before an anomaly caused loss of signal. Initial indications were that the aircraft impacted the Pacific Ocean along the planned flight path.

The Falcon HTV-2 program was an innovative research and development joint venture of DARPA and the Air Force to develop and demonstrate hypersonic technologies to help achieve a prompt global-reach capability. The X-41 involved an experimental maneuverable re-entry vehicle carrying a variety of payloads through a suborbital trajectory, and re-entering and dispersing the payload in the atmosphere. Like its HTV-1 predecessor, the system would reach Mach 22 speed, and then finish its one-hour plus mission in the Pacific Ocean. For the second glided demonstration, initially scheduled for 2008 or 2009, the Falcon HTV-2 featured a different structural design, enhanced controllability, and higher risk, performance factors during its high-speed journey.

The Defense Advanced Research Projects Agency (DARPA) and United States Air Force (USAF) conducted two experimental flight tests of the Hypersonic Technology Vehicle 2 (HTV-2). Flight testing the test bed vehicle would support US development of hypersonic technologies for a wide variety of future aerospace capabilities currently unavailable. Both flight tests would be launched from Vandenberg Air Force Base, California, using existing rocket booster systems. Following booster separation, the HTV-2 vehicle would glide at hypersonic velocities at more than 13,000 miles per hour in the upper atmosphere above the Pacific Ocean, prior to impact in the Broad Ocean Area north of the US Army Kwajalein Atoll/Ronald Reagan Ballistic Missile Defense Test Site located in the Republic of the Marshall Islands. HTV-2 would reach its destination in less than 30 minutes, a total distance from lift-off to impact of about 4,100 nautical miles.

On 28 June 2008 a sole source contract [Solicitation Number: SMC-XR-HTV-08] provided for modification of the current design to reflect weaponization, fabrication, delivery, and flight test support of a hypersonic Payload Delivery Vehicle. The requested vehicle will be manufactured in accordance with the Hypersonic Test Vehicle (HTV-2) design developed for the FALCON program by Lockheed Martin. SMC/XR required that the payload delivery vehicle share the aerodynamics, thermal protection, avionics, and structural design developed by Lockheed Martin under the FALCON program so the first two flights under the FALCON program can be used to qualify the payload delivery vehicle's structural design specifications, avionics, and flight software. The HTV-2 was then under final assembly and scheduled to fly twice in 2009. SMC/XR requires a third vehicle be procured by SMC/XR and be ready for a 2010 flight demonstration. The scope of the contract includes the redesign of the interior structural bulkheads to accommodate an 800 to 1000 lb payload, fabrication and delivery of the flight vehicle (including all necessary structural components, special tooling, software and avionics), and flight test support (including integration support at the launch facility, range support for the flight test, and data analysis associated with flight test performance).

HTV-2 incorporates an advanced aerodynamic configuration, advanced thermal protection systems, and improved guidance, navigation and control systems. The HTV-2 detailed design has been completed and an aeroshell prototype fabricated. In Phase III of the program DARPA will begin fabrication, assembly and integration of two HTV-2s and conduct two HTV-2 flight tests in 2009.

The key technical challenges and achievements of the HTV-2 program are the design of an innovative high lift-to-drag aerodynamic shape, advanced lightweight but tough thermal protection structures, materials and fabrication technologies, autonomous hypersonic navigation guidance and control systems, and an autonomous flight safety system.

Critical enabling technologies include elements necessary for hypersonic aerothermodynamics, high-temperature materials and structures, the navigation guidance and control system, and thermal protection techniques. The program demands a multidisciplinary approach and relies on expertise in such areas as aerothermodynamics, materials science, hypersonic navigation, guidance and control systems, endo- and exo-atmospheric flight dynamics, telemetry, range safety analysis, and space launch.

Leveraging technology developed under the Hypersonic Flight (HyFly) program, Falcon will address the implications of hypersonic flight and reusability using a series of hypersonic technology vehicles (HTVs) to incrementally demonstrate these required technologies in flight. The HTV-2 program will demonstrate enabling hypersonic technologies for future operational systems through rocket-boosted hypersonic flights with sufficient cross-range and down-range performance to evaluate thermal protection systems, aerodynamic shapes, maneuverability, and long-range communication for hypersonic cruise and re-entry vehicle applications.

The Defense Advanced Research Projects Agency (DARPA) launched its Falcon Hypersonic Technology Vehicle 2 (HTV-2) at 16:00 PDT on 23 April 2010. Approximately 9 minutes into the mission, telemetry assets experienced a loss of signal from the HTV-2. Preliminary review of technical data indicates the Minotaur Lite launch system successfully delivered the Falcon HTV-2 glide vehicle to the desired separation conditions. The launch vehicle executed first of its kind energy management maneuvers, clamshell payload fairing release and HTV-2 deployment. An engineering team is reviewing available data to understand this event. This flight represented many historic firsts for both the launch system and the HTV-2 vehicle. Three test ranges, six sea-based and two airborne telemetry collection assets were employed and operational on the day of launch. Technical data collected during the flight will provide insight into the hypersonic flight characteristics of the HTV-2. The first launch of the Minotaur IV Space Launch Vehicle had been scheduled to occur 20 April 2010 at noon PDT from Vandenberg Air Force Base, Calif.

The Engineering Review Board concluded the Review of HTV-2 Second Test Flight on April 20, 2012. The board found the aerodynamic design validated and new understanding of thermal material properties was gained. Following an extensive seven-month analysis of data collected from the Aug. 11, 2011 , second flight of DARPA's Hypersonic Technology Vehicle (HTV-2), an independent engineering review board (ERB) investigating the cause of a flight anomaly completed its report. The findings of the ERB validated the vehicle's aerodynamic design and uncovered new information regarding the thermal material properties of the vehicle.

"The greatest achievement from Flight Two, which the ERB's findings underscored, was that we successfully incorporated aerodynamic knowledge gained from the first flight into the second flight," said Air Force Maj. Chris Schulz, DARPA program manager, who holds a doctorate in aerospace engineering.

A technology demonstration and data-gathering platform, the HTV-2's second test flight was conducted to validate current models and increase technical understanding of the hypersonic regime. The flight successfully demonstrated stable aemdynamically-controlled fllight at speeds up to Mach 20 (twenty times the speed of sound) for nearly three minutes. Approximately nine minutes into the test flight, the vehicle experienced a series of shocks culminating in an anomaly, which prompted the autonomous flight safety system to use the vehicle's aerodynamic systems to make a controlled descent and splashdown into the ocean.

"The initial shockwave disturbances experienced during second flight, from which the vehicle was able to recover and continue controlled flight, exceeded by more than 100 times what the vehicle was designed to withstand," said DARPA Acting Director, Kaigham J. Gabriel. "That's a major validation that we're advancing our understanding of aerodynamic control for hypersonic flight." The ERB concluded that the "most probable cause of the HTV-2 Flight 2 premature flight termination was unexpected aeroshell degradation, creating multiple upsets of increasing severity that ultimately activated the Flight Safety System."

Based on state-of-the-art models, ground testing of high-temperature materials and understanding of thermal effects in other more well-known flight regimes, a gradual wearing away of the vehicle's skin as it reached stress tolerance limits was expected. However, larger than anticipated portions of the vehicle's skin peeled from the aerostructure. The resulting gaps created strong, impulsive shock waves around the vehicle as it travelled nearly 13,000 miles per hour, causing the Vehicle to roll abruptly. Based on knowledge gained from the first flight in 2010 and incorporated into the second flight, the vehicle's aerodynamic stability allowed it to right itself successfully after several shockwave-induced rolls. Eventually, however, the severity of the continued disturbances finally exceeded the vehicle's ability to recover.

According to Schulz, "HTV-2's first flight test corrected our models regarding aerodynamic design within this flight regime. We applied that data in flight test two, which ultimately led to stable aerodynamically controlled flight. Data collected during the second test flight revealed new knowledge about thermal-protective material properties and uncertainties for Mach 20 flight inside the atmosphere, which can now be used to adjust our assumptions based on actual flight data and modify our modeling and simulation to better characterize thermal uncertainties and determine how to assess integrated thermal systems."

Aerodynamic assumptions and extrapolations from known flight regimes proved inadequate when preparing for HTV-2 inaugural flight test. The data from second flight revealed that extrapolating from known flight regimes and relying solely on advanced thermal modeling and ground testing could not successfully predict the harsh realities of Mach 20 atmospheric flight.

"A group of nationally-recognized experts from government and academia came together to analyze the flight data and conduct extensive additional modeling and ground testing for this review," Schulz said. "The result of these findings is a profound advancement in understanding the areas we need to focus on to advance aerothermal structures for future hypersonic vehicles. Only actual flight data could have revealed this to us."

Moving forward, the HTV-2 program will incorporate new knowledge gained to improve models for characterizing thermal uncertainties and heat-stress allowances for the vehicle's outer shell. The remediation phase will involve further analysis and ground testing using flight data to validate new tools for this flight regime. The ERB findings and remediation phase efforts will inform policy, acquisition and operational decisions for future Conventional Prompt Global Strike initiatives executed by the Office of the Secretary of Defense, Acquisition, Technology & Logistics, Strategic Warfare directorate--the goal of which, ultimately, is to have the capability to reach anywhere in the world in less than one hour.

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