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Aerodynamic design validated and new understanding of thermal material properties
gained.
April 20, 2012
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 aerodynamically-controlled
flight 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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