DARPA Engineering Review Board Concludes Review of HTV-2 Second
Test Flight
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Engineering Review Board Concludes Review of HTV-2 Second Test Flight
April 20, 2012
Aerodynamic
design validated and new understanding of thermal material properties 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 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.
Media Queries Please direct all media queries to
DARPAPublicAffairsOffice@DARPA.mil
Posted April 21, 2012
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