Friday, August 12, 2011

Hypersonic Flight: HYFLEX deserves a closer look

The Falcon Hypersonic Technology Vehicle (HTV) team at the U.S. Department of
Defense must now prepare its formal analysis of the flight of the
Falcon HTV-2 on August 11. The loss of HTV-2 follows on the heels of
the HTV-1 failure last year and it will put quite a bit of pressure on
the team at a time when the U.S. is discussing enormous cutbacks
in defense spending including money for R&D.

Here is an excerpt from the DARPA release issued soon after the HTV-2 flight.

“Here’s what we know,” said Air Force Maj. Chris Schulz, DARPA HTV-2
program manager and PhD in aerospace engineering.  “We know how to
boost the aircraft to near space.  We know how to insert the aircraft
into atmospheric hypersonic flight.  We do not yet know how to achieve
the desired control during the aerodynamic phase of flight.  It’s
vexing; I’m confident there is a solution. We have to find it.”

“Prior to flight, the technical team completed the most sophisticated
simulations and extensive wind tunnel tests possible.  But these ground
tests have not yielded the necessary knowledge.  Filling the gaps in
our understanding of hypersonic flight in this demanding regime
requires that we be willing to fly,” said DARPA Director Regina Dugan.
“In the April 2010 test, we obtained four times the amount of data
previously available at these speeds.  Today more than 20 air, land,
sea and space data collection systems were operational.  We’ll learn.
We’ll try again. That’s what it takes.”

According to Schulz, three technical challenges exist within this HTV-2
flight regime.  They are categorized as aerodynamic; aerothermal; and
guidance, navigation and control.  And each phase of flight introduces
unique obstacles within these areas.
“To address these obstacles, DARPA has assembled a team of experts that
will analyze the flight data collected during today’s test flight,
expanding our technical understanding of this incredibly harsh flight
regime,” explained Schulz.  “As today’s flight indicates, high-Mach
flight in the atmosphere is virtually uncharted territory. ”

The first and only flight of Japan's Hypersonic Flight Experiment (HYFLEX)
happened in February 1996. Some readers might wonder why I elect to engage
in this exercise, injecting HYFLEX into the broader coverage of the HTV-2 testflight.
It is, they might argue, kind of like comparing apples to oranges.
I disagree because each of these experimental hypersonic flights -
albeit happening many years apart - involved hypersonic performance on
a sub-orbital trajectory.

A sustained air-launched hypersonic breakthrough has already been achieved
via the X-43 anyway. This is mentioned here as a matter of fact and not as a means
of triggering a broader discussion as to why the U.S. is so intent upon pursuing the
HTV solution in its quest for a "Prompt Global Strike" platform in the first place.

There is also no question that the entire HTV program involves more ambitious demonstrations including much longer flights and higher speeds than HYFLEX - Mach 20 vs Mach 15 - and yet HYFLEX flew over 700 miles in just over 3-1/2 minutes to its intended splashdown point.

The role of the two-stage J-1 booster in this HYFLEX deployment more
than 15 years ago in an attempt to bring the whole H-II Orbiting
Plane-Experimental (HOPE-X) concept one step closer to reality in this
instance was noteworthy in itself.

The dimensions of HYFLEX are often overlooked. It weighed just over
1000kg with a length of 4.4m along with a wingspan of 1.36m and a
height of just over 1m. This modest attempt by the then National Space
Development Agency of Japan (NASDA) along with the NAL and ISAS did
succeed. A malfunction of the flotation device for this spacecraft is
what sent HYFLEX to the bottom of the Pacific. Otherwise this
testflight was a remarkable event.

HYFLEX was all about demonstrating guidance and control technology -
something to keep in mind while DARPA ponders how it lost control of
both of its HTV's. Alteration of its flight trajectory using
aerodynamic forces took place in a relatively routine and predictable

Here in a nutshell is how the HYFLEX flight proceeded -

"The vehicle was launched from Tanegashima Space Center on a trajectory
with a maximum altitude of 110km. It was released from the launch
vehicle while traveling at a speed of approximately 3.9km/s and
performed a gliding right turn around Chichi-Jima Island in the
Ogasawara Islands group while flying at maximum Mach number of 15. It
finally splashed down in the Pacific using a parachute northeast of

See this report -

This same report highlighted the autonomous automatic flight control
system aboard HYFLEX.

"The vehicle's trajectory and attitude are regulated by guidance and
control laws programmed into an onboard computer, based on the
vehicle's attitude, position and velocity measured by an Inertial
Measurement Unit (IMU). Attitude control commands from the computer are
used to drive to the elevons and RCS. Guidance commands are computed
once per second, and control commands 20 times a second" the report
states. "Guidance that satisfies all flight limits, such as aerodynamic
heating rate and dynamic pressure, and depletes the kinetic energy
exactly at the destination, is performed on this plane. It can be seen
that guidance was performed properly in the experiment. Moreover,
HYFLEX demonstrated its ability to accurately reach a destination by
the fact that it splashed down only around 3km from the planned point."

Again this happened in early 1996. I intentionally bypass a discussion
here of the HYFLEX thermal protection system due to space limitations.
Any detailed discussion of flight data is also not included here for
the same reason. What exactly the Japanese and the French learned in 2003 after dropping a 500kg model of the Hope-X several times from balloons launched high over Sweden is unknown to me.

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