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October
14th, 1965 (Part
One).
"On this flight the
XB-70 proved its capability of attaining Mach 3 at 70,000
feet!" -- Al White's summary in the pilot's report for
Flight 1-17.
As AV/1 crossed the
Mach 3 threshold, her nose abruptly pitched upwards. But
even as Al White corrected, the nose just as sharply pitched
downward. Combined with the correction, the XB-70
accelerated just enough to cause a brief moment of
overpricer in the inlets. Stabilising the Valkyrie, Al White
keyed his microphone and spoke the words everyone had been
waiting for, "There's that big magic number [Mach
3]." For almost three minutes, everything appeared fine.
Suddenly, White and Cotton heard something behind them.
Although no caution lights had come on, and concerned about
damage caused by the overpricer, White decided to decelerate
and let the chase planes catch up with the XB-70. When they
did, they reported that about 2 feet of the left wing's
leading edge was missing. Fortunately, the damaged section
of the wing was far enough outboard that the debris wasn't
drawn into the engine inlets.
After 56 weeks and 17
flights, AV/1 had finally reached her goals -- but she would
never fly at Mach 3 again. The design team, alarmed with the
skin separation problems, and knowing that improvements in
AV/2 would solve the problem, decided that from that point
on, AV/1 would be limited to a maximum speed of Mach 2.5.
Mach 3 research would be left to the improved, and more
capable, second aircraft.
Initially, AV/2 (tail
number 20207) wasn't completely trouble-free. Brake chatter
continued to be a problem during low-speed taxiing, although
it was suspected that this stemmed from the lack of return
springs for the brake pads (like automobiles, the Valkyrie
was designed for the pads to lightly touch the rotors at all
times). Far more troubling, however, were problems in the
new, automatic AICS system, which would inexplicably recycle
during supersonic flight, causing an "unstart," where the
shockwave from the forward fuselage, rather than being
deflected past the inlet and under the wing, instead fell
directly into the inlet. Not only did this change in inlet
airflow cause a brief flameout on the engines, but it also
caused a dramatic loss of lift on that side, since the
XB-70s wings depended on that high-pressure air for a large
portion of their lift.
Sometimes, speed and
throat ramp settings would create a condition where the
shockwave was right at the boundary of the inlet, jumping in
and out. This "buzz" was a serious condition that had to be
rectified immediately -- left to continue for more than a
few moments, the stresses could have torn the XB-70 apart!
Following the 8th flight of AV/2, White stated in his
pilot's report that he didn't feel comfortable trying for
Mach 3 until the AICS was sorted out, which took several
additional flights.
As this was sorted
out, concerns were voiced that AV/2 might also suffer skin
separation problems under the heating caused by sustained
high-speed flight, despite the improved construction and
assembly techniques. It was decided that with AV/2, early
flights would involve "heat-soaking" at speeds less than
Mach 3. This way, unlike a quicker "dash" up to speed, the
airframe would reach the full temperature created by flight
at that speed.
Heat is the major
enemy of speed. Caused by the friction of cutting through
the air, heat has limited the top speed of modern aircraft
(such as the F-15) far more than power. Beyond Mach 2.5,
friction increases at an ever-growing rate (for comparison,
an SR-71 operating at Mach 2.2 heats up to about 275
degrees, but at Mach 3.2, skin temperatures rise to almost
900 degrees!). The same aerodynamics that gave the XB-70 so
little drag helped minimise heat buildup. The hottest
portions of the Valkyrie, her nose and horizontal splitter,
reached a temperature of only 625 degrees during Mach 3
flight, with the majority of the XB-70s skin at a
temperature of just 450 degrees! Equipment was placed in the
fuel tanks, which acted as heat sinks. As the fuel soaked up
the heat from the fuselage, it was drawn into the engines
and burned away, leaving the cooler fuel behind. At the same
time, it had to be replaced with nitrogen gas. The
temperatures inside the tanks were high enough that just two
percent oxygen would have caused the fuel to burst into
flames -- a decidedly undesirable event.
On her 15th flight,
on December 11th, 1965, AV/2 ran at Mach 2.8 for 20 minutes
(spending 41 total minutes beyond Mach 2.5) without any
indications of skin separation. Ten days later, after seven
minutes at Mach 2.9 (and 20 minutes above Mach 2.8), the oil
pump for the number four engine failed. Shutting down the
engine, White and Cotton headed back to Edwards, when an
over temperature caution came on for the number six engine,
which was shut down as well. After landing, it was
discovered that, despite the early shutdown, loss of
lubrication had ruined engine number four -- an unhappy
moment in a flight program that now only had 29 engines left
(with 12 needed just to get the two planes in the air!). The
number six engine was removed and sent off to General
Electric for rebuilding.
Engine installation
was another groundbreaking feature of the XB-70. Unlike
other airplanes, where all the engine's accessories were
bolted to the plane, and then connected to the engine
(making removal a tedious job), on the Valkyrie, everything
was bolted to the engine itself, making engine removal and
replacement a job that could be done in just a couple of
hours. Today, many military and commercial planes use this
design.
Less than 6 months
after her first flight, AV/2 reached her goal of Mach 3, on
her 17th flight (coincidentally, the same number of flights
AV/1 to reach Mach 3). Just for three minutes, then back
home for a through examination. No sign of skin damage at
all! Prudence was still the watchword, however, and AV/2
twice more poked her sleek nose beyond Mach 3 for just a few
minutes before sustaining Mach 3 for 15 minutes on her 22nd
flight.
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