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Introduction
In 1959, North
American Aviation was awarded a contract to build "Weapons
System 110" (WS-110). On paper, the project's goals seemed
insurmountable; not just the biggest craft ever to take to
the skies, but is was to be the fastest as well, to cruise
at 3 times the speed of sound, at a time when no plane had
yet flown that fast.
This is the story of
the men and the machines they built to achieve the
impossible.
The original WS-110
competition had boiled down to a competition between Boeing
and North American. Although shaped differently, both of the
original submissions had the following in common:
Takeoff weight of
1,000,000 pounds
Large detachable
portion of wing to allow for a Mach 3 "dash to
target"
Mach 3 "dash" ability
only
Were far too large to
use existing USAF facilities designed for the
B-52
Needless to say,
these proposals were rejected by the USAF, which sent Boeing
and North American back to the drawing board.
However, during the
later phases of the design process, both companies realised
that designing an aircraft to actually cruise at mach 3
wasn't really that much more demanding that designing one
that only had a "dash" capability.
Both teams went back
to the drawing boards with what were the final requirements
for the contract, which were:
Cruise Speed of Mach
3 (2,000 mph)
Cruise Altitude of
70,000 feet
A "shirt sleeve"
environment for the crew.
50,000 pound
payload
A range of 7,500
miles
Sized such that
existing runways, hangers, etc. that had already been built
for the B-52 could be
used without further
modification.
Have flying
characteristics suitable for use with average USAF line
crewmen.
Boeing's design was
essentially very similar to the later SST; that is,
essentially a delta with 3 engines (each mounted in its own
pod) under each wing.
North American
engineers pored through every aerodynamic study they could
find, looking for anything that could be applied to a large,
triplesonic bomber. They came across a forgotten NACA (now
NASA) research paper about "Compression Lift," which
involved using the shock wave generated by the nose of the
aircraft by trapping it underneath the wing, thereby
generating high pressure under the wing, and low pressure
above it, for much better lift-to-drag
characteristics.
In flight, the XB-70
could lower the outer wing sections either 25 degrees for
flying from 300 knots to Mach 1.4, or a severe 65 degrees
for speeds from Mach 1.4 to Mach 3+. Measuring just a bit
over 20 feet at the trailing edge, these wingtips are
represent the largest movable aerodynamic device ever
used.
Lowering the wingtips
had three distinct effects on the XB-70.
Total vertical area
was increased, allowing shorter vertical stabilisers than
would otherwise be needed.
The reduction in
rearward wing area countered the delta wing's inherent
rearward shift of the centre of lift as speed increased,
keeping drag-inducing trim corrections to a
minimum.
Compression lift was
30 percent more effective because the shock-wave under the
wing was better managed.
Along with the
wingtips, the six J93 engines, bomb bay, and landing gear
were all contained in a conical shape designed to enhance
shock wave management. Overall, the XB-70 has the best
lift-to-drag ratio of any manned airplane ever built, being
bettered only on the unmanned S-21 drone, an airframe
designed to be air-launched, fly at one speed and altitude,
and then self-destruct (thereby not needing to
land).
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