Facing the Heat Barrier - NASA's History Office

Facing the Heat Barrier: A History of Hypersonics
the suit maintained a constant volume when pressurized, enhancing a pilot’s freedom
of movement. Gloves and boots were detachable and zipped to this fabric. The
helmet was joined to the suit with a freely-swiveling ring that gave full mobility to
the head. Oxygen flowed into the helmet; exhalant passed through valves in a neck
seal and pressurized the suit. Becker later described it as “the first practical full-pressure
suit for pilot protection in space.” 81
Thus accoutered, protected for flight in near-vacuum, X-15 test pilots rode their
rockets as they approached the edge of space and challenged the hypersonic frontier.
They returned with results galore for project scientists—and for the nation.
X-15: Some Results
During the early 1960s, when the nation was agog over the Mercury astronauts,
the X-15 pointed to a future in which piloted spaceplanes might fly routinely to
orbit. The men of Mercury went water-skiing with Jackie Kennedy, but within their
orbiting capsules, they did relatively little. Their flights were under automatic control,
which left them as passengers along for the ride. Even a monkey could do it.
Indeed, a chimpanzee named Ham rode a Redstone rocket on a suborbital flight
in January 1961, three months before Alan Shepard repeated it before the gaze of
an astonished world. Later that year another chimp, Enos, orbited the Earth and
returned safely. The much-lionized John Glenn did this only later. 82
In the X-15, by contrast, only people entered the cockpit. A pilot fired the rocket,
controlled its thrust, and set the angle of climb. He left the atmosphere, soared high
over the top of the trajectory, and then used reaction controls to set up his re-entry.
All the while, if anything went wrong, he had to cope with it on the spot and work
to save himself and the plane. He maneuvered through re-entry, pulled out of his
dive, and began to glide. Then, while Mercury capsules were using parachutes to
splash clumsily near an aircraft carrier, the X-15 pilot goosed his craft onto Rogers
Dry Lake like a fighter.
All aircraft depend on propulsion for their performance, and the X-15’s engine
installations allow the analyst to divide its career into three eras. It had been designed
from the start to use the so-called Big Engine, with 57,000 pounds of thrust, but
delays in its development brought a decision to equip it with two XLR11 rocket
engines, which had served earlier in the X-1 series and the Douglas Skyrocket.
Together they gave 16,000 pounds of thrust.
Flights with the XLR11s ran from June 1959 to February 1961. The best speed
and altitude marks were Mach 3.50 in February 1961 and 136,500 feet in August
1961. These closely matched the corresponding numbers for the X-2 during 1956:
Mach 3.196, 126,200 feet. 83 The X-2 program had been ill-starred—it had had
two operational aircraft, both of which were destroyed in accidents. Indeed, these
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The X-15
research aircraft made only 20 flights before the program ended, prematurely, with
the loss of the second flight vehicle. The X-15 with XLR11s thus amounted to X-
2s that had been brought back from the dead, and that belatedly completed their
intended flight program.
The Big Engine, the Reaction Motors XLR99, went into service in November
1960. It launched a program of carefully measured steps that brought the fall of one
Mach number after another. A month after the last flight with XLR11s, in March
1961, the pilot Robert White took the X-15 past Mach 4. This was the first time a
piloted aircraft had flown that fast, as White raised the speed mark by nearly a full
Mach. Mach 5 fell, also to Robert White, four months later. In November 1961
White did it again, as he reached Mach 6.04. Once flights began with the Big
Engine, it took only 15 of them to reach this mark and to double the maximum
Mach that had been reached with the X-2.
Altitude flights were also on the agenda. The X-15 climbed to 246,700 feet in
April 1962, matched this mark two months later, and then soared to 314,750 feet in
July 1962. Again White was in the cockpit, and the Federation Aeronautique Internationale,
which keeps the world’s aviation records, certified this one as the absolute
altitude record for its class. A year later, without benefit of the FAI, the pilot Joseph
Walker reached 354,200 feet. He thus topped 100 kilometers, a nice round number
that put him into space without question or cavil. 84
The third era in the X-15’s history took shape as an extension of the second one.
In November 1962, with this airplane’s capabilities largely demonstrated, a serious
landing accident caused major damage and led to an extensive rebuild. The new aircraft,
designated X-15A-2, retained the Big Engine but sported external tankage for
a longer duration of engine burn. It also took on an ablative coating for enhanced
thermal protection.
It showed anew the need for care in flight test. In mid-1962, and for that matter
in 1966, the X-2’s best speed stood at 4,104 miles per hour, or Mach 5.92. (Mach
number depends on both vehicle speed and air temperature. The flight to Mach
6.04 reached 4,093 miles per hour.) Late in 1966, flying the X-15A-2 without the
ablator, Pete Knight raised this to Mach 6.33. Engineers then applied the ablator
and mounted a dummy engine to the lower fin, with Knight taking this craft to
Mach 4.94 in August 1967. Then in October he tried for more.
But the X-15A-2, with both ablator and dummy engine, now was truly a new
configuration. Further, it had only been certified with these additions in the flight
to Mach 4.94 and could not be trusted at higher Mach. Knight took the craft to
Mach 6.72, a jump of nearly two Mach numbers, and this proved to be too much.
The ablator, when it came back, was charred and pitted so severely that it could
not be restored for another flight. Worse, shock-impingement heating burned the
engine off its pylon and seared a hole in the lower fin, disabling the propellant ejec-
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