Facing the Heat Barrier - NASA's History Office

Facing the Heat Barrier: A History of Hypersonics
The X-7. (U.S. Air Force)
fuel, and obtained thrust by allowing the hot airflow to expand through a nozzle.
The turbojet achieved compression by using a rotating turbocompressor, which
inevitably imposed a requirement to tap a considerable amount of power from its
propulsive jet by placing the turbine within the flow. A ramjet dispensed with this
turbomachinery, compressing the incoming flow by the simple method of processing
it through a normal or oblique shock. This brought the promise of higher flight
speed. However, a ramjet paid for this advantage by requiring an auxiliary boost,
typically with a rocket, to accelerate it to speeds at which this shock could form. 3
The X-7 served as a testbed for development of ramjet engines as mainstream
propulsion units. With an initial concept that dated to December 1946, it took
shape as a three-stage flight vehicle. The first stage, a B-29 and later a B-50 bomber,
played the classic role of lifting the aircraft to useful altitudes. Such bombers also
served in this fashion as mother ships for the X-1 series, the X-2, and in time the
X-15. For the X-7, a solid propellant rocket served as the second stage, accelerating
the test aircraft to a speed high enough for sustained ramjet operation. The ramjet
engine, slung beneath its fuselage, provided further acceleration along with highspeed
cruise. Recovery was achieved using a parachute and a long nose spike that
pierced the ground like a lance. This enabled the X-7 to remain upright, which
protected the airframe and engine for possible re-use.
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First Thoughts of Hypersonic Propulsion
The X-7 craft were based at Holloman Air Force Base in New Mexico, which
was an early center for missile flight test. The first flight took place in April 1951,
with a ramjet of 20-inch diameter built by Wright Aeronautical. The X-7 soon
took on the role of supporting developmental tests of a 28-inch engine built by the
Marquardt Company and intended for use with the Bomarc missile. Flights with
this 28-inch design began in December 1952 and achieved a substantial success
the following April. The engine burned for some 20 seconds; the vehicle reached
59,500 feet and Mach 2.6. This exceeded the Mach 2.44 of the X-1A rocket plane
in December 1953, piloted by Chuck Yeager. Significantly, although the X-7 was
unpiloted, it remained aerodynamically stable during this flight. By contrast, the X-
1A lost stability and fell out of the sky, dropping 51,000 feet before Yeager brought
it back under control. 4
The X-7 soon became a workhorse, running off some one hundred missions
between 1955 and the end of the program in July 1960. It set a number of records,
including range and flight time of 134 miles and 552 seconds, respectively. Its altitude
mark of 106,000 feet, achieved with an airbreathing ramjet, compared with
126,200 feet for the rocket-powered X-2 research airplane. 5
Other achievements involved speed. The vehicle had been built of heat-treated
4130 steel, with the initial goal being Mach 3. The program achieved this—and
simply kept going. On 29 August 1957 it reached Mach 3.95 with a 28-inch Marquardt
engine. Following launch from a B-50 at 33,700 feet, twin solid motors
mounted beneath the wings boosted the craft to Mach 2.25. These boosters fell
away; the ramjet ignited, and the vehicle began to climb at a 20-degree angle before
leveling out at 54,500 feet. It then went into a very shallow dive. The engine continued
to operate, as it ran for a total of 91 seconds, and acceleration continued until
the craft attained its peak Mach at fuel exhaustion. It was recovered through use of
its parachute and nose spike, and temperature-sensitive paints showed that it had
experienced heating to more than 600°F. This heating also brought internal damage
to the engine. 6
Even so, the X-7 was not yet at the limit of its capabilities. Fitted with a 36-inch
ramjet, again from Marquardt, it flew to Mach 4.31 on 17 April 1958. This time
the drop from the B-50 came at 28,500 feet, with the engine igniting following
rocket boost to Mach 1.99. It operated for 70.9 seconds, propelling the vehicle to
a peak altitude of 60,000 feet. By then it was at Mach 3.5, continuing to accelerate
as it transitioned to level flight. It reached its maximum Mach—and sustained
a sharp drop in thrust three seconds later, apparently due to an engine tailpipe
failure. Breakup of the vehicle occurred immediately afterward, with the X-7 being
demolished. 7
This flight set a record for airbreathing propulsion that stands to this day. Its
speed of 2,881 miles per hour (mph) compares with the record for an afterburning
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