Facing the Heat Barrier - NASA's History Office

Facing the Heat Barrier: A History of Hypersonics
The Navaho. (Smithsonian Institution
No. 77-10905)
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Flight tests began in November
1956, with launches of complete
missiles taking place at Cape Canaveral.
The first four were flops; none
even got far enough to permit ignition
of the ramjets. In mid-July of
1957, three weeks after the first
launch of an Atlas, the Air Force
canceled Navaho. Lee Atwood, president
of North American Aviation,
recalls that Atlas indeed had greater
promise: “Navaho would approach
its target at Mach 3; a good antiaircraft
missile might shoot it down.
But Atlas would come in at Mach
20. There was no way that anyone
would shoot it down.”
There nevertheless was hardware
for several more launches, and there
was considerable interest in exercising
the ramjets. Accordingly, seven
more Navahos were launched during
the following 18 months, with the
best flight taking place in January
1958.
The missile accelerated on rocket
power and leveled off, the twin ramjet
engines ignited, and it stabilized in
cruise at 64,000 feet. It continued in this fashion for half an hour. Then, approaching
the thousand-mile mark in range, its autopilot initiated a planned turnaround
to enable this Navaho to fly back uprange. The turn was wide, and ground controllers
responded by tightening it under radio control. This disturbed the airflow near
the inlet of the right ramjet, which flamed out. The missile lost speed, its left engine
also flamed out, and the vehicle fell into the Atlantic. It had been airborne for 42
minutes, covering 1,237 miles. 15
Because the program had been canceled and the project staff was merely flying
off its leftover hardware, there were no funds to address what clearly was a serious
inlet problem. Still, Navaho at least had flown. By contrast, another project—the
Air Force’s XF-103 fighter, which aimed at Mach 3.7—never even reached the prototype
stage.
The XF-103 in artist’s rendering. (U.S. Air Force)
First Thoughts of Hypersonic Propulsion
Its engine, also from Wright Aeronautical, combined a turbojet and ramjet
within a single package. The ramjet doubled as an afterburner, with internal doors
closing off the ramjet’s long inlet duct. Conversion to pure ramjet operation took
seven seconds. This turboramjet showed considerable promise. At Arnold Engineering
Development Center, an important series of ground tests was slated to require
as much as six weeks. They took only two weeks, with the engine running on the
first day.
Unfortunately, the XF-103 outstayed its welcome. The project dated to 1951; it
took until December 1956 to carry out the AEDC tests. Much of the reason for this
long delay involved the plane’s highly advanced design, which made extensive use
of titanium. Still, the Mach 1.8 XF-104 took less than 18 months to advance from
the start of engineering design to first flight, and the XF-103 was not scheduled to
fly until 1960. The Air Force canceled it in August 1957, and aviation writer Richard
DeMeis pronounced its epitaph: “No matter how promising or outstanding an
aircraft may be, if development takes inordinately long, the mortality rate rises in
proportion.” 16
Among the five cited programs, three achieved operational status, with the X-7
actually outrunning its initial planned performance. The feasibility of Navaho was
never in doubt; the inlet problem was one of engineering development, not one
that would call its practicality into question. Only the XF-103 encountered serious
problems of technology that lay beyond the state of the art. The ramjet of the
1950s thus was an engine whose time had come, and which had become part of
mainstream design.
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