Following is excerpt from a beautiful paper by Curtis Peebles
"The X-43A Flight Research Program: Lessons Learned on the Road to Mach 10"
(full-text document available here http://hdl.handle.net/2060/20070021686)
Chapter gives quite an clear timeline of hypersonic projects considered by NASA and DoD in 1970s
I've added links to various programs already depicted at SPF to make a picture wider.
NHFRF
The development of the airframe-integrated scramjet came at a time of little
interest in such advanced concepts. The 1970s saw the end of NASA’s Apollo program,
and a halt in U.S. manned spaceflights for half a decade. The unmanned planetary
missions approved in the late 1960s and early 1970s were launched, but proposed
advanced follow-on flights failed to gain approval.
Aeronautical research experienced a similar decline. The X-15 program ended in
1968. The lifting body program was still under way in the early 1970s, with research
flights by the HL-10, X-24A, and M2-F3 vehicles, but its end was approaching. The final
lifting body was the X-24B. This was actually the X-24A fitted with a modified longnose
delta fuselage. This shape had two applications. It gave a better cross-range
capability to a vehicle re-entering from orbit, and could also be used by a hypersonic
cruise vehicle powered by advanced air-breathing engines. Flight tests of the X-24B
began in 1973 and ended in 1975; bring the lifting body program to a close.
The end of the X-15 research and the winding down of the lifting body programs
left a void in aeronautical research. The development of a new hypersonic vehicle had
wide appeal within both the Air Force and NASA. In 1972 the Air Force Flight Dynamics
Laboratory (FDL)
proposed delta-wing test vehicle that would fly at speeds between
Mach 3 to 5. The following year, the FDL offered a plan for an
Incremental Growth
Vehicle, so named because it would initially fly at Mach 4.5, but then be upgraded to
reach Mach 6, and finally Mach 9. At the same time, Langley Research Center engineers
had ideas of their own. Their initial vehicle concept was the
Hypersonic Research Facility
(HYFAC) which was designed to reach a top speed of Mach 12 – twice the X-15’s
maximum speed. This was followed in 1974 by a proposal for a less exotic vehicle, called
the
High-Speed Research Aircraft (HSRA), with a top speed of Mach 8.
All of the FDL and NASA proposed concept vehicles had provisions for testing of
air-breathing propulsion systems. The proposals differed in their research goals, however.
The FDL vehicles focused on designs that would be suitable for military roles such as
interception, reconnaissance, or strike. The NASA proposals were for long-range
hypersonic cruise vehicles and space launchers. None of the concepts received official
support to begin development.
This nearly changed with another FDL proposal, which overlapped the more
advanced concepts. At about the time the X-24B made its first flights, the FDL engineers,
who had developed the X-24B shape, began studying a hypersonic version called the
“X-
24C.” Two different vehicle concepts were proposed – one with cheek inlets for an airbreathing
engine, and a second powered by an XLR-99 rocket engine. The target speed
would be Mach 5. X-24C program costs were estimated to be around $70 million.
The advantage of the X-24C proposal was that it was largely an “off-the-shelf”
design in terms of shape, equipment, and technology. This made it a much more practical
design than the more complex FDL and NASA proposals. The X-24C gained the support
of Gen. Sam Phillips, the head of the Air Force Systems Command and a former senior
official in the Apollo program.
Both the X-15 and lifting body programs had been run as joint NASA/Air Force
efforts, so it made sense to do the same for the emerging X-24C. In December 1975, a
month after the final X-24B flight, NASA and the Air Force formed an X-24C Joint
Steering Committee. This consisted of the commanders of the Air Force Flight Test
Center and the FDL, and the directors of NASA’s Flight Test Center (now Dryden Flight
Research Center) and Langley Research Center.
Over the next several months, the group moved away from the relatively simple
X-24C concept, toward the larger, more complex vehicles proposed by Langley and the
FDL. The rationale was that a joint NASA/Air Force “research facility” should be
designed to undertake research into a wide range of “pacing technology” in hypersonic
flight, and serve as a focus for U.S. research efforts in the field. This, planners
envisioned, would combine a wide range of research goals into a single vehicle.
The Air Force wanted to test military-related technology experiments –
photography, weapons separation, radome heating, nose tip erosion, thermal protective
systems, and removable fins. Different air-breathing propulsion systems were to be
tested, including integrated rocket ramjets, as well as subsonic combustion ramjets. To do
this, designers moved away from the X-24C shape, toward a design more akin to those of
the Langley concepts. The vehicle would have a modular configuration, with a removable
center section of the fuselage, to accommodate the different experiments. With the
connection to the original X-24C vehicle now gone, the program received a new name.
The traditional X-plane designation was abandoned and replaced with the awkward
“National Hypersonic Flight Research Facility,” (NHFRF, but pronounced “nerf”).
The proposed NHFRF gained favor within NASA, as it fulfilled a number of
research and institutional needs. For the NASA Flight Research Center, the X-24C was a
continuation of the previous three decades of research into high-speed flight, revitalized
the role of aeronautics within NASA, and provided a major role for the center during the
Shuttle era. For Langley aeronautical and propulsion engineers, the vehicle was the
culmination of all their efforts in high-speed flight and air-breathing propulsion system.
They also saw it as a means to “cover the whole hypersonic waterfront and do it before
we’ve lost all the hypersonic talent we developed from the X-15 program.”
The planned performance of the NHFRF was impressive. After launch from the
B-52B mothership, the vehicle could reach a maximum speed of Mach 8 under rocket
power. It also was designed to cruise at a speed of Mach 6+ for 40 seconds. This was an
extremely demanding requirement. The project engineers envisioned construction of two
NHFRF vehicles, to be used in a 200-flight research program beginning in 1983, and
spanning a decade. This effort was estimated to cost $200 million.
Despite the support for the NHFRF by both NASA and the Air Force, however,
trouble for the project was ahead. Much of NASA’s budget was committed to the Space
Shuttle, and both technical problems and high inflation were causing the program’s costs
to balloon. At the same time, the Air Force was introducing a new generation of fighter
aircraft, against a background of funding cutbacks and poor morale. The increasing
complexity of the NHFRF had, by early 1977, raised the estimated cost to as much as
$500 to $600 million. NASA Headquarters was unwilling to foot the ever-growing bill,
and in September 1977 cancelled the agency’s participation in the project. James J.
Kramer, the acting NASA associate administrator for aeronautics and space technology,
stated that, “the combination of a tight budget and the inability to identify a pressing
near-term need for the flight facility had led to a decision by NASA not to proceed to a
flight test vehicle at this time.” The Air Force could not take over full funding of the
NHFRF, and ended its support for the effort.
Although the X-24B was the last rocket-powered research aircraft to fly, it was
the failure of the X-24C/NHFRF to receive approval that ended the thirty-year era of
rocket planes, flying to ever higher speeds and altitudes. The era had begun in 1946 with
the glide flights of the X-1, in a blaze of publicity. The era ended with a memo, and its
passing went unnoticed. The end of NHFRF dealt a hard blow to morale at the Flight
Research Center, and recriminations soon followed. Some blamed over-management.
Objections were also raised about the 40-second cruise requirement. This added
considerable cost and difficulty to development of the NHFRF. Some argued it would
have been better to build the original X-24C design rather than a more advanced and
complex vehicle designed to be all things to all people.
In retrospect, it seems unlikely that even the original X-24C concept would have
been approved. The reasons were not technical, as with the Aerospaceplane. There would
certainly have been difficulties and setbacks, especially with the more complex NHFRF
concepts, but engineers could have drawn on the technology and experience from the X-
15 and lifting body programs in solving those difficulties. The real problem preventing
the project from gaining support was that the time was simply not right for a project like
the X-24C/NHFRF. At the time, the research focus at NASA was shifting to such projects
as supercritical wing designs, winglets, and digital-fly-by-wire. While a scramjetpowered
vehicle could serve as a second-generation space vehicle, the Space Shuttle had
yet to fly; it was far too early to begin work on a replacement. Finally, the “me
generation” mentality that marked the 1970s was unfavorable to the bold challenges of
flying at high speeds and altitudes that had so captivated engineers in the previous three
decades.
Among the lessons that can be drawn from the X-24C/NHFRF is that it is not
enough for a project to be technically feasible, and that its data would be valuable. As
with the NACA’s research activities, NASA aeronautical efforts were publicly funded
projects. They had to relate to national needs. The supercritical wing designs and
winglets both offered major improvements in fuel economy; a major public and
commercial issue in the wake of the sharp increase in fuel costs in the early 1970s. Flyby-
wire technology opened new possibilities in aircraft design, in that this removed the
need for an aircraft to be inherently stable.
Over the following decades, supercritical wings became standard on business jets,
airliners, fighters, and heavy transports. Winglets appeared on sailplanes, light aircraft,
business jets, airliners, and heavy transports. Virtually all new military and many airliners
built during this period used fly-by-wire control systems. In terms of the impact on
aeronautics, the government research funding provided for these areas was justified.
The “need for speed” and ever-higher altitudes, in contrast, which had been
driving forces in both civilian and military aircraft design, were no longer critical issues.
Supersonic Transports were uneconomical to operate due to high fuel prices. Strategic
bombers now flew at low level to strike their targets, while the emerging stealth
technology was incompatible with supersonic flight. The airliners, fighters, and bombers
built in the latter part of the 20th century all flew within the speed and altitude envelopes
of aircraft built during the late 1950s and early 1960s.
Beyond these public policy issues, another factor was the lack of focus in the
research goals for the X-24C/NHFRF. Trying to accomplish a wide range of goals made
the vehicle more complex, and the heritage from the X-24B shape and systems was lost.
This resulted in rapid cost increases even before the program was approved. In contrast,
with the X-15, the original research goals were limited to high-speed/high-altitude flight.
The vehicle had enough “stretch” built into it to later accommodate scientific and
engineering experiments.
As a result of the failure of the NHFRF to gain approval, nearly a decade would
pass before a major scramjet development effort would again be made. The new effort
was to be much more ambitious, but would still suffer from the same old flaws.