A large new area of potential interest for exploration by future
research aircraft has been opened up by the advent of hypersonic air
breathing propulsion systems. These systems, which were not generally
believed feasible in the early years of the X-15 program, employ some
combination of the ramjet or scramjet for cruise, with the turbojet or
rocket for acceleration and climb. The systems of interest include space
launch vehicles, military strike or reconnaissance vehicles, and commercial
transports, with speeds extending upward to about Mach 12. Many new
problems not covered by X-15 research are found in the propulsion system,
cryogenic fuel tankage, lightweight radiation-cooled structures, and in
the piloting and operations areas. While it is not clear at present that
a new air-breathing research airplane system will prove justifiable, it
is desirable to explore and evaluate some of the possibilities.
If we look to the X-15 experience as a guide, what prime features
does it suggest? What features were most vital to the role it has played
in aerospace history? Clearly the decision in 1954 to proceed quickly
with a general research tool as opposed to a configuration fully optimized
with respect to the 1954 vision of the feature mission was vital. If the
optimized pseudo-prototype route had been followed one can see with the
advantage of hindsight that we would have picked the wrong mission, the
wrong structure, the wrong aerodynamic shapes, and the wrong propulsion.
More important, in the 3 or 4 years which all of this optimization might
have consumed we would have dissipated our technology lead time, and with
the start of the space age in 1958, in all probability the X-15 would never
have been attempted. A second basic feature of the X-15 that proved vital
was the design of the system with great latitude in performance so that it
would reach well beyond the hypersonic aerodynamic corridor into simulated
space flight.
Figure 20. Airbreathing research airplane study configuration.
A possible new research airplane system conforming to these and other
X-15 guidelines is illustrated in figure 20.
(Footnote: This is one of several systems receiving preliminary
study in the United States.)
It is a lifting-body cruise configuration designed for Mach numbers up to 12.
Its acceleration engine
is a hydrogen fueled J-2S rocket adapted from an upper-stage engine of the
Saturn vehicle. Integrated into the lower surface is a research scramjet
engine sized to paver the airplane in cruise. Following guidelines from
our X-15 experience the vehicle is kept as small as possible, aboub
25 meters in length, and it remains in the Mach 12 environment only long
enough for research purposes, about 5 minutes. As we have leaxned from
the X-15, a new hypersonic research airplane system is likely to have a
long lifetime of perhaps 15 years, during which many new unsuspected ideas
for research and changes in configuration are likely to appear as the
program develops. Accordingly, we are proposing here actually three
different vehicle arrangements. We would start the program with the
lifting-body rocket glider without the air-breathing research engine.
Later, a delta-winged version using the same subsystems would be flown.
And, finally, the integrated scranjet research engine shown here would be
installed. Provision for structural cooling schemes including direct fuel
cooling, air-film cooling, and other schemes likely to appear in these
vehicles might also be made.
The case for approval of such a new research airplane is no longer
simple as it was in 1954. A major difference is the high level of confidence
now enjoyed by the partial-simulation techniques of ground-based research
and development. The X-15 program itself, together with the successfully
developed reentry vehicle systems, has thus tended to eliminate a major
justification vhich existed in 1954. In the author's opinion no new
exploratory research airplane can ever again be successfully promoted
primarily on the grounds that it will produce unique flight data, without
which a successful technology cannot be achieved. Thus there is not likely
to be a future research airplane unless a high valuation is placed on the
other vital but less tangible contributicns - the focusing of the countless
detailed efforts in many areas, the revelation of new and unsuspected
problems for research, the overall stimulation of technology development,
and the early availability of new technology for important but initially
unforeseeable new applications. The X-15 experience affirms that the ex
ploratory research airplane is a most effective device for producing these
values.
