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OCR for page 11
POTENTIAL MILITARY HYPERSONIC APPLICATIONS
11
1.0 POTENTIAL MILITARY HYPERSONIC APPLICATIONS
The Air Force and the NASP Joint
Program Office each briefed the com-
mittee on the military aspects of a
hypersonic vehicle. Clearly a hypersonic
vehicle has several advantages. Flight
time to the target could be less than
one hour at near-orbital speeds. While
this is comparable to the flight time of
a ballistic missile, a hypersonic vehicle
could have the advantage of flexible
recall and en route re-direction. Con-
sidering the range to potential areas of
interest and targets from the interior of
the United States, the military potential
is readily apparent.
As shown in Figure 1-1, a hyper-
sonic vehicle can reach a point on the
opposite side of the earth from Omaha,
Nebraska in about one hour. The
dynamic pressure on the vehicle could be
considerably less than 1000 psf at this
flight condition.
A hypersonic vehicle can also range
from Omaha to Moscow, USSR to run
either a strike mission or a high altitude
reconnaissance mission in about 30
minutes. An ICBM can also accomplish
a strike mission in about 30 minutes,
with its warheads either reentering
ballistically or as hypersonic glide
vehicles, which can maneuver. Once
launched, however, the ICBM cannot be
recalled; the hypersonic aircraft can.
The reconnaissance mission can also be
done by other means, such as by an SR-
71 at 80,000 ft. in over three hours or
by a satellite pass over Moscow at a
predictable time. Again, the response
time of the hypersonic vehicle is
measured in minutes against many hours
for the two alternate means mentioned.
From the viewpoint of degree of
technical difficulty, the full range of
operational missions at hypersonic speed
can be divided into three areas. All
sustained (steady) flights at such speeds
will be confined to a restricted "cor-
ridor" of altitudes at each speed (Mach
number). Above this corridor the
aircraft cannot provide enough lift or
thrust; below it, vehicle heating or
structural loads are excessive. These
design factors are discussed in more
detail later. The approximate form of
the flight corridor is shown in Figure 1-
2, where the three operational areas are
defined in terms of the Mach number
and altitude range of each.
Area 1, including operations at up to
about Mach number 8, requires the least
complex hypersonic vehicles from a
technical point of view. Nevertheless, it
offers significant reductions in mission
time. Vehicles for operation in this
region appear reasonably achievable, and
the stress of the flight environment
appears to be tolerable for sensor
operation and weapon delivery. The
pattern of manned military operations in
this area will be similar to those of the
SR-71.
Area 2 involves mission accomplish-
ment from space, outside the sensible
atmosphere, between Mach numbers 20
and 25. In our view, this area encom-
passes the most attractive hypersonic
aircraft missions because of launch
flexibility, short flight time, and the
ability to take advantage of the
relatively benign environment of space
for mission accomplishment. Maneuver-
ing is another attractive feature.
The major technical challenges for
Area 2 are presented by the accelerating
climb through the atmosphere and the
subsequent re-entry. Although many of
the technical advances of the space
OCR for page 12
12
program are directly applicable, the
requirement to fly with a completely
recoverable air-breathing vehicle adds
formidable new problems. These are
addressed later in this report.
Area 3, between the two areas dis-
cussed above, involves sustained cruising
flight in the atmosphere roughly between
Mach numbers ~ and 20. This is a very
stressful flight environment with high
skin temperatures, control and maneu--
vering difficulties, ionized boundaries
through which sensors must operate, and
high infrared signatures that would make
the vehicle vulnerable to detection. For
these reasons, we have great reserva-
tions about the military utility of sus-
tained hypersonic flight in the atmo-
sphere above Mach number S.
In examining potential applications
in any of these three areas, one must
take account of some basic limitations
and restrictions on the operations of
hypersonic aircraft. Their minimum
turning radius is measured in hundreds
or even thousands of miles - propor-
tional to the square of the speed; they
can maneuver in flight with modest
energy expenditures, in contrast to the
ballistic missile, but flight path curv-
atures must be small compared to those
of current aircraft. One important
consequence is that global or near-global
range is necessary, and the plane will
often have to circle the planet to return
to base after the mission.
Another restriction is inherent in
the base support requirements associated
with cryogenic fuels. They will require
a complete departure from conventional
airport storage and distribution facilities.
For economic reasons alone, we are un-
able to envision a network of airfields
giving the flexibility that today's air-
craft enjoy. However, some mitigating
HYPERSONIC TECHNOLOGY FOR MILITARY APPLICATION
factors should be considered in address-
ing these logistic issues.
For the last 15 years or more,
hydrogen-fueled aircraft have been the
subject of serious study by NASA and
U.S. commercial aircraft companies,
primarily to enable fast, economical,
long-range flight such as supersonic
trans-Pacific flights. The airport
facilities required for liquid hydrogen
handling have received quite detailed
study, and the problems appear tractable.
The military hypersonic aircraft, in
common with these commercial concepts,
will fly farther and higher than today's
aircraft, which suggests that a much
smaller number of cryogenically-equipped
airports will be needed for satisfactory
operation. Finally, all classes of hyper-
sonic aircraft will require the same type
of base facilities, and it may be of
interest to examine the concept of a
new type of Air Force base, capable of
supporting all classes of mission, and
fully-equipped for cryogenic fuels.
It follows from these arguments that
any forecast of missions for hypersonic
aircraft must include a careful examin-
ation of the unusual support require-
ments. We suggest that as hypersonic
technology advances, periodic studies of
the logistical support requirements
should be made to give confidence in
the vehicle's military utility.
In sum, we believe that there are
clear potential advantages to the Air
Force in hypersonic air-breathing capa-
bility. These potential advantages are
sufficient to justify an intensive tech-
nology development program, including
flight vehicle research sufficient to
determine the military utility of hyper-
sonic flight at orbital or near-orbital
speeds.
OCR for page 13
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Representative terms from entire chapter:
hypersonic vehicle
