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OCR for page 11
II
Space Transportation:
Launch Systems, Propulsion, and Power
The committee concurs with NASAs view and the general consensus
that robust and reliable transport to LEO and beyond is essential to the
success of HEI. Further, it believes that the currently available launch
systems and their derivatives will not meet these criteria in the future.
Improved capabilities are required for transport of humans and high-
value cargo to and from LEO; transport of large unmanned components,
propellants, and expendables to LEO; and orbital transfer to the Moon
and Mars.
For reasons of cost and reliability, the transport of humans and other
precious cargo will require a different launch system than those for the
transport of more ordinary or bulk cargo. Because operations of the space
shuttle will continue to be labor-intensive and expensive, because the system
is not robust, and because the system probably will reach the end of its
useful life sometime between 2000 and 2010, the committee believes that a
successor to the shuttle eventually will be necessary for human transport to
orbit. For at least the next 10 years, however, the nation will necessarily rely
upon the shuttle for this role, and it is essential that the existing shuttle fleet
be maintained in a fully operable state. As indicated in the NRC report,
Post Challenger Assessment of Space Shuttle Flight Rates and Ualization, at
least one additional replacement orbiter is likely to be required every 5
to 10 years to offset inevitable attrition of the present fleet. But orbiter
replacements should not impede the emergence of a new capability for
launch to LEO. Eventually, a plan for a graceful phasing out of the shuttle
system should be prepared.
11
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12
HULL IN EXPLORATION OF SPACE
An important aspect of reducing costs, when considering the design
of new launch systems, should be the efficiency of activities on the ground
that are required to prepare vehicles and payloads for launch.
HUMAN TRANSPORTATION TO AND FROM ORBIT
The space shuttle system now appears to be operating satisfactorily,
and there is reason to believe that with continued scrupulous adherence
to proper manufacturing, maintenance, and operating procedures, it can
continue to do so. It does, however, have a limited design life, like any
high-performance system. It requires continuing refurbishment and in due
course it will require major replenishment, or it will have to be supplanted.
Continued access of humans to space will require that planning begin soon
for a new human transportation system that will first supplement and then
assume the shuttle's role in human transport to and from orbit, sometime
between 2000 and 2010. The best available technologies should be used to
produce a system that is robust, highly reliable, reasonably cost-effective,
and that has minimum requirements for ground support and preparation
for launch.
At present, the most likely configuration of the required system is a
two-stage rocket powered vehicle, with a try-back first stage, an orbiter with
substantial cross range capability, and a thermal protection system or hot
structure that allows reuse without major refurbishment. It may be that
some of the technologies being developed in the National Aerospace Plane
Program (NASP) will find application in this system.
UNMANNED LAUNCH SYSTEMS
In light of the evident requirements for lifting mass to LEO, a modern
launch system with heavy lift capability will be essential. It does not exist
in this country at this time. Therefore, a family of launch systems based
perhaps on the interagency Advanced Launch System (ALS) or similar
technologies should be defined and committed to development. The design
of these systems, although not requiring a safety rating for humans, should
still emphasize reliability and robustness over performance, as measured,
for example, by the ratio of payload weight to gross weight. The level
of reusability should be selected similarly to optimize the reliability and
robustness of the systems and to minimize cost on the basis of realistic
utilization rates. More reliable technologies can be used, at a given overall
cost, if some of the critical components are recovered. It is the committee's
view that if these criteria are met, substantial improvements in launch cost
will accrue, relative to those for current systems.
The family of launch systems envisioned is likely to accommodate the
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SPACE TRANSPORTATION
13
upper range of payload masses projected by the Department of Defense,
as well as the heavy lift requirements of the HEI. This can be done by
clustering modular liquid propulsion systems with staging appropriate to
the particular launch requirement. Key features of the required new launch
systems will be the use of modern materials and technologies. The engines
probably will operate at chamber pressures below those of the space shuttle
main engine and will be manufactured using advanced technologies, such
as precision casting, which lower costs while improving quality. Guidance
and control will take advantage of modern electronic technology to provide
fault tolerance and largely eliminate single point failures. Even with these
improvements, it will be desirable to configure the vehicles so that missions
can be completed after loss of one engine early in a launch. With the higher
performance that will be available, such robustness should be affordable.
The committee favors this approach for three reasons: First, if devel-
oped to the criteria outlined, such liquid bipropellant systems will have a
higher level of reliability than do the solid boosters utilized on the shuttle.
The engines can be test fired prior to launch, an engine-out capability is
feasible, and engine shutdown in flight is possible if a fault is detected.
Second, pollution of the atmosphere by chlorides, as occurs with solid
propellants, would be eliminated. This is likely to become an increasingly
serious issue as launch rates rise in the buildup of the HEI. Finally, the
committee believes liquid bipropellant systems have the potential for signif-
icantly lower recurring costs compared to solids. Thus, for the long term,
the committee anticipates reliance on liquid rockets.
There are several alternatives to the above strategy. One considered by
the committee is a flexible family of launchers that would use existing fully-
developed solid propellant motors in clustered arrangements, providing up
to four stages and a wide range of payloads to LEO or to higher orbits.
Such launchers are certainly feasible. Their development costs would be
lower than those of the ALS-class systems discussed above, and their
recurring costs would probably be lower than the Titan, shuttle, or Shuttle-
C. The committee has three concerns about this concept: First, with a
large number of solid motors, this system is not likely to be as reliable as
those using liquid rocket technology. Second, the recurring costs are likely
to be substantially higher than for an all-liquid system, and, finally, the
solid upper stages would present environmental problems with injection of
chlorides at very high altitudes.
The committee notes, however, that the universal launch system com-
plex being designed for this family of launchers, based on oil platform
technology, has very attractive features such as modular construction of
assembly buildings and a launch platform that can be elevated. An elevat-
able launch platform avoids the construction of flame ducts and can serve
a variety of launch vehicles. These should be considered carefully if a new
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14
HUM41V EXPLOIUTION OF SPACE
launch complex is needed. This concept could be applicable to manned or
unmanned systems and could reduce costs for the entire launch complex.
Another alternative is single-stage-to-orbit rocket launch systems,
which have been proposed many times. This option has been raised
again in the present context in a configuration that conceptually would
offer engine-out capability, a safe abort at any point in the launch, and
full recoverability. The committee's brief review of this concept has led
to the conclusion that it is founded on unrealizable assumptions regard-
ing structural weight and propulsion system weight. Dramatic advances
in single-stage structural technologies and in materials, even beyond those
anticipated in current.programs such as NASP, would be required to make
this a viable concept.
A sea-launched, two-stage, fully recoverable system with pressure-fed
engines has also been suggested, with the projection that the components
could be reused up to 25 times. In any launch system, optimistic reuse
projections can lead to attractive, but unrealistic, estimates of costs, and in
the committee's judgment, such extensive reuse is improbable due to the
effects of the marine environment. Ocean operations can pose more of a
problem than an aid. Very large pressure-fed systems are also difficult to
deal with and require very large nozzles. Further, this concept currently
lacks a credible plan for recovery of the second stage, which would reach
full orbital speed.
NUCLEAR THERMAL PROPULSION
Although the reference approaches in the NASA 90-Day Study rely on
chemical propulsion, NASA has included nuclear thermal propulsion as an
option to be considered for orbital transfer to Mars. Several possibilities
have been mentioned within this general class of systems, all of which offer
higher specific impulse than chemical rockets and employ hydrogen as the
propellant.
The alternative nuclear propulsion technologies differ in the temper-
ature to which the hydrogen is heated by the fissioning nuclear fuel; the
pressure level in the thrust chamber (which along with the temperature
determines the extent of dissociation of the hydrogen to atomic form); and
the power density assumed to be achievable in the reactor. The tempera-
ture and pressure determine the specific impulse, while the power density
largely determines the thrust-to-weight ratio of the propulsion system.
The baseline capability is taken to be the NERVA class technology,
the technical feasibility of which was demonstrated in the late 1960s. In
the NERVA program, a reactor was tested on the ground for periods
longer than required for operation, at power densities that would yield
thrust-to-weight ratios on the order of five, and at temperatures giving
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SPACE TRANSPORTATION
15
specific impulse as high as 850 seconds. This technology is available for
full-scale development. It should be evaluated for injection to Mars in
competition with hydrogen-o~ygen chemical systems. The higher specific
impulse of the nuclear rocket results in a smaller propellant expenditure
for a given total impulse, but the propulsion system weight Is higher, so that
its attractiveness depends on the velocity change needed, and whether the
system is reused. A major advantage of nuclear propulsion is its ability to
enable transfer between Earth and Mars in one-half to one-third the time
required with single-stage chemical propulsion systems. This advantage
could be critical, pending the outcome of research on human performance
in space for long periods. The use of nuclear technology in space faces
formidable barriers of public acceptance, however, especially if employed
in Earth orbit. Therefore, issues of safety are paramount in research and
development.
An advanced reactor design has been partially evaluated experimentally
that offers much higher power densities, hence much higher thrust-to-weight
ratios, than the NERVA class technology. If proven feasible, this class of
technology will make the nuclear rocket more attractive relative to chemical
propellants. The risk involved in this technology development appear very
high at present, but the committee urges a feasibility test be carried out
to determine what thrust-to-weight ratio is practically achievable. It also
recommends that the potential of the technology be reviewed by a senior
group experienced in nuclear rocket technology.
The 90-Day Study mentions gaseous-core nuclear rockets as offering
much higher specific impulse levels. A number of gaseous-core reactor
concepts were carefully evaluated in the years between 1959 and 1970,
but none was found to be technically feasible. Unless a new idea has
appeared, which is always a possibility, the committee believes the gaseous-
core nuclear rocket technology is too speculative at this time and should
be dismissed as a possibility.
If careful systems studies, using thrust-to-weight ratios and specific
impulse known to be feasible, show a significant advantage for nuclear
rockets in trip time or in weight to orbit, an in-space demonstration of
this technology should be done as soon as possible taking into account
requirements for crew, ground personnel, and public safety covering all
phases of launch and flight, including mission abort. It will not be feasible
to select the nuclear rocket as a baseline in a system architecture until such
a demonstration has been conducted.
NUCLEAR ELECTRIC POWER
The committee believes that nuclear power eventually will be essential
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HUAt4N EXPLORATION OF SPACE
for lunar and Mars bases. The NASA reference approaches incorporate
nuclear power; The Great Exploration does not.
At present, the only active technology program applicable to this need
is the SP-100 thermoelectric space reactor, which has been pursued under
a tri-agency program for several years. SP-100 was initiated in the absence
of a definite mission requirement as a general purpose space power source.
This program should be redefined in light of the requirements of the
HEI and committed to development; nuclear thermionic research should
continue to be pursued as well.
Consideration should be given to demonstration of the nuclear elec-
tric power system as the power source for an electric propulsion system,
which may have application to science missions with large launch veloc-
ity requirements. (In fact, a number of outer planet missions have been
suggested, including a Jovian system grand tour, that will require such
advanced power sources.) Here, as with the nuclear rocket, considerations
of safety must be incorporated into research, development, and demonstra-
tions and factored into assessments of overall systems performance. The
nuclear electric system might be demonstrated within these constraints by
a mission in which the system is launched to a high orbit, say 600 miles,
before it is operated. The orbit could then be raised by nuclear-electric
propulsion to geosynchronous orbit or beyond.
If safety concerns can be successfully addressed, and feasibility demon-
strated, the committee believes that use of nuclear power and propulsion
can meet many needs in the human exploration of space.
Representative terms from entire chapter:
space shuttle
