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OCR for page 8
2
International Launch Vehicle Fleet
The following brief review of the requirements for launch
vehicles and how they will be met will provide a context for
the committee's recommendations pertaining to the inter-
national launch vehicle fleet. A typical ISS launch support
manifest for the operational phase after Assembly Complete
is shown, by vehicle, in Figure 2-1. The ISS launch support
requirements can be divided into three categories: (1) crew
transport; (2) propellant resupply and ISS reboost; and
(3) logistics.
CREW TRANSPORT
According to current plans, crew transport to and from
the ISS will be provided by the Space Shuttle, the Soyuz,
and the crew return vehicle (CRV). The Soyuz spacecraft
will be launched by the Soyuz booster. The CRV will be
carried to orbit by the Space Shuttle and will return either
independently or in the Space Shuttle. Approximately five
flights per year are planned for the Space Shuttle and two per
year for the Soyuz. The CRV will have a three-year life on
orbit attached to the ISS for crew return in emergencies. The
Soyuz will provide this function until Assembly Complete
and will also supplement the CRV throughout the operational
phases of the ISS Program. At least every six months, the
docked Soyuz will be replaced by a fresh Soyuz vehicle.
The European Space Agency is also evaluating a CRV
that would be launched by Ariane 5 and would be capable of
returning six crew members and/or experiment samples and
equipment from the ISS. As yet, no budget or schedule has
been established for the development of this vehicle.
PROPELLANT RESUPPLY AND REBOOST
Propellant resupply and ISS reboost account for more than
half of the total launch mass requirement during the opera-
tional phase of the program. In the baseline plan, the reboost
function will be filled by the service module, which will be
resupplied with propellant by four Progress vehicles per year
8
launched by Soyuz expendable launch vehicles. Because of
uncertainties about the future availability of these vehicles,
NASA has developed alternatives.
ALTERNATIVES TO RESUPPLY BY PROGRESS
The committee was informed that NASA's latest assess-
ment of the availability of Progress vehicles indicates that
only three launches per year will be available during the
assembly phase of the ISS Program. Therefore, NASA has
constructed and is implementing alternatives to ensure that
the necessary propulsion and reboost capabilities will be
available. The committee considered how these alternatives
would carry over into the operational phase of the program.
NASA described the following three alternatives for deal-
ing with the uncertainties in long-term Russian support of
the ISS (McClain and Hawes, 1998~:
Option 1. Provide funding to Russia as necessary to
complete and sustain all Russian contributions.
Option 2. Provide funding to Russia for items neces-
sary to continue the ISS Program in the near term while
funding the U.S. capabilities (e.g., U.S. propulsion
module) necessary to eliminate dependence on Russian
participation, thereby establishing U.S. autonomy, in
the long term.
Option 3. Provide no funding support to Russia and
adjust the schedule of the ISS Program, as necessary,
to accommodate late Russian deliveries.
NASA has selected Option 2 as the "recommended"
option, which would entail some funding of the Russian
Space Agency for the following reasons:
· to maintain use of Russian Mission Control
· to maintain the schedule of the service module and
ensure the availability of spare parts
· to maintain uninterrupted crew return capability via
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10
ENGINEERING CHALLENGES TO THE LONG-TERM OPERATION OF THE INTERNATIONAL SPACE STATION
Soyuz until the CRV is available
· to retain Soyuz/Progress resupply vehicles until a U.S.
propulsion module, autonomous transfer vehicle
(ATV), and H-II transfer vehicle (HTV) are available
In the long term, Option 2 would establish the autonomy
of NASA's ISS operations in the following ways:
The Space Shuttle orbiters would be modified so that
they could be used in place of Soyuz/Progress to
resupply propellant (to reboost the ISS).
A U.S. propulsion module would be developed for the
ISS.
If Russian logistic support for long-term operations can-
not be guaranteed, concerns may also arise about the long-
term viability of critical Russian-built ISS components, such
as the service module, which are permanent parts of the ISS.
Under the Option 2 scenario, the orbiters would be modi-
fied to provide propellant replenishment for a U.S. propul-
sion module, thereby supporting the long-term requirements
for reboosting the ISS. Some of this propellant could be
accommodated on Space Shuttle flights already planned for
other purposes, if excess payload capability is available.
However, because of the change in the location of ISS from
a 28.5 orbit (minimum energy, due east launches from the
Space Shuttle launch site at the Kennedy Space Center) to
the higher 51.6 inclination (minimum energy, due east
launches from the more northerly Soyuz/Progress launch site
at Baikonur), the payload delivery capability of the Space
Shuttle has been reduced to 13,600 kg (30,000 lbs) for flights
to the ISS. Therefore, not many flights are likely to have
excess payload weight margin. If Space Shuttle flights must
be added to supply reboost propellant, the incremental cost
of these additional flights should be a factor in NASA's com-
parison of Option 2 to the baseline operational mode (i.e.,
support by four Soyuz/Progress flights per year with no
supplementary Shuttle flights required), even though NASA
currently includes the ISS Shuttle flights in the Shuttle
operations account rather than the ISS account.
The committee agrees that Option 3 should be rejected
but is not convinced that Option 1 should be rejected in favor
of Option 2. The choice involves a trade-off between cost
and political considerations. The committee believes the cost
to NASA might be lower if Option 1 were selected and fund-
ing were provided to enable Russians to meet their commit-
ments. NASA did not provide a cost comparison or any other
rationale for selecting Option 2. Therefore, the committee
has no data to use as a basis for comparing the relative costs
of Options 1 and 2.
Option 2 in order to reduce International Space Station (ISS)
dependence on Russia and achieve autonomy for the ISS
Program in the long term. The cost estimates should include
the following items:
the incremental cost of operating the Space Shuttle to
replace Soyuz/Progress logistics flights
the cost of developing a U.S. propulsion module and
delivering it to the ISS
the cost and risk associated with integrating a U.S.
propulsion module with the ISS this late in the ISS
program
other costs that may accrue in establishing U.S.
autonomy
· risks to the program schedule
LOGISTICS TRANSPORT
Logistics transport will be provided primarily by the
Space Shuttle at the rate of five flights per year, supple-
mented by approximately one flight per year by the Euro-
pean Space Agency's ATV launched by Ariane 5 and about
two flights per year by the HTV (provided by the National
Space Development Agency of Japan and launched by the
H-II launch vehicle). The ATV will be able to dock
autonomously with the service module, or by means of
the space station remote manipulator system (SSRMS),
to the ISS nadir port. In this location on the service module,
the ATV's thrusters will be configured correctly to
reboost the ISS. Neither the ATV nor the HTV is designed
to survive reentry.
On the whole, the baseline plan for launch vehicle sup-
port of the ISS seems adequate, if it can be implemented.
The wide variety of vehicles with different operational char-
acteristics should add robustness to the program, a major
improvement over a "Shuttle-only" supply system. However,
as the committee was informed, the baseline has already been
eroded because the original launch rate of six Soyuz/Progress
flights per year after Assembly Complete has been reduced
to four. There is no indication how long this shortfall will
continue during the operational phase of the ISS.
CREW RETURN VEHICLE
The current plan for acquiring the CRV and operating it
from 2003 to the end of the ISS Program seems adequate.
The committee's understanding is that the CRV, which will
be constructed under contract, will be based on a design
developed by NASA reflecting experience with the X-38.
The cost of development and construction of four vehicles is
estimated at $580 million; the cost of CRV operation
Recommendation. The National Aeronautics and Space throughout the program is estimated at $187 million. Both
Administration should develop a concise comparison of amounts are included in current NASA budgets. The 2003
Options 1 and 2 to document the relative costs, as well as the operational date for the CRV seems to be very optimistic,
program risks and benefits, associated with implementing however, inasmuch as a prime contractor has not yet been
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INTERNATIONAL LAUNCH VEHICLE FLEET
selected. The (REV will have to be given a high priority to
meet the ISS schedule.
The following specifications to which the CRV will be
built are consistent with long-standing requirements:
crew capacity of seven
in the case of a need to evacuate the ISS ~ the capability
to evacuate and depart the ISS in three minutes and
return to Earth within nine hours
in the case of medical emergency ~ the capability to
return personnel to a medical facility on Earth within
24 hours of a declaration of need
· lifetime on the ISS of three years
· capability of holding in orbit for nine hours after sepa-
ration from the ISS
· cross range of 450 nautical miles
The CRV mass is projected to be 12,700 kg (28,000 lbs).
The vehicle will be carried to the ISS by the Space Shuttle
and attached by the SSRMS. At the end of its three-year life
on the ISS in a standby mode, the CRV can be carried down
in the Space Shuttle payload bay or flown down either by
remote control or, occasionally, with a flight crew on board.
The CRVis designed to be used in an emergency, however,
and does not have the system redundancy required to ensure
safe repetitive operation in a piloted mode.
Recommendation. The National Aeronautics and Space
Administration should proceed with the contractor selection/
contract award process to start the flight system develop-
ment and fabrication program for the crew return vehicle.
Recommendation. The National Aeronautics and Space
Administration should evaluate returning the crew return
vehicle (CRY) to Earth by remote control at the end of its
standby period on orbit to develop experience with its
systems, performance, and reliability and to increase the
probability of a successful return from orbit in case of an
emergency. Although this would entail an increase in cost
from $20 million to $40 million because the propulsion sys-
tem would have to be refurbished after flight, operating
experience with the CRV in flight mode would support the
development of a contingency plan in the event of a vehicle
malfunction. The higher confidence level and flight experi-
ence with the system would justify the additional cost.
U.S. EXPENDABLE LAUNCH VEHICLES
A question that frequently arises is whether one or more
of the U.S. expendable launch vehicles (ELVs) could be used
to supplement the planned logistics support vehicles for the
ISS. At present, NASA has no plans to do so. Ten assess-
ments in the past eight years by different NASA groups have
consistently shown that overall ELV mission costs would be
higher than those of the Space Shuttle or of foreign ELVs
11
(Poniatowski, 1999). Therefore, NASA has concluded that
it would be less costly to use one of the logistic modules
currently under development by the international partners
(i.e., the ATV or the HTV) than to use an ELV.
The recommendations from the NASA Space Transpor-
tation Architecture Study, which focuses on meeting
NASA' s future space transportation requirements at reduced
costs, advocates the development of technology for the next-
generation reusable launch vehicle (RLV) as a replacement
for the Space Shuttle. The NASA study also recommends
that a next-generation crew and cargo transfer vehicle
(CCTV) be defined. The CCTV could be launched either by
an REV or an enhanced expendable launch vehicle (EELV).
Phase 3 of the NASA study will determine whether the reli-
ability of an EELV would be equal to, or better than, the
Space Shuttle. These studies will eventually determine the
future course of the CCTV and REV programs and will
define their role in supporting the long-term operations of
the ISS (Mulville and Freeman, 1999~.
ASSURED ACCESS TO THE ISS
The reliability of the national and international launch
systems for supply, operations, and maintenance of the
operational ISS must be considered in the context of overall
space transportation resiliency and operability (i.e., the abil-
ity of the mixed fleet and associated propulsion systems
[Soyuz, Space Shuttle, Ariane/ATV, CRV, U.S. propulsion
module, and others] to ensure access to the ISS and to ensure
the continued viability of the system as an orbiting platform).
Both the Soyuz booster and the Space Shuttle are rated for
human flight and have demonstrated very high reliability.
The CRV and the U.S. propulsion module are undeveloped
flight systems that have no record of space flight; hence their
reliability and robustness are not known. Although prob-
lems during early development may delay the full opera-
tional capability of these new space flight systems, their
reliability levels ultimately are likely to be comparable to
those of the Soyuz and the Space Shuttle.
High reliability will reduce, but not eliminate, the poten-
tial of a launch vehicle being unavailable. If the mixed fleet
is international, the availability of a launch vehicle will be a
function not only of its reliability and stand-down time in
case of a failure, but also of political considerations. In addi-
tion, if launch systems are vying for commercial business,
their availability may also be a function of commercial
launch priorities and scheduling considerations that are dif-
ferent from ISS priorities and requirements. In other words,
they may be committed elsewhere.
In a worst case scenario, Soyuz/Progress flights would
have to be replaced because of a stand-down. The most
straightforward solution would be to fill the gap with addi-
tional Space Shuttle flights. Because the Soyuz/Progress
would normally fly four flights per year, but with substan-
tially less cargo-carrying capability than the Space Shuttle,
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12
ENGINEERING CHALLENGES TO THE LONG-TERM OPERATION OF THE INTERNATIONAL SPACE STATION
several Space Shuttle flights could probably replace the four
Soyuz/Progress flights (with some adjustments to the Shuttle
mission manifest). NASA has concluded that, in the event
that Progress is unavailable, the Shuttle could perform the
reboost function and that the propellant reserves aboard the
ISS could provide the attitude control functions for one year
or more.
A significant problem would develop, however, if a Space
Shuttle stand-down occurred at the same time as the Soyuz/
Progress stand-down. The seriousness of this situation
would depend on the duration of the concurrent stand-down
of the vehicles. In this case, crew safety and return would be
ensured by a CRV. The combination of the unavailability of
Soyuz/Progress, a concurrent Space Shuttle failure, and a
concurrent failure of the CRV is considered highly unlikely.
Thus, NASA' s planning is presently focused on maintaining
the normal operations of the ISS in orbit and the timely
development and flight certification of the CRV.
ROLE OF THE INTERNATIONAL LAUNCH
VEHICLE FLEET
If both the Space Shuttle and the Soyuz were in a stand-
down mode concurrently, support of ISS crew operations
would no longer be possible. If Soyuz were unavailable for
one year, there is about a 10 to 20 percent chance that the
Shuttle would also become unavailable during that time
(based on 0.99 and 0.98 probability of success per flight,
respectively, and 10 flights during this period).
Other vehicles, particularly the ATV (propellant logis-
tics) and the HTV, are part of the ISS logistic support
baseline and could be used for noncrew-related logistics
operations. The ISS can survive without a crew, and the
ATV can dock without a crew. The contingency plan for the
concurrent stand-down case, therefore, is to evacuate the
crew and to "mothball" the ISS by moving it to a higher orbit
and providing propellant replenishment via the ATV, thereby
prolonging its life and reducing logistics flight requirements.
Although some other launch vehicles are currently opera-
tional (e.g., Atlas, Delta, Proton, Sea Launch, etc.) and some
are under development (e.g., EELV, Kistler, X-33/
VentureStar, etc.), the use of other ELVs has not been
seriously considered (those under development cannot be
seriously considered until they become operational).
Private-sector funding for a new launch vehicle that meets
ISS requirements appears to be unlikely without a signifi-
cant government subsidy. No agreements have been reached
with Arianespace to pay for the use of the Ariane launch
vehicle to support ISS operations, and no launch priorities
have been discussed. NASA suggests that a barter arrange-
ment could be worked out in the event that emergency ISS
support was requested.
Recommendation. The National Aeronautics and Space
Administration (NASA) should develop contingency plan-
ning for personnel transport and resupply during the opera-
tional phase of the International Space Station (ISS). The
assessment should identify viable options other than moving
the ISS into a high storage orbit in case of a concurrent stand-
down of the Space Shuttle, the Soyuz, and the Soyuz/
Progress vehicles. NASA's plan should accommodate new
launch vehicles that may become operational during the
operational lifetime of the ISS for both crew transport and
ISS resupply. The plan should address the relative costs of
the various options for ensuring access to the ISS.
REFERENCES
McClain, G., and M. Hawes. 1998. International Space Station Overview.
Presentation by G. McClain, Deputy Associate Administrator (Space
Station), and M. Hawes, Chief Engineer (Space Station), to the
Committee on the Engineering Challenges to the Long-Term Operation
of the International Space Station, National Research Council,
Washington, D.C., September 17, 1998.
Mulville, D.R., and D.C. Freeman. 1999. NASA Space Transportation
Architecture Study Final Recommendations. Presentation by D.R.
Mulville, Chief Engineer, and D.C. Freeman, Deputy Chief Engineer
for Space Transportation, to J. Kerrebrock and J. Greenberg, members
of the Committee on the Engineering Challenges to the Long-Term
Operation of the International Space Station, National Aeronautics and
Space Administration, Washington, D.C., June 18, 1999.
Poniatowski, K. 1999. U.S. ELV Support to the International Space Station.
Presentation by K. Poniatowski, Manager, Expendable Launch
Vehicles, to the Committee on the Engineering Challenges to the Long-
Term Operation of the International Space Station, National Aeronautics
and Space Administration, Washington, D.C., February 17, 1999.
Representative terms from entire chapter:
space station
