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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.