The shuttle is the only U.S. launch vehicle capable of carrying humans into space. As a result, it will be critical to the space station’s assembly and operation. From December 1997 to June 2002, NASA plans to use the shuttle primarily to transport station components into orbit for assembly. During this period, 27 of the shuttle’s 34 primary payloads are to be station-related. At times, only two of the four shuttles will be available for station assembly. One shuttle—Columbia—cannot provide adequate lift, and, one of the remaining three shuttles will be undergoing scheduled maintenance during some portions of the assembly schedule. Also, most station components will have to be launched in a particular sequence to provide power and structural support for other hardware. In March 1993, the President directed NASA to redesign the space station. The new configuration—renamed the International Space Station—combines the efforts of Europe, Japan, Canada, Russia, and the United States. It also increased the station’s planned orbital inclination to make it more accessible from Russian launch sites, creating the need for additional shuttle lift capacity. Easterly shuttle launches from Kennedy Space Center take advantage of the earth’s normal west to east rotation. Launches to higher inclinations such as those needed for the space station lose some of this advantage, with a resulting loss in lift capability. In November 1993, the space station program manager requested that the space shuttle program implement modifications to provide the increased lift needed to support space station assembly. The shuttle program office responded by committing the program to increasing lift capability by at least 13,000 pounds on every station flight. The lift enhancement plan—first approved in March 1994—has been amended a number of times to introduce new ideas for achieving the required lift at the least cost. The original plan identified 13,000 pounds of added lift at a cost of about $535 million. In May 1995, NASA estimated that about 16,100 pounds of lift gain would be achieved at a cost of about $444 million. Both estimates included some recurring costs for enhancement hardware, as well as costs to integrate the enhancements, and reserves to cover the possible need for additional changes. The current plan includes about 30 individual actions that involve hardware redesign, improved navigational or flight design techniques, and new operational procedures. Figure 1 depicts the percentage of added lift NASA estimates will come from these areas, based on the May 4, 1995, approved baseline. Hardware design changes account for more than one-half of the added lift. The primary redesign program is the development of a new external fuel tank—the super lightweight tank—which involves incorporating a new aluminum alloy into the tank design. This alloy will reduce the tank’s weight and change its material properties. In addition, the tank will have to accommodate a new set of design loads created by the mix of hardware and flight design changes. Other development programs necessary to support the space station include various orbitermodifications and improved main engines. The super lightweight tank program has experienced some early development problems that could affect its performance. Shortly after beginning development of this tank, technical concerns about the properties of the new material were raised. An independent review of the program was performed, and based on its results, NASA adopted a more rigorous test plan for the tank and modified the tank’s production strategy. More recently, the uniqueness of the new metal caused delays in manufacturing a test article. NASA believes these early concerns have been resolved, but it recognizes that uncertainty with the development and manufacturing of the new material could ultimately reduce the amount of lift gain projected for the new tank. The main engine improvements are expected to make the engines heavier than the current engines. However, the new engines are expected to be more efficient, thus needing less propellent. They are also expected to permit occasional use at higher than normal thrust levels. Early test results indicated that the engines would not achieve all of the efficiency originally expected. NASA made additional modifications, and it now expects to achieve most of the originally predicted performance. However, as of May 1995, shuttle program officials still considered the engine development status to represent a threat to the lift gain expected from the enhancement program. An independent shuttle management review team also expressed concerns with these two programs. In its report, the team (1) concluded that the new tank had the potential for problems during development and manufacturing and (2) questioned using the improved engines for increased thrust capability. In addition to hardware redesign, NASA plans to incorporate flight design and operation enhancements. These enhancements include the use of more advanced navigational tools as well as software changes to create a more efficient trajectory. The effect of achieving greater efficiency during ascent is that less propellent would be needed. The most significant operational change involves the deletion of some of the contingency fuel, water, oxygen, and other consumables. NASA protects each mission by ensuring that there are sufficient quantities of consumables to continue the mission in the event of unexpected problems such as difficulties in docking and retrieving payloads. In the past, it has been NASA’s policy to cover nearly every possible contingency. The new policy reduces the amount of consumables by about 4,000 pounds per flight. According to NASA, the revised approach will still ensure that individual unexpected problems can be handled without jeopardizing the mission. However, the reduction in consumables increases the risk of mission failure if a combination of unexpected events occurs. Under the new policy, for example, it might not be possible to perform a second rendezvous with the station, if necessary, and, as a worst case, it could be necessary to jettison a payload before landing. NASA believes the increase in risk is minimal and cites the new policy as a means to reduce weight, increase lift, and save money. In addition, it notes that the maximum reduction in consumables will only be necessary on the heaviest of station flights. According to the program director, this change helped make it possible to terminate two of the more expensive enhancements—development of a lightweight booster and extended motor nozzle—at a savings of about $35 million. To support the first shuttle space station launch, all of the enhancement programs must be integrated and recertified into the shuttle system within a demanding schedule. NASA has developed a systems integration plan identifying the major events and schedules associated with the shuttle enhancement program, as currently approved. The plan describes over 200 individual events related to the development and integration of shuttle lift-increasing modifications. The events began in early calendar year 1994, and they will end with the first space station flight, which is scheduled for December 1997. The single most critical event is the delivery of the super lightweight tank, and, according to the chief engineer of the shuttle integration office, it is on a very success-oriented schedule that has already experienced some delays. While the tank’s critical design review has already been held, the final set of design loads are still being updated. Thus, many design and environment definition activities will occur in parallel. If any of the assumed design loads substantially change, additional certification cycles may have to be conducted. However, there is no schedule or budget margin that allows for major adjustments because the first tank is to be delivered only 2 to 3 months before the first launch. Based on its launch history and projected budget, the shuttle may not be able to meet the demanding launch requirements of the space station’s assembly schedule. To meet the station’s “assembly complete” milestone, shuttle officials have designed a very compressed launch schedule. During certain periods of the station’s assembly, clusters of shuttle flights are scheduled to be launched within very short time frames. The schedule calls for five launches within a 6-month period in fiscal year 2000 and seven flights during a 9-month period in fiscal year 2002. On two other occasions, three launches are scheduled in a 3-month period. This schedule equates to about 1 launch per month, or a rate of up to 12 flights a year for these periods. In addition, on two occasions, the schedule calls for launches of two missions with less than 35 days separating them. While NASA has achieved similar launch rates a few times, it will have fewer processing personnel during the space station era. The space station’s flight rate frequency cannot be met unless the orbiter is processed in 20 to 30 days less than standard. To process the orbiter at this rate, shuttle personnel will have to work overtime. However, according to operations officials, budget constraints could make it difficult to fund overtime. Because the schedule is so compressed at times, there is very little margin for error. According to shuttle and station officials, there is little flexibility in the schedule to meet major contingencies, such as late delivery of station hardware, or technical problems with the orbiters. Between December 1991 and September 1994, 9 of 22 shuttle flights slipped from the planned launch dates established 6 months before launch. The shuttle program maintained its annual flight rate, in part, by launching payloads out of sequence. However, during station assembly, most payloads must be launched in the established sequence. The Shuttle Program Director told us that he recognizes the launch schedule is tight and that if a significant delay occurs with any station flight, subsequent flights are likely to slip also. The shuttle program will be attempting to accomplish the demanding station assembly schedule with fewer resources than were available in the past. For example, to reduce operating costs, NASA has reduced the shuttle processing workforce at Kennedy Space Center by 1,400 people, or 20 percent, since 1992. According to a February 1995 internal workforce review, schedule risk already exists in areas such as engine testing, crew training, and flight software development, and NASA plans further funding cuts in the future. According to shuttle processing officials, NASA will reduce the shuttle processing workforce by another 900 people, or 15 percent, through fiscal year 2000. NASA continues to review all elements of shuttle operations to improve processes and increase efficiency and believes that these savings are achievable.