In 1972, President Nixon approved the development of the space shuttle. The shuttle—the world's first reusable space launch vehicle—was intended to provide routine, economical access to space. It would be capable of delivering a variety of government and commercial satellites to low Earth orbit and would serve as a platform for numerous human-related space activities. Plans called for a multiple-orbiter fleet that could fly many times a year, thus bringing down the cost of launching payloads into space. A massive development program culminated in the first launch of space shuttle Columbia in 1981.
In the early years of shuttle operations, every successful mission showed that the shuttle was indeed a very versatile spacecraft. However, some systems had to be modified in response to problems and anomalies experienced during the initial flights. Initial upgrades included improvements to the external tank insulation, the replacement of several thousand insulation tiles with insulation blankets, and modifications to the wheel brakes and auxiliary power units (APUs). Despite these and many other improvements, it became clear that the shuttle's extensive requirements for refurbishment and maintenance would make it difficult for the program to achieve high flight rates and low launch costs.
After 24 successful shuttle flights, the 1986 Challenger accident stunned the nation and caused the National Aeronautics and Space Administration (NASA) to reevaluate many fundamental design features of the shuttle vehicle, as well as its entire operations support system, in order to reduce risk. During the nearly two-and-one-half year recovery period following the accident, more than 200 changes were made to the shuttle system, including a major redesign of the solid rocket motor joints and the addition of a limited crew escape capability.
Maintenance and flight procedures were also significantly modified, and substantial structural improvements were made to the launch pad, the external tank, and the solid rocket booster.
More than a billion dollars was spent on these changes before the shuttle returned to flight, and funding in subsequent budgets was allocated for several major follow-up improvements. Upgrades planned for incorporation after the shuttle's return to flight included the advanced solid rocket motor (ASRM), a 14-inch diameter disconnect valve for the main engine propellant line, high-performance carbon wheel brakes, redundant high-speed nose wheel steering, and modifications to the shuttle's software to increase the chances that aborts during ascent will be successful. Virtually all changes were approved based on their capability to improve system reliability or operational safety. Some, including the ASRM, the 14-inch propellant disconnect valve, and a high-performance escape system, were later canceled because of unanticipated technical problems and/or high costs.
At the same time, the role of the shuttle in the nation's space endeavors was being reassessed at the national policy level. A December 1986 National Security Decision Directive stated that “NASA shall no longer provide launch services for commercial and foreign payloads unless those spacecraft have unique, specific reasons to be launched aboard the Shuttle.” In 1991, a new National Space Launch Strategy restated the shuttle restriction and added “as the nation is moving toward development of a new space launch system, the production of additional space shuttle orbiters is not planned.” It also stated:
By continuing to operate the Shuttle conservatively, by taking steps to increase the reliability and lifetime of existing orbiters, and by developing a new launch system, the operational life of the existing orbiter fleet will be extended (White House, 1991).
In compliance with these policy statements, NASA phased out shuttle launches of most commercial and defense payloads and initiated steps to improve the reliability of the shuttle and cut its operating costs to help fund new launch technologies with NASA's shrinking budget.
In 1992, NASA undertook a new initiative to assess and improve the safety and reliability of the shuttle. Based on the results of a limited 1988 quantitative risk assessment of the shuttle launch phase for the Galileo mission (General Electric, 1988) and building on the space shuttle main engine (SSME) project's attempts to improve the engine's safety margins, the program prioritized potential upgrades according to their ability to address the perceived predominant risk contributors. High on the list of proposed upgrades were new high-pressure fuel and oxidizer turbopumps, a two-duct powerhead (main injector), and a redesigned main combustion chamber. A new, more reliable, main engine heat exchanger, an upgraded APU, a health monitoring system for the main engines, and an upgrade to the orbiter cockpit displays were also given high priorities. Total funding
for these upgrades—most of which have been or are being implemented—was approximately $1.5 billion. (The health monitoring system for the main engine is still in the early research and development (R&D) stage.)
While these safety and reliability improvements were under development, the shuttle program was undergoing substantial cuts in contractors and civil service personnel and was implementing many changes to reduce the program's operational budget. In a five-year period, the shuttle budget was cut from $3.5 billion per year to $2.9 billion per year (in real-year dollars), while the flight rate of six to eight shuttle launches per year was maintained.
Three events in 1993 resulted in additional upgrades to the space shuttle system. A new program in which the shuttle would rendezvous and dock with the Mir space station required several modifications to the orbiter, including the development of a new payload bay airlock/docking system. The ASRM program was canceled, depriving the shuttle of approximately 5,000 kg of additional payload capacity. Finally, the planned orbit for the newly restructured International Space Station (ISS) was moved to a 51.6 degree inclination. The new orbit was compatible with Russian launch facilities but reduced the amount of payload the shuttle could deliver to the ISS by more than 5,000 kg.
In order to address these decrements, the shuttle program embarked on a campaign to improve the shuttle's payload capability significantly so that it could meet the ISS program requirements. The largest upgrade was a super lightweight tank, a $200 million program that increased the payload the shuttle could deliver to the ISS by 3,500 kg. Additional upgrades, including lightweight crew seats, adjustments to trajectory and propellant reserves, and many minor weight reductions throughout the orbiter, increased payload capacity by approximately another 4,000 kg.
The national policy debate about the possible replacement of the shuttle took another step forward with the National Space Transportation Policy of August 5, 1994 (White House, 1994). This policy charged NASA to “provide for the improvement of the space shuttle system, focusing on reliability, safety, and cost-effectiveness” and also to “be the lead agency for technology development and demonstration for next-generation reusable space transportation systems, such as the single-stage-to-orbit concept.” Following the release of this policy statement, NASA initiated the X-33 and X-34 prototype demonstration programs to test technologies for low-cost, highly reliable access to space. To free its scarce resources for the new programs, NASA decided to further reduce the cost of the shuttle program where possible, consistent with flight safety.
In February 1995, the Space Shuttle Management Independent Review Team issued a report (known as the Kraft report) recommending a freeze of the space shuttle configuration to cut costs (NASA, 1995). According to the report, “freezing the current vehicle configuration, hardware, and software will stabilize the program and allow reductions in cost.” The Kraft report also recommended that “future changes should be minimized and [should] concentrate on making the
vehicle more reusable and operational.” Following the release of this report, the shuttle program effectively stopped approving new changes to the space shuttle's design, unless the changes were required for approved missions or necessary for safety reasons, to avoid obsolescence, or to meet new environmental regulations.
In keeping with a new national space policy (White House, 1996), NASA planned for the shuttle design freeze to continue until the end of the decade, at which time a decision would be made as to whether developments in the X-series vehicles were likely to result in a replacement for the shuttle. If at the end of the decade a shuttle replacement appeared to be imminent, the shuttle design would essentially remain frozen until the shuttle was replaced by an operational reusable launch vehicle. If, however, no replacement vehicle were on the horizon, a program for major renovations and upgrades would be initiated to extend the shuttle's viability to 2020 and beyond. In the latter case, upgrades would be aimed at reducing life cycle costs, alleviating obsolescence, and supporting activities beyond the ISS.
To further reduce operational costs, NASA also began to transfer responsibility for some elements of the Space Shuttle Program to the private sector. In September 1996, NASA signed a contract making United Space Alliance (USA) the prime contractor for space shuttle operations. USA corporation—a joint venture between Boeing and Lockheed Martin—is now primarily responsible for operations, including launching, landing, refurbishing, logistics, and sustaining engineering. NASA retains control over the development of upgrades and plans to continue its overall management of the shuttle program for the foreseeable future, even as day-to-day operations for individual elements of the program come under USA corporation's authority.
During fiscal year 1997, NASA, still not sure if a timely shuttle replacement would become available, lifted the configuration “freeze” and authorized the shuttle program to dedicate the majority of its reserves each year to a new upgrade program. At approximately $100 million per year, this program funds minor modifications to reduce obsolescence, support missions, improve safety, and reduce costs, as well as advanced studies of potential major upgrades in preparation for a decision to continue operating the shuttle beyond 2012. Chapter 2 of this report describes this program. Chapter 3 assesses the process by which NASA prioritizes and selects proposed upgrades, and Chapter 4 presents a top-level technical assessment of key proposed upgrades.
General Electric Astro Space Division. 1988. Final Safety Analysis Report II for the Galileo Mission. Document 87SDS4213. Valley Forge, Pa.: General Electric Astro Space Division.
NASA (National Aeronautics and Space Administration). 1995. Report of the Space Shuttle Management Independent Review Team. February 1995. Washington, D.C.: National Aeronautics and Space Administration.
White House. 1986. National Security Decision Directive 254. United States Space Launch Strategy. December 27.
White House. 1991. National Space Launch Strategy. NSPD-4. July 10.
White House. 1994. National Space Transportation Policy. NSTC-4. August 5.
White House. 1996. National Space Policy Fact Sheet. September 19.