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Shuttle Saw Many Improvements Over the Years
By Mike Wright and Jim Owen
Reprinted from the April 14, 2011 Marshall Star

For more than 30 years, improvements in the Marshall Space Flight Center's space shuttle propulsion system -- the external tank, solid rocket boosters and solid rocket motors and the space shuttle main engine -- made the shuttle safer and better.

Each element underwent upgrades that improved performance, reliability and safety in a relentless pursuit of improvement.

Space shuttle external tank
  • In the early 1980s, the External Tank Project office implemented a redesign called "light-weight tank," reducing the structural weight from 76,000 to 66,000 pounds -- a significant accomplishment given that the tank flies to orbital velocity with the shuttle orbiter, and each pound saved results in an additional pound available for payloads. The first light-weight tank was launched in 1983.
  • In the early 1990s, a composite nosecone, manufactured at the Marshall Center, was added to eliminate the need for thermal protection material and a possible debris source.
  • During the mid-1990s, additional payload capability was required to meet International Space Station payload requirements. The External Tank Project again implemented a block redesign called "super light weight tank," which removed an additional 7,500 pounds of structural weight by using a light-weight aluminum lithium alloy. This significant accomplishment enabled partnership with the Russian Space Agency for assembly of the space station in a high-inclination orbit. The first flight of the super light-weight tank was in 1998.
  • After engineers completed proof pressure tests of an aluminum lithium test article to verify the tank design at Marshall, NASA Administrator Dan Goldin phoned astronaut Shannon Lucid aboard the Russian Mir in 1996, telling Lucid, "Marshall just completed the qualification test on the aluminum lithium tank. We have a 200 extra pound margin … to take payloads up to the International Space Station." This was a significant step toward the first launch of components to assemble the International Space Station.
  • Following the Columbia accident in 2003, the External Tank Project implemented significant design and processing improvements to reduce ascent debris risk. The bipod fitting that attached the tank to the orbiter was redesigned, heaters were added at strategic locations to reduce ice formation and foam ramps were redesigned or removed. A camera was added to provide video during ascent flight, enabling enhanced post flight assessment of ascent debris performance. These changes resulted in the risk due to ascent debris being reduced by more than a factor of 100, a remarkable achievement.
  • In a spectacular Independence Day display, space shuttle Discovery lifted off from the Kennedy Center July 4, 2006, on its STS-121 mission to the space station -- the first of the year and the second Return to Flight mission -- and successfully tested shuttle safety improvements. The 13-day mission also produced never-before-seen, high-resolution images of the shuttle during and after launch.
  • Friction stir welding, a process that uses a rotating pin tool to soften, stir and forge a bond between two metal plates, was implemented in the external tank manufacturing process. Because the method does not melt the material as fusion-welding techniques do, the weld has excellent mechanical properties and exhibits very little shrinkage or distortions even in long welds. The first friction stir welded tank flew in 2009.
  • Another improvement during the program was the cryogenic insulation system, which served the dual purpose of thermal protection from ascent heating. The protection system was successfully reformulated several times to comply with environmental regulations.
  • In 2005, Hurricane Katrina devastated the Gulf Coast and significantly damaged NASA's Michoud Assembly Facility in Louisiana, where the tanks were manufactured. Recovery from this natural disaster was a significant achievement and production was resumed within a month. During the program, 139 tanks were manufactured for flight and testing, concluding with a remarkable record of continuous improvement.
Reusable Solid Rocket Motor
The space shuttle solid rocket motor, the first and only human-rated solid rocket motor, also underwent significant redesign and upgrades during the life of the program. Improvements began in 1983 with the high performance motor that included increasing the motor chamber pressure, reducing the nozzle throat, increasing the nozzle expansion ratio and modifying the propellant grain-inhibiting pattern.

These enhancements resulted in increasing the payload capacity by 3,000 pounds. The first high-performance motors were flown on STS-8 in 1983. Following the space shuttle Challenger accident in 1986, the redesigned solid rocket motor incorporated significant safety improvements and flight performance was a remarkable success. Other modifications included:
  • Motor case and nozzle joint sealing systems were redesigned, with the addition of numerous features to add sealing system redundancy and robustness, including improved case metal hardware with a capture feature and third O-ring. An innovative insulation feature called a "j-leg," which prevents gas intrusion within the joint, was added.
  • Heaters were added to thermally condition sealing systems.
  • Processing controls, subscale and full-scale ground-scale testing and a thorough post-flight assessment process added to system safety.
  • Keys to process control included chemical fingerprinting to assess the consistency of delivered materials required for manufacturing the motor and implementation of process failure modes and effects analysis.
  • A ground-testing program included small motor testing at Marshall and the contractor facility in Utah. Full-scale motor testing also was conducted in Utah to assess material and design changes and special instrumentation to characterize performance.
  • In 2003, a space shuttle solid rocket motor was tested and pushed beyond typical launch performance boundaries. The five-segment test motor test, which ran for 118 seconds and generated more than 3.6 million pounds of thrust, performed flawlessly.
  • By the end of the program, 52 static motors had been tested.
  • Post flight assessment provided thorough evaluation of hardware condition, the ability to identify any items of interest and initiate corrective actions if needed. This capability was unique among solid rocket motors, as the space shuttle solid rocket motors were recovered for reuse.
  • Other innovative design improvements included use of a carbon fiber rope thermal barrier material in the nozzle joints implemented in 2008.
  • Improved resiliency O-ring material implemented in 2009.
  • An innovative intelligent pressure transducer was flown to assess pressure oscillations for future motor designs.
Solid Rocket Boosters
The solid rocket boosters -- which integrate subsystems needed for ascent flight, entry and recovery of the combined booster and motor system -- also were significantly improved during the past 30 years. Improvements were made in the thrust vector control, auxiliary power unit, avionics, pyrotechnic, the range safety system, parachutes, thermal protection, forward and aft structures and recovery systems.

The space shuttle solid rocket boosters were the only recoverable and refurbishable system ever developed and flown. During the program, recovery and post-flight operations were a challenging and essential component of accomplishing flight operations, achieving reliability and implementing continuous improvement. Challenges included subsystem integration and designing for severe loads, including water impact. Several of the subsystems evolved during the program through design changes:
  • The parachute system, essential for booster recovery, was redesigned with larger parachutes in 1983.
  • The booster thermal protection system evolved from a Marshall sprayable ablator initially used in 1982 to a Marshall convergent coating first used in 1996.
  • The aft skirt structure was modified in 1998 to add a bracket to increase structural safety.
  • The external tank attach ring was redesigned in 1988 from a 270-degree to a full 360-degree ring, and in 2006 the material was changed to improve structural margins.
  • The solid rocket boosters provided stand-alone data acquisition systems beginning in 1996 to record flight accelerations and recovery loads.
  • Enhanced data acquisition systems to support the post-Columbia Return to Flight external tank modifications flew in 2005; these systems also were used in 2008 to acquire pressure oscillations and structural response data for use by future programs.
  • Cameras were added in 1996 to observe parachute performance, and again in 2005 to observe ascent debris performance.
  • The command receiver and decoder, an essential feature of the range safety system, was redesigned in 2007.
  • Frangible nuts, used in the space shuttle pad holddown and release system, were redesigned in 2008. They featured an innovative pyrotechnic design to ensure proper timing during liftoff.
  • Environmental compliance was implemented for coatings, thermal protection systems and for post-flight operations.
  • Booster separation motors were redesigned and first flew in 2008. This was accompanied by conducting a substantial ground-test program to certify the new motors for flight.
  • The thrust vector control system auxiliary power unit incorporated a fuel pump redesign in 2011.
Space Shuttle Main Engine
The amazing space shuttle main engine implemented evolutionary upgrades during the 30-year program, resulting in a four-fold improvement in predicted reliability, many improvements in component life and reduced maintenance time.
  • The space shuttle main engine transitioned from its first manned orbital flight configuration to a phase II configuration in 1983. The phase II engine logged 231 engine flights and included improvements to the controller to increase memory, main injector improvements, turbine blade improvements within the turbopumps and additional nozzle insulation.
  • The engine transitioned to Block I configuration in 1995 with significant changes including a two-duct powerhead, an alternate high-pressure oxidizer turbopump featuring ceramic ball bearings, a single tube heat exchanger and improved hot gas sensors.
  • The Block I featured the new liquid oxidizer turbopump that Pratt & Whitney developed using a casting process that eliminated all but six of the more than 300 welds in the previous turbopumps.
  • The Block I included a new two-duct powerhead that improved fluid flows within the engine to decrease pressure and loads.
  • The Block I engine included a new single-coil heat exchanger that eliminated seven weld joints inside the engine to reduce wear, maintenance and post-flight inspections.
  • The Block I incorporated new ball bearings made of silicone nitride, a type of ceramic material, instead of steel. This new bearing material was 30 percent harder and 40 percent lighter than the old steel bearings and provided an ultra-light smooth finish to decrease operating friction.
  • A Block IIA configuration flew in 1998 with a large throat main combustion chamber, ceramic ball bearings in the low pressure oxidizer turbopump and improvements in the main injectors. The large throat combustion chamber reduced internal pressures in the space shuttle main engine, leading to reduced operating environments and improved component life.
  • The Block II configuration added a new high-pressure fuel turbopump in 2001. The advanced turbopump featured precision castings intended to significantly reduce welds in the pump. Engineers aimed their work at increasing pump operating margins and facilitating fabrication, assembly and maintenance.
  • The Advanced Health Management System, which flew the first time in 2006, interrogated the high-pressure turbomachinery during flight to detect precursors to a problem, and in the event of a problem, was capable of shutting an engine down prior to failure.
  • Marshall's Hardware Simulation Laboratory provided hardware and software verification for all main engine controller upgrades and software changes throughout the life of the program.
  • Development and operational ground testing, conducted at Stennis Space Center and Santa Susana Field Laboratory in California, was a key element of achieving success.
  • Marshall initiated a space shuttle main engine technology test bed program to test fire highly instrumented space shuttle main engine components in 1988.
  • The space shuttle main engine reached a significant milestone in 2004 when it surpassed one-million seconds of successful test firings and launches. The space shuttle main engine compiled a remarkable record of demonstrated reliability and successful flight operations.
Integration efforts performed at the Marshall Center during the 30-year program included propulsion system performance assessment, evaluation of natural environments including the ground lightning monitoring system and day-of-launch winds assessments, computational assessment of liftoff debris risks, imagery analysis, and many technical disciplines contributing to anomaly resolution and risk assessment. The Huntsville Operations Support Center provided real-time, day-of-launch support for all shuttle missions.

At the Marshall Center, all technical disciplines improved capability during the 30-year program. Design, analysis, test, materials and processing capabilities improved with application of advanced technologies, combined with the significant technical challenges posed by the Space Shuttle Program. For example, computational fluid dynamics emerged as an analysis and design tool -- unheard of in the 1970s but routinely used in aerospace design today. Many disciplines contributed to safety of flight improvements with new technology including computer-aided design and manufacturing, non-destructive evaluation techniques, failure analysis and fracture control methods and liquid and solid propulsion systems analysis. The Marshall Center also provided the material used for space shuttle wing repair, first flown on STS-114 in July 2005. The repair material was dubbed "NOAX," for non-oxide adhesive experiment, to be used if needed during a mission to repair the orbiter wing leading edge prior to reentry.

During the program a number of project management processes were implemented and improved. Technical performance, schedule accountability, cost control, and risk management were effectively managed and implemented. Perhaps the most important aspects of success were related to relationships developed with the prime contractors, which evolved to become very successful government/industry partnerships. Safety and mission assurance, evaluation of hazards and assessment of risk became routine, and continuous processes were evaluated as part of each mission's certification of flight readiness.

The 30-year flight program demonstrated reusability; accomplished deployment, servicing, retrieval and repair of satellites, including the remarkable recovery of the Hubble Space Telescope mission; demonstrated improved extravehicular capabilities; significantly advanced robotics; and accomplished servicing and/or assembly of two space stations, culminating with a magnificent International Space Station. We accomplished research in microgravity in the areas of materials science, biotechnology and human physiology. We launched great observatories to spectrally map the universe and launched missions to study planets and the sun. We studied the Earth's atmosphere, mapped the Earth's surface and studied space environments. Our missions of exploration and discovery have forever advanced human knowledge.

The propulsion elements provided a remarkable, high-performance, reusable rocket engine; evolved to provide highly reliable, large solid rocket motors; provided a fully integrated, recoverable and reusable booster; and provided a structurally efficient propellant tank with truly significant debris risk reduction. Integration of the propulsion system enabled reliable Earth-to-orbit performance with very small margins.

We finished strong. The final 22 flights were not easy, but were a fulfilling contribution to human spaceflight, worthwhile work. STS-135 is a superb "completion of mission."

In a message to employees following the STS-135 landing July 21, Marshall Director Robert Lightfoot said, "Indeed, we stand on the shoulders of giants from Apollo. But as I look back from the vantage point of STS-135, the last flight of the Space Transportation System, I realize that I walked every day among giants at Marshall. Generations for years to come will stand on your shoulders as they reach for the next step in our mission to explore space."

Wright is the Marshall Center historian. Owen is chief engineer of the Shuttle Propulsion Office.