More than four years since three of its kind were last fired in anger, on 8 July 2011, to deliver Atlantis’ STS-135 crew towards orbit—on the poignant final flight of the Space Shuttle Program—Aerojet Rocketdyne’s famous RS-25 engine was granted a new lease of life yesterday (Monday). NASA has awarded the Sacramento, Calif.-based propulsion manufacturer a $1.16 billion, nine-year contract to restart its production, prior to the inaugural voyage of the Space Launch System (SLS) booster on Exploration Mission (EM)-1 in November 2018. Better known to history as the Space Shuttle Main Engine (SSME), the revival of the RS-25 production line will see Aerojet Rocketdyne modernize the oxygen-and-hydrogen-fueled powerplant to make it more affordable and expendable for the SLS, as well as implementing fewer parts and welds and certifying it to higher thrust levels.
Utilized for more than three decades to launch 135 shuttle crews, the RS-25 was a major player in the delivery of most of the elements of the International Space Station (ISS), as well as helping to enable nine dockings with Russia’s Mir space station and transporting three-quarters of NASA’s fleet of “Great Observatories”—namely the Hubble Space Telescope, the Compton Gamma Ray Observatory and the Chandra X-ray Observatory—into orbit. Despite suffering five on-the-pad abort situations between June 1984 and August 1994, and weathering a single, in-flight shutdown on Mission 51F in July 1985, the engine has proven exceptionally reliable and has been substantially upgraded over the years, leading to its last shuttle-era incarnation, the RS-25D (or “Block II”). First trialed alongside a pair of earlier “Block IIA” engines aboard Atlantis on STS-104 in July 2001, the RS-25D later flew as a complete, three-engine set for the first time on STS-110 in April 2002. It incorporated a robust High-Pressure Fuel Turbopump (HPFT) and enabled a Full Power Level (FPL) of up to 109 percent, as well as exhibiting greater strength and reliability over its predecessors, through the elimination of several welds, the use of a casting process for its turbopump housing and stronger components and bearings.
Under President George H.W. Bush’s Constellation Program, the RS-25D might subsequently have powered the core and second stages of the Ares V rocket, but these roles were ultimately taken by the RS-68 and J-2X engines. The Constellation Program, of which the Ares Program was an integral part, was later canceled by President Barack Obama in early 2010. Then, in the twilight years of the shuttle era, a number of options were considered for the remaining RS-25D inventory to be sold or donated to U.S. museums or universities, but when NASA unveiled the SLS program in September 2011 the venerable old engine was revived to boost the new rocket’s core stage. “SLS is America’s next-generation heavy-lift system,” said Julie van Kleeck, vice president of Advanced Space & Launch Programs at Aerojet Rocketdyne. “This is the rocket that will enable humans to leave low-Earth orbit and travel deeper into the Solar System, eventually taking humans to Mars.”
As part of the SLS design, four RS-25Ds were to be mounted at the base of the core, although the engine promises to pump out more thrust than it did during its shuttle heritage. Typical shuttle launches saw the engine reach 104.5 percent—or 491,000 pounds (222,700 kg) of propulsive yield—but during its first four SLS launches, between November 2018 and the early part of the next decade, it is expected to attain 512,000 pounds (232,240 kg), thereby hitting 109 percent of rated performance. Subsequent advancements anticipate that when the original shuttle-era RS-25D inventory is exhausted, a new RS-25E will be introduced, capable of up to 521,700 pounds (236,640 kg) of thrust. “The new engines will be certified for flight at 111-percent thrust,” NASA’s Cheryl Warner told AmericaSpace. “The shuttle-era engine was ground-tested at this level, but never fully certified for flight at the higher thrust level.” However, the “D” and “E” descriptors are no longer in active use and all engines are now simply referred to as RS-25s, but the 16 existing flight engines are referred to as “adaptation engines” to distinguish them.
“Part of NASA’s strategy to minimize costs of developing the SLS rocket,” the space agency explained in yesterday’s announcement, “was to leverage the assets, capabilities and experience of the Space Shuttle Program, so the first four missions will be flown using 16 existing shuttle engines that have been upgraded.” It was the availability of these 16 existing flight assets that proved a major factor in selecting the RS-25 for SLS core duties.
Those 16 RS-25Ds—together with two development engines for ground testing—were initially stored at NASA’s Stennis Space Center, near Bay St. Louis, Miss. Described earlier this year by Steve Wofford, manager of the SLS Liquid Engines Office at the Marshall Space Flight Center (MSFC) in Huntsville, Ala., the powerplant is “the most efficient engine of its type in the world”, with “a remarkable history of success and a great experience base that make it a great choice for NASA’s next era of exploration”. All told, the SLS Program currently has an inventory of 14 veteran shuttle-era engines, together with a pair of “rookie” RS-25Ds, including Engine 2063, whose assembly was completed at Stennis in May 2015, and which is currently targeted to fly aboard the second SLS launch, Exploration Mission (EM)-2 in the 2021-2023 timeframe.
Also this year, RS-25D hot-fire testing on Stennis’ A-1 test stand resumed for the first time since 2009. In January, it was fired-up for 500 seconds, after which scheduled work was undertaken on the facility’s high-pressure industrial water system, before additional tests got underway in May and June, all intended to gather critical data on the engine controller unit—its computerized “brain”—and better understand how the RS-25D will respond to colder liquid oxygen temperatures and larger inlet pressures due to the taller SLS core stage liquid oxygen tank and higher vehicle acceleration, as well as significantly greater nozzle heating, due to the presence of four clustered engines. One significant change in the engine design, as it matured from the shuttle to SLS programs, is the insulation on the exterior of the nozzle. “The SLS thermal environment will be more severe than on the shuttle,” NASA told AmericaSpace, “so we developed new ablative insulation to protect the nozzles.”
This aggressive phase of testing continued through the summer months and also included the longest-ever test of the RS-25D in support of the SLS Program, lasting a marathon 650 seconds, together with demonstrations at various power levels, up to 109 percent. These activities culminated, earlier this month, when Engine 2059 became the first “flight” engine to be raised into the A-1 stand for an intensive series of certification tests in the first part of 2016 for actual SLS operations.
“While we are using proven Space Shuttle hardware with these engines, SLS will have different performance requirements,” Mr. Wofford explained in June. “This is a whole new ballgame. We need way more power for these engines to be able to go farther than ever before when it comes to human exploration.” Beginning with the November 2018 maiden voyage of the “Block I”, 70-metric-ton variant of SLS—capable of delivering a payload of 154,320 pounds (70,000 kg) into low-Earth orbit—on the unpiloted EM-1 circumlunar mission, the 16 RS-25D engines will power four of these gigantic boosters into the heavens, including the crewed EM-2. However, unlike the shuttle, which returned to Earth with its engines intact and capable of refurbishment and reuse, the SLS core will be expendable, meaning that the RS-25s will be discarded at the end of each mission.
“Engineers often use the phrase “form, fit and function” to describe a design,” NASA told AmericaSpace. “The RS-25 has the same overall form, fit and function as the Space Shuttle Main Engine, with a few notable exceptions. We are benefiting from the relaxed life requirements—the new SLS RS-25 is an expendable engine, not a reusable engine—to pursue a number of affordability options to make the engines easier to produce and cost less.”
The heritage of the engines to be flown on the EM-1 mission in November 2018 stretches through more than a decade of shuttle operations, including Mir docking flights, the return-to-space voyage of John Glenn and ISS assembly and maintenance. According to NASA, the first SLS booster will be powered aloft by Engines 2045, 2056, 2058 and 2060. Behind those numbers are some of the shuttle program’s most dramatic and historic flights. Engine 2045 first flew aboard STS-89—the second-to-last Mir rendezvous and docking mission—in January 1998, as well as Glenn’s STS-95 in October 1998, and eventually closed out its shuttle career by powering STS-135 uphill in July 2011. Engine 2056 previously supported STS-109, Columbia’s last wholly successful voyage, as well as both the STS-114 and STS-121 Return to Flight (RTF) missions in July 2005 and July 2006. Engine 2058 helped to propel Discovery into space on her swansong, STS-133, in February 2011, whilst Engine 2060 was also aboard Atlantis for the STS-135 ascent.
In parallel with NASA’s announcement of the $1.16 billion, nine-year contract, Aerojet Rocketdyne revealed yesterday that the restarted RS-25 production line has been “significantly improved and made more efficient” since the end of the shuttle era. “The RS-25 engines designed under this new contract will be expendable with significant affordability improvements over previous versions,” explained Jim Paulsen, Aerojet Rocketdyne’s vice president for Program Execution, Advanced Space & Launch Programs. “This is due to the incorporation of new technologies, such as the introduction of simplified designs, 3D printing technology called “additive manufacturing” and streamlined manufacturing in a modern, state-of-the-art fabrication facility.”
Improvements in materials, the use of five-axis milling machines and digital X-rays are all expected to confer substantial benefits onto the revived engine. Combined with better processes, the RS-25D’s main combustion chamber will be produced at half of its previous expense and schedule, whilst flex-hoses will replace complex articulating joints at a mere fraction of their previous cost and nozzle jackets will benefit from fewer piece-parts and thus a minimal number of welds. Key components will be simplified with a “dramatically reduced part-count and number of welds” and the RS-25 itself will be certified to a higher operational thrust level. In addition to the design simplification, ongoing Value Stream Mapping (VSM) analysis—which “were proven effective” during the shuttle program—has identified significant cost and schedule benefits by eliminating inefficiencies, redundancies or waste in the production process flow.
Yesterday’s contract award, NASA noted, “restarts [Aerojet Rocketdyne’s] production capability, including furnishing the necessary management, labor, facilities, tools, equipment and materials required for this effort”, as well as “implementing modern fabrication processes and affordability improvements and producing hardware required for development and certification testing”. The contract—which runs through 30 September 2024—also provides for a potential future modification which would allow NASA to order an additional six RS-25 engines. “Four of the six engines to be delivered under the second phase of the contract will be used for a potential fifth mission,” NASA told AmericaSpace, “and the remaining two are dedicated as contingency flight engines.”
SLS RS-25 Test Fire August 2015 / Stennis Space Center
RS-25 – The Ferrari of Rocket Engines
Aerojet Video on Restarting RS-25 Development
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