The mighty RS-25 rocket engine

NASA's Marshall Space Flight Center intends to buy 18 additional RS-25 rocket engines from Aerojet Rocketdyne in Canoga Park, Calif., to support the agency's new Space Launch System (SLS). The estimated value of this proposed action is $2.2 billion with an estimated period of performance from date of execution through July 15, 2028.

“RS-25” is the generic designation for the staged combustion, liquid hydrogen/liquid oxygen rocket engine system previously known as the Space Shuttle Main Engine (SSME) and it is the established core stage engine for the new SLS rocket. This proposed effort will be based on a previously existing production line for an engine system with thirty years of human spaceflight history. It is not a new engine development effort. The purchase of 18 additional RS-25s will fullfill the SLS Program engine requirements that are beyond the scope and the period of performance of the current contract, which provides six RS-25 flights engines with a period of performance through Sept. 30, 2024.

The new SLS vehicle uses a “stage-and-a-half” configuration that ignites the four core stage engines seconds before liftoff and then ignites the solid motors (boosters) at liftoff. The boosters burn out approximately two minutes into the flight while the core stage engines continue to burn until the desired cutoff point is achieved. This basic configuration is flexible for both early demonstration flights and for ultimately evolving the SLS vehicle to a configuration with a capability to lift 130 metric tons to low-earth orbit in support of future exploration missions.

NASA's strategy for minimizing the cost for development of the new SLS vehicle is to leverage the assets, capabilities and experience of the Space Shuttle Program. Early SLS flights will utilize 16 RS-25 engines from the Space Shuttle Program with necessary refurbishment and adaptations for SLS. The availability of 16 flights assets was one factor in selecting the RS-25 for the SLS architecture along with the demonstrated performance and extensive experience with this engine. These 16 assests can be used for the first four flights of SLS, with four engines per stage.

In addition to the 16 engines, six new engines were previously procured under the current contract to provide engines for the fifth SLS flight and two risk mitigation spare engines.

For the additional 18 RS-25 engines, it is estimated that each unit will take five years to fabricate and assemble. While it will be the goal of this procurement action to reduce this cycle time, the timeline of five years matches the documents Aerojet Rocketdyne historical norm for this engine.

Aerojet Rocketdyne designed, developed, and matured the RS-25 engine system as the SSME over the past forty-plus years, and has been the only source utilized for the design, development, manufacture, refurbishment, recycle, testing, and flight operations of the RS-25 for the life of the Space Shuttle Program. Further, Aerojet Rocketdyne is the contractor currently responsible for adapting the residual Space Shuttle RS-25 hardware for use as part of the SLS Program. No other contractor has this accumulated knowledge with respect to hands-on technical experience and programmatic history of this engine.

Aerojet Rocketdyne manufacturing is performed at three facilities; machining, welding, assembly and test of subassemblies at the Canoga Park California Strategic Fabrication Center, turbopump assembly operations at the West Palm Beach, Florida facility, and final assembly and test at the NASA Stennis Space Center in Mississippi. While the NASA Stennis Space Center is a Government-owned facility, the other two facilities are Aerojet Rocketdyne facilities.

Aerojet Rocketdyne (and its predecessor companies) is the only contractor in this country to design and build large liquid hydrogen/liquid oxygen rocket engines for human spaceflight. They designed and built the first liquid hydrogen/liquid oxygen engines to ever fly, the RL10, first launched in 1963. They designed and built the J-2 engine used for the second stage of the Saturn IB vehicle and the second and third stage of the Saturn V vehicle. Furthermore, they designed and built the world's largest liquid hydrogen/liquid oxygen production engine, the RS-68, for the Delta IV vehicle in support of the Department of Defense. And most recently, they restarted production of six new RS-25 flight engines for the SLS Program.

For the past forty-plus years the RS-25 was, and remains today, the highest performing large staged combustion liquid hydrogen engine in the world. It is a unique engine with unique capabilities that took substantial and prolonged effort to develop and certify for human spaceflight. An attempt to develop a new engine with a new contractor (or the RS-25 engine with a new contractor) would require significantly more engine hot fire testing to certify for flight.

The RS-25 engine design carries with it four decades of development and production activity and three decades of flight experience. As a staged-combustion liquid hydrogen engine, the RS-25 engine design is also the most advanced and complex engine ever built and flown. With over one million seconds of accumulated hot-fire test time and the equivalent of over four hundred human spaceflights, the RS-25 design, production processes, and operational procedures have incorporated within them thousands of lessons learned.

The overall period of performance for the proposed activity in support of the current SLS Program flight manifest is approximately nine years. To support an August 2025 launch of a fifth mission, the first four engines must be delivered no later than July 2025. Based on historical data it takes a lead-time of approximately five years to fabricate and assemble the engine. Based on an ATP of July 2019, the first of four engines would need to be delivered in July 2024. Assuming the engines are delivered every three months, the fourth engine would be delivered in April 2025, with about three months contingency in the schedule. The last of the 18 engines would deliver in the first quarter of fiscal year 2028.