J-2X

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The J-2X was a U.S.-built, liquid-fueled rocket engine designed for NASA’s Ares programs and, later, the Space Launch System (SLS). It ran on liquid oxygen and liquid hydrogen and produced about 1,307 kN (294,000 lbf) of thrust in vacuum, with a specific impulse of 448 seconds. The engine weighed around 2,470 kg (5,450 lb), making it heavier than its predecessor, the J-2.

The J-2X was intended to power upper stages of the Ares I and Ares V rockets and, later, the Earth Departure Stage for SLS Block 2. It was based on the older J-2 design but was redesigned to be more efficient and easier to manufacture. Changes included removing beryllium, using a centrifugal turbo pump, different chamber and nozzle expansion ratios, a channel-walled combustion chamber, redesigned electronics, and a gas generator and supersonic main injector inspired by the RS-68. The project aimed to be cheaper and simpler than the RS-25 engine.

NASA awarded Pratt & Whitney Rocketdyne (now Aerojet Rocketdyne) a $1.2 billion contract in July 2007 to design, develop, test, and evaluate the J-2X. Testing began at the Stennis Space Center, with component tests in 2007–2008 and gas-generator tests in 2008 and 2010. Full hot-fire testing began around 2011. However, in 2013 development was paused due to funding limits, shifting mission plans, and the selection of an alternative upper-stage design for SLS. By 2022, components were auctioned, signaling that the program had effectively ended.