A liquid-fueled engine, which once formed part of the most powerful rocket ever brought to operational status in human history, will kick off an ambitious series of tests in the coming weeks at NASA’s Stennis Space Center. The second J-2X development engine, designated No. 10002, will shortly be transferred to the A-1 Test Stand for an inaugural round of gimbaling evaluations as part of efforts to someday incorporate it into the Space Launch System (SLS) heavy-lift booster. “The upcoming test series is not only a critical step forward, but important to the Stennis test team as well,” said Gary Benton, manager of the J-2X test project at the Hancock County, Miss., installation. “This test series will help us increase our knowledge of the J-2X and its performance capabilities. In addition, the series will help us maintain the high skill level of our team as we look ahead to continued J-2X testing.”
Although current plans envisage that a set of RS-25D powerplants of Space Shuttle Main Engine heritage will propel the SLS’ first stage, it is expected that Pratt & Whitney Rocketdyne’s J-2X will ultimately form the backbone of the Earth Departure Stage in the fully-evolved form of the new booster. According to NASA’s conservative manifest, this final form of the SLS is not expected to fly until at least 2032, but will be capable of lofting up to 130 metric tons of payload into low-Earth orbit. (This is substantially higher than the 118-metric-ton capacity of the Saturn V.) Known as the “Block II,” this beefed-up form of the SLS will form the cornerstone of the agency’s human spaceflight aspirations and support the eventual (though somewhat nebulous) goal of reaching an asteroid or Mars, sometime in the mid-2030s.
Yet all grand endeavors begin with incremental steps. The first aim of the upcoming Stennis tests will be to verify and demonstrate the J-2X’s capabilities. Already, the No. 10001 development engine has been successfully “hot-fired,” and a total of 34 tests have already been undertaken, including a full-flight-duration firing of 500 seconds on only its eighth test. (This is the earliest full-flight-duration firing ever performed on a rocket engine during its test phase in U.S. history.) Engineers will compare data from a series of hot-fire tests from the 10001 and 10002 engines and will vary liquid hydrogen and oxygen inlet pressures and temperatures to assess their impact on overall performance.
The heritage of the J-2X extends back almost five decades, to the era of the Saturn V and its immediate predecessor, the Saturn IB, which boosted the Apollo astronauts on missions into low-Earth orbit and outward to the Moon in 1968–75. The latter, the IB, saw service throughout that period, lofting Apollo 7, together with three Skylab crews and the astronauts of the Apollo-Soyuz Test Project, and a J-2 provided the impetus of its S-IVB second stage. When the Saturn IB flew for the first time, the J-2 became the first rocket engine in history to be capable of restarting in space, and this was successfully demonstrated on the unmanned AS-203 mission in July 1966. Astronauts Gus Grissom and Wally Schirra, prime and backup commanders for Apollo 1, the first scheduled manned launch of the Saturn IB, nicknamed the booster “the big maumoo.”
However, it was during the Saturn V heyday that the engine literally reached its greatest heights. Five J-2s adorned its S-II second stage—which, at 82 feet tall, represented the largest cryogenically-fueled rocket stage ever built—with a single J-2 on its S-IVB third stage. The latter supported the so-called “Translunar Injection” (TLI) burn, three hours after launch, which propelled the Apollo spacecraft out of Earth’s gravitational well and onto a course for rendezvous with the Moon.
The eventual success of the J-2 in its endeavor to enable humanity’s first visit to another celestial body did not come about in the absence of difficulty. During the unmanned Apollo 6 flight in April 1968, two of the engines on the S-II abruptly shut down, just four minutes into a planned six-minute firing, and this required the others to burn for 59 seconds longer to compensate for the power loss. Since the failed engines were on opposite sides of the S-II, their loss balanced each other out and the Saturn V did not tumble, but the glitch offered a hairy reminder that playing with fire—and particularly playing with liquid hydrogen and oxygen, together with an enormous beast of a rocket—was by no means routine.
It is no understatement to declare that the performance of the J-2 during the 1968–69 timeframe had a direct impact upon the effort to plant American bootprints on the lunar surface. In the wake of the Apollo 6 incident, engineers found that frost forming on propellant lines when the J-2 was fired at ground temperatures offered extra protection against ruptures. However, this frosting did not occur in the vacuum of space … and this pointed to a potential failure point. A great deal of attention was paid to the engine in the latter half of 1968, until, in October, the crew of Apollo 7—astronauts Wally Schirra, Donn Eisele, and Walt Cunningham—rode the Saturn IB perfectly into orbit. As the first stage fell away, and the J-2-fed S-IVB second stage took over, four words from Schirra made all of the engineers’ efforts worthwhile. She was “riding like a dream.”
That dream ultimately went far beyond any previously imagined in human history, and in July 1969 the crew of Apollo 11 rode the Saturn V, with J-2s propelling its second and third stages, to accomplish the first piloted landing on the Moon. Five more landings were achieved by December 1972. The Skylab space station was boosted into low-Earth orbit in May 1973, marking the final flight of the Saturn V. And today, exactly four decades later, the engines that helped make these grand dreams a reality are ready to show us again what they can do. Last month, Jason Rhian wrote about efforts to include an upgraded version of the F-1 engine—of which five fed the Saturn V’s first stage—into the SLS design, and the forthcoming J-2X tests offer a tantalising shred of hope that, this time, the effort to send humans beyond Earth orbit may bear fruit.
Still, there remains a long road ahead. The contract to design and develop the J-2X—worth some $1.2 billion—was awarded by NASA to Pratt & Whitney Rocketdyne in July 2007, and since then its progress has been rapid and impressive. Within a year, it completed a successful shakedown of its gas generator, and the engine itself was hot-fired for a full flight duration of 500 seconds in November 2011. A little over six months later, a marathon test of its “powerpack”—the upper segment of the engine, including the gas generator, liquid hydrogen, and oxygen turbopumps, as well as related valves and ducting—was performed. This won rave reviews as the longest firing ever conducted at Stennis’ A Test Stand; at 1,150 seconds, it easily eclipsed a Space Shuttle Main Engine test of 1,075 seconds in August 1989.
Although the J-2X will not ride an SLS until at least the early 2030s, the first generation of the booster is presently less than five years away—according to NASA’s most recent manifest—from its maiden launch. In December 2017, it will carry the unmanned Exploration Mission (EM)-1 on a circumlunar trial of the Orion spacecraft, equipped with a European-built service module, and between two and four years after that event, a crew of four will embark on humanity’s first venture beyond Earth orbit in almost five decades when they circumnavigate the Moon.
“This is a very exciting time for the country and NASA as important achievements are made on the most advanced hardware ever designed for human spaceflight,” said Associate Administrator for the Human Exploration Operations Directorate William Gerstenmaier, last year. “The SLS will power a new generation of exploration missions beyond low-Earth and the Moon, pushing the frontiers of discovery forward. The innovations being made now, and the hardware being delivered and tested, are all testaments to the ability of the U.S. aerospace workforce to make the dream of deeper Solar System exploration by humans a reality in our lifetimes.”