When humans next chart a course beyond Earth orbit, setting sail for an asteroid, the moons of Mars or the Red Planet itself, they will do so atop the mammoth Space Launch System (SLS) and will ride one of the most powerful rockets ever brought to operational status. In its final ‘evolved’ form, the SLS will boast an Earth Departure Stage, propelled by a trio of cryogenic engines whose heritage extends back almost five decades. The J-2X is derived from the same powerplant which once boosted Apollo astronauts to the Moon and it recently underwent a record-breaking test of the critical components which will someday enable exploratory missions deeper into space than ever before. At the same time, NASA and Boeing last week finalised the requirements needed to create blueprints for the SLS ‘core’ stage, ahead of real hardware production. These enormous strides serve to demonstrate that America’s new heavy-lifter is steadily progressing towards a greater level of maturity than was ever reached by its ill-fated predecessor, the Constellation Program’s Ares V.
The J-2X ‘powerpack’, recently tested by prime contractor Pratt & Whitney Rocketdyne at NASA’s Stennis Space Center in Mississippi on 8 June, represents the upper segment of the engine. It includes the gas generator, liquid oxygen and hydrogen turbopumps and related ducts and valves to feed the systems which will produce engine thrust. By testing it and throttling it in the absence of the main combustion chamber, injector and nozzle, engineers were better able to evaluate its turbomachinery under a range of conditions to determine durability, performance and safety margins. In doing so, they also managed to achieve a new record for Stennis’ A Test Complex. At 1,150 seconds – more than 19 minutes – the powerpack demonstration was the longest firing ever conducted at this individual stand, surpassing a 1,075-second test of a Shuttle main engine in August 1989.
Video courtesy: NASA
“This is the longest and the most complex J-2X test profile to date,” said NASA’s Mike Kynard, the SLS liquid engines element manager for SLS. “By combining as many test objectives as we can, we aim to get the most out of every opportunity and work as affordably and efficiently as possible, while maintaining a reasonable level of risk.” It is a remarkable accomplishment, when one considers that the J-2X is the first human-rated liquid engine for exploration beyond Earth orbit to be produced since the Apollo era. In its final form, it will weigh 5,450 lb and three of its kind will power the Earth Departure Stage of the fully-evolved Block II variant of the SLS, which will be capable of lofting a payload of up to 130 metric tons.
The $1.2 billion contract to design and develop the J-2X was awarded to Pratt & Whitney Rocketdyne by NASA in July 2007 and within weeks the construction of a test stand at Stennis got underway. Over the course of the following year, nine tests of Apollo-era J-2 engine components were performed and the J-2X’s gas generator underwent its first successful shakedown in September 2008. The engine was later hot-fired for 500 seconds in November 2011, leading up to this month’s marathon test of its powerpack.
Of course, the SLS comprises far more than the J-2X. Its huge core stage, which will stand more than 200 feet tall and measure 27.5 feet in diameter, will provide much of the muscle needed to deliver payloads – including NASA’s Orion Multi-Purpose Crew Vehicle – into low-Earth orbit. Last week, engineers from SLS prime contractor Boeing and from the Marshall Space Flight Center in Alabama, which is managing the rocket for NASA, formally reviewed the requirements, design concepts and production approaches which will advance the core stage to the level of blueprint creation and hardware construction. In the words of Tony Lavoie, manager of Marshall’s SLS stages element, the review represented “the first major checkpoint for our team”, placing NASA and Boeing “right on track to deliver the core stage for the SLS Program”.
That core is fed by liquid oxygen and hydrogen and draws on the design of the Shuttle’s External Tank, with the aft section adapted to accommodate its main propulsion system and the upper section modified to accept an inter-stage structure. The first few missions of the core will be powered by between three and four RS-25D Shuttle main engines, of which the SLS Program currently holds an inventory of 16, whilst subsequent flights will utilise uprated RS-25Es. Side-mounted onto the core will be a pair of Solid Rocket Boosters, again drawing on a Shuttle-era heritage, which prime contractor ATK Thiokol plans to subject to initial qualification tests next year.
“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 William Gerstenmaier, the space agency’s associate administrator for the Human Exploration Operations Directorate. “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 US aerospace workforce to make the dream of deeper Solar System exploration by humans a reality in our lifetimes.”
‘In our lifetime’ can often be perceived as a blurred concept, particularly when one considers NASA’s worst-case budgetary scenario for the conduct of SLS missions. Although the earliest variant of the rocket, capable of lofting a payload of up to 70 metric tons, is due to fly its maiden mission in five years’ time, the fully-evolved 130-metric-ton vehicle, with its J-2X-propelled Earth Departure Stage, is not expected to debut until the early 2030s. As the schedule presently stands, Exploration Mission-1 (EM-1) will occur in December 2017, with the SLS launching an unmanned Orion spacecraft around the Moon. This will be followed by a 10-14-day crewed voyage (EM-2) to lunar orbit, perhaps in August 2019…only weeks after the 50th anniversary of Neil Armstrong and Buzz Aldrin’s historic steps on the Sea of Tranquillity. Both flights will utilise an Interim Cryogenic Propulsion Stage (iCPS), probably based upon the second stage currently used by the Delta IV rocket.
In the absence of a firm exploration roadmap from NASA, subsequent flights are more uncertain, with President Barack Obama’s oft-noted mission to a near-Earth asteroid pencilled in for somewhere in the mid-2020s, together with proposals for voyages to geostationary orbit, low lunar orbit and the lunar surface itself and even Mars and its moons, Phobos and Deimos. Last December, Boeing advanced a plan for an Exploration Gateway, with components based on already extant International Space Station hardware, lofted into one of the Earth-Moon Lagrange Points to enable future deep space exploration. Whether or not the Gateway would be an SLS payload remains to be seen; certainly, current projections envisage the use of an Evolved Expendable Launch Vehicle or perhaps SpaceX’s Falcon 9. Nevertheless, the technical and managerial advances made in recent weeks and months towards the development of America’s new heavy-lift launcher carry great promise for the future human exploration of the Solar System.