WASHINGTON — Lacking the funds to complete a flight-ready J-2X upper-stage engine before 2021, NASA will procure a commercial cryogenic rocket engine for the first two flights of the Space Launch System (SLS), the agency said.

“The 70 metric-ton SLS configuration will include an interim cryogenic propulsion stage to enable the Orion trans-lunar mission,” Jennifer Stanfield, a NASA spokeswoman at the Marshall Space Flight Center in Huntsville, Ala., said in a Nov. 22 email. “Acquisition planning is in progress to evaluate the various commercial options available to NASA.”

She would not say which systems NASA was considering, but noted that “affordability is one of the driving objectives of SLS.” The only U.S. cryogenic upper-stage engine in service is the RL-10 that Pratt & Whitney Rocketdyne builds for United Launch Alliance’s Atlas 5 and Delta 4 rockets.

SLS and its companion crew capsule, the Lockheed Martin-built Orion Multi-Purpose Crew Vehicle (MPCV), comprise the next U.S. government-owned deep human spaceflight system. NASA is developing SLS and MPCV partly by using contracts awarded under the canceled Constellation program, the agency’s previous attempt to create a successor to the space shuttle. Lockheed is building MPCV under one such contract, and Pratt & Whitney Rocketdyne of Canoga Park, Calif., is working on the J-2X as part of another.

The maiden flight of SLS is scheduled for 2017. In its debut, a 70-metric-ton variant of the rocket will boost an unmanned MPCV to the Moon and back. A second flight in 2021 would aim to repeat the feat with a crewed MPCV. Both launches would feature an SLS with a core stage powered by a cluster of four or five RS-25D engines Pratt & Whitney Rocketdyne originally built for the space shuttle.

NASA recently proposed adding $130 million to Pratt & Whitney Rocketdyne’s $1.5 billion J-2X contract to cover work the company would need to do to support use of the existing RS-25D engines for four or five initial SLS flights.

Because NASA is anticipating flat budgets for SLS for the next several years, the agency now plans to put J-2X development on a four-year hold after a battery of tests on the engine wrap up some time in the 2013-2014 timeframe, Stanfield said.

Earlier this month, after the first of four J-2X development engines completed a 500-second test firing, SLS Deputy Project Manager Jody Singer said that the J-2X would not fly until after 2021.

Ron Ramos, Pratt & Whitney Rocketdyne vice president of exploration and missile defense, said certifying the J-2X to carry humans would take six months to a year. Integrating a flight-ready J-2X with an SLS stack would take an additional year or two, he told Space News in a phone interview.

Ramos said that at NASA’s urging, Pratt & Whitney Rocketdyne studied a number of possible program changes aimed at preserving the 2017 SLS/MPCV mission under a flat NASA budget.

Stalling J-2X development is “one of many scenarios we are preparing for,” Ramos told Space News Nov 18.

NASA’s 2012 budget, which was signed into law Nov. 18 , includes $1.8 billion for SLS.

NASA’s spending plan for 2013 and beyond is not due to be released until February when President Barack Obama presents his annual budget proposal to Congress.

However, NASA officials said the space agency is proceeding under the assumption that it will be dealing with flat budgets for the foreseeable future.

“Given the constrained budget environment, NASA is unable to develop all necessary components for the long-term exploration goals, such as the upper stage and the J-2X, in parallel,” Bill Hill, assistant deputy associate administrator for explorations systems development, wrote in a Nov. 22 email to Space News.

Hill reiterated that NASA is still planning to fly a 70-metric-ton SLS in 2017 and 2021, and added that “we believe that our current, flat-line funding profile offers sufficient resources to accomplish both missions.”

Dan Leone is a SpaceNews staff writer, covering NASA, NOAA and a growing number of entrepreneurial space companies. He earned a bachelor’s degree in public communications from the American University in Washington.