NASA is considering dropping the Space Shuttle Main Engine (SSME) from its heavy-lift launch vehicle plans and using the cheaper-to-manufacture RS-68 engine instead.

Daniel Dumbacher, deputy director of the Exploration Launch Office at NASA's Marshall Space Flight Center, Huntsville, Ala., told reporters following his presentation at the Goddard Memorial Symposium here March 14 that a formal trade study is under way to examine the cost, schedule and performance merits of the SSME and RS-68. At present those two engines are NASA's first choice for the main stage engines that would power the planned heavy-lift cargo launcher NASA intends to build to boost payloads on their way to the Moon.

Dumbacher said the trade study would be completed this spring. "It's got to be done in the next month or so because it plays a factor in how we do our budget planning," he said.

If NASA goes with the RS-68, then the SSME would have no obvious future beyond the space shuttle program, which is slated to end in 2010.

NASA had previously planned to use a modified version of the SSME to power the Crew Launch Vehicle's upper stage, but decided earlier this year instead to go with an updated version of the J-2 engine that powered the upper stage of the Saturn 5 rocket.

Dumbacher said the J-2 decision figured into NASA's decision to take another look at the RS-68 for the heavy-lifter's core stage.

"Since NASA made the decision to use the J-2X for the [Crew Launch Vehicle], we have been looking at options which might allow us to use RS-68 propulsion for the [cargo launch vehicle] instead of the SSME," he said.

Because going with the RS-68 could affect the space shuttle program's fly-out plans, Dumbacher said the decision would not be the Exploration Systems Mission Directorate's alone to make. "These kinds of decisions are agency-level decisions," he said.

The RS-68 is the largest liquid oxygen-liquid hydrogen booster in existence, capable of producing 650,000 pounds of thrust at sea level. It was built by Pratt & Whitney Rocketdyne for Boeing's Delta 4 rocket. The SSME also also built by Pratt & Whitney Rocketdyne and has been in service for more than 25 years, powering every space shuttle on its way to orbit. Each SSME is capable of producing just under 420,000 pounds of thrust at sea level.

For the better part of a year, NASA's working assumption has been that the heavy-lift vehicle would be powered by a cluster of five SSMEs. That was the recommendation of the 2005 Exploration Systems Architecture study.

While human-rated and highly reliable, the reusable SSMEs are expensive to manufacture. NASA and its contractors are looking at whether an expendable version of the SSME could be produced more cheaply, but NASA officials have privately expressed doubts about just how much cost can be squeezed out of the roughly $50 million per copy engine.

The RS-68, at roughly $14 million per copy, is cheaper to manufacture, but it does not have the SSME's track record.

John Mitchell, a spokesman for De Soto, Calif.-based Pratt & Whitney Rocketdyne declined to comment, saying the company does not release cost information about its engines.

Sources familiar with the Exploration Systems Architecture Study trade analysis that recommended the SSME as the best choice for the heavy-lift rocket, said that while the RS-68 might be cheaper to build, it also offers less performance and would have to be human-rated before trusting it to launch Moon- and Mars-bound hardware worth billions of dollars.

Although the heavy-lifter is expected to enter service around the middle of the next decade as a dedicated cargo launcher, Dumbacher said NASA wants to preserve the option of eventually using the heavy-lifter to launch humans.

The RS-68 engine can produce more total thrust than the SSME, but the RS-68 has a much lower specific impulse, making it something of a fuel hog.

While it is certainly possible to use a cluster of RS-68 engines to power a rocket capable of boosting 125 metric tons of payload, sources familiar with the trade analysis said the rocket would probably have to be bigger than an SSME-equipped heavy-lifter because of the extra fuel it would need to carry to get the job done. One possible solution for keeping an RS-68-powered rocket from growing too tall, even for the 52-story Vehicle Assembly Building at Kennedy Space Center, would be to build it wider and add an extra stage. And that, said sources familiar with the Exploration Systems Architecture Study's trade analysis, could mean that NASA would not be able to reuse tooling, equipment and facilities it already has on hand for manufacturing and working with the space shuttle external tank.

"Some people think it will cost less to use the RS-68 because the engines are so much cheaper," said a source knowledgeable about the earlier analysis. But taking into account the "total integrated picture," the source said, the life-cycle cost of the RS-68 could be higher.

Dumbacher said NASA is asking all these types of questions in order to come up with the right answer.

"This is the time to be asking these questions," he said.