The initial propulsion work in support of NASA's bid to return to the Moon and go on to Mars will focus primarily on adapting space shuttle systems and developing methane-fueled engines, a technology with which the United States has little experience.

The space shuttle main engine and solid rocket boosters are the basis for two new launchers NASA intends to develop, one for lofting an astronaut-carrying capsule known as the Crew Exploration Vehicle (CEV), and a heavy lifter for Moon-bound cargo loads. As currently envisioned, the CEV and other elements of the Moon and Mars exploration architecture would rely on engines fueled by a mixture of liquid oxygen and methane, NASA officials said.

Relying on a mix of old and new technology will help NASA limit risks in its propulsion development work, said Steve Cook, deputy director for space transportation at NASA's Marshall Space Flight Center in Huntsville, Ala. Marshall is in charge of developing the rockets NASA will need to return to the Moon by 2018 and go on to Mars.

"Historically when we've gone into a program like this, we've had engine developments that start right off the bat," Cook said. "Instead we're saying, 'let's focus our new propulsion development on much smaller-scale efforts. Let's also have back up plans in case it doesn't work out.' It's a balanced risk versus reward that we've laid out here."

NASA's propulsion plan also addresses the need to keep the shuttle propulsion work force intact until the last shuttle orbiter has flown.

Breathing New Life

NASA's strategy drew praise from the company that stands to benefit the most from NASA's planned propulsion work.

"The whole initiative [NASA Administrator Mike Griffin] has introduced since he took over breathes new life into the propulsion industry," said Byron Wood, president of Pratt & Whitney Rocketdyne of Chatsworth, Calif., the dominant U.S. maker of liquid-fuel rocket engines. "Before we were kind of in a going-out-of-business mode."

Wood added that NASA's emphasis on heritage systems will help Pratt & Whitney Rocketdyne, among others, ensure a safe fly-out of the space shuttle manifest.

"I think [Griffin] came up with a concept that helps preserve the skills by taking advantage of what's available in the shuttle program and adapting it to the new program so you give the existing [work force] a future and utilize their years of experience to make the future a success."

NASA hopes to field the 25-metric-ton CEV and its launcher, known as the Crew Launch Vehicle, by 2012 to support the international space station program. The rocket is based on the space shuttle solid rocket booster built by ATK Thiokol of Brigham City, Utah, and uses a variant of the space shuttle main engine as an upper stage.

That launcher also would be used starting in 2018 for Moon missions, lofting the CEV into low Earth orbit to rendezvous with an Earth Departure Stage and Lunar Lander, both of which would be launched on a heavy-lift cargo vehicle. The heavy-lift rocket, which NASA intends to start developing around 2011, is expected to be powered by two space shuttle solid rocket boosters and a cluster of five space shuttle main engines.

Advancing Methane Propulsion

The new methane-fueled engines would be used to power the CEV and the ascent stage of the Lunar Lander. NASA envisions using methane-fueled technology for both the main maneuvering engines and attitude- and reaction-control thrusters on the CEV.

Cook said NASA intends to initiate before year's end an advanced development effort for methane propulsion. The effort, he said, would be run by NASA's Glenn Research Center in Cleveland. Industry sources said Glenn is expected to issue a draft request for proposals for methane research in the coming weeks.

Cook, who was in charge of the launch vehicle portion of NASA's recently unveiled exploration plan, said NASA's decision to develop a pressure-fed methane engine was driven by a look ahead at what would be needed for manned missions to Mars. While methane is a less efficient propellant than liquid hydrogen, it is easier to store for long stretches and is readily available on Mars, making it possible for NASA to meet future propellant needs by taking advantage of martian resources.

Further, NASA sees the Moon as a proving ground for systems needed to explore Mars.

The United States has never flown a methane-fueled engine, Cook said, but has studied methane propellants closely enough over the years to know that it can be done. Some of this work was done by the major U.S. propulsion houses.

'Achievable' Engine

Both Pratt & Whitney Rocketdyne and its main rival, GenCorp Aerojet of Sacramento, Calif., are interested in developing methane engines.

Jim Long, Aerojet's director of business development, said the tricky part of methane engines primarily has to do with the storage and handling of the fuel.

"We believe that from an engineering standpoint that a [liquid oxygen]-methane engine is achievable with most of the risk being cost and schedule," Long said. "The greater part of the technical risk is in the tank and feed system."

Wood said more guidance is needed from NASA on the methane-fueled propulsion system, but that industry is up to the task. "We have been working with methane and talking about methane for 40 years," Wood said. "There has never been an engine produced that runs on methane."

Pratt & Whitney Rocketdyne and Aerojet are likely to face competition from at least two small upstarts that have been working on methane engine technology: XCOR Aerospace of Mojave, Calif., and Orion Propulsion of Madison, Ala.

"We have been looking pretty seriously at it both for auxiliary propulsion and for main propulsion for about two years now," XCOR President Jeff Greason said. "We got interested in it when we were doing a design study for [the U.S. Defense Advanced Research Projects Agency] on a low-cost upper stage."

Greason said XCOR decided late last year to build and test a small methane-fueled thruster "on our own dime to get some experience with the propellant." XCOR expects to be ready to start test firing the small thruster in early 2006, he said.

XCOR also submitted a proposal to NASA last year to develop, build and test a 10,000-pound to 15,000-pound variable thrust liquid oxygen-methane engine. NASA awarded XCOR a contract for a separate proposal to build and test a composite tank for cryogenic fuels, but passed on its methane engine proposal.

Orion Propulsion, meanwhile, has been doing component-level work on small methane thrusters under a Marshall contract, according to a company press release.

Recognizing that the methane engine is fraught with the usual technical and schedule risks, NASA has decided to devote some effort to a hypergolic system as a backup. Hypergolic fuels, such as hydrazine, were used during the Apollo program and are still used by the space shuttle for in-space maneuvering. But hypergolic propellants are not the most efficient propellants and are extremely toxic, requiring cumbersome and costly handling procedures.

Upper Stage Work

Next up on NASA's list of near-term propulsion needs, Cook said, is an upper stage engine for the Crew Launch Vehicle. Rather than design a new engine, NASA's plan calls for using the space shuttle main engine modified to start while aloft rather than on the ground.

Here again, NASA's choice was made with an eye on the future. NASA intends to use a cluster of five space shuttle main engines to power the main stage of its heavy-lift rocket. "Since that won't start until 2011," Cook said, using the shuttle engines for the Crew Launch Vehicle "keeps the [space shuttle main engine] line going" until then.

Cook said he expects the space shuttle main engine air-start program to be in component level testing by the end of the year, with larger-scale testing getting under way at Stennis Space Center in Mississippi "about a year from now."

"The whole program is about a three-year effort," he said.

Wood said adapting the space shuttle main engine to serve as the Crew Launch Vehicle's upper stage is "a very doable thing." He said the company took a close look in early 1990s at what it would take to start the engine in flight.

"The tests we did back then indicated that it would not be a big hurdle for the [space shuttle main engine] to achieve," Wood said. "But it's a matter of getting started and having time to run the verification test program to satisfy everybody that it can meet the mission requirements."

Wood said the space shuttle main engines that NASA flies today could be used for the Crew Launch Vehicle once the shuttle program wraps up.

"When the shuttle is retired I can immediately take the engine assets out of the shuttle program and fly them on these vehicles," Wood said.

NASA has 12 complete engines at Kennedy Space Center in Florida, Wood said. In addition, the agency has several test articles and older variants of the engines, and those plus hardware available at Pratt & Whitney Rocketdyne may bring up to 30 the number of engines that could be assembled relatively quickly in support of the exploration effort, he said.

NASA also needs to make some modifications to the four-segment solid rocket booster that forms the main stage of the Crew Launch Vehicle. ATK Thiokol says the changes needed are minor.

Michael Kahn, ATK Thiokol vice president of space launch systems, said the only foreseen changes have to do with the difference between the way the boosters separate from the shuttle orbiter -- they fall off to the side -- and the way they will separate from the Crew Launch Vehicle's upper stage. He said there will need to be some changes to the booster's aft skirt.

He also said Thiokol would have to take a careful look at the booster's existing parachute system since the boosters would be falling farther and faster when used for the Crew Launch Vehicle than when used for the shuttle.

The heavy-lift cargo vehicle NASA wants calls for two five-segment solid rocket boosters that will require a significant development effort. Kahn said the sooner NASA gets started on the five-segment booster, the better.

Looking further out into next decade, NASA needs an engine for the Earth Departure Stage it intends to use to send the Crew Exploration Vehicle and Lunar Lander on their way to the Moon. NASA intends to use the J2-S or an equivalent engine. Built by Pratt & Whitney Rocketdyne, the J2-S is a never-flown variant of the engine that powered the second- and third-stages of the Saturn 5 rocket.