NASA's decision to drop the development of liquid oxygen-and-methane-fueled engines from its Crew Exploration Vehicle (CEV) program is viewed as an opportunity lost by proponents of the clean-burning propellant.

When NASA rolled out its detailed space exploration plans in September, the U.S. space agency touted methane's performance advantages over more conventional propellant choices and emphasized the potential for producing fuel for the methane engine on the surface of the Moon and Mars. This capability, in theory, would help lower the cost of sustained operations in either location.

But NASA and its contractors are now proceeding with the CEV development under the assumption that it will include a propulsion system that consumes hypergolic propellants instead of gaseous methane and liquid oxygen. Industry sources said specifying a hypergolic system for the CEV service module makes it highly probable, if not certain, that NASA will go with the same type of propulsion for its lunar lander's ascent module, which previously also called for a methane engine.

Hypergols such as the nitrogen tetroxide and monomethyl hydrazine combination the space shuttle burns to maneuver on orbit are considered highly reliable and easier to store than other propellants. But they offer lower performance than methane and other so-called green propellants and are highly caustic, requiring painstakingly careful -- and therefore expensive -- handling by workers on the ground who have to be extremely careful to avoid potentially lethal exposure.

Scott Horowitz, NASA associate administrator for space exploration, said the decision to drop the methane-engine requirement from the CEV program came down to changing assumptions about the performance advantages and technical risk. There are no methane-fueled space propulsion systems in service today. Hypergolic systems, on the other hand, were used on board the Apollo command and service modules and the lunar landers.

Mike Hecker, NASA's Constellation Program director, said the agency is nonetheless still interested in methane and other green propellants.

"We have decided that for the baseline CEV lox-methane is no longer the propulsion system we will use," he said in a Jan. 27 interview. "However, that does not mean we will not continue to support some sort of green solution or a propellant solution" that supports in situ resource utilization as NASA refers to systems that would make use of natural resources at a place like the Moon or Mars to manufacture fuel or other necessities.

Hecker said it is still possible that future versions of the CEV and even the lunar lander could use a green propellant system.

The United States has only limited experience with methane engines. Most of the work being done is happening at small companies including Orion Propulsion, a Madison, Ala.-based company that last September test fired a 100-pound liquid-oxygen-methane reaction control thruster, and Mojave, Calif.-based Xcor Aerospace, which test fired a 50-pound thruster in November. Xcor Aerospace President Jeff Greason said he can understand NASA's decision to go with a lower-risk hypergolic system for CEV -- after all, he said, the higher propulsion performance of methane is not required to reach the international space station.

"I wouldn't necessarily do a lot of second guessing of NASA's decision," Greason said. "I feel much more strongly about the question whether NASA should continue to mature [liquid oxygen]/methane technology to the point where it is ready for flight than whether NASA should require it on [the very first CEV]."

Greason said that NASA should continue to invest in methane propulsion because inserting the technology into its exploration plans promises to pay huge dividends in terms of added engine performance, lower propellant weight and easier, safer handling on the ground. Eventually, stores of the propellant could be produced on Mars from water vapor and the carbon dioxide that is abundant in the planet's atmosphere. Even on the Moon, Greason said, NASA at least could produce liquid oxygen, which is 80 percent of the methane propellant combination.

"You're not going to find pools of hydrazine anywhere in the solar system," Greason said. Xcor, like many other companies, worked through Christmas to respond to several requests for proposals that NASA's Glenn Research Center in Cleveland issued late last year for a methane-engine development program for the CEV.

When NASA dropped the CEV methane requirement, the awards were put on hold. Rich Christensen, Glenn's deputy director, said the agency was exploring the possibility of funding some low-level methane propulsion work.

Greason said he would like to see NASA do more than that.

"The No. 1 thing that NASA should be doing is carrying on the technology maturation work that they set out to do," he said.

Greason said the $40 million methane propulsion effort Glenn initiated last year was set up to yield two different working and firing CEV-service-module-sized engines and two separate working and firing reaction-control engines.

"That's a heck of a lot of insurance against weight and performance shortfalls in the program for not a lot of money," he said.

Orion Propulsion President Tim Pickens also was disappointed to see NASA drop the methane requirement and put the Glenn-led effort on hold.

"I thought it was pretty visionary of NASA to consider methane because it confirmed their role as a wellspring of technology and innovation," Pickens said. "It had a nice selling point to it for why this was different from Apollo."

Orion is a small company that was started in 2003 and has been helping other small firms design and test propulsion systems. Pickens said Orion was supporting four teams, one of which was led by "one of the big primes," in their efforts to capture some of the NASA methane money.

"With the termination of the large engine program, that's several million dollars of lost opportunity for Orion because Orion would have been testing the big rocket engines here at our test facility near Huntsville," he said.

Pickens, however, said Orion's methane-propulsion work won't stop just because NASA dropped it as a CEV requirement. "For us, methane work is going to continue," he said, mentioning a $70,000 Small Business Innovative Research grant it has from NASA's nearby Marshall Space Flight Center.

David Riseborough, a research engineer at C&Space, said his firm did not go after any of the NASA Glenn money even though recently he and his colleagues successfully tested a thrust chamber for the 22,000-pound Chase-10 liquid-oxygen-methane engine they hope to roll out by November.

C&Space is based in Seongnam City, South Korea, and because of that Riseborough said he did not think the company would be a welcome addition to any team competing for NASA funding.

C&Space was created in 2004, but work on the Chase-10 has been going on since 1996, when the project was initiated by Hyundai's Rotem division. When the project was closed down, the engineers working on acquired rights to the engine and started C&Space.

Riseborough said C&Space, with the support of a South Korean electronics company, is developing the Chase-10 for the emerging suborbital spaceflight market.

C&Space also is working with U.S.-based start-up AirBoss Aerospace on a suborbital space vehicle concept and has other deals in the works, Riseborough said.

As for NASA's propulsion needs, he said C&Space is open to working with U.S. companies to license and produce the engine for NASA.

"If it were possible, we would be very happy to sell the engine to NASA one way or the other," he said.

Comments: bberger@space.com