NASA has adopted a streamlined hardware-development approach to its lunar exploration plan that emphasizes greater commonality between the shuttle-derived vehicles that will launch crew and cargo to the Moon late next decade.

Rather than develop separate upper-stage engines for each vehicle, NASA will go with an updated version of the Apollo-era J-2 engine for both. The previous plan called for using an air-ignited version of the space shuttle main engine for the Crew Launch Vehicle upper stage.

Similarly, the agency has dropped plans to use a four-segmented solid-rocket booster as the main stage of the crew launcher, opting instead to build a more-powerful five-segmented version that also will help propel the planned heavy-lift cargo launcher.

Scott Horowitz, NASA associate administrator for exploration systems, said the newly approved changes are meant to economize the development and production of the hardware needed for Project Constellation, the U.S. space agency’s effort to replace the space shuttle and return to the Moon by 2020.

“Now we only require the development of a single upper-stage engine, a single development to get to a more mass-producible [space shuttle main engine] and we are only going to keep in the stable a single version of the solid-rocket booster,” Horowitz said in a Jan. 20 interview.

Plans still call for using a simplified version of the space shuttle main engine as the main stage of the heavy lifter. Horowitz said that in lieu of the air-start effort, NASA and it contractors will focus on finding ways to make the reusable engines more economical to produce since they will be treated as expendable hardware on the heavy-lifter.

Horowitz pointed out that the Exploration Systems Architecture Study that NASA commissioned last spring to help shape its space transportation plan concluded that the J-2/five -segment booster combination entailed more up-front expense and technical risks, but would be cheaper over the long-run. NASA, intent on minimizing the gap between retiring the shuttle and fielding its replacement, the Crew Exploration Vehicle (CEV), selected what it deemed to be the quicker approach. This entailed using the space shuttle solid-rocket booster essentially as is, with four segments, and modifying the space shuttle main engine to start at altitude.

But NASA has since shifted gears, opting for the approach that offers the potential of saving money over the long haul.

Horowitz said the amount of technical risk involved in the new approach is essentially the same as the old approach since NASA also has dropped the requirement for a liquid oxygen/methane engine on the CEV and the lunar lander’s ascent module.

“We basically just traded one critical path for another,” he said.

ATK Thiokol of Promontory, Utah, which builds the space shuttle solid-rocket boosters, tested a five-segmented version of the motor on the ground in 2003 . In addition, NASA has done some component-level work on the J-2 engine as recently as 1998.

Horowitz declined to discuss the near-term budget implications of NASA’s new approach prior the release of the agency’s 2007 spending request Feb. 6.

However, he said even with the additional up-front booster and engine development, the CEV could still enter service within two years of the shuttle’s planned 2010 retirement.

“We can actually still start test-flying the rocket in the 2009 time frame … and still have a rocket available to launch crew in the 2012 time frame,” he said.

Other approved changes to Project Constellation, according Horowitz as well as other sources familiar with the new plan, include reducing the diameter of the CEV from 5.5 meters to 5 meters for additional weight savings and using existing Russian docking hardware for missions to the international space station. Previous plans called for using a U.S.-developed system.

NASA also has decided not to use methane-fueled engine technology for the CEV.

According to Project Constellation documents obtained by Space News and individuals briefed on the changes, NASA instead intends to go with more-familiar hypergolic propulsion technology, which is used on the space shuttle and on satellites for orbital station keeping.

In announcing the Project Constellation architecture in September, NASA Administrator Mike Griffin said the agency had chosen the methane engine for the CEV service module and the lunar lander’s ascent stage because of its performance advantages over hypergolic systems and to gain operational experience with a propellant that could be produced on Mars and possibly the Moon. But Griffin also cautioned that the methane approach was not a sure thing.

“We will be carrying as a backup in the program the use of hypergolic propellants on the service module and on the lunar ascent stage such that, if the [liquid-oxygen] -methane technology development does not work out as we expect, we will have a system that will work, ” Griffin said.

NASA spokesman Kelly Humphries said analyses performed since September by NASA engineers and CEV contactor teams showed that the performance advantages of methane propulsion were not as great as previously thought, especially given the additional technical risk involved in developing a new propulsion system.

NASA’s CEV propulsion change came as a disappointment to Glenn Research Center, which had been given the lead role in developing a methane engine and already had solicited proposals from industry to get started on some of the necessary components.

Richard Christiansen, deputy director of the Cleveland-based NASA field center, said Jan. 11 that he had been instructed by Project Constellation officials to hold off for now on making any contract awards. “We are looking into the potential for some of the work to continue as technology projects,” he said.