NASA is moving toward using the same venerable rocket engine to power both its planned robotic lunar lander and the generic descent module that will deliver crew and supplies to future Moon bases.

Ultimately, the first U.S. spacecraft to land on the Moon since 1972 could wind up looking a lot like the planned Lunar Surface Access Module, NASA's 21st century follow-on to the Apollo-era Lunar Excursion Module. A throttleable version of the RL10 rocket built by Pratt & Whitney Rocketdyne since 1959 is baselined as the descent engine for the new lunar module.

The agency assigned the job of setting requirements for the descent module to Marshall Space Flight Center, which has been studying a robotic lander large enough to handle the RL10 since last year. For trade studies on options for a planned robotic precursor to the human lunar lander, Marshall and the Johns Hopkins University Applied Physics Laboratory (APL) proposed a 9-metric-ton spacecraft as a way to test the technology needed to use a big cryogenic engine to land on the Moon.

CONTINUING THE flight-test aspect of the Marshall/APL concept, the robotic lander might use an early flight of the planned Crew Launch Vehicle, also being developed at Marshall (AW&ST Jan. 23, p. 47).

No final decisions have been made on the descent engine for the robotic lander, according to Jeff Hanley, who manages the Project Constellation exploration vehicle development effort. But if the new Marshall lander office decides to go with the RL10 for the robotic precursor, the move would be in keeping with a trend in NASA's exploration effort to minimize separate development efforts in the push back to the Moon. The agency already dropped development of two new variants of the space shuttle main engine in favor of upgrades to other rocket engines that are less expensive and have more applications in a back-to-the-Moon architecture (AW&ST May 29, p. 33).

"We [must make] sure that what we do in the robotic program is feeding useful information to the Constellation program," says Scott Horowitz, associate administrator for exploration systems.

Horowitz has shifted responsibility for robotic precursors--the planned 2008 Lunar Reconnaissance Orbiter and the follow-on lander--from Ames Research Center to Marshall and assigned Marshall to develop the descent module for the multipurpose lunar lander to lower humans, habitats and other hardware to the Moon's surface (AW&ST June 5, p. 19).

A big cryogenic engine like the 16,500-22,300-lb.-thrust RL10 can deliver a lot of cargo to the lunar surface, but the robotic lander trade studies identified several concerns. First, cryogenic storage technology must be developed to prevent the engine's liquid-oxygen and liquid-hydrogen propellants from boiling off during the 3-4 days it will take to get to the Moon. Controlling the rocket in a landing against only one-sixth the gravity that the DC-XA faced on Earth may also be difficult.

"An engine can be too large to land, so one of the challenges that we have is how deeply can you throttle that," said Horack during the original robotic lander trade studies in December 2005. "Can you get a fine enough touch on the controls so that you're not overthrusting? You have to have a very delicate thrust, and delicate thrust control, to land. And these are large rocket engines designed to put out a lot of thrust. So there's an engineering technical challenge to sort of expand the range over which you can step on the gas."

The lift capability of the engine would give flexibility to the team headed by Horowitz deputy Doug Cooke that is developing a strategy for lunar surface operations that should be finished by year's end. Plans call for the same descent module to deliver everything humans need on the surface of the Moon, and Marshall's earlier work would start testing that basic module with the first robotic lunar landing.

"THIS LANDER IS NOT your dad's lunar module," Hanley says. "It's a new system. It has to be a multi-use system, because we're not putting just a crew module on the surface. We have lots of equipment to put on the surface--habitats, power stations, pressurized and unpressurized rovers, science equipment, all of the piece parts that come with a significant presence on the surface."

It remains to be seen if the crew that lands on the lunar surface will control its own descent, but the other payloads that grow out of the surface-strategy work will rely on robotics to land. NASA plans to use Johnson's human-spaceflight experience to develop the lander crew module.

Griffin and the other exploration executives who presented the new exploration "work packages" for the 10 NASA field centers denied that Congress influenced the agency's decisions. In addition to shifting the robotic and lunar-lander work from Ames to Marshall, the new plan assigns management of the Crew Exploration Vehicle service module to Glenn Research Center. Ames will continue to work on the Lunar Crater Observation and Sensing Satellite that will ride the Lunar Reconnaissance Orbiter to the Moon (AW&ST Apr. 17, p. 26), as well as materials for thermal protection systems.

Development work on the orbiter will remain at Goddard Space Flight Center. The Jet Propulsion Laboratory will use its experience operating robots on Mars and elsewhere to help Marshall with its lander planning, and the Dryden Flight Research Center will oversee testing of the CEV abort system--a solid-fuel rocket that would pull the CEV to safety, like the escape rockets used in the Mercury, Gemini and Apollo programs. Other centers will draw on their experience and heritage to make contributions to the exploration effort, Griffin says.