Today's RL10s can throttle down to 20 percent of their full thrust, but the LSAM engines will have to do better: 10 percent. That shouldn't pose a problem, thinks Connolly, but the achievement still requires some development work, and NASA may want to test these highly throttleable engines on robotic landers scheduled to begin visiting the moon as early as 2011.
Because the descent engines will be bigger and more powerful than the ones that flew on Apollo, engineers have had to consider the effects of the stronger blast on the lunar surface. When Pete Conrad piloted his lander to a touchdown in the Ocean of Storms during the November 1969 Apollo 12 mission, the rockets kicked up so much dust that for the last two minutes of his descent, he could hardly see anything below him. Could the debris from a more powerful blast fly up and hit the lander?
Connolly's team studied the problem, even watching old videos to analyze how the dust scattered. Most of it blew horizontally away from the landing site, not straight up. Conclusion: Rocket thrust shouldn't be a danger, at least for the LSAM itself. Some still worry about blast effects on previously landed modules, since the vehicles will probably touch down close to one another.
The single engine on the ascent stage also will use the same propellants used for Apollo, even though the combination wasn't NASA's first choice. The Architecture team originally wanted to use liquid methane fuel, in one of the lunar program's few nods to an even more distant future. Someday, when astronauts land on Mars, they'll need to live off the land as much as possible. Methane could theoretically be extracted from the atmosphere and turned into rocket fuel. So if the LSAM used methane engines, NASA could get early practice with a technology applicable to Mars.
The trouble is, no one has flown a methane rocket in space. A French-Russian demonstration project called Volga concluded last year that the technology looked promising, and a couple of companies have tested small-scale engines. But that's about it. Methane engines would have been one of the bigger leaps in the Architecture, and in the end NASA decided it was too big of one. The ascent engine absolutely, 100 percent, no kidding, has to work perfectly on the moon. Otherwise the astronauts are stranded.
Before trusting methane engines on the moon landers, NASA would have wanted years of experience flying them on the CEV. But with the CEV's debut planned for as early as 2010, there isn't time to develop such an important technology. So, reluctantly, NASA gave up on methane.
The decision to go with hydrogen fuel -- and lots of it, since the LSAM will need to shift the plane of its orbit as much as 90 degrees to reach polar landing sites -- influences other aspects of the design. The most striking example is the size of the fuel tanks. Hydrogen takes up more volume than denser fuels, so the descent stage tanks alone will be taller than the entire Apollo descent stage.
The 1960s lander was "a marvel of minimization" that strained the ingenuity of its builders, says Connolly. "They had to whittle away at every piece of metal on the vehicle to make it as light as possible." Current designers have more mass and volume to play with, so the LSAM will be substantially roomier than its predecessor, more like an RV than a minivan.
Connolly's team hasn't devoted much thought to amenities, but he predicts a few improvements that will mean a lot to future astronauts. They won't have to relieve themselves in bags, for example -- the LSAM will have some kind of toilet, even if it isn't as elaborate as the one on the space station. It might be something like the small, portable job the Russians have used on their Soyuz spacecraft for years. And whereas the Apollo astronauts ate strictly cold fare during their time on the moon, the LSAM should have a food warmer.
Last year, a group of astronauts and engineers at Johnson received a modest NASA grant to brainstorm what accommodations might be needed for long-term living on the moon. The group built a mockup of a lunar habitat in one of the center's warehouse -- like buildings in Houston, and invited a few veteran astronauts and scientists with Antarctic experience to attend their Lunar Habitation Systems Workshop. Three-time shuttle astronaut Mario Runco led the exercise.