The One-Pound Problem
All the Mars Ascent Vehicle has to do is deliver 16 ounces of rocks in a container the size of a grapefruit to Martian orbit. If only it were as easy as it sounds.
- By Tony Reichhardt
- Air & Space magazine, November 1999
(Page 2 of 6)
The MAV wasn't the only headache. The big sample-collecting rover was having its own development troubles, and getting it ready by 2001 looked like a long shot. Meanwhile, the human exploration office at NASA, realizing that sample return was the only train leaving for Mars, proposed adding an experiment to the 2001 mission that would benefit future astronauts: a prototype propellant plant to extract oxygen and hydrogen from the Martian environment and make fuel for the rocket. The only catch was that the money had to come from JPL's fixed budget, which was now strained to the breaking point.
By the time Bill O'Neill, a manager who had come from the Galileo Jupiter mission, took over as head of the sample-return project early last year, "a lot of issues were out of control," he says. "As things unraveled, all the senior managers kind of pitched in and said, 'We've got to figure out a way to fix this.' " One of the things O'Neill did was hold a pair of workshops in spring 1998 at the Embassy Suites hotel in Arcadia, just down the freeway from JPL. His notion was to invite fresh ideas from inside and outside the team on how best to do a sample return.
Brian Wilcox wasn't working on the project--his expertise is rover design--and he was a bit self-conscious venturing outside his turf. But he asked to present a concept for a Mars rocket he'd had in the back of his mind for years, based on work his father had done at China Lake in the 1950s. "I was thrilled to be able to step forward and say 'Hey, I've got another way. And by the way, I've got a 19-minute videotape that shows how it can be done.' "
The video shows Howard Wilcox sporting a bow tie and crewcut, standing at a blackboard circa 1958, and looking for all the world like Mister Wizard as he chalks out the basics of a then-secret project called NOTSNIK. Wilcox and his team attempted--some think they succeeded in--launching a two-pound satellite to Earth orbit from under the wing of a Douglas Skyray fighter (see "The China Lake Launches," Feb./Mar. 1997). One of their innovations was to launch NOTSNIK's last rocket stage, the one attached to the satellite, with its nozzle pointing in a direction opposite from the other stages. When it reached space it coasted for half an orbit, spinning all the while to maintain its orientation, so that on the opposite side of Earth its "backward" nozzle was pointing in the right direction to boost the satellite to its final orbit. (Think of the spacecraft as a car on a ferris wheel, its nozzle pointed down on its downward arc. With its attitude held stable, its nozzle will continue to point down as the craft begins to climb and can accelerate it in the direction of its forward motion.) It was a clever way to avoid putting guidance electronics on the last stage and save precious weight in a system that measured performance in ounces.
What Wilcox proposed to the JPL workshop wasn't the theoretical pencil-size Mars rocket he and his father had once dreamed up, but it was close. The "Mini-MAV," as it was dubbed, weighed a mere 40 pounds and stood only three feet high. It could deliver seven ounces--200 grams--of Martian rock to orbit, a significant step toward the payload the team now hopes for. The first stage would be set spinning rapidly on a turntable just before launch, the second stage would spin up with small thrusters, and the third stage would pull its little NOTSNIK trick to point itself in the right direction. No active guidance at all. The Mini-MAV wouldn't need it, because another key assumption was changing at around the same time. Sample caching would now be done in Martian orbit instead of on the surface. A dumb--that is, imprecise--rocket could just lob the sample canister into space, where a smart orbiting spacecraft--supplied by France and sent to Mars by the powerful Ariane 5--would find it, rendezvous with it, capture the canister, and send it back to Earth.
Scrapping the guidance system was "a very big deal," says Mark Adler, because it alone weighed tens of pounds compared to less than a pound for the sample itself. Any mass removed from the top of the rocket had tremendous leverage on the whole system. The ratio was eight to one: For every pound cut from the payload, you'd save eight in motor and fuel at the bottom.
At first, Mini-MAV appeared to be one of those rare engineering miracles that offers nothing but advantages. It was so lightweight that you could now get away with using lower-performance solid-rocket engines instead of liquid engines, with all their fussy plumbing and sensitivity to the cold. And solid rockets, with their relatively simple technology, could be ready by 2003, while the large liquid MAV couldn't. Finally, NASA could return samples from both cached sites, one in 2003 and one in 2005.
That summer, Wilcox gave presentations on his idea to other groups hashing out the Mars architecture, and no one shot it down. In fact, with each level of review the enthusiasm only mounted. One veteran space consultant whom O'Neill had hired to help rethink the sample-return mission was so jazzed about Mini-MAV that he canceled a long-planned European vacation to keep working on it. Adler was excited too, but cautious. "My initial thought was Gee, that's a good idea. I wonder if it works." A couple of small teams at JPL were asked to give it a closer look.