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
When Brian Wilcox was growing up in the 1950s and '60s, his father was in the space business, and for a kid whose hobby was model rocketry, what could be better than that? Howard Wilcox worked on early launch systems for the Naval Ordnance Test Station in China Lake, California, then later for General Motors, where he designed robots that could have scouted landing sites for the Apollo astronauts (but never did). In those visionary days, when the rules of spaceflight were still being written, Brian and his father would have long talks about what kind of launch vehicles we would someday need on Mars, and whether the small model rockets he and his friends built could--in principle--reach Earth orbit.
The answer, sadly, was no. A former physics professor at Berkeley, Howard Wilcox knew it wasn't gravity that dictates the minimum size for a launcher. "My father explained to me at a very young age that the only limitation was the atmosphere," Brian recalls. "If you didn't have an atmosphere, you could make them small." A terrestrial rocket has to push through a plug of air equivalent to a 30-foot column of water, and physics dictates that the smallest vehicle capable of moving all that atmospheric mass without paying a penalty in momentum is about 30 feet long.
On Mars, though, the thin atmosphere is equivalent to only about four inches of water. Many years later, working as an engineer at NASA's Jet Propulsion Laboratory, Brian Wilcox would participate in a key series of workshops to brainstorm ideas for a Mars sample-return mission, the most ambitious planetary project in history. One of the critical questions facing the workshop attendees, he remembers, was: What was the smallest rocket that could possibly leave the surface of Mars and make it into space? "And I knew that answer because I'd discussed it with my father many times. The answer is: about the size of a pencil."
Howard Wilcox didn't live to see his son's triumph on July 4, 1997, when, as a member of JPL's Mars Pathfinder team, Brian helped pull off the first spacecraft landing on Mars in more than two decades. Pathfinder was a watershed event for NASA. Its $265 million price tag proved that planetary missions didn't have to be expensive to be fun, and it gave planners confidence that the agency's goal to return samples of Martian soil to Earth by the middle of the next decade--which had just received a huge boost from the discovery of signs of possible fossil life in a Martian meteorite (see "Pieces of the Rock," Apr./May 1997)--was feasible for around $1 billion.
By the following spring, however, the confidence had all but evaporated. The sample-return mission--in fact the whole Mars exploration "architecture," which called for launching at least one spacecraft to the planet at every two-year opportunity--was running into severe financial and technical problems and was on the verge of meltdown.
The plan at the time of the Pathfinder landing called for three missions directed toward the goal of getting samples back from Mars. The first two would send large, well-equipped rovers to Mars in 2001 and 2003 to dig and drill rock samples from different sites, then "cache" them in sealed containers on the surface to await pickup. The third would dispatch a smaller "fetch rover" in 2005 to whichever of the two caches looked more promising scientifically. The little rover would trundle over to the chosen sample, bring it back to the lander, and transfer it to a rocket called the Mars Ascent Vehicle (MAV), which would send it back to Earth.
The scheme quickly ran into difficulties, though, that threatened to break the project's budget and schedule. The package launched to Mars in 2005--which included the lander, the fetch rover, the rocket, and miscellaneous other equipment--would have to fit inside a medium-size Delta launcher to make the mission affordable. And designing a lightweight Mars rocket that could live within that limit was turning into a very tough challenge.
The MAV was to be liquid-fueled, so it needed exotic propellant that wouldn't freeze during the Martian night. It required fancy plumbing, miniaturized components, and lightweight motors and fuel tanks. "It needed a whole bunch of stuff to try and get this thing down to a low mass," recalls Mark Adler, chief engineer for the sample return mission. "That was all extremely expensive." The price for the MAV ballooned to $120 million at a time when the entire project was budgeted for only $200 million a year. And, at 1,300 pounds, the thing was way overweight.