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 3 of 6)
Doug Caldwell, a young engineer who had left the computer industry six years earlier to join JPL, took over as the new head of the MAV office in September 1998. Before that, working on the Deep Space 1 technology demo mission, he had heard "a crazy idea of a spinning rocket that will weigh only 30 or 40 kilograms," and was well aware of the buzz surrounding it. Outside JPL, scientists and NASA managers who the year before had watched in horror as the sample-return mission nearly collapsed were now talking up Mini-MAV "like it's going to completely save the world," says Caldwell. And he thought to himself, Yeah, but you don't actually have to build it!
He thought the idea was cool too, and still does. But he knew that "whenever someone throws out something brand new, it almost always looks better" than what you've got in front of you. And the study teams were already beginning to find cracks in the Mini-MAV miracle. "A lot of the simplifications turned out to be oversimplifications," says Caldwell.
For starters, the rocket's conjectured 45-pound weight hadn't included the turntable on which it rested, the "igloo" covering needed to keep it warm, or other sundry equipment not on the rocket itself. Add another 35 pounds right off the bat.
The NOTSNIK had done without a guidance system partly by using fins to stabilize itself in the atmosphere, but the Martian air was too thin to help here. Plus, the NOTSNIK engineers hadn't been picky about where their satellite wound up, as long as it made it into orbit. The Mini-MAV couldn't be quite that loose--it had to come within striking distance of the Mars orbiter.
"God created guidance systems for a purpose," says Caldwell. So the engineers admitted defeat and added active control--thrusters, inertial position sensors, computers, and power--to the first stage, which pushed the weight up from 80 pounds to almost 300.
Another issue the Mini-MAV design hadn't addressed was how the sample canister would keep the Martian dirt pristine. This was important not only to scientists, who wanted uncontaminated samples, but to NASA's "planetary protection" watchdogs, who had to guarantee to the public that no dangerous Martian bug--a remote possibility, but not entirely out of the question--would be returned to Earth. The sample-return project hadn't yet come to grips with the planetary protection dilemma when Brian Wilcox proposed his Mini-MAV. By the time it did, the sphere that could hold the Martian dirt and keep it sterile weighed ten times more than the sample itself.
This creeping weight gain was fairly typical for the early design phase of a space engineering project, says Caldwell. "Push the system here and it bulges out there with some kind of problem--too much heat, or cost, or weight." It was mainly that damn eight-to-one mass penalty. When anyone suggested adding another pound or two to the sample canister, he says, "I wanted them to know how painful it is."
By the spring of this year, the Mini-MAV had ballooned up to a 375-pound, plain old MAV, which stood about as tall as a person. It was still a lot better than the old liquid-fueled MAV, which had trimmed down to 600 pounds but appeared to be stuck there. Even so, with all the commotion that had been made over the Mini-MAV, Caldwell couldn't help feeling like the guy who's been given a perfect handoff, only to fumble it inches from the goal line.