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 5 of 6)
"Our job would be a lot easier if we could power the landers with nuclear batteries the way Viking did," says O'Neill. "But it's politically no longer acceptable." So three months is all the rover will have to do its job, and some question whether that's long enough to collect 500 grams of material from 20 different sites. "We'll be hard pressed to get enough stuff to fill up the cache boxes," says O'Neill. Steven Squyres of Cornell University, one of the lead scientists for the sample return, shares his concern. "This mission is going to be like that old supermarket sweepstakes, where you've got 90 seconds to go through the aisles, grabbing everything you can," he says.
Returning more than 500 grams could mean that some of the stuff gets tossed quickly into an undifferentiated bin at the end of the rover's lifetime, which isn't as appealing to scientists. And this loose dirt would have to be balanced in some way so it didn't slosh around when the MAV lifted off.
That, plus the general penalty for adding mass to the sample return canister, made even a few extra grams of material worth arguing about. JPL now feels comfortable with the 500-gram requirement, but "We don't know how we could get from 500 to 1,000 grams," says O'Neill. The difference is about the weight of a slim hardbound book.
On an unseasonably cold April day in Pasadena, with patches of snow still clinging to the peaks of the San Gabriel mountains that abut the JPL campus, around 50 engineers and technical managers from the space industry gathered, at Caldwell's invitation, for an all-day briefing on MAV. In coats and ties, unusual attire for JPL, they sat mostly silent while O'Neill tried to get them as pumped up as government contractors are allowed to get.
"The first-ever launch from another planet--what a great thing to be able to participate in!" he enthused, kicking off the meeting with a quick Vu-Graph walk-through of the sample-return mission. Bringing back a piece of another world, he continued, was "one of the few remaining firsts in planetary exploration," a historic endeavor on a par with Sputnik or Apollo.
Caldwell, introducing himself as the "MAVman," explained the reason for the meeting: JPL wanted an outside opinion. Most of the $60 million to be spent on the rocket system would go to contractors once actual fabrication began. Right now, though, he was looking for a reality check before going any further. "Do we have the right architecture?" he asked.
The MAV design was holding at about 375 pounds, and mission planners were hoping to drop to 350. It still called for three solid rocket stages: a guided first stage, spinning second and third stages, plus that additional NOTSNIK back-end-first approach on the third. One area in which the team needed advice was on the fuel composition for the solid rockets, because spinning had certain drawbacks. A key reason for rejecting the spinning first stage early on was that it required something like 500 rpm for accurate pointing. But even with a slower spin rate for the upper stages, people were concerned about the buildup of aluminum slag. Solid rockets on Earth typically are made with 16 to 18 percent aluminum. The more aluminum, the more oomph. But when the fuel burns, it produces aluminum oxide slag, which in a spinning rocket would throw off the balance like an uneven load of laundry in a washing machine. The answer appeared to be to cut the aluminum content way back, to one or two percent, but the exact formulation would have to be determined. Caldwell wanted a new team to look just at propulsion.
Some of the people attending the April briefing worked for companies--notably Lockheed Martin, part of which used to be Martin Marietta--that had been fiddling with Mars rocket concepts for at least 20 years. Most of these were old-style behemoth designs unsuited to the more economy-minded NASA of the 1990s. Later, back in his office, Mark Adler pointed to one of the old artist's conceptions showing a big, heavy rocket with an elaborate gantry--on Mars. "This is part of a ten-billion-dollar sample-return mission that never flew," he chuckled. "I can't imagine why not."