“We all got pretty excited,” recalls Reed. He and colleagues at JPL and in industry worked on various Marsplane designs based on Mini-Sniffers and sailplanes. The grandest came from an aeronautical engineer named Abe Kerem. It weighed about 1,200 pounds, had a wingspan of about 70 feet, and used a distinctive inverted-V tail. “He likes that inverted V-tail,” says Reed. In fact, Kerem’s innovation later was incorporated into military unmanned aerial vehicles (UAVs) that evolved into today’s Predator, a medium-altitude, long-endurance reconnaissance aircraft that Reed sees as “an outgrowth of this Mars airplane.”
The Mars vehicles differed in shape and engine, but they all shared the unusual feature of starting from the top of the atmosphere, not the bottom. During the long voyage through interplanetary space they would be folded up inside an aeroshell like the one that contained the Viking landers. Upon entering the Martian atmosphere the aeroshell would be slowed by a parachute, then would peel away. The still falling aircraft would deploy itself, its wings and tail unfolding as it fell. (In the case of a sailplane-size aircraft, that’s a lot of unfolding.) The engine would fire, and powered flight would begin.
The other end of the flight seemed as though it would be simpler—at first. “Originally we were just proposing crashing at the end of the mission,” recalls Reed. “But then we got the scientists on board and they said, ‘Oh no, we don’t want to crash. We’d like to use the airplane after it lands.’ ”
So Reed found a way to convert a sailplane to vertical flight and land it with a rocket. “I took a Schweizer sailplane and rigged the tailplane so it would pop up,” he recounts. “We towed it up to 10,000 feet and pulled the lever, and [it] came down almost perfectly at a 70-degree angle. The wing goes into a deep stall at a high angle of attack, and it stays controllable—it comes down like a parachute.” A Mars aircraft could do the same, switching on hydrazine thrusters in its belly at the last minute to make a rocket-cushioned soft landing just as the Viking probes had done. When it was time to lift off again, the same rockets would kick it back into the sky. Reed’s scheme allowed a Mars airplane to fly to a selected landing site, stay there while the scientists back home went over its data, then go on to a second site newly identified as interesting.
The final mission concept that evolved from this work in 1978 was deeply ambitious. Larry Lemke, who has worked on Mars aircraft designs at NASA’s Ames Research Center near San Francisco, remembers that the plan called for three spacecraft to enter Martian orbit at around the same time, each carrying four Mars airplanes in Viking-style aeroshells. Each spacecraft would require a space shuttle to deliver it to Earth orbit, and the three launches would have to take place within a period of less than a month to make use of the limited window of opportunity for Mars missions. (Back then, NASA still aimed to fly a shuttle every week or so.) The squadron of twelve aircraft, which might carry a variety of scientific instruments, would fly down to the surface one by one, some revisiting sites of interest spotted by earlier missions. With each aircraft capable of flying perhaps 3,500 miles before landing (1,800 if it used fuel to land, take off, and land again), the mission had the potential to completely circle Mars and explore at least a dozen sites close up, investigating the planet in more detail than ever would have been possible before.
By the time the shuttle actually started to fly—once every few months, if NASA was lucky—it was clear such a grandiose and expensive project would never get off the ground. Mars aircraft were taken off the agenda. But it turned out you didn’t have to be thinking about Mars to do useful work on the problem. The key issue involved, in aeronautical terms, is a low Reynolds number. Essentially, this describes the way in which a fluid (air, in this case) flows, and depends on the density of the fluid, the speed of the airflow, and the chord of the aircraft wing. Flying at high altitudes, at slow speeds, or with small wings all translate to low Reynolds numbers. Work on high-altitude research aircraft and on human-powered airplanes like Paul MacCready’s Gossamer Albatross taught engineers new tricks that could be applied on Mars. In fact, at least one existing aircraft—the very-high-altitude, solar-powered Pathfinder UAV built by MacCready’s company AeroVironment—would in principle be nearly capable of flying on Mars, if you could get it and its support team there.
Because of these advances in other fields, by the time NASA’s planetary exploration program started to pick up again in the early 1990s, it could draw on more expertise relevant to Mars aircraft than ever before. In 1992, at the first workshop devoted to NASA’s Discovery program of low-cost planetary missions, a Mars aircraft proposal was put on the table by John Langford, whose company, Aurora Flight Sciences, had developed high-altitude aircraft for NASA to use in programs much like the environmental impact study that had led Dale Reed to design the Mini-Sniffer. Langford had also managed the Daedalus project to build the human-powered aircraft that a cyclist flew 74 miles across the Sea of Crete in 1988.
As the Discovery program developed, more ideas for Mars airplanes surfaced. Larry Lemke’s team at the Ames center came up with a craft that was basically a scaled-down version of the Reed Marsplane of the 1970s. Weighing about 400 pounds, it would fly for six hours or so, land, study the surface, then take off a month later for more cruising. The Ames people even had a target in mind: Gusev Crater, which, evidence suggests, may have once been a lakebed. Water inside the crater might have been warmed by a large volcano more than 100 miles to the north. Many researchers—especially at Ames, where the crater has a particularly passionate set of advocates—think Gusev could hold traces of past Martian life.
Another Ames proposal, done in cooperation with planetary scientist Mike Malin’s small company (see “Getting the Picture,” Aug./Sept. 1999), was MAGE, a mission that used a graceful flying wing with a pusher propeller to carry a suite of geophysical instruments over the Martian canyons. At the same time, a team involving AeroVironment, JPL, and others suggested an even simpler mission, which flew a series of small gliders rather than a single powered aircraft. “We ended up in a situation where we more or less had to choose between carrying a propulsion system and carrying a scientific payload,” says Carlos Miralles of AeroVironment. Flying several vehicles instead of one added resilience. “You can tolerate failures, you can target them independently, you can cover a larger total range and get more diversity than if you are stuck with one airplane trying to fly for a long period of time,” he says. Six gliders would have been popped down at different sites in Valles Marineris. Although each would have flown for at most 60 miles, together they might have provided data on the whole length of the canyon system.
Clever as they were, these ideas were slightly ahead of their time. In November 1998, after the Discovery review panels worried about the risk involved in using unproven technologies, NASA turned down both MAGE and the fleet of gliders. But the December 2003 Wright centennial, which happens to coincide with a favorable launch opportunity for reaching Mars, had already begun to generate a buzz for Mars airplanes. Both proposals had used the name Kitty Hawk—MAGE viewgraphs even had the word proudly emblazoned on the wing.