How to build the first extraterrestrial airplane.
- By Oliver Morton
- Air & Space magazine, January 2000
(Page 2 of 5)
The Mini-Sniffer thus solved two of the problems facing potential Mars airplanes. It worked in very thin air (though not as thin as that on Mars) and it generated all its power with onboard fuel. This mattered because the Martian atmosphere, such as it is, is composed almost entirely of carbon dioxide. Jets and internal combustion engines wouldn’t work there, but Reed’s hydrazine steam engine would do just fine. What’s more, it could use a fuel that any Mars-bound spacecraft would likely carry anyway.
The fact that the Martian atmosphere is mostly carbon dioxide was also, in a small way, a bonus. At any given pressure, carbon dioxide is denser than the air on Earth, which would increase a wing’s lift. The biggest plus of all, though, was the low gravity on Mars, which reduces the wing loading on an aircraft, allowing it to get by with less lift. All these factors suggested to Reed and to the NASA engineers who approached him in 1978 that Mars flight might indeed be feasible.
“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.