How to build the first extraterrestrial airplane.
- By Oliver Morton
- Air & Space magazine, January 2000
As a monument to the aerospace century, the idea could hardly be bettered. A hundred years to the day after the Wright brothers took off from Kitty Hawk, the first aircraft built for another planet would fly through the light pink skies of Mars. A technology that had girdled and transformed one world—Antarctica is the only continent without contrails regularly dissecting its sky—would begin afresh on a second.
Right from the start, the idea stirred powerful emotions. NASA Administrator Dan Goldin announced in early 1999 his agency’s plan to send a little robot aircraft, one of a proposed series of Martian “micromissions,” to arrive at Mars on December 17, 2003. More than just an adventure, it would have the benefit of bringing together the normally separate aeronautics and space sides of NASA’s house. Unfortunately, what seemed an appealing interdisciplinarity ended up making the project’s infancy more troubled, with different factions in NASA fighting over who should do what. At one point responsibility was divided between two research centers on opposite coasts. “It was like the wisdom of Solomon,” says one observer, “except that they actually did cut the baby in half.”
Now the baby is back in one piece. The Mars Airplane program is being run by NASA’s Langley Research Center in Hampton, Virginia, which by the time you read this should have selected an industry-led team to build the first extraterrestrial aircraft. Even with no further bureaucratic infighting, it will be a challenge. Launch in November 2002 makes the development schedule tight, the budget (though not yet fixed) will be low, and no one has ever done such a thing before. Success will be a triumphant 20 minutes of data. Failure will be a far-off, unheard crash.
Even if this particular invention never reaches Mars, or is postponed to some less historic date, the Kitty Hawk anniversary has already served to focus attention on the argument that someday, the exploration of Mars will require flight. If a human expedition ever gets under way (“And you and I know that it will,” says Joel Levine, the Mars Airplane project scientist, with commendable faith), powered flight could vastly increase its scope. If unable to fly, Martian pioneers will be able to explore the vicinity of their landing site using rovers that cover perhaps a state’s worth of territory. With airplanes, they will be able to explore a world. “If we do our work properly,” says Marsplane pioneer Dale Reed of NASA’s Dryden Flight Research Center in California, “we should have a two-seater airplane available when the astronauts get there 15 to 20 years from now. That’s what this whole effort should be leading to.”
The idea is not new. Nearly half a century ago, Wernher von Braun described Mars landings using hypersonic gliders—Chesley Bonestell painted one sitting on the dusty Martian plain like a silver arrow. Von Braun might not have bothered, though, if he had known what we know now about the Martian atmosphere. Before the Space Age, it was understood to be thin. Just how thin wasn’t appreciated until the first spacecraft flybys in the 1960s. The pressure at the planet’s “datum”—the notional surface that serves as a sea level on sealess Mars—turns out to be only about six millibars, or six thousandths of the atmospheric pressure at Earth’s sea level. Even well below the datum, in the heart of the vast canyon system known as Valles Marineris or in the depths of the Hellas basin, it never climbs much above one percent of Earth’s sea level pressure. There is simply not much aero for an aeronautical engineer to work with.
But if the planet’s atmosphere was disappointingly thin, the fascinating surface revealed by NASA’s three Mars orbiters of the 1970s—Mariner 9 and Vikings 1 and 2—more than made up for the letdown. Some of the landscapes are astonishing: volcanoes the size of countries, canyons that could stretch across continents, flood channels through which a sea could drain in a matter of days. This was clearly a place worth exploring.
After the Viking landings in 1976, aircraft came to be seen as an exciting way of carrying the exploration forward. The pioneers of the Space Age had most admirably solved the problem of reaching other planets, but hadn’t been able to move around once they got there. The Viking program, for example, dispatched extremely sophisticated machines to a world millions of miles away, where they inspected only a few square yards of the surface. The attraction of spacecraft that could investigate larger areas at higher resolutions than you could achieve from orbit was obvious. So engineers at the Jet Propulsion Laboratory in Pasadena, California, the center that handles most of NASA’s planetary science, began to think about airplanes. Their thinking soon led them to Dale Reed.
While NASA’s planetary probes were opening up the solar system, Reed was concentrating on a completely different, if also rather futuristic, problem—the development of supersonic airliners on Earth. One worry, then and now, was that these high fliers might do all sorts of damage to the stratosphere. NASA therefore started a program to measure the environmental impact of supersonic flight by sampling the wake of an SR-71 traveling through the stratosphere at Mach 3. That required another aircraft that could get up to 70,000 feet and take the samples. To meet the requirements, Reed designed the Mini-Sniffer, a small, remote-controlled vehicle powered by a unique hydrazine engine. Hydrazine blows itself apart in the presence of the right catalyst, a trait that has long made it a popular fuel for spacecraft thrusters. Reed’s design used heat given off by this reaction to run a little steam engine; that engine in turn drove a propeller.
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.
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.
Edgar Choueiri, a plasma physicist at Princeton University, had also noticed that the Wright anniversary coincided with a Mars launch window, and mentioned it to Norm Augustine, the former Lockheed Martin CEO who had moved to Princeton. Augustine became an enthusiastic proponent of the idea, talking it up to Dan Goldin and others in the space agency, who saw its potential as a Mars Pathfinder–like source of national excitement.
And so it was that the NASA budget came to include a bold, if modestly funded, new project: the Mars Airplane.
To the people at Ames, JPL, Langley, Dryden, Aurora, AeroVironment, and other places who had been thinking about Mars aircraft, the most striking thing about the proposal was how small the vehicle was. The Mars micromissions are parasites lifted to Earth orbit by a European Ariane 5 rocket. While going about its everyday business of launching communication satellites two at a time, the Ariane 5 has enough oomph left over to put very small payloads into highly elliptical orbits around Earth. From there, with the help of a couple of lunar swing-bys to pump up their velocity, such spacecraft can go on to Mars. French and American researchers have all sorts of tentative plans for little orbiters, penetrators, and communications relay satellites that could travel to Mars this way, all based on or carried by a standard “microspacecraft” that the Mars Airplane will be the first to use.
Because of the Ariane constraints, the aeroshell will be at most 30 inches across, meaning that the first Mars aircraft has to fit inside a container the size of a large wok. Even though the airplane will be small, weighing maybe 40 pounds, fitting it in such a tight space will require some clever origami. The designs that Langley worked on last summer had five separate folds. The outer segments of the wings fold in across the body. The vertical rudder flattens itself down onto the tailplane. And the two booms that attach the body to the tail assembly have to bend too (the booms fit on either side of the casing for the parachute, which sits in the small of the aeroshell’s back).
A difficult job is made even more difficult by the Martian atmosphere. On Earth, aerodynamic pressure is used to bend a folded aircraft into shape. On Mars, according to AeroVironment’s Miralles, who was part of a team bidding this fall to build the Mars Airplane, you have to use springs. A test model that AeroVironment developed while working on the glider project was able to spring itself into shape in only a second while falling. But springs add mass, and mass is one of the things the Mars Airplane is short of. Ease of folding was one reason why the MAGE design used a flying wing with no tail. It’s unlikely that any of the teams bidding to build NASA’s Marsplane will offer a flying wing, however, because there’s an associated lift penalty that makes it infeasible for a very small aircraft. The ultimate solution to the unfolding problem therefore remains unclear.
Langley’s official request for proposals, released in September, listed the things that the aircraft should be able to do—demonstrate powered flight and certain maneuvers, carry a small instrument package including some cameras, and so on—but didn’t say how they should be done. Contractors who bid on the Mars Airplane were left to decide for themselves what shape would be best, what sort of power source to use, and other specifics.
That didn’t mean, however, that the Langley team didn’t have its own ideas. Engineers at the center spent most of last summer working on preliminary ideas and running tests in wind tunnels. This early work suggested that a rocket engine would be the best way to go—a hydrazine engine, already proven in spaceflight and capable of delivering two or three pounds of thrust. According to Joel Levine of Langley, the chief scientist on the program, it seems simpler and more certain than using a more complex motor and a propeller that would have to work in that terribly thin air. The incorporation of a rocket also gave one of Langley’s preliminary designs a pleasingly otherworldly look—a flattened teardrop of a body with top-mounted, swept-back wings and tail booms kinked like a downhill skier’s knees to lift the forward-swept tail out of the rocket’s hot exhaust plume. The choice between conventional and swept wings involves another tricky aerodynamic tradeoff. At low Reynolds numbers, there is a pronounced “separation bubble” between the smooth flow of air over the front of the wing and the turbulent flow at the back. At high speeds—the Mars aircraft needs to fly at about 250 mph—this bubble stretches, eventually reaching a point where the flow no longer attaches at all and the wing stalls. Sweeping the wings may help achieve the desired reattachment, as might various other tricks, such as putting knobby “turbulators” to disturb the flow on the upper surface just behind the leading edge. Split flaps on the trailing edge can also help by generating lift without reattachment. But due to its tight fit inside the aeroshell, sweeping the Mars Airplane’s wings has the down side of decreasing their total area, and area is important when you need every ounce of lift you can scrape together. “We barely have enough lift to make this go,” says Juan Cruz, the project's chief engineer at Langley.
Getting enough lift will be particularly important at the beginning of the flight. After its release from the aeroshell, the airplane will be in a dive. “On Earth,” says Cruz, “the same airplane would pull out of that dive in a few hundred feet. On Mars it’s going to take anywhere from [2 to 5.6 miles]. We just don’t have enough extra lift to bring it around.” According to Miralles at AeroVironment, it’s this first dive that will be the moment of truth. The longer it goes on, the faster the aircraft will be going and the closer it will come to the speed of sound (in Mars’ cold, thin atmosphere, that’s only 520 mph). High Mach numbers make the separation problem worse, so you want to get out of the dive as fast as possible. AeroVironment learned a few helpful tricks in this regard when designing propellers for its high-altitude UAVs, but Miralles was reluctant to talk about them this fall, while the NASA contract was still up for grabs.
Once the Mars Airplane levels out—assuming it does—low lift will still cause problems, and will make changing direction a chore. Cruz estimates the turning circle will be more than three miles in radius. So even though the aircraft will be speedy, there won’t be any hot-dogging. “It will be like flying in an airliner where you sit and watch the terrain just going by,” says Miralles. “You’ll be closer to the terrain, but it will be the same sensation.”
Leisurely though it may feel, this will be a purposeful flight. “The reason people build airplanes is not because an airplane can take you anywhere, but because an airplane can take you somewhere you want to go,” Cruz says. “So we want the airplane to demonstrate that it can hold a heading and then change that direction to some pre-selected second heading. And then return to the original heading.” All of this will have to be done with pre-programmed maneuvers, because Earth will be too far away to transmit advice, even at the speed of light.
Demonstrating that the airplane can fly straight and level, turn when required, and ride out whatever turbulence may occur is the project’s primary goal. But the aircraft also has a 4.5-pound science payload consisting of cameras and—if room allows—a spectrometer for assessing the mineral content of rocks and a magnetometer (one of the great surprises to come out of the current Mars Global Surveyor mission is that some regions on the planet appear to have strong magnetic fields). The cameras will be able to spot details on the surface as small as six inches across. “If Mars has rabbits, we’ll see them,” jokes Levine.
Unfortunately, we on Earth won’t be able to watch live video of the mission because the aircraft can’t carry a radio powerful enough to send back that much data. Instead, it will send signals to the spacecraft that brought it to Mars, which will store them and send them back to Earth over a period of days. This is not a perfect arrangement, not least because the carrier spacecraft will be in line-of-sight communication with the aircraft for only 20 minutes of a flight that might last longer. But it should allow some images to come back, with at least one transmitted in real time. There will be some sort of show for Earthlings after all.
Some planetary scientists look on at all this unimpressed. They worry that the Mars Airplane will cost more than currently envisioned—$60 million was one outside panel’s estimate—and that what is basically a technology mission will start to eat into NASA’s science budget. Some would rather have used the first micromission opportunity for a communications relay satellite that could benefit other Mars exploration spacecraft with more ambitious research agendas. And some just don’t think it can be made to work. One scientist cites the Monty Python sketch about the difficulties involved in teaching sheep to fly: “The thing is, they don’t so much fly as plummet.”
But you can bet that if the Mars Airplane does fly, scientists will soon be queuing up to make use of its descendants’ ability to explore the Martian landscape. Once landing and takeoff are mastered—this aircraft will not try either—scientific instruments could routinely be sent to many sites during the same mission, making the investigations that much more productive. And aircraft could do things that no lander (unless extremely lucky) could ever do, like sniff out molecules given off by things living on or under the surface, if they exist. Because such molecules would be local, scarce, and short-lived, says Levine, they would probably be undetectable from orbit. But a search by aircraft (Dale Reed’s Mini-Sniffer again) could well find them. And although the existence of underground life would likely only be firmly established by drilling holes, a sniffer could at least show you where to drill.
The Mars Airplane is the first word in Mars aircraft design, not the last. “This is just the beginning of a generation of airplanes that will fly in the atmospheres of other planets,” says Levine. After all, if you can fly in the near vacuum of Mars, you can fly more or less anywhere. “Over the next 30 years we’re going to have many planes going to Mars, planes flying below the cloud layer on Venus to study the surface for the first time in visible wavelengths; we’ll study the organic haze on Titan; we’ll be sending planes to Jupiter and Saturn and looking under the clouds.” He points out that a recent report from the National Research Council concluded that mobility is not just important for solar system exploration, it’s essential. And mobility is just what airplanes promise.
The vehicles that make good on that promise will have all sorts of shapes and sizes. “There’s not one right way to make a Mars airplane, any more than there’s one right way to make an Earth airplane,” says Larry Lemke of NASA’s Ames center. Big sailplane-like vehicles may be good for some types of remote sensing. But if plans to manufacture rocket fuel from ingredients in the Martian atmosphere pay off, point-to-point mobility might be achieved with aircraft that use sheer speed to get around the difficulties of flying through a thin atmosphere, just as the SR-71 does on Earth. For other purposes, aerodynamically shaped dirigibles might be the way to go. The relative density of Mars’ carbon dioxide atmosphere makes lighter-than-air flight attractive; so does the fact that hydrogen, a better working fluid in every way than the helium used on Earth, will not burn in carbon dioxide. Give a big arrowhead dirigible a flat top and cover it with thin-film solar cells to generate power, and it could fly around Mars forever. Someday. Perhaps.
The Mars Airplane will bequeath technology to these far-off projects, but that may not be its major contribution. The Wright brothers changed not just the way we travel around the world but also the way we see it. Today all the images we have of Mars, save for those of three rocky landing sites, come from looking down at the planet. This orbital viewpoint, while wonderfully revealing, can’t help but turn Mars into a scientific specimen, a data set, a planet to study rather than a world to experience. The Mars Airplane will let us look out, not down, to distant horizons and what lies beyond them. It will let us watch our shadow moving on the rocks below as we fly through the sky. The camera in its rudder will show us the delicate banking of the aircraft’s wings as it heads off in directions no one has ever followed before. Long before human pilots fly over the Red Planet, these pictures may rekindle the romance of a new world in the audience back home.