How to build the first extraterrestrial airplane.
- By Oliver Morton
- Air & Space magazine, January 2000
(Page 4 of 5)
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.