Little wing


NASA scientists developing an aircraft to fly in the Martian atmosphere believe they have solved a long-standing aerodynamic mystery. Al Bowers, the chief scientist who is leading the project at NASA’s Armstrong Flight Research Center in California, said he hoped to prove the details of the discovery during a wind tunnel test of a “revolutionary” flying wing design at the end of August at NASA’s Langley Research Center.

The flying wing, known as Prandtl-m (Preliminary Research Aerodynamic Design to Land on Mars – and a tribute to German aerodynamicist Ludwig Prandtl) is intended to ‘piggyback’ on a future mission to the Red Planet. The aircraft would then be released to glide in a low-level reconnaissance flight, and gather data on potential landing sites for a manned mission.

In a departure from previous proposals for a Martian aircraft, Prandtl-m is intended to be a lightweight and relatively cheap vehicle, made of flexible material so that it can be ‘rolled up’ and stowed neatly inside another spacecraft for the ride to Mars. The platform selected for the mission is the flying wing – a design that offers many advantages in the circumstances, says Bowers, but also presents some interesting aerodynamic challenges, which he is keen to overcome.

Look, no tailfin

Speaking to Aerospace Testing International in August, Bowers, who has worked as an aerodynamicist at NASA for more than 20 years, said his original line of inquiry had been to solve the long-standing problem of how this type of aircraft – and similarly birds – manage to maneuver without a vertical tailfin. He believes he has found the answer in the course of his work on Prandtl-m, and also demonstrated it in flight.

“It turns out that flying wings are a rather difficult problem,” Bowers said. “This was in fact my original question, and along the way we believe we have figured out how it is that birds are able to maneuver without a vertical tail. We have had some difficulty in convincing the aeronautical community that we have solved this problem. So one of the things we have done is that we have paid for a wind tunnel model and a wind tunnel test at NASA Langley in three weeks [at the end of August]. So three weeks from now I will be in a wind tunnel with the engineers and researchers at NASA Langley with one of these designs, and I can have them generate the data for me.”

While Bowers says a journal article on his findings is pending, he adds that the key to the problem was to change the span load of the wing in flight: “We changed the span load, and in the process of changing the span load you change many of the other aerodynamic characteristics in ways that are unexpected.”

Prandtl-m is supported by NASA’s Flight Opportunities Program at Armstrong. After the wind tunnel, Bowers hopes to conduct three flight tests at high altitude beginning in October 2015, where the Earth’s atmosphere is similarly thin to that of Mars close to the surface.

Mars aircraft have been proposed as many as eight times in the past, says Bowers. “I have participated on a number of proposals previously, and mostly they were very large vehicles that would carry extensive scientific payloads.” Now his team is taking a different approach: to produce a vehicle quickly that is also small and cheap, but still capable of gathering useful information. “We could use it to photograph proposed human landing sites on Mars. We might be able to get superior photographs of the landing sites and provide some of the landing site information for the Human Exploration and Operations (Mission Directorate) at NASA.”

Currently, the Mars Reconnaissance Orbiter (MRO), flying at 300km above the planet’s surface, can produce images of only 1m resolution horizontally and 3m vertically. Prandtl-m would improve on this by several orders of magnitude. “A field of 85cm boulders, which would not be such a great landing site, is not entirely visible from the Mars Reconnaissance Orbiter. Just doing the math, with our little airplane, we can get down to about 50mm resolution.”

The key to the project is the flying wing platform, says Bowers, which will offer relatively good stability in the thin Martian atmosphere. “We tried to pick something that was very stable,” he explains. “That was really the piece of the puzzle that I had. It is also a fairly simple aircraft, which didn’t require a large boom or tail, and one of the other things that we discarded along the way was a propulsion system. Those are all very desirable things, which help out in many ways – the propulsion system particularly because of the amount of time you have to gather information. But the complexity of adding those things is difficult.”

Roll play

Another challenge is to transport the airplane to Mars and deploy it successfully above the surface. As always, mass and volume are critical factors in space flight, but again the flying wing design offers an innovative solution. Bowers and his team plan to roll up the aircraft and place it inside the aeroshell of another major mission, where it will act as a piece of the essential ballast. This will then be ejected over the planet at the end of the flight. The aircraft will therefore need to be light and extremely flexible, yet capable of unrolling into a rigid and highly aerodynamic shape. Bowers and his team have been trying a range of different composite materials that are both flexible and capable of being ‘pressed out’ to form a flat aerofoil.

“We are experimenting with S-glass, E-glass, Kevlar, and carbon fibre, and carbon fibre is functioning most beautifully in this regard,” reveals Bowers. “One thing we do not know is what are the creep characteristics, particularly for long durations at extreme cold temperatures.”

The plan is to place the rolled-up aircraft inside a stack of three CubeSats, measuring 30cm by 10cm; this could then be used as ballast on a larger vehicle. The idea was inspired by missions such as Curiosity, which was flown to Mars in 2011 with 57kg of tungsten on board as ballast. This was necessary because the volume and position of the rover within the aeroshell of the spacecraft caused the center of gravity to be offset.

“To get the center of gravity and the inertias back to the very center for the cruise portion to Mars, you had to put ballast along one rib,” explains Bowers. “So they built these slugs of tungsten that were put inside the aeroshell, and then the aeroshell could be spun until it was spin-stabilized during its cruise to Mars. Just before it got to Mars, Curiosity did a de-spin and ejected all the ballast, which fell onto the Martian surface. So we thought that we could be the ballast.”

The next step, therefore, would be to deploy the airplane from the CubeSat as it is falling toward the planet’s surface “and survive”. For this, the team will rely on another new piece of technology developed at NASA’s Ames Research Center in California: a giant parachute called the Exo-Brake, which is designed to de-orbit small spacecraft.

“They have a very large parachute that they are putting on CubeSats that they are deploying from the International Space Station. They are using the atmosphere at the altitude of the ISS in orbit to de-orbit the CubeSats and bring them back [to Earth]. This is being done without any thermal protection and is exactly what we want to do. We would love to use their technology on a 3U CubeSat, and as we are descending through the Martian atmosphere, about 5,000m above the surface, we would eject our little airplane and fly away.”

Flight testing

The three flight tests, the first of which is planned for October 2015, will be concerned with this stage of the mission: the successful deployment into the Martian atmosphere from the CubeSat and then navigation by autopilot to a way point.

In October, the flying wing will be released from a balloon at an altitude of 30,000m. The focus of this test will be the autopilot technology, which on Mars will rely on a digital terrain map created by data gathered by the MRO. Bowers hopes that by pattern matching between the map and the images it gathers itself, the autopilot should be able to guide the airplane over a series of target areas.

“We should be able to navigate with the images we are gathering for the actual experiment,” he says. “The terrain map is coming out of another piece of technology that we developed here, which was our ground collision avoidance. Our truth model, of course, on Earth will be the GPS data. So we are going to see if it can do the pattern matching when we are doing the balloon drop, and then see how accurate that navigation is for us.”

Bowers says he suspects the first balloon drop will be over Oregon. For the second balloon drop, the aircraft will be carried to 40,000m inside a 3U CubeSat. The CubeSat will then be dropped on a drogue chute before the airplane is then deployed. Bowers hopes to use a sounding rocket for the third mission to deploy the satellite at near-orbital altitude. The Exo-Brake parachute would be used to bring the CubeSat back to an altitude of 40,000m and then deploy the aircraft. At that altitude, the Earth’s atmosphere approximately matches that of Mars close to the surface. “By the time we get down to 25,000m, the experiment is over, and after that you are trying to get home as quickly as possible.”

Even if the multi-stage deployment can be pulled off as planned, how can such a lightweight and flexible vehicle carry any useful sensing equipment that might be required to make such a mission worthwhile?

There are two issues here, responds Bowers. First is the weight of any experiment. However, on this count, the 38% gravity on Mars is helping out. If the planned payload is 900g on Earth, on Mars it will be an actual weight of 340g. “All the experiments here on Earth are being flown at about the 340g mass,” he says.

The second, more difficult problem, adds Bowers, is volume. “Micro-electronics are amazing, but they have not quite got to the point where these parts vanish. We are struggling a bit with that because of the drag that they generate. Right at the moment it is the miniaturization. Getting them small enough so that they don’t dramatically affect the aerodynamics of the airplane is our current challenge. We can get the wing to fly very well by itself, then we slowly grow the size of the device on the bottom and the performance degrades considerably. It is very easy to overwhelm the aerodynamics of the wing and make it unflyable.”

Among the various experiments that might be included on Prandtl-m, apart from the photographic mission, are a radiometer and a dropsonde to measure atmospheric pressure, temperature and wind, but these depend on successful miniaturization.

The aircraft would have a range of 30-40km, and perhaps fly over a series of targets in a 10-minute mission. After that, it would not have sufficient power to transmit any of its findings directly back to Earth, so the data would be relayed via the orbiter or one of the surface rovers.

If Prandtl-m successfully completes the third flight test, Bowers believes there is a good chance that NASA headquarters will give permission for it to ride to Mars on board a rover mission, perhaps even as early as the beginning of the next decade. 

George Coupe is an engineering and technology writer based in the UK.

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