Mars Airplane Design
by Steve Smith
March 23, 2001
We were pretty excited when we were asked to help design an airplane to fly on Mars. This was a chance to use our aeronautics knowledge on a completely new challenge with some very special problems. Several of the team members had some experience with flight at very low atmospheric pressure from working on airplanes to fly at extremely high altitudes on Earth. But we had never tried to design an airplane that had to meet some really special requirements. It was my job as the Project Leader to understand the requirements and limitations on the design and make decisions about how we would meet the requirements.
The first requirement was that our airplane would have to fold up inside a small container that would carry it to Mars and protect it during a fast, hot atmospheric entry. this container, called an aero-shell, has a special heat shield called a thermal protection system on it to protect it and its contents from the intense heat during entry. You can read about other researcher's work on thermal protection systems at http://quest.arc.nasa.gov/aero/events/thermal.html Once the aeroshell was slowed down and descending in the lower atmosphere, it opened a parachute, and then the heat shield was released. At this point, our airplane would be dropped out of the aeroshell, and unfold and start to fly.
We were very worried about getting the airplane to unfold correctly. suggested that the fewer the folds, the better. So we picked a design with just 3 folds: two wing folds and a fuselage fold. Some other people have tried designs with many more folds, as many as ten. But they did not always unfold properly.
Even with the airplane folded up, it still had to be pretty small to fit in the aeroshell. Not only that, but the mission planners gave us a maximum weight limit for the airplane, because the spacecraft that was to carry our airplane and aeroshell to Mars could only carry so much weight. Part of the weight allowance was used for guidance and communication equipment on the spacecraft, basically a computer, a radio, and an antenna. Then, there was the weight of the aeroshell itself, with its heat shield and parachute. So we were given a mass allowance of 20 kg for the airplane. On Earth, 20 kg weighs 44 pounds, but on Mars, it would only weigh about 15 pounds because there is less gravity on Mars. The inside diameter of the aeroshell was about 0.65 meters, or just over 2 feet. So with the airplane all folded up, it had to be less than 2 feet in any direction. With the wings unfolded, the wing span would be about 6 feet, since we had two folds.
Another challenge that we did not know very much about was all the equipment that the airplane would have to carry. The scientists that want to use an airplane to explore Mars told us the kind of instruments they would like to carry, but they did not know very much about how small they could be made. They also did not know how we would get the information from the instruments sent back to Earth. So I formed a group to concentrate on the instrumentation and data communication back to Earth.
The mission planners told us that the best way to get the information sent back to Earth was to first send it to the spacecraft that took us to Mars, and then that spacecraft would relay the information back to Earth. But this is really not very good, because after the spacecraft drops the aeroshell off on the way to Mars, it just keeps flying off into space. It is only in a good position to relay data from Mars for about 20 minutes. So even if our airplane could fly for longer than 20 minutes, no information would come back from it after the spacecraft flew past Mars.
The instrumentation and data communication group decided what instruments the airplane would carry, and what kind of radio and antenna would be needed, and what the data-management computer would need to store the data and send it back to the spacecraft. This group also studied the flight control computer problems too. In order to fly all by itself on Mars, the airplane needs some sensors and a computer to work as an autopilot. Once this group told the airplane design group what the instruments were, how much they weighed, and how big they were, then the airplane design could really begin.
We also had to decide how we would power the airplane. For our first airplane, we decided it would be simple to use an electric motor and batteries to power the airplane using a propeller. The other ideas we considered were using a rocket, and using a special turbine motor that burns rocket fuel but turns a propeller. The turbine motor would be similar to a jet engine, except that with no oxygen in the atmosphere, it won't work like a jet on Earth that breaths intake air. Instead, we would use rocket fuel that has its own oxygen mixed in. In the long run, this choice is probably the best. But no such motor exists, so for now, we would use an electric motor, and when a new motor becomes available, we could switch.
We had to make sure there was room in the fuselage for all the sensors, instruments, antenna, computer, plus the motor and batteries for the airplane. We were careful to position the various items in the fuselage so that the center of gravity was in the right place to get the airplane to balance for stability and trim.
While all this was happening, other people in the group were studying the best wing shapes and airfoils. Others were estimating how much the wings and fuselage would weigh, and what the folding mechanisms would be like. You can read Andy Hahn's, Field Journal to get more ideas on what this part of the project was like. Finally, we put everything together and evaluated the performance of our design. To do that, we had to be sure it would make enough lift to carry its weight, estimate the drag of the airplane, make sure there was enough thrust to equal the drag, and then see how long the batteries would last. After a few tries, we found we could fly for more than 20 minutes, so our airplane was a success. We also decided that we would try to make it fly longer using the turbine motor and rocket fuel. We found that it could fly much longer, almost two hours. So in the future, this will be a good way to power airplanes on Mars.
Now, with the new planetary aeronautics website, you can try to design your own Mars airplane. You can make choices about what instruments to carry, what kind of fuel and propulsion system to use, what the wings should look like. The web applet will tell you if your design will work, and how long it will fly. See how long a flight time you can get! Good Luck.