The 2003 Mars Plane Mission
by Andrew Hahn
November 7, 2000
Most people don't know that NASA is actually made up of several centers, with varying amounts of overlap in terms of responsibility and capability. When our Administrator, Daniel Goldin, decided that it would be neat to send a powered airplane to Mars to commemorate the 100th anniversary of the Wright brothers' first flight, he asked groups at Langley, Dryden, Glenn, and Ames for proposals. This journal covers the early efforts of the Ames proposal. Initially, Dave Kinney and I were tapped to do a feasibility study for Ames. We were told to start at 4:00 p.m. on Friday, 5 February 1999 and needed "something" to support our management in a meeting at Headquarters the following Thursday. It was a long weekend. Dave and I had done many conceptual designs before, but nothing like this. The deadline was very short, the task wasn't well defined, and the vehicle operated under conditions that were foreign to us. In short, we weren't quite sure where to begin. On top of that, all of our design tools pretty much assumed that airplanes flew on earth. Mars has a different gravitational constant (g), a very different atmosphere, and no oceans. Really fundamental questions like how much does it weigh, how much lift or drag does it have and how high up is it couldn't be answered with our existing design codes. We had to start from scratch, questioning everything we thought we knew and we had to do it fast.
Right away, we decided to use the metric system for everything. Normally, we use the English measure system because a great deal of our basic design data are in English units and over the years, we have gotten a feel for the scale of answers we get, which alerts us to really big errors. Unfortunately, this project is particularly sensitive to keeping mass and force distinct, which English units are less conducive to doing, and the scale of the plane meant that our wealth of experience was not going to be applicable. So, there was a potentially bad outcome if we stayed with English units and no really good reason to stick with them. As we found out later, mixing units was definitely bad, causing a probe to crash, but also one of the other groups had sporadic errors that happened to be off by the ratio of the Earth's and Mars' gravitational constants, an indication that mass and force were being confused.
We also decided to prototype a new design code for Extraterrestrial Flyers in a spreadsheet because we didn't have any time to do really sophisticated analysis, the task didn't require really sophisticated analysis, we didn't think we could find all the "g's" in our really sophisticated analysis, and this problem was so uncharted that we needed the flexibility in calculation flow that spreadsheets are really good at.
We did get some guidance from the space folks that the maximum mass (not weight) allowable for the Mars Plane was about 24 kg. and that it had to fit into a re-entry shell of about one meter diameter. They also told us that the gravity was about 3/8 that of Earth's and that the atmosphere where we wanted to fly was similar to the Earth's at 100,000 feet altitude.
We then had to make a number of assumptions about speed, airfoils, materials, propulsion, and range to see how easily an airplane could be made that met even the most basic requirements. What we found out was that the most important design constraints were the mass and volume available on the space vehicle that carries the plane to Mars. Early results indicated that we wanted the biggest plane that we could fit into the re-entry vehicle, which in turn wanted to be the biggest we could fit onto the launch vehicle, the Ariane 5. Our initial concept looked very much like a radio control model airplane that folded up into a compact re-entry shell and while it looked feasible, we wanted more. This prompted our looking at some very strange ideas for fitting a flight vehicle into a re-entry vehicle. We did a qualitative look at a folding wing with only two hinges, a folding wing with eight hinges, a cable braced roll up wing, a flexible membrane sailwing (kind of like a bat wing) and a parafoil (like a square parachute). In the end, we wound up choosing the folding wing with the fewest hinges for a lot of practical reasons.
Given our assumptions, it appeared that the most attractive mission from the standpoint of performance feasibility was the relatively low altitude Canyon Flyer. It turned out that the Mars scientists liked the same mission because they were very interested in getting a look at the walls of the huge Valles Marineris over as long of a stretch as they could. As an added bonus, the primary scientific instrument, a video camera, would provide really cool pictures for the public as well as document the really risky deployment from the re-entry shell. It was beginning to look like things were coming together.
In just five days, Dave and I had concluded that the Mars Plane mission was difficult, but doable. We had given our management enough information for them to make important, early decisions, and started a relationship with the space side of NASA. Over the next nine months, many more people were called in to flesh out the design and make an integrated proposal. Specialists in aerodynamics, communications, power systems, structures, missions, science, fabrication, and contracting turned our simple study into something quite impressive.
We found that our initial assessment was a little optimistic, but that as we found problems, our people were able to minimize the impact through clever, detailed solutions. Even so, there were several critical areas of uncertainty that could only be managed through very sophisticated analysis and testing. Cost and schedule would have been very tight and, in the end, might have required either pulling resources from other projects to "do it right" or taking on too large of a risk of failure and so the project was killed.