DemoSat Metrover

10 October 2003

        On Saturday August 2nd, Metropolitan State College of Denver flew a payload on an edge of space high altitude balloon. The mission was sponsored by the Colorado Space Grant Consortium and was funded by NASA. The overall mission, dubbed DemoSat, was to have Colorado Post secondary science and engineering students develop ideas and strategies that might be used by NASA on future Mars missions. To that end, each school chose a specific mission and worked to send that mission up on an edge of space, high altitude balloon.

        Metropolitan State College of Denver chose the RoverSat mission. The payload was a small, autonomous rover whose mission was to deploy from its carrier upon landing following ascent and decent. After exiting the carrier the rover was to move around at the landing site and take photographs.

        The mission experienced mixed results. Some aspects of the mission were successful, while others were not. This report will explain the areas of the mission that did succeed and the areas of the mission that did not. Possible reasons for those failures will be described as well. Additionally this report will provide some suggestions for future undertakings of this same mission that will enhance the likely-hood of mission success.

• Camera operation. The original mission parameters did not require that photographs be taken during the flight, only upon landing. The MetRover team decided, however, that the camera should attempt to take photographs during the flight with the goal of obtaining images of earth from the edge of space. The camera functioned as intended, as well as the electronic timer and the wiring connections to the camera, and the rover did bring back images of the earth from an altitude of nearly one hundred thousand feet.

• Rover carrier. Two vital aspects of the carrier design functioned very well. The two areas where the carrier was successful were: ensuring that the rover was properly oriented upon landing, and that it had a secure method of attachment that minimized the risk of the carrier becoming disconnected from the payload string with a resulting loss of the carrier and rover.
  • In order to ensure the proper orientation of the rover for exit following landing, the carrier was designed to land one of two specific sides, and it did in fact come to rest on one of the intended sides, and thus had the rover in proper orientation.
  • Additionally, the carrierÕs payload attachment method utilized an internal routing of the tether line round the inner perimeter of the carrier. This internal routing prevented the carrier from becoming disconnected from the payload stack. Also of importance to this design element was that the tether not interfere with the rover. Having the line routed around the perimeter eliminated any possible interference with the roverÕs operation.

• Door latch mechanism. Premature exit of the carrier by the rover was a concern of the team, therefore the carrier door had to have a sturdy and reliable latching mechanism in order to stay closed during flight. In order to accomplish this, the rover had a micro servo mounted to it that latched the door closed until landing. The latching mechanism did hold the door closed during the entire flight and the rover stayed in the carrier as intended. The carrier door did stay closed during the flight and the rover did not exit while in flight.

• Heater Circuit. The rover was expected to experience significantly cold temperatures, as low as Ð40 deg C. In order to keep the onboard batteries alive to operate the rover following landing, a small heater was on board. Following recovery, the batteries were found to still have some power available.

• Wheel construction. The rover was equipped with aluminum wheels that were designed and manufactured by the team members. The wheels were made as light as possible but they still had to be able to survive the impact of the carrier with the ground upon landing. At the time of recovery there was no damage to any of the wheels.

• Rover Drive Motors. The rover utilized separate left and right drive motors. Following recovery, the roverÕs motors were tested and found to be fully operable. The conclusion is that the cold experienced during flight did not affect the motor windings, and the impact of landing did not affect the wiring connections from the controller to the motors.

• Carrier Window. In order for the camera to take photos of the flight, a clear plastic window was installed in the carrier that the camera could photograph through. The inside of the carrier was bright white in color. This allowed the sunlight which entered the though the window to be reflected around inside the carrier. This caused a reflection of the camera itself in the window. The cameraÕs auto-focus feature attempted to focus on this reflection of a ÒnearbyÓ object. The result was the outline image of the cameraÕs lens in many of the photos, and the far away images are blurry and difficult to make out.

• Camera Mounting. The method of mounting the camera to the rover consisted of several small square pieces of balsa wood super-glued in a stack, and then glued to the carbon fiber of the rover chassis. Another piece of carbon fiber was then glued to the other side of the stack of wood and then the camera was mounted to this piece of carbon fiber with a small bolt. Upon landing impact the balsa wood failed. The failure occurred in the piece of wood that was glued directly to the rover chassis; and the failure occurred parallel and very near the glue joint. This failure resulted in the camera pointing in a direction that made the photos taken after landing useless. Balsa wood was used for this design element because it is lightweight, relatively easy to work with, and readily available in the necessary size. Unfortunately balsa wood does not hold up well under the landing impact conditions experienced by the rover. One element of good that came from this situation, however, is that the wood did not fail completely and the camera did remain attached to the rover.

• On-board Programming of the Rover Drive and Door Latch Mechanism. At the time of recovery, the rover was found still inside its carrier with the door still firmly latched. Approximately two hours later, the roverÕs drive and door latching mechanisms were tested under more controlled circumstances at which time it operated as intended. The electronics specialist of the team suspected that a loose connection somewhere in the timer circuit caused the program to reset sometime during the flight. This accidental reset caused the roverÕs programming to start over and thus be somewhere in mid-program when recovered at the landing site.

• Carbon Fiber Chassis. The rover was constructed of two flat pieces of carbon fiber, which then sandwiched a layer of extruded polystyrene foam. At the rear of the flat chassis pieces were two thin arms that sandwiched plastic blocks, which were the rear wheel axle mounts. These thin arms were cut too narrow. One of these arms failed at the time of landing. The failure occurred where an attaching screw went through the arm and into the plastic block that held the axle for the rear wheel. The reasons for the failure were two fold. The first and most obvious reason was that the carbon fiber was cut too narrow and as a result did not allow enough material on either side of the screw hole. The second reason for the failure is that the corner of the carrier that impacted the ground upon landing was the source of the force that caused the failure. The carrier did not have a Òcrumple zoneÓ in this location as originally intended. The result of not having this Òcrumple zoneÓ is that the full impact of landing was transmitted to the rover causing the narrow attachment point of the carbon fiber to fail. It is unclear if the failure would have occurred if either situation had been different; wider carbon fiber arm or carrier Òcrumple zone.Ó

• Finalize the design sooner. This teamÕs ongoing enthusiasm for the project ended up costing too much time. The rover and carrier were constantly being ÒredesignedÓ with new and innovative ideas. The downside to all of those ideas and plans was that the final design was decided upon much later than it should have, resulting in insufficient time for building and testing.

• Increased Testing. One of the results of not having the design finalized earlier in the life of the project is that testing time is minimal. Increasing the testing time should give a clearer indication of such problems as the programming resetting inadvertently. This would allow these types of faults to be found and repaired significantly earlier in the process.

• Stronger Rover Chassis. The overall design use of carbon fiber is felt to be sound, however the dimensions for the critical mounting areas need to be increased. This should prevent such failures as were experienced during landing by this teamÕs rover.

• Stronger Camera Mounting. Use of a material other than balsa wood for the camera mount would likely result in a stronger mount. This stronger mount should be able to withstand the high stresses of landing impact. Perhaps even a completely new design of mount would be in order, as weight is critical and stronger materials would also tend to be heavier that the balsa wood.

• Reduce Glare Inside of the Carrier. Use of matte black paint or ink on the interior surfaces of the carrier should sufficiently reduce any reflected light. This is expected to reduce or eliminate the likelihood of the camera trying to focus on its reflection in the carrier window.

• Fixing the Camera's Focus for Distance. A further enhancement of the photographic capabilities of this mission would be to set the camera to only take photos on distant objects. This action, combined with darkening the interior of the rover should result in much improved photographs. A possible downside to this suggestion would be that any photos taken at the landing site might not be in focus. This is due to these photos generally being of much closer objects. The ideal alternative is to take over control of the auto-focus feature of the camera during flight and fixing the focal length for infinity, then returning the auto-focus feature to its normal operation at the time of rover deployment to get nearby photos to be in focus as well.

• Do Not Underestimate Project Requirements. This team is made up of several individuals who are very Òhands onÓ and like to physically build the components involved with such a mission as was undertaken. One of the difficulties that was discovered was that some of the project areas were taken too lightly as they seemed ÒeasyÓ to accomplish. One example is the carrier. Early on in the project, the carrier seemed like the least complicated, easiest to build component of the mission; it is a box that will hold the rover. In reality, the carrier turned out to be just as complex and demanded as much design and building time as any other individual aspect of the project.

• Do Not Stop Working on Subsystems Until Ready to Launch. This seemingly simple suggestion comes from getting a prototype of the carrier built and then shelving it because one important aspect of it worked reasonably well and thinking that the remaining details were insignificant and could wait until later. This situation occurred for this team fairly early in the project. It became a very real problem when the launch date was fast approaching and the carrier was found to need quite a bit more work than originally thought. Continuing to work on the details of any subsystem, even while other areas of the mission were being worked out, would be a much wiser use of time than wrestling with it just scant weeks or even days before launch.

        The overall experience of working on this DemoSat project has been invaluable to every member of this team. As a group we made significant personal sacrifices to see that the mission was as successful as possible. Each member of this team learned valuable lessons along the way. No member of this team came away from the experience with anything less than pride in the completion of the rover and carrier and seeing them be launched on the balloon. After the recovery, each team member was expressing enthusiastic interest in participating in such an endeavor in the future; and although not every aspect of the mission went as planned, the team felt that it was an overall success.

        As a group, this team extends a sincere thank you to those at CSGC and NASA who made this project possible, and we hope that you can find some useful aspect of what we have done to help make going to Mars, or other space flight missions a success. We also hope that you will be interested in pursuing such projects again in the future as they will likely prove very valuable to all those who are involved.