Specification and Design of a CO2-Powered Flying Mars Exploration Vehicle
Mentors:
Dr. Robert Ash & Dr. Colin Britcher
Department of Aerospace Engineering
Students:
John J. Miller, David Covington, Bradley Dupont, Brian Meagher, Nelson Gosnell,
Grant Jennings and Ian Johnson
Aerospace/Computer Sci./Electrical/Mechanical Departments
OldDominionUniversity
Abstract
The goal of the CO2-Powered Mars Science Vehicle project is to specify and create a preliminary design for a reusable, compressed-gas, rocket-powered vehicle, which can operate repetitively in the Martian atmosphere during an extended exploration mission. To design this vehicle, two things need to be accomplished:
First, the Martian environment and atmospheric conditions need to be understood—densities, temperatures, wind currents, thermal energies, etc.Understanding the range of environmental conditions for which the vehicle must operate is one key to a successful design.
Second, after the operating theater has been characterized, a robust and scientifically useful Mars exploration vehicle needs to be designed. This vehicle should be capable of traveling through the Martian atmosphere, then without damaging its frame, successfully land at random locations on the surface of Mars. Once on the ground, the vehicle should be able to orient and then launch itselfback up into the Martian atmosphere to continue exploring and collecting data from the surrounding environment.
The scope of this project will be directed toward creating a first-pass design: an initial proposal for a vehicle capable of traversing large distances (compared with the Mars Exploration Rovers) by utilizing atmospheric flight. After researching and documenting the background for the vehicle—this includes studying the previous work in the area of Martian flight, as well as fully characterizing the Martian atmosphere—studies will be conducted to determine the optimum vehicle configuration with respect to flight range, weight and size, useable payload options and systems needed for navigation and for communication between the vehicle and the Earth. We will attempt to define optimum operating conditions in terms of flight trajectory altitude, cruise speed, operational flight range, and airframe landing, launch and maneuverability dynamics. All these components and variables will then be combined establishing a framework or baseline design upon which others can expand, develop, and refine viable designs for future Mars exploration vehicles.
Table of contents
Abstract
Table of contents
List of Figures
List of Tables
Introduction
1Structures
1.1Introduction
1.2Research of Previous Pojects
1.3Development of Design
1.4Conclusion/Future Work
2Propulsion
2.1Introduction
2.2Background
2.3Constraints and Assumptions
2.4Data Gathered
2.5Model of Propulsion System
3Flight Control
3.1Introduction
3.2Flightpath Determination
3.3Flightpath Control
4Power Systems
4.1Introduction
4.2Power generation
4.3Heating element
4.4Power storage
5Communications
5.1Considerations
5.2Recommendations
Conclusion
Status Report
References
I.Structures References
II.Propulsion References
III.Controls References
IV.Power Systems References
V.Communication References
List of Figures
Figure 1: Phase Diagram2 of CO2
Figure 2: Thrust Function of Isothermal Propulsion System Model
Figure 3: Graph of Lithium-Polymer battery weight vs. battery power
Figure 4: Gantt Chart
List of Tables
Table 1: Isothermal Propulsion System Model:
Table 2: InGaAs/Ge Type Photovoltaic Cell Capabilities
Introduction
Mars is a fascinating planet: Having altitudedifferences of up to 40 km, and diurnal temperature swings of 90̊ F.Mars sports Olympus Mons, largest mountain in the galaxy, andValles Marineris, a 7k-deep rift, whose length extends across a fifth of the planets circumference. Mars has been known since prehistoric times, and has been extensively studied with ground-based observatories. The first spacecraft to visit Mars was Mariner 4 in 1965. Several others followed including Mars 2, the first spacecraft to land on Mars and the two Viking Landers in 1976. Ending a long 20 year hiatus, Mars Pathfinder landed successfully on Mars on 1997 July 4. In 2004 the Mars Expedition Rovers "Spirit" and "Opportunity" landed on Mars sending back geologic data and many pictures; they are still operating after more than a two years on Mars. Three Mars orbiters (Mars Global Surveyor, Mars Odyssey, and Mars Express) are also currently in operation.However, there is a problem with the current means of Martian exploration, if the two Mars Exploration Rovers currently operating on the surface continue at their current rates it will take more than one million years to explore the Martian surface.Thus, it can be seen that there is a need to develop small exploration vehicles, with renewable propulsion systems that can be used to transport robotic instruments over large distances over relatively short periods of time. If a large number of these small robotic surface probes could then be moved about the Martian surface to increase the rate of surface exploration.
One possible approach for providing this type of propulsion is to exploit Mars’ cold night temperatures and the high percentage of CO2 in the atmosphere (95.3%).Aided by the low nocturnal temperatures it is possible to freeze out carbon dioxide (it freezes at 140 K at Mars ambient pressures).Collecting the dry ice in a tank, and then during the day, heat the dry ice to approximately 300 K, causing the dry ice to become a pressurized fluid at almost 4,000 psi. That pressurized gas could then be used as a propellant to lift the payload off the surface.
Using these unique properties of the Martian atmosphere, a lightweight, reusable Mars exploration vehicle could be designed. The focus of this is projectto begin researching feasibility issues and design challenges associated with building such a system. To research and design such a craft, the project has been divided up into five separate sub-categories.
Structures: This element of the project is focused on providing a platform that will contain and transport the vital components and scientific instruments necessary for the exploration of Mars
Propulsion: Provides a renewable and efficient means for the platform to traverse the Martian surface at relatively high speeds.
Flight Control: Supplying the navigation of the Martian exploration vehicle; allowing it to detect and avoid any obstacles or hazards, which may be presented to the vehicle.
Power Systems: Collecting, and delivering the energy necessary for navigation, guidance, and the scientific payload.
Communication: Providing the means for Earth to talk to, and receive information from the vehicle, while it is operating on the Martian surface.
1
1Structures
1.1Introduction
The objective of the structural design element of this project is to provide a platform from which an expanded range robotic exploration system can be deployed on the surface of Mars. This platform should offer a level of re-usability, range, and speed not yet achieved with any other exploration concept. While doing this, the platform should be robust enough to withstand the extreme working conditions found on the Martian surface. At the same time, the design should be simple enough to guard against failure.
A list of parameters and requirements will be tabulated in order to facilitate the selection of our prototype design. A key requirement is that the craft should be able to cover as much of the surface as possible. The craft should have the capability of being programmed to go to any destination desired. The craft, if airborne, should have the ability to loiter over a target for some period of time. If airborne, the craft will require a system for landing and re-launching. The craft should not be "tied down" to any one location on the planet. The main propulsion of this craft is to be provided by compressed CO2(one focus of this project is a cryo-cooler that can produce pressurized gas when dry ice is converted to a fluid phase). Most importantly, the craft should provide a platform for conducting scientifically important research.
1.2Research of Previous Pojects
To prepare for the task of designing a Mars vehicle, previous vehicle design concepts have been studied. Those designs covered a wide range of capabilities and complexities, ranging from non-buoyant inflatable platforms (imagine a large beach ball) to Martian rotorcraft (robotic helicopters). Each of these designs has inherent strengths and weaknesses. The discussion that follows will highlight the strengths and weaknesses identified during the preliminary phase of this project. It should be noted that while these vehicles have gone through a great deal of research and development, none of them fully achieve the desired goals of this project.
The simplest design researched for this project was the non-buoyant inflatable device. This vehicle can be compared to a giant toy beach ball with a sensor package mounted inside. Research on this idea has been conducted by NASA and North CarolinaStateUniversity under the name "Tumbleweed". The tumbleweed has been developed and tested to determine the usefulness of such a device at Mars. It was found that given a steady wind, such as the one present on the Martian surface, the tumbleweed could be capable of rolling along indefinitely on the surface, collecting data as it went. The system is extremely simple and nearly failure proof. The main issue with this system is its lack of guidance. The tumbleweed was designed to travel at random with no way of guiding it from one point to another. Also, as long as the wind is blowing, the tumbleweed is moving. This gives no opportunity to loiter at any particular location. While thissystem does allow for simplicity and has very few failure points, its lack of control prohibits its use.
A ballistic or “hopper” system is a rocket-propelled vehicle that could be launched from one point to another. This would be another fairly simple system, having few moving parts. The science and systems associated with this type of device have been proven numerous times on Earth and on the Moon. The biggest problem with a system like this is the amount of energy needed to loiter over an area. Also, if attention were restricted to compressed carbon dioxide propulsion, range and sortie duration (the amount of time the vehicle can remain in motion) would be limited by the amount of CO2 propellant on board, which in turn is limited by the size of the vehicle. Because of these limitations, the ballistic vehicle was not identified as a strong candidate.
A buoyantgas inflatable device is similar to a weather balloon or blimp. Its strongest advantage is the fact that it simply eliminates some of the design issues stated previously. For example, the craft could in principle be designed such to remain airborne indefinitely and therefore avoid launch and landing difficulties during its entire missionlife. Also, since the craft never lands, there is the possibility to loiter over a target for as long as needed. There were two major disadvantages to this system. The first dealt withthe Martian atmosphere. Since this is a buoyant craft, the nearly 30 km altitude range associated with Mars surface topology, and the strongly varying atmospheric conditions play a significant role in the success of the mission. It was found that the atmosphere density variations resulting from the Mars diurnal cycle are significant and therefore a buoyant craft's lift would change by a significant amount when compared with similar systems on Earth. One solution to this problem was to suspend part of the craft's weight on a long cord that could be drug along the ground as the buoyant force changed. This concept was rejected based on the reliability associated with scientific instruments thatwould be drug on the ground intermittently. The other issue with this ideas deals with materials. At this time, there are no suitable skin materials for the balloon envelope. Lightweight materials that are suitable for space systems cannot withstand the temperature extremes and solar radiation associated with the Martian atmosphere. A buoyant inflatable device therefore was not selected as the baseline design(See reference 2)
AMartian rotorcraft, such as a helicopter or autogyro concept is another interesting design concept. This type of vehicle has been studied extensively and holds promise for various Mars missions. Benefits include the ability to takeoff and land vertically andthe ability to cover large distances and loiter at will. While offering very complete coverage of the design parameters, this idea too has severe shortcomings. First, because of the large variations in Mars surface elevation and the low atmospheric density—even at mean zero altitude—rotorcraft propulsion systems require a great deal of energy (helicopters cannot be flown on Earth at altitudes of 30,000 ft and the nominal Mars surface corresponds to a terrestrial altitude greater than 130,000 ft.) All previously developed Martian rotorcraft designs rely on hydrazine monopropellant propulsion systems and hydrazine cannot be made from local Mars resources. Therefore, in order to refuel and recharge batteries, the rotorcraft would need to return to a centrally located landing base, thus restricting its exploration range to the immediate vicinity of the base. Furthermore, materials do not yet exist which could withstand the loads placed on rotating components and still be light enough for a Mars application. (See references 3 & 4)
Finally, a fixed wing aircraft has evolved as our design focus. A variety of research organizations have pursued Martian airplane concepts. The most notable among these is the ARES project from NASA. This platform allows for a wide variety of potential science missions. An aircraft of this type would be able to accommodate the large fluctuations in Martian surface atmospheric conditions. An ARES-type vehicle can cover large distances in a short period of time. The aircraft fly in a type of holding pattern, loitering over an area for some period of time. The flight trajectory can be planned from Earth using satellite image mapping, enabling the exploration oftargets as they are presented to scientists on Earth. The platform is a very good starting point for evaluating a cryo-cooler based compressed carbon dioxide propulsion system, and the fixed wing platform offers everything this project needs except for a very crucial set of requirements; take off and landing. Fixed wing aircraft require a flat, fairly smooth surface on which to land. Even more space is usually required to takeoff. (See references 5 – 8)
1.3Development of Design
After carefully considering all of the above design options, a fixed wing aircraft was deemed the preferred option for achieving the desired design parameters prescribed for this project. It has been the most extensively researched and the most highly developed. There are only two unexplored primary design elements—provisions for multiple take offs and landings. This project will concentrate on that aspect of the vehicle system design. To date, several system concepts have been identified. The concepts under consideration will be discussed.
A vertical take-off and landing (VTOL) airplane is one option. This idea was explored by a student design team at the University of Texas at Arlington (UTA, reference 5). UTA's approach utilized a large propeller/rotor mounted on the front of the craft. The efficiency of such a device, or rather its lack of efficiency, makes it impractical. If power were to be used to land the vehicle, the amount of energy needed would be comparable to the amount needed to launch. In effect having a powered landing would cut the endurance of any mission in half. Also, this type of system would introduce opportunities for failure that probably could not be overcome. In other words, if the vertical landing system failed, the mission would likely end.
One unique re-launch method examined is that of the self-righting launch system. This so-called "weeble rocket" concept like the weighted, egg-shaped toy weeble characters, this design concept would right itself without any assistance regardless of its landing orientation) would require careful management of the overall vehicle proper shape and center of gravity. Using this approach, after the vehicle landed it would stand on its tail in its desired launch position automatically. The weeble rocket idea does notoffer any landing advantages and places significant constraints on the overall system geometry.
Another, somewhat more complicated concept, appears to offer good solutions to the problems of landing and re-launching, while providing some protection against failure. To land, a system of flaps and airbrakes similar to those used on a sailplane could be deployed. This would reduce the forward speed of the aircraft, which would reduce the terminal impact energy. The entire wing units could be used as flaps in some configurations. Just prior to landing, the entire wing could be rotated very rapidly to a vertical position, maximizing the drag and vehicle deceleration rate. Then, to re-launch, the center section of the aircraft, which contains the propulsion system, would rotate to thisvertical position and lock into the wings. When the CO2 rocket lifts the craft off the ground, gravity and aerodynamic forces could be employed to reconfigure the vehicle into its flight geometry and lock the wing and fuselage positions. The most likely failure would bethe mechanism that shifts the position of the wings and center section. This type of failure could be overcome by designing the craft such that the wings natural tendency is to be in the vertical position and either the wing or the center section could be powered into either a vertical or horizontal launch position.