Engineering topics
Spencer, David A.
Publications and source records attributed to Spencer, David A..
Landing Site Dispersion Analysis and Statistical Assessment for the Mars Phoenix Lander
The Mars Phoenix Lander launched on August 4, 2007 and successfully landed on Mars 10 months later on May 25, 2008. Landing ellipse predicts and hazard maps were key in selecting safe surface targets for Phoenix. Hazard maps were based on terrain slopes, geomorphology maps and automated rock counts of MRO's High Resolution Imaging Science Experiment (HiRISE) images. The expected landing dispersion which led to the selection of Phoenix's surface target is discussed as well as the actual landing dispersion predicts determined during operations in the weeks, days, and hours before landing. A statistical assessment of these dispersions is performed, comparing the actual landing-safety probabilities to criteria levied by the project. Also discussed are applications for this statistical analysis which were used by the Phoenix project. These include using the statistical analysis used to verify the effectiveness of a pre-planned maneuver menu and calculating the probability of future maneuvers.
Aerobraking Cost and Risk Decisions
Four missions have successfully employed aerobraking at Venus and Mars to reduce the spacecraft orbit period and achieve the desired orbit geometry. The propellant mass reductions enabled by the aerobraking technique allow the use of smaller launch systems, which translate to significant savings in launch costs for flight projects. However, there is a significant increase in mission risk associated with the use of aerobraking. Flying a spacecraft through a planetary atmosphere hundreds of times during months of around-the-clock operations places the spacecraft in harm's way, and is extraordinarily demanding on the flight team. There is a cost/risk trade that must be evaluated when a project is choosing between a mission baseline that includes aerobraking, or selecting a larger launch vehicle to enable purely propulsive orbit insertion. This paper provides a brief history of past and future aerobraking missions, describes the aerobraking technique, summarizes the costs associated with aerobraking, and concludes with a suggested methodology for evaluating the cost/risk trade when considering the aerobraking approach.
Design of the ARES Mars Airplane and Mission Architecture
Significant technology advances have enabled planetary aircraft to be considered as viable science platforms. Such systems fill a unique planetary science measurement gap, that of regional-scale, near-surface observation, while providing a fresh perspective for potential discovery. Recent efforts have produced mature mission and flight system concepts, ready for flight project implementation. This paper summarizes the development of a Mars airplane mission architecture that balances science, implementation risk and cost. Airplane mission performance, flight system design and technology maturation are described. The design, analysis and testing completed demonstrates the readiness of this science platform for use in a Mars flight project.
Aerobraking cost/risk decisions
this paper provides a brief history of past and future aerobraking missions, describes the aerobraking technique, summarizes the costs associated with aerobraking, and concludes with a suggested methodology for evaluating the cost/risk trade when selecting the aerobraking approach.
Navigation Flight Operations for Mars Pathfinder
On July 4, 1997, Mars Pathfinder became the first spacecraft to land on the surface of Mars in 21 years. Pathfinder was launched on December 4, 1996 and spent seven months en route to the red planet. This report discusses the navigation flight experience for the Mars Pathfinder interplanetary cruise. In particular, orbit determination and maneuver design and execution results are presented. Special emphasis is given to the navigation role in the days and hours leading up to and including the Entry, Descent, and Landing (EDL) phase.
Navigation Strategy for the Mars 2001 Lander Mission
The Mars Surveyor Program (MSP) is an ongoing series of missions designed to robotically study, map and search for signs of life on the planet Mars. The MSP 2001 project will advance the effort by sending an orbiter, a lander and a rover to the red planet in the 2001 opportunity. Each vehicle will carry a science payload that will Investigate the Martian environment on both a global and on a local scale. Although this mission will not directly search for signs of life, or cache samples to be returned to Earth, it will demonstrate certain enabling technologies that will be utilized by the future Mars Sample Return missions. One technology that is needed for the Sample Return mission is the capability to place a vehicle on the surface within several kilometers of the targeted landing site. The MSP'01 Lander will take the first major step towards this type of precision landing at Mars. Significant reduction of the landed footprint will be achieved through two technology advances. The first, and most dramatic, is hypersonic aeromaneuvering; the second is improved approach navigation. As a result, the guided entry will produce in a footprint that is only tens of kilometers, which is an order of magnitude improvement over the Pathfinder and Mars Polar Lander ballistic entries. This reduction will significantly enhance scientific return by enabling the potential selection of otherwise unreachable landing sites with unique geologic interest and public appeal. A landed footprint reduction from hundreds to tens of kilometers is also a milestone on the path towards human exploration of Mars, where the desire is to place multiple vehicles within several hundred meters of the planned landing site. Hypersonic aeromaneuvering is an extension of the atmospheric flight goals of the previous landed missions, Pathfinder and Mars Polar Lander (MPL), that utilizes aerodynamic lift and an autonomous guidance algorithm while in the upper atmosphere. The onboard guidance algorithm will control the direction of the lift vector, via bank angle modulation, to keep the vehicle on the desired trajectory. While numerous autonomous guidance algorithms have been developed for use during hypersonic flight at Earth, this will be the first flight of an autonomously directed lifting entry vehicle at Mars. However, without sufficient control and knowledge of the atmospheric entry conditions, the guidance algorithm will not perform effectively. The goal of the interplanetary navigation strategy is to deliver the spacecraft to the desired entry condition with sufficient accuracy and knowledge to enable satisfactory guidance algorithm performance. Specifically, the entry flight path angle must not exceed 0.27 deg. to a 3 sigma confidence level. Entry errors will contribute directly to the size of the landed footprint and the most significant component is entry flight path angle. The size of the entry corridor is limited on the shallow side by integrated heating constraints, and on the steep side by deceleration (g-load) and terminal descent propellant. In order to meet this tight constraint it is necessary to place a targeting maneuver seven hours prior to the time of entry. At this time the trajectory knowledge will be quite accurate, and the effects of maneuver execution errors will be small. The drawback is that entry accuracy is dependent on the success of this final late maneuver. Because propulsive maneuvers are critical events, it is desirable to minimize their occurrence and provide the flight team with as much response time as possible in the event of a spacecraft fault. A mission critical maneuver at Entry - 7 hours does not provide much fault tolerance, and it is desirable to provide a strategy that minimizes reliance on this maneuver. This paper will focus on the Improvements in interplanetary navigation that will decrease entry errors and will reduce the landed footprint, even in the absence of aeromaneuvering. The easiest to take advantage of are Improvements In the knowledge of the Mars ephemeris and gravity field due to the MGS and MSP'98 missions. Improvements In data collection and reduction techniques such as "precislon ranging' and near-simultaneous tracking will also be utilized. In addition to precise trajectory control, a robust strategy for communications and flight operations must also be demonstrated. The result Is a navigation and communications strategy on approach that utilizes optimal maneuver placement to take advantage of trajectory knowledge, minimizes risk for the flight operations team, is responsive to spacecraft hardware limitations, and achieves the entry corridor. The MSP2001 mission Is managed at JPL under the auspices of the Mars Exploration Directorate. The spacecraft flight elements are built and managed by Lockheed-Martin Astronautics in Denver, Colorado.
The Mars Surveyor 2001 Lander: A First Step Toward Precision Landing
As the robotic exploration of Mars continues, science objectives have driven mission and flight system development towards the use of precision landing technology such that small surface features, such as craters, can be investigated. In addition, the surface rendezvous elements of human exploration missions will require landing accuracy that is greatly improved over that achievable with ballistic flight. With improved approach navigation and hypersonic maneuvering technologies, the MSP'01 Lander is taking the first significant step toward precision landing on Mars. This advance requires both the ability to generate lift during the atmospheric flight and an on-board guidance algorithm to direct a three-axis control system. Many configuration options were examined to generate the required lift, with an afterbody-mounted deployable flap emerging as the lightest-weight solution. Five candidate guidance algorithms have been developed and submitted to the MSP'01 Project for evaluation. Through high-fidelity simulation, each of these algorithms has demonstrated the ability to greatly improve upon the landed accuracy provided by ballistic flight. As a result, the science community should expect to be within 10 km of the specified landing target. In fact, depending on the selected aeroshell L/D, a 5-km precision landing goal is achievable with greater than 90% confidence.
Mars Pathfinder Atmosphere Entry Trajectory Design
The Mars Pathfinder spacecraft will enter the Martian atmosphere directly from the interplanetary trajectory, at a relatively high velocity. The design of the nominal entry trajectory, and the accurate determination of potential trajectory dispersions, is necessary for the development of the Pathfinder Entry, Descent, and Landing (EDL) System.
Computation and Application of the TOPEX/POSEIDON Orbit Event File
The Orbit Event Program is an analytical software tool that searches for the occurence of various satellite geometries relative to the earth, sun, Deep Space Network stations, and the Tracking and Data Relay Statellite System.
Computation and Application of the TOPEX/Poseidon Orbit Event File
During TOPEX/POSEIDON, mission design process, the need was identified for sofware capabl of generating a predicted set or orbitala events for use in view period scheduling planning.