 (NASA/JPL-Caltech/University of Arizona) |
Flight Systems Engineering for Robotic Spacecraft
Matthew J. Hart and Frank F. Donivan
The Jet Propulsion Laboratory (JPL) and NASA's Goddard Space Flight Center frequently draw on The Aerospace Corporation's capabilities across a wide range of tasks and applications to meet the nation's space, Earth science, and planetary exploration objectives.
Aerospace plays an important role in assuring mission success for NASA, which is most clearly illustrated through the successful launch and operations of complex planetary science spacecraft including Deep Impact, Mars Exploration Rover, Dawn, and Earth orbiters, such as the Hubble Space Telescope and the Tracking Data Relay Satellite System (TDRSS). Aerospace's customers include two of NASA's preeminent centers for spacecraft development, the Jet Propulsion Laboratory (JPL), located in Pasadena, California, and the NASA Goddard Space Flight Center (GSFC), located in Greenbelt, Maryland. JPL was established by the California Institute of Technology before World War II, and developed the first U.S. satellite, Explorer 1, which launched in 1958. The first robotic spacecraft sent to the moon and across the solar system were also created at JPL.
GSFC develops and operates many of NASA's Earth observation, astronomy, and space-physics missions as well as the TDRSS for communications with the space shuttle, International Space Station, and science missions such as the Hubble Space Telescope. Throughout its history, Aerospace, which has a long-standing relationship with both institutions, has helped with the evaluation of advanced mission concepts, development, testing, launch, flight, and anomaly resolution of high-profile NASA missions.
Conceptual Development of Science Missions
Conceptual development for a scientific space mission begins with engineers taking very early ideas and giving them form. NASA's scientific requirements drive the concept design, which includes an initial spacecraft and launch vehicle sizing, instrument payload definition, mission design, and concept of operations. Throughout the advanced-study process, mission concepts are developed, refined, and evaluated for scientific merit, technical and programmatic risk, and cost.
Most early concepts are rejected because they are deemed too risky or unaffordable. Concepts approved for further study are reviewed through concurrent engineering methods and tools. Concurrent engineering is a process whereby engineering tasks are done in parallel so that changes to one area are quickly conveyed to other affected areas.
At JPL, the Project Design Center is the hub of conceptual development activity, and Team X is the collaborative engineering team that accomplishes the work. Aerospace has been a partner in Team X since its inception in the early 1990s. Aerospace employees are trained to support Team X, and two or three typically support each study. Team members specialize in spacecraft subsystem conceptual design, systems engineering, mission design, and programmatic assessment, including cost and risk.
Each designer and analyst on Team X has a set of tools linked to other tools in a common database. When a design change is made to one area, it is immediately communicated to other affected subsystem areas. Engineers can then modify their own designs as necessary. Team X might converge on concept designs for two or three different mission concepts in a week, generating configuration drawings of the spacecraft and instruments, master equipment lists, power and mass budgets, mission design and operations details, and an estimate of the development and operations cost.
The Mars rovers were designed to be 30 percent larger than the previous Mars Pathfinder rover and lander, but had to fit within the same volume as their smaller and less capable predecessor. Mass growth threatened the project throughout its development. In the end, the rover and lander touched down on Mars weighing almost 50 percent more than Pathfinder. (NASA/JPL) |
When Team X was formed, Aerospace engineers had been working on an integrated concurrent engineering model tool that could be used by one system engineer to accomplish an entire mission concept design. JPL, at the same time, was developing a design team where individual subsystem design engineers representing all areas of a spacecraft design would meet to iterate on a design. Taking both approaches into consideration, Aerospace developed the initial tool framework for Team X by breaking the integrated concurrent engineering model into subsystem-level tools for use by each team member. Aerospace later adopted the collaborative engineering team concept similar to that used at JPL for its own Concept Design Center, which provides a similar function for Air Force and national security space customers.
Aerospace provides design expertise to countless Team X studies, tackling challenging mission concepts such as Europa, and Titan orbiters; Mars rovers and landers; missions to Venus, Mercury, and the sun; and missions beyond the boundaries of the solar system. Aerospace also helps to improve the core design tools and the databases behind them.
Concept Development to Support NASA Space System Architectures
In addition to helping formulate a broad spectrum of science missions for NASA, Aerospace is often asked to help conceptualize new spacecraft that must operate within an existing spacecraft and ground system architecture.
The NASA TDRSS provides tracking and data acquisition services between crewed and uncrewed low Earth orbiting (LEO) spacecraft and NASA control and data processing facilities. LEO satellites can only maintain contact with a given ground station for 15 minutes or less. Using TDRSS satellites at nine positions in the geostationary belt, LEO spacecraft can transmit and receive data without interruption, for their full orbit. The current constellation of nine satellites (seven active plus two spares) provides the communications link for a large suite of space missions including the space shuttle, the International Space Station, the Hubble Space Telescope, Atlas II vehicles during launch, the commercial Sea Launch enterprise, and Earth and deep-space science missions as well as the McMurdo and South Pole stations.
Aerospace technical experts have worked shoulder to shoulder with the GSFC project engineers in the concept, design, and requirements definition of the new spacecraft. Aerospace develops and maintains models of communications traffic requirements and loading on the full constellation, and advises NASA in the selection and timing of new vehicles to address new or changing requirements. Aerospace also maintains a high-fidelity reliability model for each spacecraft and provides predictions and assessments of vehicle failure scenarios, and replenishment strategies, for the TDRSS fleet.
Mission Selection
Not every advanced mission concept makes it through Team X to the next stage, but for those that do, the competition to get to this next step is fierce. NASA decides whether it will assign a specific mission to JPL or any other of the field centers based on the required engineering expertise and the development risk.
Traditionally, high-risk missions have been directed to organizations with core competencies in certain areas. For example, complex robotic planetary orbiters, and surface landers, such as the Mars rovers, have been assigned to JPL. High-risk astrophysics missions, on the other hand, have been assigned to Goddard Space Flight Center.
A large part of NASA's space science portfolio is competitively procured through NASA's Announcement of Opportunity process. Innovative proposals may come from university, industry, government, federally funded research and development centers, and international partners.
NASA's competitive mission program includes a wide range of solar system, planetary, and Earth science programs. These include Discovery and New Frontiers, Mars Scout, Earth System Science Pathfinder, and Explorer. Aerospace is frequently asked to help NASA in the selection of proposed missions by evaluating the technical, cost, and management aspects of each. Aerospace also performs independent cost estimates for NASA centers as well as specific flight projects.
Once NASA gives the go-ahead for a mission, Aerospace is often asked to perform mission assurance, systems engineering, and technical assistance during the preliminary and critical design phases. Recent examples of this include work on the Mars Exploration Rovers, the Juno mission to Jupiter, GRAIL, and the James Webb Space Telescope.
The Mars rovers Spirit and Opportunity have been exploring the surface of Mars for five years. Aerospace provided systems engineering and mission assurance support to many elements of the project. (NASA/JPL) |
Mars Exploration Rover
The Mars rovers were developed at JPL at a cost of approximately $820 million, including launch. The mission timeline was ambitious, requiring development and launch of a system in 3 years. This time frame was achieved through parallel development activities and tight synchronization of the system and lower-level requirements and design, all of which required heightened attention to systems engineering and mission assurance.
Aerospace provided risk management planning and execution, systems mission assurance support, and integrated requirements management to maintain consistency between requirements at the mission level and lower flight system or subsystem levels. The primary mission assurance focus early in the development phase was the creation of system-level failure modes effects and criticality analysis. This gave the project-management team visibility into the overall risk of the system design as it matured and highlighted areas where additional attention was required.
Aerospace formed a small team of engineers who worked in parallel with the system and subsystem design engineers. They systematically addressed each of the vehicle subsystems, identifying failure modes, classifying root causes, and documenting the criticality to mission success of each potential failure and the likelihood of its occurrence. Planned mitigations and preventive measures were identified and factored into the final mission risk. These data were regularly gathered across all areas of the flight system by interviewing key design engineers, analyzing design changes, and documenting the implications of design updates and potential failures on interrelated subsystems.
The disciplined process of inquiry, assessment, reporting, and analysis was conducted across all subsystem areas within the flight system team during the preliminary and critical design phases, and was continually updated as changes to higher-level requirements resulted in modifications or additions to the lower-level subsystem designs. A failure modes database was used to drive design changes to improve reliability and reduce risk, inform the fault protection design and implementation, and help with anomaly resolution during operations. This database complimented the project's fault-tree analysis and provided additional insight into how and why failures may occur and what preventative measures could be adopted through robust design.
At the system level, Aerospace established a second team, again working in parallel with the JPL design team, to ensure that requirement changes adopted through the project's change control board were correctly flowed down and implemented at lower levels. Tight synchronization of requirements changes approved by the change control board to implementation in the design was critical to keeping the project moving toward its launch date. Aerospace engineers developed and maintained the system-level requirements database that captured these changes as they occurred and confirmed that they had been flowed down correctly and were properly implemented in the design.
Aerospace wrote the original risk management plan for the rover project, defining the approach to capturing, documenting, evaluating, and mitigating project risks. Aerospace personnel also managed the risk management system during the design phase. Aerospace offered design guidance in evaluating the options for diversity in functionality and the use of redundant design elements. Aerospace also developed reliability models for alternate designs and collected and applied failure-rate data at the component level. Lastly, Aerospace performed launch vehicle mission design trade studies and authored the target specification for the launch vehicles.
The primary goal of the Juno mission is to better understand the formation, evolution, and structure of Jupiter. Concealed beneath a dense cover of clouds, Jupiter safeguards secrets to the fundamental processes underlying the early formation of the solar system. (NASA) |
Juno Mission to Jupiter
Juno will be the first mission to Jupiter from Earth in 20 years. Scheduled for launch in 2011 and arrival at Jupiter in 2016, the Juno spacecraft will spend a little more than a year probing Jupiter's atmosphere and magnetosphere in hopes of learning how that planet was formed and implications for the early evolution of the solar system.
Aerospace is providing independent mission assurance and systems engineering support to the Juno project management team, addressing a variety of design and operational challenges, including the application of parts and materials expertise to ensure successful operation in an extremely cold, high-radiation environment.
Spacecraft power is a critical design challenge for Juno. The sun is little more than a bright star at Jupiter. At that distance, solar arrays—which are commonly used to power spacecraft orbiting Earth or Mars—typically do not generate enough energy to run a spacecraft, or its onboard science and communications instruments. Previous missions to Jupiter and the other outer planets solved this problem by carrying their own power source, which used the thermal energy produced through the radioactive decay of plutonium to generate electricity.
Solar cell efficiency has improved measurably in the last 20 years, enabling the development of arrays able to operate further from the sun. However, even with these advances in technology, Juno requires about 45 square meters of active solar array area to generate the 450 watts of power needed to operate at Jupiter. In fact, Juno's solar arrays are so large that they cannot all be switched on until the vehicle reaches a safe distance from the sun between Mars and Jupiter. Otherwise, the arrays would produce so much energy that they would damage the spacecraft's electrical power system.
The Juno vehicle is a spin-stabilized spacecraft, with the spacecraft bus and science instruments at the hub and three large solar array wings projecting outward. The spacecraft's configuration is reminiscent of the rotor on an old-fashioned windmill. Juno's solar cells will operate under low-intensity, low-temperature conditions, similar to those used for the Dawn mission and the European Space Agency mission Rosetta. Solar cell efficiency increases with lower temperature but decreases with lower light intensity, and the low light intensity from the sun is the challenge. Aerospace solar power experts are working with JPL engineers to ensure that Juno's solar arrays will generate the required power during orbital operations.
Living within Juno's electrical power allocation and not allowing it to grow is a primary design consideration for this mission. Solar array area grows about 25 times faster for Juno than it does for Earth orbiting missions, so every watt of additional power required by the spacecraft or science instruments has a large impact on the solar array size, which then ripples back into the spacecraft bus design, and ultimately the launch vehicle selection. Finding ways to improve the overall array efficiency through careful evaluation and selection of the solar cells and to manage power usage once in orbit at Jupiter are crucial to the success of the mission.
Another challenge for the Juno project is designing for operations in Jupiter's high-radiation environment. Juno will receive a total ionizing dose in its 13-month mission orbiting Jupiter similar to that received by an Earth orbiting geosynchronous satellite in 10–12 years. This, combined with the extremely cold temperatures and exposure to plasma at Jupiter, creates challenges for the spacecraft, its instrument electronics and surfaces, and materials and coatings on the exterior of the spacecraft.
Most of the sensitive electronics are inside a tantalum vault mounted on top of the spacecraft bus, which helps protect them from the outside radiation environment. However, there are instrument detectors, antennas, components, coax cabling, and surface coatings that will be exposed. Aerospace is applying lessons learned from radiation effects on Earth orbiting spacecraft to the Juno project to help in evaluating and selecting electronic parts and materials. Aerospace is also providing expertise in evaluating the electromagnetic environment that will be induced by the spacecraft bus and instruments, and is helping manage the spacecraft charging and potential electromagnetic interference through testing, evaluation, and appropriate design selection.
GRAIL, the Gravity Recovery and Interior Laboratory, will study the interior, or composition, of the moon. Aerospace is helping assure that the mostly single-string spacecraft are optimized for reliability. (NASA) |
Going to the Moon with GRAIL
The Gravity Recovery and Interior Laboratory, or GRAIL, will fly twin spacecraft in tandem orbits around the moon to study its interior structure and gravity field. GRAIL has in its lineage the XSS-11, which was a small experimental spacecraft launched in April 2005 and managed by the Air Force Research Laboratory, with technical oversight provided by Aerospace. Aerospace is using the experience and mission assurance principles successfully applied to the XSS-11 spacecraft on GRAIL. Aerospace had a number of critical roles on the XSS-11 project, including project system engineer and flight director, mission design and safety lead, and launch vehicle integration and operations lead.
Like XSS-11, GRAIL is mostly a single-string system, which means it does not have redundancy or backup systems to protect against failure. The onboard scientific instrument must measure minute changes in acceleration using a radio link between the two spacecraft. If one of the two spacecraft fails, the mission is lost. Aerospace is now performing mission assurance studies to assess the reliability of the GRAIL design during its preliminary design phase. These studies allow GRAIL engineers to quickly assess what impact adding select redundancies has on the reliability of the mission.
James Webb Space Telescope
The James Webb Space Telescope, successor to the Hubble Space Telescope, is an ambitious project to place a 6.5 meter diameter infrared detecting telescope at the second Lagrange point approximately 1.5 million kilometers from Earth, opposite the sun. The mission will allow scientists to examine the history of the universe from the first glow of the "Big Bang" to the formation of stars, galaxies, and planetary systems. Not only must the segmented beryllium mirror be deployed automatically like a flower opening its petals, but the telescope will be shielded from light and heat from the sun, Earth, and moon by a five-layer "sunshield" approximately the size of a tennis court. Detection of very faint sources in the infrared part of the spectrum requires that the near-infrared detectors be maintained at a temperature of 37 degrees above absolute zero and that the mid-infrared detector be maintained at a temperature of seven degrees above absolute zero.
Because of its distance from Earth, the James Webb Space Telescope will not be serviceable by astronauts (unlike Hubble). Thus, the onboard thermal control system, used to cool the infrared detectors and shield the telescope assembly from the sun, must operate for many years without benefit of servicing or replacement. GSFC, the center developing the telescope, asked Aerospace to review requirements and the design of the cryocooler and sunshield. The design of the sunshield, consisting of thin membranes made from a polymer-based film, will be examined along with supporting equipment, and its ability to withstand degradation from radiation and charged particles.
Dawn Propellant Tank
Aerospace is often asked to apply its experience from Air Force, industry, and national security space programs to troubleshoot problems that occur late in the development phase of robotic missions when the majority of the flight hardware is built and options such as redesign or rebuild are limited.
The Dawn spacecraft was launched in 2007 on a journey to study Vesta and Ceres, the largest two main-belt asteroids. Because of schedule and resource constraints, the flight propellant tank was fabricated and installed into the spacecraft core structure before the flight spare and qualification tanks completed their fabrication and test. After installation, the flight spare tank unexpectedly failed an in-process screening test, and the qualification tank failed a postqualification program burst test. The failures of the flight spare and qualification tanks called into question the flightworthiness of the flight tank. Aerospace supported an independent review of the flight tank, through an assessment of the fabrication and test process, inspection, and nondestructive testing of existing hardware articles. The findings of this assessment informed NASA's own risk assessment, and supported the conclusion that the original flight tank was adequate for completing the mission.
Glory is a low Earth orbit scientific research satellite designed to collect data on the properties of aerosols and black carbon in the Earth's atmosphere and climate system. It will also collect data on solar irradiance for understanding long-term effects on Earth's climate. (NASA) |
Glory Instrument Development
The Glory mission, scheduled to launch in March 2009, will collect data on the properties of aerosols and black carbon within Earth's atmosphere and climate system, and will collect data on solar irradiance for long-term effects on Earth's climate. An onboard instrument, the Aerosol Polarimetric Sensor, has been an extremely challenging instrument to construct, and Aerospace has assisted with identification and elimination of stray light within the instrument that would corrupt the detector with spurious signals. Aerospace electro-optical experts have been able to identify the source of the light as reflections off internal surfaces and worked with NASA and the vendor to eliminate the contaminating light. Aerospace was asked to use its specialized estimating tools to conduct independent cost estimates of the total project, since resolution of the engineering problems caused the mission to exceed its original predicted cost.
In-Plant Monitoring for TDRS
Aerospace has a significant role in the development of the next two members of the TDRS constellation, K and L. Similar to its role with the earlier spacecraft, Aerospace will be involved with development, testing, and launch. At NASA's request, Aerospace established an office to provide full-time technical expertise at the Boeing Satellite Systems plant in El Segundo, California. Aerospace will monitor and report on design, development, integration, testing, and launch of TDRS K and L.
Ground system modernization for the operational TDRS system is another challenge recently addressed by Aerospace. The TDRS spacecraft downlink their data to a dedicated satellite ground station complex at White Sands, New Mexico, where it is then distributed to users. The complex operates around the clock on crewed missions such as the space shuttle that cannot tolerate interruptions in service, making upgrades to the ground system extremely challenging. The system has been operated and maintained by a skilled team of engineers and technicians for more than 30 years. However, today it is almost impossible to find replacement parts for the first-and second-generation computers and applications code written in languages no longer supported, nor taught, in today's computer science classes.
Assuring Launch Readiness
Planetary missions are particularly challenging in terms of achieving launch readiness. Earth and Mars align for a short time every 26 months, such that a Delta II launch vehicle can be used to carry a spacecraft to Mars. Similar situations exist for launches to other planets, to varying degrees. In all cases, the motion of the planets limits how often it is possible to launch and the length of the launch period. Daily launch opportunities are often constrained to short periods of hours or minutes per day, after which Earth's rotation and the location of the launch site will no longer allow the launch vehicle to reach its target.
The ability to successfully launch on any given day depends on many technical and operational factors, as well as uncontrollable events, such as the weather or errant incursions of aviation or marine vehicles into the keep-out zone surrounding the launch site. Aerospace has examined U.S. launch history, focusing on launch delays and root causes, to provide an assessment of launch-readiness requirements for NASA/JPL.
Technical glitches can also occur on the launchpad after the spacecraft is integrated onto the launch vehicle, where it is often difficult to troubleshoot problems. One of the Mars rovers experienced a ground fuse failure while on the launchpad shortly before launch. Aerospace was asked to support the JPL team assessing the risks to the mission and helped develop a solid technical rationale for going forward with the launch.
Conclusion
Aerospace's role in supporting NASA robotic missions takes many forms and spans the project lifecycle from conceptual design to launch and operations. Aerospace brings unique technical skills and capabilities to address the many challenges of these one-of-a-kind missions of scientific space and Earth exploration. Aerospace support to these missions benefits the nation through applications of lessons learned across a broader diversity of programs. NASA and the DOD are enhanced by mission success for our nation's most critical missions of exploration.
