No full-text available
To read the full-text of this research,
you can request a copy directly from the author.
Enabling outer planet exploration: performance and feasibility of nuclear thermal propulsion for rendezvous missions
... Finally, the last module of the systems engineering model is cost estimation. In the paper, the mission cost is determined as a function of launch vehicle cost plus spacecraft dry mass using the historical data of NASA's planetary science missions cost versus spacecraft dry mass [28]. Considering that the NTP system is currently in the development phase, the cost estimate of the propulsion system is beyond the scope of this study. ...
Nuclear thermal propulsion (NTP) has emerged as a promising technology for enhancing the capabilities of robotic missions in space exploration. This paper investigates and analyzes the tradeoffs and sensitivity associated with utilizing NTP systems for robotic missions to the outer planets. The engine trade results for Jupiter and Neptune rendezvous missions using expendable configuration have demonstrated that the thrust range of 12.5–15 k-lbf (55.6–66.7 kN) can enable new frontiers and flagship-class missions with minimum initial mass in low Earth orbit. The specific impulse sensitivity analysis has shown that using a 13 k-lbf (57.8 kN) engine with an [Formula: see text] as low as 850 s can enable flagship-class missions to the gas giants in a direct transfer trajectory.
Nuclear thermal propulsion (NTP) has emerged as a promising technology for enhancing the capabilities of robotic missions in space exploration. This paper investigates and analyzes the tradeoffs and sensitivity associated with utilizing NTP systems for robotic missions to the outer planets. The engine trade results for Jupiter and Neptune rendezvous missions using expendable configuration have demonstrated that the thrust range of 12.5–15 k-lbf (55.6–66.7 kN) can enable new frontiers and flagship-class missions with minimum initial mass in low Earth orbit. The specific impulse sensitivity analysis has shown that using a 13 k-lbf (57.8 kN) engine with an [Formula: see text] as low as 850 s can enable flagship-class missions to the gas giants in a direct transfer trajectory.
Mission concepts using Nuclear Thermal Propulsion for planetary science missions have demonstrated the enhanced capability in delivering higher payload mass and/or reduce trip by a factor of two or more when compared with traditional chemical propulsion powered missions. To perform high-fidelity mission analysis and simulate realistic mission scenarios, a model-based approach is implemented on the Spacecraft Integrated System Model. The implementation of model-based systems engineering allows the execution of integrated descriptive model and requirements verification. The system model is also used to perform trade space exploration for various planetary science mission concept architectures. This paper apprises the current progress on the NTP mission model development and simulation of a conceptual NTP powered lunar rendezvous mission. Requirements and descriptive models are integrated with the physics based analytical tools to visualize and verify the mission requirements along with the parametric analysis for spacecraft arrival conditions during the lunar capture maneuver.
This paper discusses the current challenges of exploration of outer planets and proposes a nuclear thermal propulsion (NTP) system for future deep space exploration missions. The mission design problem with respect to the NTP system is presented where it is proposed that NTP-powered missions need to integrate the requirements and constraints of mission objective, spacecraft design, NTP system design, and launch vehicle limits into a self-consistent model. The paper presents a conceptual NTP-powered rendezvous mission to Neptune that uses a single high-performance–class commercial launch vehicle to deliver over 2 mT of useful payload in a direct transfer trajectory with total trip time being under 16 years.
Nuclear Thermal Propulsion (NTP) can enable new class of planetary science missions for deep space exploration. To perform high-fidelity mission analysis an integrated mission model for an NTP powered spacecraft in currently being developed. In this paper, we extend our initial model-based framework and demonstrate mission analysis using an integrated modeling approach. The integrated model enables in performing parametric analysis and optimizing results by linking key system parameters across analysis tools. The paper demonstrates an example NTP demo mission using the modeling approach described.
Highly efficient nuclear thermal propulsion (NTP) can enable a new class of planetary science missions for deep space exploration. This paper presents Jupiter and Saturn rendezvous missions using an NTP system that will focus on end-to-end trajectory analysis. The complexities of Earth escape and planetary orbital insertion using finite burn maneuvers are highlighted. With respect to the mission design problem, NTP-powered missions need to integrate the requirements and constraints of mission objectives, spacecraft design, NTP system design, and launch vehicle limits into a self-consistent model. Using a single high-performance-class commercial launch vehicle with a lift capability to low Earth orbit of 22 metric tons, an NTP-powered mission can deliver NASA’s large strategic science missions in a direct transfer trajectory to Jupiter in 2.1 years and to Saturn in 4.7 years.
Data returned from the Cassini–Huygens mission have strengthened Enceladus, a small icy moon of Saturn, as an important target in the search for life in our solar system. Information gathered from Cassini to support this includes the presence of a subsurface liquid water ocean, vapor plumes and ice grains emanating from its south polar region, and the detection of essential elements and organic material that could potentially support life. However, several outstanding questions remain regarding the connectivity of plume material to the ocean and the composition of the complex organic material. Herein we introduce Tiger, a mission concept developed during the 2020 Planetary Science Summer School at NASA's Jet Propulsion Laboratory. Tiger is a flyby mission that would help further constrain the habitability of Enceladus through two science objectives: (1) determine whether Enceladus's volatile inventory undergoes synthesis of complex organic species that are evidence for a habitable ocean, and (2) determine whether Enceladus's plume material is supplied directly from the ocean or if it interfaces with other reservoirs within the ice shell. To address the science goals in a total of eight flybys, Tiger would carry a four-instrument payload, including a mass spectrometer, a single-band ice-penetrating radar, an ultraviolet imaging spectrograph, and an imaging camera. We discuss Tiger's instrument and mission architecture, as well as the trades and challenges associated with a habitability-focused New Frontiers–class flyby mission to Enceladus.
This paper presents a literature review of technical papers and reports on mission studies conducted for robotic missions using Nuclear Thermal Propulsion (NTP) system for interplanetary missions. The paper outlines the basic principle and design of a nuclear thermal engine including performance assessment of small and large class NTP engines. Robotic mission concepts using NTP to various destinations such as Europa, Pluto and Ice giant planets are reviewed. The paper highlights the knowledge gaps in mission concept studies and focuses on various areas requiring detailed analysis to fulfill those knowledge gaps.
The future of human exploration missions to Mars is dependent on solutions to the technology challenges being worked on by the National Aeronautics and Space Administration (NASA) and industry. One of the key architecture technologies involves propulsion that can transport the human crew from Earth orbit to other planets and back to Earth with the lowest risk to crew and the mission. Nuclear thermal propulsion (NTP) is a proven technology that provides the performance required to enable benefits in greater payload mass, shorter transit time, wider launch windows, and rapid mission aborts due to its high specific impulse and high thrust.
Aerojet Rocketdyne (AR) has stayed engaged for several decades in working NTP engine systems and has worked with NASA recently to perform an extensive study on using low-enriched uranium NTP engine systems for a Mars campaign involving crewed missions from the 2030s through the 2050s. Aerojet Rocketdyne has used a consistent set of NASA ground rules and they are constantly updated as NASA adjusts its sights on obtaining a path to Mars, now via the Lunar Operations Platform-Gateway. Building on NASA’s work, AR has assessed NTP as the high-thrust propulsion option to transport the crew by looking at how it can provide more mission capability than chemical or other propulsion systems.
The impacts of the NTP engine system on the Mars transfer vehicle configuration have been assessed via several trade studies since 2016, including thrust size, number of engine systems, liquid hydrogen stage size, reaction control system sizing, propellant losses, NASA Space Launch System (SLS) payload fairing size impact, and aggregation orbit.
An AR study activity in 2018 included examining NTP stages derived from Mars crew mission elements to deliver extremely large cargo via multiple launches or directly off the NASA SLS. This paper provides an update on the results of the ongoing engine system and mission trade studies.
The major post-Cassini knowledge gap concerning Saturn's icy moon Titan is in the composition of its diverse surface, and in particular how far its rich organics may have ascended up the "ladder of life." The NASA New Frontiers 4 solicitation sought mission concepts addressing Titan's habit-ability and methane cycle. A team led by the Johns Hopkins University Applied Physics Laboratory (APL) proposed a revolutionary lander that uses rotors to land in Titan's thick atmosphere and low gravity and can repeatedly transit to new sites, multiplying the mission's science value from its capable instrument payload.
Catalog maintenance for Space Situational Awareness (SSA) demands an accurate and computationally lean orbit propagation and orbit determination technique to cope with the ever increasing number of observed space objects. As an alternative to established numerical and analytical methods, we investigate the accuracy and computational load of the Draper Semi-analytical Satellite Theory (DSST). The Standalone version of the DSST was enhanced with additional perturbation models to improve its recovery of short periodic motion. The accuracy of DSST is, for the first time, compared to a numerical propagator with fidelity force models for a comprehensive grid of low, medium, and high altitude orbits with varying eccentricity and different inclinations. Furthermore, the run-time of both propagators is compared as a function of propagation arc, output step size and gravity field order to assess its performance for a full range of relevant use cases. For use in orbit determination, a robust performance of DSST is demonstrated even in the case of sparse observations, which is most sensitive to mismodeled short periodic perturbations. Overall, DSST is shown to exhibit adequate accuracy at favorable computational speed for the full set of orbits that need to be considered in space surveillance. Along with the inherent benefits of a semi-analytical orbit representation, DSST provides an attractive alternative to the more common numerical orbit propagation techniques.
This paper proposes an innovative Nuclear Thermal Rocket (NTR) concept of the Korea Advanced NUclear Thermal Engine Rocket (KANUTER) for future space applications. KANUTER consists of an Extremely High Temperature Gas cooled Reactor (EHTGR) utilizing hydrogen propellant, a propulsion system, and an electricity generation system. KANUTER has the notable characteristics of high efficiency, being compact and lightweight, and having bimodal capability. The high efficiency comes from the extremely high temperature of the reactor loaded with an ultra heat-resistant ternary carbide fuel. The compact and lightweight EHTGR results from the thermal neutron spectrum core adopting efficient moderators. The EHTGR could be bimodally operated in a propulsion mode of 100 MWth and an electricity generation mode of 100 kWth equipped with a dynamic power conversion system. The preliminary design study theoretically estimates the reference performance such as a thrust of 19,500 N, a thrust-to-weight ratio of 5.1, a specific impulse of 944.5 s, and an electric power output of about 15 kWe with a radiator size of 3.4 m2/kWe. In addition, the small and compact engine with relatively low power can be applied to various space missions by clustered arrangements enhancing total thrust and system redundancy for the survivability of a spacecraft.
NASA LeRC was selected to lead nuclear propulsion technology development for NASA. Also participating in the project are NASA MSFC and JPL. The U.S. Department of Energy will develop nuclear technology and will conduct nuclear component, subsystem, and system testing at appropriate DOE test facilities. NASA program management is the responsibility of NASA/RP. The project includes both nuclear electric propulsion (NEP) and nuclear thermal propulsion (NTP) technology development. This report summarizes the efforts of an interagency panel that evaluated NTP technology in 1991. Other panels were also at work in 1991 on other aspects of nuclear propulsion, and the six panels worked closely together. The charters for the other panels and some of their results are also discussed. Important collaborative efforts with other panels are highlighted. The interagency (NASA/DOE/DOD) NTP Technology Panel worked in 1991 to evaluate nuclear thermal propulsion concepts on a consistent basis. Additionally, the panel worked to continue technology development project planning for a joint project in nuclear propulsion for the Space Exploration Initiative (SEI). Five meetings of the panel were held in 1991 to continue the planning for technology development of nuclear thermal propulsion systems. The state-of-the-art of the NTP technologies was reviewed in some detail. The major technologies identified were as follows: fuels, coatings, and other reactor technologies; materials; instrumentation, controls, health monitoring and management, and associated technologies; nozzles; and feed system technology, including turbopump assemblies.
Safety must be ensured during all phases of space fission system design, development, fabrication, launch, operation, and shutdown. One potential space fission system application is fission electric propulsion (FEP), in which fission energy is converted into electricity and used to power high efficiency (Isp > 3000 s) electric thrusters. For these types of systems it is important to determine which operational scenarios ensure safety while allowing maximum mission performance and flexibility. Space fission systems are essentially non-radioactive at launch, prior to extended operation at high power. Once high power operation begins, system radiological inventory steadily increases as fission products build up. For a given fission product isotope, the maximum radiological inventory is typically achieved once the system has operated for a length of time equivalent to several half-lives. After that time, the isotope decays at the same rate it is produced, and no further inventory builds in. For an FEP mission beginning in Earth orbit, altitude and orbital lifetime increase as the propulsion system operates. Two simultaneous effects of fission propulsion system operation are thus (1) increasing fission product inventory and (2) increasing orbital lifetime. Phrased differently, as fission products build up, more time is required for the fission products to naturally convert back into non-radioactive isotopes. Simultaneously, as fission products build up, orbital lifetime increases, providing more time for the fission products to naturally convert back into nonradioactive isotopes. Operational constraints required to ensure safety can thus be quantified. (authors)
This paper provides a summary of the 2007 Mars Design Reference Architecture 5.0 (DRA 5.0), which is the latest in a series of NASA Mars reference missions. It provides a vision of one potential approach to human Mars exploration, including how Constellation systems could be used. The strategy and example implementation concepts that are described here should not be viewed as constituting a formal plan for the human exploration of Mars, but rather provide a common framework for future planning of systems concepts, technology development, and operational testing as well as potential Mars robotic missions, research that is conducted on the International Space Station, and future potential lunar exploration missions. This summary of the Mars DRA 5.0 provides an overview of the overall mission approach, surface strategy and exploration goals, as well as the key systems and challenges for the first three concepts for human missions to Mars.
Nuclear Thermal Propulsion (NTP) has emerged as a promising technology for enhancing the capabilities of robotic missions in space exploration. This paper aims to investigate and analyze the trade-offs and sensitivity associated with utilizing NTP systems for robotic missions to the outer planets. The engine trade results for Jupiter and Neptune rendezvous mission using expendable configuration has demonstrated that the thrust range of 12.5 klbf to 15 klbf can enable flagship class missions with minimum IMLEO. The specific impulse sensitivity analysis has shown that using a 13 klbf engine with as low was 850 s can enable flagship class missions to the Gas giants in a direct transfer trajectory.
NASA has recently identified Nuclear Thermal Propulsion (NTP) as a critical technology for the human exploration of Mars. Recent studies have also demonstrated its unique capabilities towards enabling deep space exploration robotic missions with higher payload mass and/or reduced trip times. This game changing technology can also provide the efficient cargo and crew transportation through cislunar space for sustained human presence on the Moon. The NTP systems capability of generating both high thrust and high specific impulse (over twice of the best chemical propulsion systems) can enable a variety of lunar missions (rendezvous and round-trip) which can play critical role in the next phase of lunar exploration. This paper presents the applications of the NTP for sustainable cislunar exploration and identifies the most efficient architectures and engine thrust class for host of lunar missions.
Recently, NASA has pushed for returning humans to the moon sustainably with In situ resource utilization being the central focus. In its permanently shadowed regions, the moon has an abundance of water and ammonia, which are two potential alternative propellants to hydrogen in nuclear thermal propulsion (NTP) engines. Using Aerojet Rocketdyne Mars mission architectures and University of Alabama in Huntsville NTP engine models, this research analyzed the impacts of using water or ammonia as propellants on mission architectures. For a human mission to Mars originating in the lunar distant retrograde parking orbit, when comparing the baseline hydrogen vehicles to vehicles using water or ammonia, the effects of increased propellant density by an order of magnitude despite a 62% decrease in specific impulse will still require an average of 53% of the launches for vehicle assembly, 59% of the dry mass, 25% of the propellant volume for the conjunction-class mission, and 50% of the propellant volume for the opposition-class mission when comparing against the baseline hydrogen vehicle. Due to the oxidative nature of water, which results in 29% of the engine life compared with that of hydrogen and ammonia, ammonia outperforms water in terms of the total number of missions per engine block by an average factor of 6.8. Because ammonia’s specific impulse is 40.6% that of hydrogen, the propellant mass is increased by a factor of 2.9 and longer burn times result in an average of a 10% decrease in the total number of missions that the hydrogen system can achieve. Since the changes to the vehicle architecture are more extreme and in favor of ammonia than the total mission capability, for reusable vehicles using NTP technology ferrying humans to Mars while utilizing in situ propellants from the moon, this research recommends ammonia as the propellant of choice.
Recently, NASA’s Artemis Program has considered sustainably returning humans to the moon with in situ resource utilization as the central focus and considers nuclear thermal propulsion (NTP) to be a viable propulsion system candidate for deep space missions. This program also considers the moon to function as a staging area for deep space missions due to its strategic orbit around Earth. The Lunar Crater Observation and Sensing Satellite discovered the abundancy of water, ammonia, and other volatiles in permanently shadowed regions on the moon. Because NTP can theoretically use any fluid as a propellant, water and ammonia are two potential alternative propellants to hydrogen that do not require postprocessing to be used directly in situ. Alternative propellant NTP engine models were developed to examine the performance of water and ammonia. It was found that bleed cycle architectures are infeasible with current turbine materials, but expander cycles have acceptable turbine inlet temperatures with marginal performance differences from bleed cycles. At reactor power levels comparable to hydrogen NTP engines, water doubles the thrust while ammonia increases it by 70%, although at a much lower specific impulse. The largest contributor to the life of the engine was the maximum temperature of the SiC cladding for water and the maximum temperature of the fuel for ammonia where higher values resulted in shorter engine life.
Chemical propulsion has been the primary propulsion system from the beginning of space age for interplanetary robotic missions1. However, the chemical propulsion system has shown its limitations towards the exploration of outer planets and beyond due to its low energy density and low specific impulse. An outer planet mission using only chemical propulsion system would not be possible without multiple gravity assists or by utilizing a super heavy lift launch vehicle. On the other hand, the advancements in the Nuclear Thermal Propulsion (NTP) system using Low Enriched Uranium (LEU) NTP engine system design have demonstrated the improved performance towards payload mass and short transit time2. High thrust and high specific impulse (over twice the best chemical propulsion engine) NTP system can enable missions which have been limited due to the large ΔV requirements. An NTP propelled spacecraft with high ΔV would reduce the trip time by up to a factor of two or more when compared with a chemical propulsion system spacecraft requiring multiple gravity assists.
This work examines the capability of an NTP powered spacecraft for Jupiter rendezvous mission and discusses the advantages in terms of payload delivery and trip time in comparison to the conventional chemical propulsion powered spacecrafts.
View Video Presentation: https://doi.org/10.2514/6.2021-3598.vid A model-based approach can enhance the conceptual mission design process for NTP enabled robotic missions. This paper presents the early steps taken on the implementation of Model-Based Systems Engineering (MBSE) application with final goal to have rapid integrated system analysis capability. The paper describes the NTP mission model and process of deriving mission requirement diagrams. The modeling approach has been implemented on a conceptual mission design process of an NTP powered robotic mission to Neptune.
This paper presents a Jupiter rendezvous mission using nuclear thermal propulsion system which will focus on end-to-end trajectory analysis and highlight the complexities of Earth escape and planetary orbital insertion using finite burn maneuvers. The detailed mission design includes spacecraft design, launch schedule, delta-V requirements and multi body high-fidelity finite thrust trajectory analysis. Spacecraft’s initial low Earth parking orbit is considered to be circular at 1000 km in altitude and at 28 degrees inclination. The high fidelity trajectory analysis of the nuclear thermal propulsion spacecraft for a rendezvous mission has been divided into three phases - Earth escape, coasting and orbit insertion phase.
If humans are to reach beyond this planet and establish permanent outposts at Mars or any other solar system body, advanced propulsion will be needed. Optimum advanced propulsion needs high thrust to operate within the deep gravity well of a planet and also needs to provide high propulsive efficiency for rapid travel and reduced total spacecraft mass.
One prominent propulsion technology that meets the “optimum” criteria has been researched and tested in the past: it is Nuclear Thermal Propulsion (NTP). NTP, whether designed to provide the thrust to move a spacecraft between orbits or operate as a dual-mode system that provides power and propulsion capability, provides a strong architectural benefit to exploration missions and reusable in-space transportation systems. NTP provides smaller vehicle systems due to its specific impulse (ISP) capability being twice that of the best cryogenic liquid rocket propulsion. The higher ISP also permits reduced trip times when optimizing for the higher delta-V capability for round-trip missions from Earth to Mars. Aerojet Rocketdyne (AR) is working with NASA and other industry partners to improve the design and reduce the cost of NTP engine systems.
AR has been working with NASA and members of industry to perform mission analysis and to design engines and vehicles for Low Enriched Uranium (LEU) approaches for NTP beginning in 2016. LEU can provide a path to a more affordable “new generation” of NTP. The LEU NTP design can offer mission architecture stages or elements that can be used for both Lunar and deep space exploration missions. The NTP designs enable packaging of various NTP stage designs (e.g., crew Mars vehicle stage elements, cargo stage derivatives, a deep space stage with payload, Lunar stage elements) on the NASA SLS Block 2 using the 8.4-meter fairing.
This paper will present the evolution of the conceptual vehicle and engine design performed under the NASA MSFC LEU NTP program. The program has gone through six Design Analysis Cycles (DACs). The mission to be performed is shown and how the mission requirements are translated into vehicle design choices is described. Inputs from various NASA Mars campaign studies and further component studies from NASA were used in the vehicle design. The significant mission analysis that led to the DAC 1 vehicle and engine design is described along with the implications of the analysis results on the individual design choices. The design changes and why they occurred is presented for each of the subsequent DACs.
Cambridge Core - Texts in Political Thought - Hegel: Elements of the Philosophy of Right - edited by Allen W. Wood
This article offers a summary of past efforts in the development of Nuclear Thermal Propulsion systems for space transportation. First, the generic principle of thermal propulsion is outlined: A propellant is directly heated by a power source prior to being expanded, which creates a thrusting force on the rocket. This ena-bles deriving a motivation for the use of Nuclear Thermal Propulsion (NTP) relying on nuclear power sources.
Then, a summary of major families of NTP systems is established on the basis of a literature survey. These families are distinguished by the nature of their power source, the most important being systems with radioi-sotope, fission, and fusion cores. Concepts proposing to harness the annihilation of matter and anti-matter are only touched briefly due to their limited maturity. For each family, an overview of physical fundamentals, technical concepts, and – if available – tested engines’ propulsion parameters is given.
Europa, the fourth largest moon of Jupiter, is believed to be one of the best places in the solar system to look for extant life beyond Earth. “Because of this ocean’s potential suitability for life, Europa is one of the most important targets in all of planetary science” (2011 Planetary Decadal Survey). The proposed Europa Clipper mission would investigate Europa's habitability by characterizing the three “ingredients” for life as we know it: liquid water, chemistry, and energy. Europa Clipper would consist of a spacecraft equipped with a payload of NASA-selected scientific instruments, to execute numerous flybys of Europa while in Jupiter orbit. A key challenge is that the flight system must survive and operate in the intense Jovian radiation environment, which is especially harsh at Europa. The innovative design of this multiple-flyby tour is an enabling feature of this mission: by minimizing the time spent in the radiation environment the spacecraft complexity and cost has been significantly reduced compared to previous mission concepts. The spacecraft would launch from Kennedy Space Center (KSC), on a NASA supplied launch vehicle, no earlier than 2022. The proposed mission would be formulated and implemented by a joint Jet Propulsion Laboratory (JPL) and Applied Physics Laboratory (APL) Project team.
Recent exploration architectures are considering capability based approaches that use various propulsion technologies that need flight qualification. When human exploration missions come to fruition, rapidity of mission flight time and minimization of the number of flight elements will be important to reduce risk and cost. Nuclear thermal propulsion (NTP) is a technology that enables rapidity of transit and minimizes the number of spacecraft systems and launch vehicles to enable robust exploration. NTP has been proven scientifically and many engineering challenges have been addressed in past ground testing. The final validation and verification that still remains is to prove NTP in a flight demonstrator. Current efforts are focused on reducing demonstrator cost via application of proven liquid rocket components and fuel element design technology evolution. Pratt & Whitney Rocketdyne has been working with NASA Glenn Research Center to conceptualize the reactor design requirements and propulsion system design relative to a small NTP with scalability to human exploration spacecraft systems. Studies performed in 2011 and continuing through 2012 have defined the reactor neutronics, thermodynamic cycle, and component mass for a small NTP and the scalability of the design for larger (e.g., 25,000lbf) propulsion systems. Part of these studies has shown this small NTP can be flight demonstrated using current Delta IV and Atlas V expendable launch vehicles and provide improved capability for robotic exploration missions. Nomenclature
NASA's Radioisotope Power Systems (RPS) Program began formal implementation in December 2010. The RPS Program's goal is to make available RPS for the exploration of the solar system in environments where conventional solar or chemical power generation is impractical or impossible to meet mission needs. To meet this goal, the RPS Program manages investments in RPS system development and RPS technologies. The current keystone of the RPS Program is the development of the Advanced Stirling Radioisotope Generator (ASRG). This generator will be about four times more efficient than the more traditional thermoelectric generators, while providing a similar amount of power. This paper provides the status of the RPS Program and its related projects. Opportunities for RPS generator development and targeted research into RPS component performance enhancements, as well as constraints dealing with the supply of radioisotope fuel, are also discussed in the context of the next ten years of planetary science mission plans.
The nuclear thermal rocket (NTR) represents the next “evolutionary step” in high performance rocket propulsion. Unlike conventional chemical rockets that produce their energy through combustion, the NTR derives its energy from fission of Uranium-235 atoms contained within fuel elements that comprise the engine's reactor core. Using an “expander” cycle for turbopump drive power, hydrogen propellant is raised to a high pressure and pumped through coolant channels in the fuel elements where it is superheated then expanded out a supersonic nozzle to generate high thrust. By using hydrogen for both the reactor coolant and propellant, the NTR can achieve specific impulse (Isp) values of ~900 seconds (s) or more - twice that of today's best chemical rockets. From 1955-1972, twenty rocket reactors were designed, built and ground tested in the Rover and NERVA (Nuclear Engine for Rocket Vehicle Applications) programs. These programs demonstrated: (1) high temperature carbide-based nuclear fuels; (2) a wide range of thrust levels; (3) sustained engine operation; (4) accumulated lifetime at full power; and (5) restart capability - all the requirements needed for a human Mars mission. Ceramic metal “cermet” fuel was pursued as well, as a backup option. The NTR also has significant “evolution and growth” capability. Configured as a “bimodal” system, it can generate its own electrical power to support spacecraft operational needs. Adding an oxygen “afterburner” nozzle introduces a variable thrust and Isp capability and allows bipropellant operation. In NASA's recent Mars Design Reference Architecture (DRA) 5.0 study, the NTR was selected as the preferred propulsion option because of its proven technology, higher performance, lower launch mass, versatile vehicle design, simple assembly, and growth potential. In contrast to other advanced propulsion options, no l- rge technology scale-ups are required for NTP either. In fact, the smallest engine tested during the Rover program - the 25,000 lbf (25 klbf) “Pewee” engine is sufficient when used in a clustered engine arrangement. The “Copernicus” crewed spacecraft design developed in DRA 5.0 has significant capability and a human exploration strategy is outlined here that uses Copernicus and its key components for precursor near Earth object (NEO) and Mars orbital missions prior to a Mars landing mission. The paper also discusses NASA's current activities and future plans for NTP development that include system-level Technology Demonstrations - specifically ground testing a small, scalable NTR by 2020, with a flight test shortly thereafter.
The high precision mean element (semianalytic) satellite theory developed at Draper Laboratory is more efficient than conventional numerical methods and more accurate than the current generation of analytic theories. This efficiency, along with its portability to a variety of computing environments makes the semianalytic theory a natural choice for maneuver planning applications. These applications will become more important in the future as the capability of individual platforms to maneuver, the number of platforms in space, and the requirements for rapid response to requests for data all increase. The application of semianalytic satellite theory to an Earth Observation satellite in an orbit similar to that expected for LANDSAT 6 is investigated. Orbit constraints such as sun synchronous, repeat ground track, frozen orbit, and non-impulsive maneuver capabilities are included in this analysis. Applications of maneuver planning to past and future satellite missions that include at least two of the listed orbit constraints are discussed. Since atmospheric drag is the primary uncertain disturbing acceleration to the nominal satellite orbit, upper and lower limits of a density confidence interval were determined. Two methods were analyzed; it was found that using forecast and actual solar flux and geomagnetic activity data from the years 1986-1990 resulted in a conservative but realistic confidence interval. The upper limit is utilized to compute the time of the orbit adjust burn and the lower limit of the density is used to calculate the magnitude of the orbit adjust burn. These limits are necessary so that the ground track boundaries are not exceeded.
We have conducted studies of a revolutionary new concept for conducting
a Europa Sample Return Mission. Robotic spacecraft exploration of the
Solar System has been severely constrained by the large energy
requirements of interplanetary trajectories and the inherent delta V
limitations of chemical rockets. Current missions use gravitational
assists from intermediate planets to achieve these high-energy
trajectories restricting payload size and increasing flight times. We
propose a 6-year Europa Sample Return mission with very modest launch
requirements enabled by MITEE. A new nuclear thermal propulsion engine
design, termed MITEE (MIniature reacTor EnginE), has over twice the
delta V capability of H2/O2 rockets (and much greater when refueled with
H2 propellant from indigenous extraterrestrial resources) enabling
unique missions that are not feasible with chemical propulsion. The
MITEE engine is a compact, ultra-lightweight, thermal nuclear rocket
that uses hydrogen as the propellant. MITEE, with its small size (50 cm
O.D.), low mass (200 kg), and high specific impulse (~1000 sec), can
provide a quantum leap in the capability for space science and
exploration missions. The Robotic Europa Explorer (REE) spacecraft has a
two-year outbound direct trajectory and lands on the satellite surface
for an approximate 9 month stay. During this time, the vehicle is
refueled with H2 propellant derived from Europa ice by the Autonomous
Propellant Producer (APP), while collecting samples and searching for
life. A small nuclear-heated submarine probe, the Autonomous Submarine
Vehicle (ASV), based on MITEE technology, would melt through the ice and
explore the undersea realm. The spacecraft has approximately a three
year return to Earth after departure from Europa with samples onboard.
Spacecraft payload is 430 kg at the start of the mission and can be
launched with a single, conventional medium-sized Delta III booster. The
spacecraft can bring back 25 kg of samples from Europa. Europa, in the
Jovian system, is a high priority target for an outer Solar System
exploration mission. More than a decade ago the Voyager spacecraft
revealed Europa as a world swathed in ice and geologically young. NASA's
Galileo spacecraft passed approximately 500 miles above the surface and
provided detailed images of Europa's terrain marked by a dynamic
topology that appeared to be remnants of ice volcanoes or geysers. The
surface temperature averages a chilly -200° C. The pictures appear
to show a relatively young surface of ice, possibly only 1 km thick in
some places. Internal heating of Europa from Jupiter's tidal pull could
form an ocean of liquid water beneath the surface. More recently,
Ganymede and Callisto are believed to be ocean-bearing Jovian moons
based on magnetometer measurements from the Galileo spacecraft. If
liquid water exists, life may also. NASA plans to send an orbiting
spacecraft to Europa to measure the thickness of the ice and to detect
if an underlying liquid ocean exists. This mission would precede the
proposed Europa Sample Return mission, which includes dispatching an
autonomous submarine-like vehicle that could melt through the ice and
explore the undersea realm. Because of the large energy requirements
typical of these ambitious solar system science missions, use of
chemical rockets results in interplanetary spacecraft that are
prohibitive in terms of Initial Mass in Low- Earth Orbit (IMLEO) and
cost. For example, using chemical rockets to return samples from Europa
appears to be technically impractical, as it would require large delta V
and launch vehicle capabilities. On the other hand, use of nuclear
thermal rockets will significantly reduce IMLEO and, subsequently,
costs. Moreover, nuclear thermal rockets can utilize extraterrestrial
resources as propellants, an option not practical with chemical rockets.
This "refueling" capability would enable nuclear rockets to carry out
very high-energy missions, such as the return of large amounts of
extraterrestrial material to Earth. The Europa missions considered in
this proposal will be restricted to starting from LEO only after being
placed in a stable orbit by a launch vehicle. This simplifies and eases
the safety issues and mitigates political concerns. High propulsive
efficiency of the MITEE engine yields the benefits of reduced transit
time and a smaller launch vehicle.
Several topics are presented in viewgraph form which together encompass
the preliminary assessment of nuclear thermal rocket engine clustering.
The study objectives, schedule, flow, and groundrules are covered. This
is followed by the NASA groundrules mission and our interpretation of
the associated operational scenario. The NASA reference vehicle is
illustrated, then the four propulsion system options are examined. Each
propulsion system's preliminary design, fluid systems, operating
characteristics, thrust structure, dimensions, and mass properties are
detailed as well as the associated key propulsion system/vehicle
interfaces. A brief series of systems analysis is also covered
including: thrust vector control requirements, engine out possibilities,
propulsion system failure modes, surviving system requirements, and
technology requirements. An assessment of vehicle/propulsion system
impacts due to the lessons learned are presented.
The parallel special perturbation catalog maintenance software "SpecialK" being introduced at Naval Space Command uses an eighth order Gauss-Jackson predictor-corrector for numerical integration. This integrator comes from legacy code which is not extensively documented. A study of the theory upon which the integrator is based and the method by which the integrator is implemented in the software is necessary, to determine if speed or accuracy improvements can be made. This paper presents some results from the study, including a discussion of a variable step size method that had been implemented in the ancestor code that proves to be ineffective.
An analytical solution is given for the motion of an artifical Earth satellite under the combined influences of gravity and atmospheric drag. The gravitational effects of the zonal harmonicsJ
2,J
3, andJ
4 are included, and the drag effects of any arbitrary dynamic atmosphere are included. By a dynamic atmosphere, we mean any of the modern empirical models which use various observed solar and geophysical parameters as inputs to produce a dynamically varying atmosphere model. The subtleties of using such an atmosphere model with an analytic theory are explored, and real world data is used to determine the optimum implementation. Performance is measured by predictions against real world satellites. As a point of reference, predictions against a special perturbations model are also given.
A new compact ultra light nuclear reactor engine design termed MITEE (MIniature Reac Tor EnginE) is described. MITEE heats hydrogen propellant to 3000 K, achieving a specific impulse of 1000 seconds and a thrust-to-weight of 10. Total engine mass is 200 kg, including reactor, pump, auxiliaries and a 30% contingency. MITEE enables many types of new and unique missions to the outer solar system not possible with chemical engines. Examples include missions to 100 A.U. in less than 10 years, flybys of Pluto in 5 years, sample return from Pluto and the moons of the outer planets, unlimited ramjet flight in planetary atmospheres, etc. Much of the necessary technology for MITEE already exists as a result of previous nuclear rocket development programs. With some additional development, initial MITEE missions could begin in only 6 years.
The Nuclear Europa Mobile Ocean (NEMO) mission would land on the surface of Europa, and deploy a small, lightweight melt probe powered by a compact nuclear reactor to melt down through the multi-kilometer ice sheet. After reaching the sub-surface ocean, a small nuclear Autonomous Underwater Vehicle (AUV) would deploy to explore the sub-ice ocean. After exploration and sample collection, the AUV would return to the probe and melt back to the lander. The lander would have replenished its H2 propellant by electrolysis of H2O ice, and then hop to a new site on Europa to repeat the probe/AUV process. After completing the mission, the NEMO spacecraft would return to Earth with its collected samples. The NEMO melt probe and AUV utilize enriched U-235 fuel and conventional water reactor technology. The lander utilizes a compact nuclear thermal propulsion (NTP) engine based on the cermet fuel and high-temperature H2 propellant. The compact nuclear reactors in both the NEMO melt probe and AUV drive a steam power cycle, generating over 10 kW(e) for use in each. Each nuclear reactor's operating lifetime is several years. With its high-mobility and long-duration mission, NEMO provides an ideal platform for life detection experiments.
The Cassini mission to Saturn will start a second phase in the exploration of the Saturnian system, after the historical Voyager flybys of Saturn in 1980 and 1981. The Cassini primary mission is scheduled to be launched in October 1997 by the Titan IV/Centaur. Cassini uses four planetary gravity-assist flybys to gain the energy necessary to reach Saturn in July 2004. This arrival date at Saturn provides a unique opportunity for a flyby of Saturn's outer satellite Phoebe on the final approach. This paper provides an overview of the processes involved in the interplanetary trajectory design and analysis of the Cassini mission to Saturn.
A thermally optimized in-space zero boil-off densified cryogen storage system model is developed. The Cryogenic System Design Tool is introduced and is used to model a spherical liquid hydrogen tank with active cooling and passive insulation systems. The model is used to investigate the effects of fluid storage temperature, multilayer insulation (MLI) thickness, and actively cooled shields on the overall storage system mass, cryocooler input power, and system volume. A validation of the Cryogenic System Design Tool is presented. The model predicts that a zero boil-off densified liquid hydrogen storage system minimizes the overall storage system mass and volume for nearly the same cooling input power as that of a normal boiling-point liquid hydrogen storage system.