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At least 649 records · Page 36
All sourcesNASA NTRSDOE OSTI
NASA NTRS2015

The Importance of the Cathode Plume and Its Interactions with the Ion Beam in Numerical Simulations of Hall Thrusters

Hall2De is a first-principles, 2-D axisymmetric code that solves the equations of motion for ions, electrons, and neutrals on a magnetic-field-aligned grid. The computational domain downstream of the acceleration channel exit plane is large enough to include self-consistently the cathode boundary. In this paper, we present results from numerical simulations of the H6 laboratory thruster with an internally mounted cathode, with the aim of highlighting the importance of properly accounting for the interactions between the ion beam and cathode plume. The anomalous transport of electrons across magnetic field lines in Hall2De is modelled using an anomalous collision frequency, νanom, yielding νanom approximately equal to omega ce (i.e., the electron cyclotron frequency) in the plume. We first show that restricting the anomalous collision frequency to only regions where the current density of ions is large does not alter the plasma discharge in the Hall thruster as long as the interaction between the ion beam and the cathode plume is captured properly in the computational domain. This implies that the boundary conditions must be placed sufficiently far as to not interfere with the electron transport in this region. These simulation results suggest that electron transport across magnetic field lines occurs largely inside the beam and may be driven by the interactions between beam ions and electrons. A second finding that puts in relevance the importance of including the cathode plume in numerical simulations is on the significance of accounting for the ion acoustic turbulence (IAT), now known to occur in the vicinity of the cathode exit. We have included in the Hall2De simulations a model of the IAT-driven anomalous collision frequency based on Sagdeev's model for saturation of the ion-acoustic instability. This implementation has allowed us to achieve excellent agreement with experimental measurements in the near plume obtained during the operation of the H6 thruster at nominal conditions (300V, 20A) and chamber background pressure of approximately 1.5 x 10(exp -5) Torr. In addition, the numerical results obtained with the latter approach exhibit less sensitivity to background pressure than previous attempts at explaining the features of the plasma properties in the near plume.

Lopez Ortega, Alejandro↗
NASA NTRSDate not supplied

Synthetic Data Generation for 3D Mesh Prediction and Spatial Reasoning During Multi-Agent Robotic Missions

In-space assembly operations require accurate reasoning over the pose, location, and structural organization of both the autonomous agents and assembly materials. In a full six-degree-of-freedom space, an accurate understanding of the full three-dimensional structure of the object of interest greatly enriches information for pose estimation and collision planning. Current methods of predicting pose estimation require a priori understanding of the shape of the object. Additionally, visual information in the space environment is impacted by variations in contrast and illumination. Using synthetic data allows us to rapidly generate large datasets with in varying environments and lighting conditions. This work details the generation of synthetic data used to explore the use of a region-based convolutional neural networks to detect objects of interest and predict a voxel-based three-dimensional mesh in order to understand their full three-dimensional shape. This mesh provides useful spatial information during in-space assembly operations without requiring either the complexity of maintaining models over the progress of building an object or observations from multiple angles. The generated meshes are then compared to that of ground truth in order to measure its performance.

James Ecker↗
NASA NTRS1995

Operation of the computer model for direct atomic oxygen exposure of Earth satellites

One of the primary causes of material degradation in low Earth orbit (LEO) is exposure to atomic oxygen. When atomic oxygen molecules collide with an orbiting spacecraft, the relative velocity is 7 to 8 km/sec and the collision energy is 4 to 5 eV per atom. Under these conditions, atomic oxygen may initiate a number of chemical and physical reactions with exposed materials. These reactions contribute to material degradation, surface erosion, and contamination. Interpretation of these effects on materials and the design of space hardware to withstand on-orbit conditions requires quantitative knowledge of the atomic oxygen exposure environment. Atomic oxygen flux is a function of orbit altitude, the orientation of the orbit plan to the Sun, solar and geomagnetic activity, and the angle between exposed surfaces and the spacecraft heading. We have developed a computer model to predict the atomic oxygen exposure of spacecraft in low Earth orbit. The application of this computer model is discussed.

Bourassa, R. J.↗
NASA NTRSDate not supplied

Fe3 Traffic Simulator Overview

NASA Ames developed a first of its kind air traffic simulation tool known as Flexible engine for Fast time evaluation of Flight environments (Fe3). The patent-pending technology is a robust simulation tool that provides the capability of statistically analyzing the high-density and low-altitude traffic system. With this capability, stakeholders can study the impacts of critical components (such as wind, surveillance, communication, collision, avoidance, traffic rules, energy consumption, etc.) in the low-altitude high-density traffic system, gain insights and help define requirements, policies, and protocols for a safe and efficient traffic system, and assess operational risks and optimize fight schedules.

Flexible engine for Fast time evaluation of Flight↗
NASA NTRS2000

Trajectory Design and Control for the Compton Gamma Ray Observatory Re-Entry

The Compton Gamma Ray Observatory (CGRO) controlled re-entry operation was successfully conducted in June of 2000. The surviving parts of the spacecraft landed in the Pacific Ocean within the predicted footprint. The design of the maneuvers to control the trajectory to accomplish this re-entry presented several challenges. These challenges included timing and duration of the maneuvers, fuel management, post maneuver position knowledge, collision avoidance with other spacecraft, accounting for the break-up of the spacecraft into several pieces with a wide range of ballistic coefficients, and ensuring that the impact footprint would remain within the desired landing area in the event of contingencies. This paper presents the initial re-entry trajectory design and the evolution of the design into the maneuver sequence used for the re-entry. The paper discusses the constraints on the trajectory design, the modifications made to the initial design and the reasons behind these modifications. Data from the re-entry operation are presented.

Hoge, Susan↗
NASA NTRS1998

U.S. Civil Rotorcraft Accidents, 1963 through 1997

The U.S. National Transportation Safety Board (NTSB) has recorded 8,436 rotorcraft accidents during the period mid - 1963 through the end of 1997. Review and analysis of the NTSB summary narrative for each accident has been completed. In addition, FAA (Federal Aviation Administration) counts of the growing registered rotorcraft fleet over this period has obtained. Taken together, a large and informative data base is now available, which indicates that the accident rate (on a per airframe basis) has changed very little since the mid 1970s. The data base, even in the summary form provided by this paper, offers suggestions for safer designs and improved flight operations. For analysis purposes, each accident has been placed in one of 21 top level categories as defined by the NTSB. Analysis of this grouping shows that 70 percent of rotorcraft accidents are associated with four categories. The accident count in these top four categories are: (1) 2,408 Loss of engine power (2) 1,322 In flight collision with object (3) 1,114 Loss of control (4) 1,083 Airframe/component/system failure or malfunction. Single engine rotorcraft dominate these accident statistics because of their sheer numbers over the study period. One-third of the loss of engine power accidents with these aircraft is fuel/air mixture related and fuel exhaustion is a common event. This appears to be the case whether a piston or turbine engine is installed. This paper provides similar study results in the other major mishap categories. It shows that both minor and major design and flight operations changes can -- and should -- be made to reduce rotorcraft accidents in the future. The paper outlines these changes and suggests how they may be made.

Harris, Franklin D.↗
NASA NTRS1974

Barium cloud evolution and striation formation in the magnetospheric release on September 21, 1971

The joint NASA-Max Planck Institute Barium Ion Cloud (BIC) Experiment on September 21, 1971 involved the release of 1.7 kg of neutral barium at an altitude of 31,500 km at a latitude of 6.93 deg N. and a longitude of 74.40 deg W. A theoretical model describing the barium neutral cloud expansion and the ion cloud formation is developed. The mechanism of formation of the striational features observed in the release is also discussed. Two candidate instabilities, which may contribute to striation formation, are examined. The drift instability stemming from the outwardly directed drag force exerted on the ions by the outstreaming neutrals is rejected on the grounds that the ion density is too low during the collision-dominated phase of the cloud expansion to support this kind of instability. The joint action of Rayleigh-Taylor and flute instabilities plausibly accounts for the observed striational structure. This same mechanism may well be operative at times of sudden injection of plasma into the inner magnetosphere during geomagnetic storms and may thus contribute to the formation of field-alined inhomogeneities which serve as whistler ducts.

Adamson, D.↗
NASA NTRS1980

Annual survey of spaceflight safety systems - 10th supplement /Survey period: July 1979-June 1980/

The survey summarizes 15 papers whose topics represent significant safety developments reported between July 1979 and June 1980 in connection with spaceflight. The majority of the papers address the Space Shuttle mission (control-system software modification; abort procedures; Vandenberg AFB launch operations; assembly and repair of the Orbiter's Thermal Protection System; flight hardware). One paper describes a procedure aiding Space Transportation System (STS) payload organizations in complying with safety requirements established by NASA. Spacecraft cabin environments, collision avoidance in near-earth space, and intercept and rendezvous charts are other topics of discussion. The survey also includes two papers outlining the European and Chinese approaches to astronaut selection and training.

Brown, N. E.↗
DOE OSTI2021

Formation and reconfiguration of tight multi-lane platoons

Advances in vehicular communication technologies are expected to facilitate cooperative driving in the future. Connected and Automated Vehicles (CAVs) are able to collaboratively plan and execute driving maneuvers by sharing their perceptual knowledge and future plans. In this paper, an architecture for autonomous navigation of tight multi-lane platoons traveling on public roads is presented. Using the proposed approach, CAVs are able to form single or multi-lane platoons of various geometrical configurations. They are able to reshape and adjust their configurations according to changes in the environment. The proposed architecture consists of two main components: an offline motion planner system and an online hierarchical control system. The motion planner uses an optimization-based approach for cooperative formation and reconfiguration in tight spaces. A constrained optimization scheme is used to plan smooth, dynamically feasible and collision-free trajectories for all the vehicles within the platoon. The paper addresses online computation limitations by employing a family of maneuvers precomputed offline and stored on a look-up table on the vehicles. The online hierarchical control system is composed of three levels: a traffic operation system (TOS), a decision-maker, and a path-follower. The TOS determines the desired platoon reconfiguration. The decision-maker checks the feasibility of the reconfiguration plan based on real-time information about the surrounding traffic. The reconfiguration maneuver is executed by a low-level path-following feedback controller in real-time. The effectiveness of the approach is demonstrated through simulations of three case studies: (1) formation reconfiguration (2) obstacle avoidance, and (3) benchmarking against behavior-based planning in which the desired formation is achieved using a sequence of motion primitives.

42 ENGINEERING↗
DOE OSTI2022

Characterizing Minimum Admissible Separation Distances in Heavy Duty Vehicle Platoons

Heavy duty (HD) vehicles platooning under highway operating conditions, have been projected to provide significant fuel economy (FE) gains based on aerodynamic drag reduction of the platooning vehicles. Realizing these benefits under real-world operating conditions have several challenges. This paper (the second as part of series) quantifies the minimum admissible separation distance between platooning vehicles in HD commercial vehicle (CV) applications. This distance must be set up such that no collision occurs with leading vehicles under all deceleration conditions. In this paper key interactions to characterize this distance are demonstrated. These include, road surface conditions, grade, inter-vehicle communication delays, air-brake lag time, vehicle speed, weight, and aerodynamic drag. It is seen that characterizing the vehicle and road dynamics will be critical in establishing required deceleration or stopping distances for each vehicle in a platoon. The results of the study also necessitate vehicle control systems that can dynamically adjust not only the separation distances but also the braking reference points based on road conditions. The narrative provides application decision personnel with a well-defined set of admissible separation distance impact factors to consider as part of their architecture selection process.

Sujan, Vivek↗
DOE OSTI2023

MARBLES (Multi-scale Adaptively Refined Boltzmann LatticE Solver) [SWR-23-37]

MARBLES (Multi-scale Adaptively Refined Boltzmann LatticE Solver) is an open-source computational fluid dynamics package powered by the lattice Boltzmann equations and built on AMReX. In the lattice Boltzmann method, local collisions between meso-scale fictitious particles drive the governing equations which enables MARBLES to easily simulate flow around complex and/or moving geometry without the generation of a body-conforming mesh. Using AMReX data structures and operations ensures a high level of computational performance and parallel scaling on heterogenous architectures while also naturally supporting locally enhanced grid resolution and fidelity through automatic mesh refinement. New domains and problem definitions are easily specified through an input file with examples and guidance on all options and variables provided in the MARBLES documentation.

Henry de Frahan, Marc↗
NASA NTRS1991

Advanced mechanisms for robotics

An overview of applied research and development at the Goddard Space Flight Center (GSFC) on mechanisms and collision avoidance skin for robots is presented. The work on robot end effectors is outlined, followed by a brief discussion of robot-friendly payload latching mechanisms and compliant joints. This is followed by discussions of the collision avoidance/management skin and the GSFC research on magnetorestrictive direct drive motors. A new project, the artificial muscle, is introduced. Each of the devices is described sufficiently to permit a basic understanding of its purpose, capabilities, and operating fundamentals. The implications for commercialization are discussed.

Vranish, John M.↗
NASA NTRS1994

Investigating and implementing enhancements to the simulation of short-term collision hazards

A software tool, the Relative Collision Matrix (RCM), has been developed to provide a quick-look representation of the shortterm collision hazard to space systems from a fragmentation event in Earth orbit. The software performs multiple fragmentation simulations of space objects to quantify the probability of collision for a satellite or a constellation of satellites nearby. Previously, the results were displayed in a color matrix format which showed the relative hazard of each constellation. The RCM can be used for scientific research and operational assessments even though it was designed for test and evaluation applications. Because of its successful use as an analytical tool, the capabilities of RCM are being extended by enhancing the orbital hazard analysis routines, developing ballistic trajectory hazard analysis routines, and expanding the breakup modeling. Improvements are also being made to the RCM's usability and presentation quality by developing a graphical user interface and by providing graphical animated and nonanimated output.

Huth, Jeffrey S.↗
NASA NTRS1997

AIAA Survivability Technical Committee Draft

A relatively new area of interest in aerospace systems survivability is the growing threat of spacecraft penetration by orbital debris. Orbital debris, or "space junk", is composed of the man-made remnants of non-functioning spacecraft still orbiting the Earth. NASA estimates that there are currently over 100,000 orbital debris particles 1 centimeter in diameter or larger that cannot be tracked by existing radar, with the population growing at approximately 4% per year in low earth orbits. With an average velocity of over 8.7 km/sec, these projectiles can penetrate and disable many vulnerable spacecraft systems. Since the likelihood of spacecraft penetration increases with spacecraft surface area, large spacecraft (such as the International Space Station) and communication satellite fleets (such as Iridium) have begun to adopt survivability enhancement strategies similar to those employed by combat aircraft. Collision avoidance maneuvers are commonly practiced by the Space Shuttle and are planned by the International Space Station to decrease their susceptibility to impact by trackable orbital debris; likewise, improved shielding, internal equipment placement, and improved crew operations following penetration can reduce the vulnerability of spacecraft to loss following orbital debris impact. Computer simulations such as the Manned Spacecraft and Crew Survivability (MSCSurv) program at the NASA-Marshall Space Flight Center have recently been developed to quantify and reduce the likelihood of crew or spacecraft loss following orbital debris penetration. The AIAA Survivability Technical Committee is working to enable the transfer of military-developed survivability technologies to help the aerospace industry cope with this growing threat.

Shipman, Jim↗
NASA NTRSDate not supplied

The Viability of See and Avoid for Urban Air Mobility Operations

Urban Air Mobility (UAM) is an emerging aviation concept that could supplement today’s ground and air transportation systems. For UAM, it is generally assumed that the private sector will manage separation and not rely on the Federal Aviation Administration air traffic control system. To date, discussions of initial operations focus on using the visual abilities of the pilot to “see and avoid” (SAA) other aircraft. Decades of research on SAA has demonstrated that it is inadequate for reliable detection of aircraft that might pose a collision risk. The literature on multi-object tracking is also reviewed for findings on how well humans can visually track objects. The research shows that humans have limited resources for tracking and that this may be affected by object characteristics and cognitive skills. The conclusion is that SAA is a risky method for avoiding midair collisions. It is recommended that flight deck displays and automated collision avoidance systems be implemented for all UAM aircraft at the outset of their introduction.

urban air mobility↗
NASA NTRS2023

The Viability of See-and-Avoid for Urban Air Mobility Operations

Urban Air Mobility (UAM) is an emerging aviation concept that could supplement today’s ground and air transportation systems. For UAM, it is generally assumed that the private sector will manage separation and not rely on the Federal Aviation Administration air traffic control system. To date, discussions of initial operations focus on using the visual abilities of the pilot to “see and avoid” (SAA) other aircraft. Decades of research on SAA has demonstrated that it is inadequate for reliable detection of aircraft that might pose a collision risk. The literature on multi-object tracking is also reviewed for findings on how well humans can visually track objects. The research shows that humans have limited resources for tracking and that this may be affected by object characteristics and cognitive skills. The conclusion is that SAA is a risky method for avoiding midair collisions. It is recommended that flight deck displays and automated collision avoidance systems be implemented for all UAM aircraft at the outset of their introduction.

urban air mobility↗
DOE OSTI2024

Radiation induced athermal diffusivity in uranium mononitride

Uranium mononitride (UN) is one of the ceramic nuclear fuel alternatives to oxide fuel considered for light water reactors and advanced reactor designs. Properties like self- and fission gas diffusivity need to be better understood, given that they influence key fuel performance phenomena such as fission gas swelling and release. In particular, the radiation induced athermal (D 3 ) diffusivity remains challenging to accurately predict and has only been sparsely characterized in UN, despite its importance as it likely governs diffusion at the low temperatures this high-thermal-conductivity fuel form may operate. Molecular Dynamics simulations are used to estimate the mean square displacement induced by a primary knock-on atom (PKA) with a given kinetic energy. These results are combined with the PKA energy distributions obtained from binary collision approximation calculations to obtain the displacement due to a particular fission fragment. Finally, this is combined with experimental fission fragment yields to determine the displacement due to an average fission event and, thus, express the athermal diffusivity as a function of the fission rate density. These results are in excellent agreement with available experimental data. In conclusion, a particular importance is given to the understanding and the quantification of the variability of these results.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
NASA NTRS1977

Optimal design of mixed-media packet-switching networks - Routing and capacity assignment

This paper considers a mixed-media packet-switched computer communication network which consists of a low-delay terrestrial store-and-forward subnet combined with a low-cost high-bandwidth satellite subnet. We show how to route traffic via ground and/or satellite links by means of static, deterministic procedures and assign capacities to channels subject to a given linear cost such that the network average delay is minimized. Two operational schemes for this network model are investigated: one is a scheme in which the satellite channel is used as a slotted ALOHA channel; the other is a new multiaccess scheme we propose in which whenever a channel collision occurs, retransmission of the involved packets will route through ground links to their destinations. The performance of both schemes is evaluated and compared in terms of cost and average packet delay tradeoffs for some examples. The results offer guidelines for the design and optimal utilization of mixed-media networks.

Huynh, D.↗