11115 research outputs found
Sort by
Simulation of Debris Events in Cislunar Orbits Near the Moon
No full textSpacecraft breakup events are simulated in the vicinity of the Lunar Gateway’s planned near-rectilinear halo orbit and a 70,000 km radius lunar distant retrograde orbit similar to that used during the Artemis I mission. For each scenario, realistic Monte Carlo simulations consisting of 5000 breakup events and over a million debris fragments enable a statistical evaluation of the longevity of debris in the pre-breakup orbit, the locations of lunar surface impacts, and the conjunction risks to other spacecraft such as the Gateway. Conjunction risks are quantified in terms of the likelihood of observing close approaches within specified distances. The near-rectilinear halo orbit simulations focus on modeling a scenario in which an object suffers a breakup event shortly after deployment from the Gateway, and deployment strategies to reduce the risk to the Gateway are identified, such as a minimum deployment change in velocity of 4 m/s and deployment only at perilune. Conjunctions within 5 km were rare, but approaches within 500 km occurred in the majority of simulation runs, which may generate concern given tracking limitations in cislunar space. Debris was not stable in the vicinity of the near-rectilinear halo orbit but could remain stable in the distant retrograde orbit for at least 10 years. The results of this study provide quantifiable insight into the risk of close approaches between debris and cislunar spacecraft such as the Gateway and identify strategies that can be used to develop operational plans to mitigate this risk
Temperature dependence of the nSpec fieldable neutron spectrometer
No full textExcerpt: Fieldable instruments can encounter a variety of environments during their use. A recent fieldable fast neutron spectrometer, nSpec, has been exploring alternate materials for further ruggedizing the system. One of these materials, EJ276D, is a plastic scintillator which provides several benefits over the current liquid scintillators used in nSpec, including an operating temperature range that is more closely matched to the limits of the electronics in the system. In the following work, the temperature dependence of EJ276D is studied from -10 to 50 °C. The temperature dependence of the calibration and resolution of the detector were characterized using basic gamma-ray sources, while the characterization of the light output quenching for neutron interactions was done utilizing the Los Alamos Neutron Science Center (LANSCE)
Investigation of Stability and Control Shortcomings of the North American X-15
Full text linkThere is growing interest in the design of maneuvering high-speed aircraft to fly within or at the edge of the atmosphere. We identify and develop novel quasi-static vehicle screening methodologies, suitable for use during preliminary design, to better predict an incipient loss of control due to the dynamic effects of feedback. We validate these metrics by reverse-engineering Neil Armstrong’s 1962 loss of control and inadvertent atmospheric skip while piloting the X-15. In 1962, then-extant flight dynamics screening methods did not forecast likely troubles. We assemble and refine a collection of predictive metrics which operate upon basic quasi-static aerodynamic data and predict the confluence of lateral/directional stability and controllability issues which plagued the flown mission. These tools, which leverage McRuer’s “equivalent stability derivative” approach, enable future engineers to make proactive design changes which can avoid lateral/directional instabilities developing at high speeds
Cooperative Pursuit-Evasion Games with a Flat Sphere Condition
No full textIn planar pursuit-evasion differential games considering a faster pursuer and a slower evader, the interception points resulting from equilibrium strategies lie on the Apollonius circle. This property is instrumental for leveraging geometric approaches for solving multiple pursuit-evasion scenarios in the plane. In this paper, we study a pursuit-evasion differential game on a sphere and generalize the planar Apollonius circle to the spherical domain. For the differential game, we provide equilibrium strategies for all initial positions of the pursuer and evader, including a special case when they are on the opposite sides of the sphere and on the same line with the center of the sphere when there are infinitely many geodesics between the two players. In contrast to planar scenarios, on the sphere we find that the interception point from the equilibrium strategies can leave the Apollonius domain boundary. We present a condition to ensure the intercept point remains on the boundary of the Apollonius domain. This condition allows for generalizing planar pursuit-evasion strategies to the sphere, and we show how these results are applied by analyzing the scenarios of target guarding and two-pursuer, single evader differential games on the sphere
Methods for Analyzing Transforming Orbits
No full textIn the spatial Circular, Restricted Three-Body Problem (CR3BP) there exist quasi-periodic orbits that undergo long duration periodic changes in the shape of the orbit. These long duration transformations may initially appear as though the motion is chaotic; however, over time these transformations repeat with a predictable pattern. Similar to any periodic orbit in the CR3BP, these transforming orbits can be defined by a position and velocity vector, but these six terms do not allow one to easily visualize and understand the transformation of the orbit. An efficient way of describing the motion of a transforming orbit is necessary in order for these orbits to be evaluated for their usefulness for conducting different mission types. This paper introduces two new variables called the Scaled Position Difference and Transformation Indicator to analyze the behavior of transforming orbits. This paper presents a standardized method for describing transforming orbits to enable them to be more easily discussed and considered for future missions. This paper also uses a chaos indicator to analyze these transforming orbits. The chaos indicator can provide additional insight into how the orientation of the orbit changes over time
An Optimal Strategy for Off-Board Proximal Sensing of a Target: Part 1
No full textThe time-optimal strategy for a pursuer, endowed with a guided off-board sensor, is found to place the range-limited sensor platform onto a non-maneuvering target. Leveraging optimal control theory, a feedback control strategy is obtained for the pursuer using the indirect optimal control method. The target is assumed to have constant speed and move slower than the off-board sensor and the pursuer. The limited-range off-board sensor is assumed to be faster than the pursuer and the target. The pursuer is assumed to have unlimited range and is faster than the target but slower than the off-board sensor platform. All agents\u27 communication, location, and heading are assumed known by all; the scenario assumes communication between the pursuer and off-board sensor is ideal. This work presents how the optimal control approach can be leveraged as well as highlighted in example simulations and an accompanying discussion of those results
Semiclassical Approach to Computing Vibrationally Resolved Ionization Cross Sections for Molecular Nitrogen
No full textDistribution Comparisons of EAC Cost Growth for Aircraft Work Breakdown Structure Elements
Full text linkThis article analyzes and investigates the distribution of cost growth of the Estimate at Completion (EAC) for the Work Breakdown Structure (WBS) elements of approximately 60 historical United States Acquisition Category I Research, Development, Test and Evaluation aircraft programs. Using the method of maximum likelihood in conjunction with the Akaike Information Criterion, the authors suggest that both the lognormal and Weibull distributions provide relatively good fit to EAC cost growth, with the lognormal slightly edging out the Weibull. As a summarized finding, the authors present their empirical results for the mean, coefficient of variation (CV), the 15th and 85th percentiles by WBS element. With an overall mean of 1.36, 0.80, and 3.1 for the mean, CV, and 85th percentile, respectively, these results suggest a high likelihood of cost growth in budgets. To account for this possible underestimation, the authors propose higher cost factors to consider or to use as a crosscheck