Years to decade-long cyclic orbital period changes have been observed in several classes of close binary systems including Algols, W Ursae Majoris and RS Canum Venaticorum systems, and the cataclysmic variables. The origin of these changes is unknown, but mass loss, apsidal motion, magnetic activity, and the presence of a third body have all been proposed. In this paper we use new CCD observations and the century-long historical record of the times of primary eclipse for WW Cygni to explore the cause of these period changes. WW Cygni is an Algol binary whose orbital period undergoes a 56 year cyclic variation with an amplitude of ≈ 0.02 days. We consider and reject the hypotheses of mass transfer, mass loss, apsidal motion and the gravitational influence of an unseen companion as the cause for these changes. A model proposed by Applegate, which invokes changes in the gravitational quadrupole moment of the convective and rotating secondary star, is the most likely explanation of this star's orbital period changes. This finding is based on an examination of WW Cygni's residual O−C curve and an analysis of the period changes seen in 66 other Algols. Variations in the gravitational quadrupole moment are also considered to be the most likely explanation for the cyclic period changes observed in several different types of binary systems.

Metal additive manufacturing (AM) encapsulates the myriad of manufacturing processes available to meet industrial needs. Determining which of these AM processes is best for a specific aerospace application can be overwhelming. Based on the application, each of these AM processes has advantages and challenges. The most common metal AM methods in use include Powder Bed Fusion, Directed Energy Deposition, and various solid-state processes. Within each of these processes, there are different energy sources and feedstock requirements. Component requirements heavily affect the process determination, despite existing literature on these AM processes (often inclusive of input parameters and material properties). This article provides an overview of the considerations taken for metal AM process selection for aerospace components based on various attributes. These attributes include geometric considerations, metallurgical characteristics and properties, cost basis, post-processing, and industrialization supply chain maturity. To provide information for trade studies and selection, data on these attributes were compiled through literature reviews, internal NASA studies, as well as academic and industry partner studies and data. These studies include multiple AM components and sample build experiments to evaluate (1) material and geometric variations and constraints within the processes, (2) alloy characterization and mechanical testing, (3) pathfinder component development and hot-fire evaluations, and (4) qualification approaches. This article summarizes these results and is meant to introduce various considerations when designing a metal AM component.

In 1970, Hiltner & Mook reported the results of the first multiyear study of the optical emission from Sco X-1. They found that the Sco X-1 B-magnitude histograms changed from year to year. Subsequent multiwavelength campaigns confirmed the variable nature of these optical histograms and also found that the X-ray and optical emissions were only correlated when Sco X-1 was brighter than about B = 12.6. Models had suggested that the optical emission from this source arose from X-rays reprocessed in an accretion disk surrounding the central neutron star. It was therefore difficult to explain why the optical and X-ray fluxes were not more closely correlated. In 1994 and 1995, two new simultaneous optical and X-ray campaigns on Sco X-1 were conducted with the Burst and Transient Source Experiment on the Compton Gamma Ray Observatory and the 1 m Yale telescope at Cerro Tololo Inter-American Observatory. Using these data and models by Psaltis, Lamb, & Miller, it is now possible to provide a qualitative picture of how the X-ray and optical emissions from Sco X-1 are related. Differences in the B-magnitude histograms are caused by variations in the mass accretion rate and the relatively short time period typically covered by optical investigations. The tilted-C pattern seen in plots of the simultaneous X-ray and optical emission from Sco X-1 arises from (1) the nearly linear relation between the optical B magnitude and the mass accretion rate in the range 13.3 ! B ! 12.3 and an asymptotic behavior in the B magnitude outside this range, and (2) a double-valued relation between the X-ray emission and mass accretion rate along the normal branch and lower flaring branch of this source.

A first generation Nuclear Cryogenic Propulsion Stage (NCPS) based on Nuclear Thermal Propulsion (NTP) is currently being developed for Advanced Space Exploration Systems. The overall goal of the project is to address critical NTP technology challenges and programmatic issues to establish confidence in the affordability and viability of NTP systems. The current technology roadmap for NTP identifies the development of a robust fuel form as a critical near term need. The lack of a qualified nuclear fuel is a significant technical risk that will require a considerable fraction of program resources to mitigate. Due to these risks and the cost for qualification, the development and selection of a primary fuel must begin prior to Authority to Proceed (ATP) for a specific mission. The fuel development is a progressive approach to incrementally reduce risk, converge the fuel materials, and mature the design and fabrication process of the fuel element. A key objective of the current project is to advance the maturity of CERMET fuels. The work includes fuel processing development and characterization, fuel specimen hot hydrogen screening, and prototypic fuel element testing. Early fuel materials development is critical to help validate requirements and fuel performance. The purpose of this paper is to provide an overview and status of the work at Marshall Space Flight Center (MSFC).

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