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Bill Doggett

Publications and source records attributed to Bill Doggett.

Relocatable 10 kW Solar Array for Lunar South Pole Missions

Deployable, relocatable, free-standing solar arrays are being developed to provide modular power for future lunar South Pole missions. Major design requirements for these arrays will be low mass, compact launch stowage, and highly reliable deployment and retraction. This paper presents a novel conceptual design for a10 kW solar array to address these requirements referred to as the Relocatable Solar Array (RSA).Simply stated, the concept is a pair of solar cell blankets freely hanging from a horizontal cross arm supported by a vertical, slender, telescoping mast resting on a deployable tripod base. A major factor in simplifying the design is that the force exerted by lunar gravity is used to deploy and maintain extension of the hanging array blankets. A second major factor in achieving the desired low mass and high volumetric efficiency is that the array operates in the vacuum, low-gravity, lunar environment with no deployed vibration frequency requirement. Such a low-load environment enables use of extraordinarily slender and low mass structural members to support the hanging array blankets. This concept was developed, in part, to serve as a NASA reference solar array concept against which other proposed arrays can be compared.

Lunar Surface Power

Effect of Micrometeoroid and Orbital Debris Impact on Modal Response of Persistent Assets

Protecting the infrastructure and payload components of persistent assets (PAs) from damage caused by MicroMeteoroid and/or Orbital Debris (MMOD) impacts is a significant design consideration. The impact resistance to MMOD will need to be provided by the robustness of the structural configuration such that the asset does not suffer loss of function or catastrophic failure. Finite element modelling (FEM) can be used to investigate the effects of MMOD impact on the overall structural behavior of the PA. The models are being used here to investigate the structural response to ‘missing’ truss members, and parametric studies are being performed to identify the worst-case scenario of an impact catastrophically damaging a truss element. These models will be used to determine which truss members are the most critical to the structural performance, and thus, which may require a higher degree of protection or robustness to ensure they can withstand an MMOD impact without causing catastrophic effects on the entire structure. Initial results have been generated indicating a reduction in the global frequency due to missing truss members as a result of MMOD impact.

in-space assembly (ISA)

TriTruss Packaging and Deployment Trade Study

An architecture and feasibility study of next generation In-Space Assembled Telescopes (iSAT) concluded that robotic in-space assembly of modular components is necessary to enable large (>15-meter diameter) primary apertures. The iSAT study recommended that the foundational structure be assembled from modular TriTruss modules that are packaged for launch, deployed on-orbit and robotically assembled into the final configuration. This paper will describe and summarize the results of a trade study that investigated viable concept of operations (ConOps) for the packaging and deployment (P&D) of individual planar TriTruss modules that could be assembled to form a large aperture iSAT. The ideal TriTruss P&D concept is defined as one that: allows for efficient packaging; has sufficient geometric versatility to be launch vehicle independent; provides a stiff and lightweight structure; has low mechanical complexity; and has component modularity. The P&D concept should allow for prelaunch subsystem or utility integration if required. The concept would be kinematically simple and be robotically deployed using a minimum number of specialized tools. In this first phase of an ongoing more comprehensive trade study, concepts were proposed and then evaluated based on initial metrics representing features of an ideal P&D concept. The P&D concepts evaluated are categorized as: core collapse, face collapse, and erectable structures. Sub-scale models were constructed to help understand the kinematics and mechanical complexity required to enable P&D. Based on a weighting scale, the most promising candidate P&D concepts have selected and will undergo more rigorous structural design, analysis, and testing in the study’s next phase. The ultimate goal of the comprehensive trade study will be to recommend a single TriTruss design and associated P&D concept that will be built and evaluated at NASA Langley Research Center’s In-Space Assembly Laboratory.

in-space assembly (ISA)

Tessellation and Numerical Simulation of the in-Space Assembled Telescope (iSAT) Reflector

A novel tessellation method has been developed to allow the use of uniform truss modules to form curved or doubly curved surfaces. To illustrate the method, the structural support for the doubly curved in-Space Assembled Telescope primary reflector is developed. The method intentionally leaves gaps at the nodes between identical connecting modules; the gaps are relatively small, on the order of 0.5% of the lattice characteristic cell size. Uniquely shaped nodal connectors called multi-nuts are generated to fill in the nodal gaps and connect adjacent modules. With optimization techniques, the size of these nodal gaps can be minimized to thereby minimize the incurred structural efficiency losses. Finite element analyses of the primary reflector were performed, and the structural efficiency losses amounted to a natural frequency reduction of 5%. The benefits of having uniform truss modules outweigh the structural penalties as it will greatly simplify assembly and lower manufacturing costs.

Tessellation

Near-term Persistent Platform Orbital Testbed: Three Candidate Architecture Options

On-orbit Servicing, Assembly, and Manufacturing (OSAM) will revolutionize the space industry by transforming the concept of operations of space systems and enabling new, radically different system implementations. These new implementations will benefit from a novel persistent asset design paradigm which focuses on evolvable designs that are tailored to the operational environment, not the launch environment. In addition, the ability to launch sub-systems independently enable future persistent assets to economically expand in capability and size, achieving cost effective and productive operations lasting for decades like terrestrial observatories. With few exceptions (International Space Station, Hubble Space Telescope, Mission Extension Vehicle customers), current space systems are not visited once they are operational. Leveraging emerging low cost commercial launch provides the ability to repeatedly and routinely revisit space systems. Thus, revolutionary new approaches for space system design are possible, creating completely new opportunities for small businesses and accelerating the growth of already established space industries. To usher in the revolutionary new operational paradigm, two things are needed. First, to build confidence in the technology and new paradigm, there must be a leading example, a bellwether persistent asset, that demonstrates the reliability and maturity of the new persistent asset paradigm (where repeated visits are common). Second, in order to rapidly advance and validate OSAM capabilities, an efficient means is required to conduct tests in the space environment. A persistent platform testbed satisfies both these needs. The space environment exhibits a plethora of characteristics that are difficult and costly to accurately simulate for a full system in a terrestrial laboratory, such as near zero gravity, a wide range of ionizing radiation types, atomic oxygen, and micro-meteoroids and space debris traveling at high velocity. In addition, since persistent assets range in mass from a few grams to several metric tons, it is difficult to accurately simulate interactions between these systems and visiting vehicles (that also exhibit a wide range of varying masses and capabilities). These interactions include the transmission of forces and/or exchanging mass (in the form of instruments, fuel, robotic assets, etc.). Thus, a rapid, versatile and cost efficient in-space testing capability that includes a persistent test platform and a surrounding in-space test zone is needed to mature technologies through experimentation. The testbed can provide common services, such as: power, thermal control, vibration isolation, data transmission between experiments and terrestrial experimenters, station-keeping, pointing, and robotic agents that can be leveraged by customer experiments. The onboard robotic agents can be used to provide payload handling services, such as: assembly, change out or upgrade, relocation, connecting/disconnecting utilities, inspection, repair or servicing, etc. Since the persistent platform cost will be amortized over many hosted payloads, its services can eventually be offered at a price much lower than if one were to design a unique and dedicated spacecraft and mission for those few experiments. The key to achieving an effective testbed is providing efficient cost effective access and infrastructure to a variety of commercial, academic and government customers coupled with extensibility, in the capability of an individual persistent platform test bed or replication of the test bed in a different operational regime. Three potential options for implementing a test bed were developed and evaluated in this study.

Persistent Platform

On-orbit Servicing, Assembly and Manufacturing (OSAM) Near-Term in-Space Developmental Test Persistent Platform: ESPA-Star Based Implementation

Roundtable Session on In-Space Developmental Test: Springboard for Rapid Development. Presentation is a brief description of the On-orbit Servicing Assembly & Manufacturing (OSAM) national initiative & using standard EELV Secondary Payload Adapters (ESPAs) as the core building block to construct an ESPA-Star based implementation for an iSDT persistent platform in space.

Persistent Platform

Ideas For Infusing In-Space Servicing, Assembly and Manufacturing Concepts into Nuclear Electric Propulsion Architectures

NASA is currently investigating nuclear electric propulsion (NEP) for human Mars transport within the space nuclear propulsion portfolio. NEP spacecraft have the following characteristics, they: 1) include very large structures (~100-meter length); 2) are comprised of many components/modules; and 3) have very long lifetimes (e.g., 50 years for fuel rods). Thus, NEP spacecraft can be classified as a “persistent asset,” which is any zero-g or planetary surface system that benefits from in-space assembly (ISA) or multiple visits for servicing, repairs, and upgrades. NEP spacecraft will benefit from taking advantage of, and incorporating, In-space Servicing, Assembly, and Manufacturing (ISAM) capabilities in the spacecraft architecture from the onset, enabling system maintenance, repair, and evolution. ISA has a long history of being proposed for, and studied as, a means for achieving large systems in space. More recently, the benefits of ISA have been recognized by NASA, the Department of Defense (DOD), other government agencies, and commercial space companies, and thus, ISAM is being actively pursued at a national level. Past and current strategies for achieving large structures in space have relied largely on two strategies; the first is to launch monolithic structures (designed to meet launch vehicle requirements for payload size and mass) that are docked or berthed to other monolithic structures on-orbit to form a larger structure (e.g., the International Space Station [ISS]); the second is folding and packaging large structures to fit inside a payload fairing and deploying the full-sized structure (unaided) once on-orbit (e.g., the James Webb Space Telescope [JWST]). To date, conceptual architecture studies performed for NEP spacecraft capable of human-rated Mars transport have only included a combination of the two previously mentioned strategies. This paper will propose ideas for infusing ISAM strategies into NEP vehicle architectures that leverage existing and near future technologies and enable the resulting NEP systems to be realized in a more time- and cost-efficient manner.

in-space assembly

Ideas for Infusing In-Space Servicing, Assembly, and Manufacturing Concepts into Nuclear Electric Propulsion Architectures

NASA is currently investigating nuclear electric propulsion (NEP) for human Mars transport within the space nuclear propulsion portfolio. NEP spacecraft have the following characteristics, they: 1) include very large structures (~100-meter length); 2) are comprised of many components/modules; and 3) have very long lifetimes (e.g., 50 years for fuel rods). Thus, NEP spacecraft can be classified as a “persistent asset,” which is any zero-g or planetary surface system that benefits from in-space assembly (ISA) or multiple visits for servicing, repairs, and upgrades. NEP spacecraft will benefit from taking advantage of, and incorporating, In-space Servicing, Assembly, and Manufacturing (ISAM) capabilities in the spacecraft architecture from the onset, enabling system maintenance, repair, and evolution. ISA has a long history of being proposed for, and studied as, a means for achieving large systems in space. More recently, the benefits of ISA have been recognized by NASA, the Department of Defense (DOD), other government agencies, and commercial space companies, and thus, ISAM is being actively pursued at a national level. Past and current strategies for achieving large structures in space have relied largely on two strategies; the first is to launch monolithic structures (designed to meet launch vehicle requirements for payload size and mass) that are docked or berthed to other monolithic structures on-orbit to form a larger structure (e.g., the International Space Station [ISS]); the second is folding and packaging large structures to fit inside a payload fairing and deploying the full-sized structure (unaided) once on-orbit (e.g., the James Webb Space Telescope [JWST]). To date, conceptual architecture studies performed for NEP spacecraft capable of human-rated Mars transport have only included a combination of the two previously mentioned strategies. This paper will propose ideas for infusing ISAM strategies into NEP vehicle architectures that leverage existing and near future technologies and enable the resulting NEP systems to be realized in a more time- and cost-efficient manner.

in-space assembly