Thesis: Ph. D., Massachusetts Institute of Technology, Department of Aeronautics and Astronautics, 2014. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 155-165). / Hierarchical advanced composite structures comprised of both nano- and micro-scale fibers are currently being studied as next-generation materials for multifunctional aerospace applications. Carbon nanotubes (CNTs) are an attractive reinforcing fiber for aerospace composites due to their scale and superior specific stiffness and strength, as well as their potential to enhance multifunctional properties. Nano-scale fibers can address current challenges in composites such as relatively weak through-thickness properties that occur due to matrix-rich regions, including those found at interlaminar ply interfaces, that are prone to delamination and lead to overall reductions in mechanical properties. Existing technologies such as stitching, z-pinning, and braiding provide through-thickness reinforcement; however, these improvements come with simultaneous reductions in in-plane properties. CNTs provide an alternative fiber reinforcement, though currently the literature reveals that laminate mechanical property enhancements are lower than expected. Investigations into how CNTs affect laminate properties have stalled due to difficulties with producing quality laminates and controlling CNT orientation and dispersion. In this work, manufacturing routes of a nano-engineered composite are developed to provide consistent control over laminate quality while placing aligned CNTs (A-CNTs) in the polymer matrix in the interlaminar and intralaminar regions. Manufacturing techniques are developed for growing aligned CNTs on a three-dimensional woven microfiber substrate and infiltrating the fuzzy fiber plies with polymer to realize the Fuzzy Fiber Reinforced Plastics (FFRP) architecture. These FFRP laminates show < 1% void fraction for a viscous marine epoxy system via hand lay-up and effectively void free (<< 1%) laminates for an aerospace epoxy system via infusion. The influence of the A-CNTs on manufacturability is quantified by assessing permeability and compressibility of the fuzzy fiber plies. Less than an order of magnitude decrease in permeability independent of CNT loading is observed (up to 3.6% volume fraction), demonstrating compatibility of the fuzzy fiber plies with both polymer matrices and both manufacturing routes. By contrast, fuzzy fiber ply compressibility increases linearly with CNT loading such that target composite volume fractions of - 50% mnicrofiber volume fraction can only be achieved with added external pressure in ranges typically available in composite production. The mechanisms of Mode I fracture toughness enhancement in FFRP laminates are elucidated experimentally by varying the type of epoxy and length of A-CNTs. Reinforcement effectiveness is found to vary from reduced initiation toughness to 100% increase in steady-state fracture toughness, depending upon the interlaminar fracture mechanisms. Toughness enhancement is less than expected based on idealized fiber pullout models, and is attributed to multiple and competing modes. Fractography reveals toughening mechanisms for both aerospace and marine epoxy laminates at several length scales, from the pull-out of A-CNTs to microfiber tow breakage. The toughening behavior and magnitude of steady-state toughness improvement is found to be highly dependent on the type of epoxy. In the more brittle aerospace epoxy system, modest improvement (~ 33%) in steady-state toughness with long (~ 19 microns) A-CNTs occurs because the cohesive interlaminar matrix failure mode around woven tow features is unchanged and toughening only occurs via increased fracture surface area through CNT pullout and rough epoxy fracture. The tougher marine epoxy allows much larger (up to 100%) steady-state toughness enhancement with A-CNTs by significantly adding instances of microfiber breakage and pullout along with CNT pullout from the epoxy. Varying the CNT length begins to reveal how the geometrical (re)arrangement of microfibers through tow swelling and changes in woven ply nesting affect the crack propagation path and subsequent interlaminar toughness. Fracture of A-CNT polymer nanocomposites isolates CNT-polymer effects from the microfibers and shows no increase in initiation toughness from the A-CNTs, but does confirm the role of CNTs in increasing fracture surface area post crack initiation, i.e., steady-state toughening. This work establishes the dependence of fracture toughness on A-CNT length and polymer type for the FFRP architecture. Future work includes quantifying the contribution of CNT pullout from the matrix on the laminate fracture behavior via modified standard tests for fracture initiation and toughness. Preliminary multifunctional investigations of the FFRP architecture indicate several other promising directions of future work, including damage sensing. Based on new understanding in this work on boh manufacturing and reinforcing mechanisms at work in FFRPs, mechanical and multifunctional enhancement of aerospace composites, particularly carbon fiber FFRP, are enabled. / by Sunny S. Wicks. / Ph. D.

Thesis: S.M., Massachusetts Institute of Technology, Department of Aeronautics and Astronautics, 2017. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 100-109). / The relatively high mass-specific strength and stiffness of carbon fibers (CFs) has established CF reinforced plastics (CFRPs) as the benchmark material for next-generation aerospace structures. While CFRPs with radially-grown aligned carbon nanotubes (CNTs), termed fuzzy fiber reinforced plastics (FFRPs), have exhibited enhanced inter- and intralaminar mechanical properties on model FRP systems, these results have not been replicated for aerospace-grade CFRP due to challenges in manufacturing. This thesis reports a scaled (weave- vs. tow-level) manufacturing method of fuzzy woven CFRPs designed to yield dense and aligned CNT coverage on the fibers, and retain the fiber tensile and interface properties. These challenges were explored through mechanical testing, in addition to numerical reactive computational fluid dynamics (CFD) CNT growth models. Single fiber tensile tests for fuzzy fibers from aerospace-grade CF weaves showed no reduction in tensile strength compared to baseline (as received) fibers. Continuously monitored single fiber composite fragmentation testing revealed a 34% decrease in fiber-matrix interfacial shear strength (IFSS) for sized (polymer coating on fibers) fuzzy fibers, attributed to thermally induced sizing transformations during CNT growth, whereas the fuzzy de-sized fibers exhibited no reduction in IFSS. The CFD model demonstrated gas depletion trends correlated to the areas of substandard growth and a high sensitivity to the surface-to-volume ratio of the porous woven substrate. Retained CF properties supports this facile, scaled manufacturing method's ability to disperse CNTs uniformly on CF weaves to create a laminate-level fuzzy CFRP towards enhanced mechanical and multifunctional properties. With continued CNT growth modeling efforts, further scaling of this fuzzy CFRP architecture could be integrated into commercial manufacturing processes. / by Hayden K. Cornwell. / S.M.

Thesis: S.M., Massachusetts Institute of Technology, Department of Aeronautics and Astronautics, 2017. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 187-189). / Recent studies have shown that distributed spacecraft missions, or constellations, can offer similar performance to monolithic satellite missions for lower cost and less risk. Additionally, recent developments in and implementation of electric propulsion (EP) technologies further the case for the use of constellations because they enable operational possibilities otherwise unavailable to satellites with chemical thrusters by reducing costly fuel requirements. Through more efficient fuel usage, EP allows for wide-scale rendezvous of satellites for refueling/maintenance as well as constellation reshuffling and orbit raising to recover system performance after losing a satellite. With these constellation-wide maneuvers at an operator's disposal, distributed spacecraft missions will be able to operate longer and will have more flexibility to adapt and respond to malfunctions in the constellation. This thesis analyzes the performance gains of distributed spacecraft missions that utilize EP by analyzing satellite constellations at both microscopic and macroscopic levels - first, by understanding how payloads of different types, when combined with higher power requirements for EP systems, impact and influence an individual satellite's design and mass, and then exploring how, within a 2D orbital plane, this individual satellite can use its greater endurance to move within the network and influence entire constellation performance. Together, these different levels of understanding provide the necessary framework to effectively design and analyze robust and effective constellations, regardless of mission type. A case study of the OneWeb global internet mission demonstrates that use of currently available electric propulsion technologies can save up to 3000 kg per plane over chemical thrusters and can completely eliminate the need for spare satellites for lifetime failure rates of up to 10%. / by John Kyle Graham. / S.M.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 1974. / Bibliography: p. 565-568. / by Greg L. Zacharias. / M.S.

Massachusetts Institute of Technology. Dept. of Aeronautics and Astronautics. Thesis. 1970. M.S. / MICROFICHE COPY ALSO AVAILABLE IN AERO LIBRARY. / Ten blank pages included in paging. / Bibliography: p. 289. / by Louis Anthony D'Amario [and] Stephen Patrick Synnott. / M.S.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 1991. / Includes bibliographical references (leaves 135-138). / by David Thomas Roberts. / M.S.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 2003. / Includes bibliographical references (p. 75-76). / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / The impact of geometric variability on compressor performance is investigated using a compressor repeating-stage model based on well-known correlations for profile losses, endwall blockage, deviation, and the onset of stall. Previous computations with a quasi-two dimensional cascade analysis code are used to link geometric variability to performance deviations. Performance variability is then introduced probabilistically through random perturbations to tip clearances, profile losses and turning. For the variation input, at design incidence, the mean efficiency is found to decrease by 1%, mostly due to the mean shift in profile losses, and the mean pressure rise is reduced by 2.5%, mostly because of the mean shift in turning. A parametric study for compressor stages of different designs shows a lower degradation of mean performance and a lower performance variability for stages which have higher work coefficient, lower degree of reaction, and higher blade aspect ratio. It was found that the influence of blade profile effects was well represented, but the impact of tip clearance variation was not well captured when compared to three-dimensional computations. It is concluded that to address the effects of tip clearance variability, emphasis should be placed on development of models which both can include the alteration of end-wall displacement thickness within the compressor stage and are appropriate for probabilistic description. / by Antoine Vincent. / S.M.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 2010. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 109-115). / With demand for aviation projected to grow by 5% per year over the next 20 to 25 years, policy makers must not only consider ways to ensure that the air transportation system can accomodate significant growth, but also how their policy decisions will affect the environment. Because environmental issues are becoming increasingly apparent, the sustainability of policy measures will likely constrain responses to this potential increase in aviation demand. Policy makers will need to consider various trade-offs that come with policy decisions, and find ways to balance the demands of the air transport system with the need to reduce the environmental impact of aviation. This thesis assesses the environmental impacts of implementing a policy scenario, which employs both operational and technological improvments to the air transport system. The impacts are presented in both physical and monetary metrics using the Aviation environmental Portfolio Management Tool, to allow for a comparison of trade-offs among different environmental effects. This thesis discusses the limitations of this particular scenario, while also providing an overview of policy-making models, and the observed weaknesses in current policy-making processed involving technical data. In particular, it identifies the mismatch between needs of those involved in the policy-making process, and the information provided by analysts, which can be an obstacle to developing credible and objective support for a policy proposal. It finally provides suggested methods for improving the relationship between different groups involved in developing policy to allow for better informed decision-making, and a more fluid policy-making process. / by Alice Fan. / S.M.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 2002. / Includes bibliographical references (p. 181-185). / Optimal performance and optimal allocation of resources, such as pointing accuracy and onboard fuel utilization, are of primary concern in the design and operation of precision pointing spacecraft. Ironically, internal spacecraft instruments and reaction control actuators often act as sources of narrowband disturbances and impede the optimal performance of these precision systems. The fundamental objective of this work is the development of an active control methodology capable of rejecting disturbances with narrowband nonstationary spectral distributions with particular focus on spacecraft reaction wheel induced disturbances. A closed-form symbolically parameterized optimal feed-forward disturbance rejection methodology for flexible systems has been developed. The methodology combines disturbance modeling for narrowband disturbances together with quasi-stationary optimal control to yield a parameterized feed-forward control architecture. In the case of the reaction wheel disturbance rejection problem, the symbolic optimal control gains are parameterized in terms of wheel spin rate, enabling continuous and analytically exact gain adjustments as a function of the measurable scheduling parameter. The methodology was shown to be compatible with loop-shaping control design methods such as frequency-weighted optimal control. This quasi-stationary disturbance rejection methodology has been generalized and applied to the nonstationary reaction wheel imbalance problem. The nonstationary formulation involves expanding the reaction wheel's angular states in terms of a general series representation. Bessel functions and their properties are employed to define an equivalent finite-dimensional quasi-stationary disturbance signal. / (cont.) The effectiveness of the methodology has been experimentally demonstrated on a highly compliant system with non-collocated sensors and actuators. Experimental results show peak performance yielding nearly a 40 dB improvement over conventional broadband control with improved performance across a wide range of frequencies. / by Sean P. Kenny. / Ph.D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 2004. / Includes bibliographical references (leaves 51-57). / (cont.) days) post-implantation. Histological and immunohistochemical analyses showed severe fibrous and glial scar formation in Groups I and III, less fibrous scarring in Group II and very little scar manifesting in Groups IV and V. A quantitative analysis of myelinated axons in the center of the explants corresponded with the assessment of scar as a physical barrier to competent axon growth. Groups I and III exhibited the least regenerated axons, Groups IV and V the most. The findings also validated the effectiveness of the dorsal barrier in promoting spinal cord regeneration. Overall, the combination of wrap membrane and dorsal barrier (Group V) proved most effective in creating a hospitable environment for regenerative success. / Traumatic injury to the adult mammalian spinal cord results in varying degrees of lost motor and sensory nerve function. Damaged axons of the central nervous system (CNS) exhibit a severely limited regenerative capacity; paralysis induced by severe trauma is generally permanent. Previous studies have attempted to simulate the peripheral nerve environment, where axonal regeneration is spontaneous, through the implantation of peripheral nerve graft tissue, exogenous growth factors or prosthetic devices. Such intervention has demonstrated the ability of central nerve axons to regrow over significant distances and partially restore distal limb function. The current work aims at evaluating the efficacy of two distinct collagen implants towards promoting spinal cord regeneration. The experimental spinal lesion is a 5mm mid-thoracic gap created by transections at T7 and T9 and removal of intermediary cord and peripheral roots. The two implants offered different entubulation schemes; one implant was a thin walled tube composed of Type I bovine collagen, the other a commercially available bilayered membrane composed of Types I and III porcine collagen. Whereas the tube was fitted directly into the spinal lesion, the membrane was wrapped around the cord stumps like a tubular bandage. Five experimental groups defined the current research: Groups I and II received no implant, Groups III and IV were implanted with tubes, and Group V was implanted with the membrane wrap. A secondary aim of the research was to validate the use of a dorsal barrier in further reducing scar infiltration to the wound. This additional collagen membrane was simply draped over the implant (or lesion) of Groups II, IV and V. Mid-thoracic spinal cord sections were explanted from all groups 4 weeks (28 / by Sajjad S. Matin. / S.M.