908 research outputs found

    Comparison of the solar sail with electric propulsion systems

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    The propulsive efficiencies of the solar sail and of electric propulsion systems are compared on the basis of specific impulse. It is shown that the solar sail is more efficient at one a.u. for mission durations greater than about two months, that the advantage is increased for missions toward the inner planets, and that the same conclusions are reached when the comparison is based on maximizing the momentum transferred to the payload. Other factors that will influence the choice of a propulsion system for a specific mission are mentioned

    NASTRAN cyclic symmetry capability

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    A development for NASTRAN which facilitates the analysis of structures made up of identical segments symmetrically arranged with respect to an axis is described. The key operation in the method is the transformation of the degrees of freedom for the structure into uncoupled symmetrical components, thereby greatly reducing the number of equations which are solved simultaneously. A further reduction occurs if each segment has a plane of reflective symmetry. The only required assumption is that the problem be linear. The capability, as developed, will be available in level 16 of NASTRAN for static stress analysis, steady state heat transfer analysis, and vibration analysis. The paper includes a discussion of the theory, a brief description of the data supplied by the user, and the results obtained for two example problems. The first problem concerns the acoustic modes of a long prismatic cavity imbedded in the propellant grain of a solid rocket motor. The second problem involves the deformations of a large space antenna. The latter example is the first application of the NASTRAN Cyclic Symmetry capability to a really large problem

    Latin Laguage in History

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    Considerations in the design of large space structures

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    Several analytical studies of topics relevant to the design of large space structures are presented. Topics covered are: the types and quantitative evaluation of the disturbances to which large Earth-oriented microwave reflectors would be subjected and the resulting attitude errors of such spacecraft; the influence of errors in the structural geometry of the performance of radiofrequency antennas; the effect of creasing on the flatness of tensioned reflector membrane surface; and an analysis of the statistics of damage to truss-type structures due to meteoroids

    Wall structure changes in low-loss magnetic bubble materials

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    Transitions between underdamped and overdamped radial motion in magnetic bubble domains are investigated in a low-loss rare-earth garnet material. Three distinct types of domain wall structures, which are present during underdamped motion, have been identified. Bubble walls were subjected to a bias field pulse (H) and tested for underdamped motion sometime (τ) later. The first type of structure follows the form H = H' exp (τ\τ_0), with 170 nsec < τ_0 < 270 nsec for the first transition, and is not statically stable. Transitions associated with the second type are characterized by a constant critical angle Ψ_c between the magnetization in the middle of the wall, and the plane of the wall. For the first transition, Ψ_c = 230°, and for the second, Ψ_c = 370°. These structures are statically stable. The third type of structure is not statically stable, and H is independent of τ. The first and second wall structure types are associated with multiple transitions while the third only exhibits a single transition

    Modelling strain distributions in ion-implanted magnetic bubble materials

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    The detailed properties of strain distributions in Ne+, B+, and He+ implanted magnetic bubble garnet materials are accounted for by calculating the nuclear energy loss as a function of depth. The calculation is based on stopped ion distributions for ZnS, with suitable corrections made for differences in material density. The constant of proportionality K between strain and nuclear energy loss, and the density ratio l are determined for each ion by comparing calculated strain distributions with experimental results obtained previously using an x-ray diffraction technique. It is found that K is roughly the same for all three ions, 0.016±0.003 (eV/Å3)^−1, and that the average value l = 0.79±0.08 is consistent with the actual density ratio l = 0.72. Good agreement is found in additional examples of both single and multiple implants (±10% relative error). Finally, a procedure for selecting the incident energies and dosages required to produce a uniformly strained layer for bubble device applications is described, and then demonstrated by achieving a 0.4-um-thick layer with (1.07±0.09)% strain, using a double B+ implant
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