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  • 1
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    A three-dimensional dynamic analysis and libration study of a tethered satellites system (1986)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: By a three-dimensional Lagrangian formulation, an exact set of ten coupled nonlinear second-order differential equations has been derived for a system with an extensible tether connecting two end satellites of distributed mass. The effects of tether mass, small orbital eccentricity, central body oblateness, aerodynamic drag force, and solar radiation pressure are also included in the formulation. By linearizing the exact differential equations, the in-plane (orbital plane) differential equations are found to be decoupled from the out-of-plane ones. The characteristic equation of the in-plane differential equations is derived and some associated stability constraints are shown.
    Keywords: ASTRODYNAMICS
    Type: AAS PAPER 85-340
    Format: text
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    NASA TECHNICAL REPORTS
  • 2
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    An exact solution for orbit view-periods from a station on a tri-axial ellipsoidal planet (1986)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: This paper presents the concise exact solution for predicting view-periods to be observed from a masked or unmasked tracking station on a tri-axial ellipsoidal surface. The new exact approach expresses the azimuth and elevation angles of a spacecraft in terms of the station-centered geodetic topocentric coordinates in an elegantly concise manner. A simple and efficient algorithm is developed to avoid costly repetitive computations in searching for neighborhoods near the rise and set times of each satellite orbit for each station. Only one search for each orbit is necessary for each station. Sample results indicate that the use of an assumed spherical earth instead of an 'actual' tri-axial ellipsoidal earth could introduce an error up to a few minutes in a view-period prediction for circular orbits of low or medium altitude. For an elliptical orbit of high eccentricity and long period, the maximum error could be even larger. The analytic treatment and the efficient algorithm are designed for geocentric orbits, but they should be applicable to interplanetary trajectories by an appropriate coordinates transformation at each view-period calculation. This analysis can be accomplished only by not using the classical orbital elements.
    Keywords: ASTRODYNAMICS
    Type: AIAA PAPER 86-2068
    Format: text
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  • 3
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    Co-apsidal autonomous terminal rendezvous in Mars orbit (1979)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: The autonomous (or unmanned) Mars Orbit Rendezvous may be divided into four phases: approach, terminal, station-keeping, and docking. The paper outlines a feasible concept for the terminal rendezvous phase. A co-apsidal terminal rendezvous technique is described which utilizes the advantages of aligning the lines of apsides of two coplanar elliptic orbits of slightly different periods but of the same apoapsis distance. An initial maneuver nulling the inclination between the orbits is followed by another maneuver to align the two lines of apsides. A proper phasing maneuver is then executed at the coapsidal point, and the final rendezvous maneuver is performed at the same coapsidal point to complete the terminal rendezvous transfer. Cases are formulated in order of complexity with a view to illustrate the basic ideas of this technique. Pertinent calculations are also presented.
    Keywords: ASTRODYNAMICS
    Type: AAS PAPER 79-167 , American Astronautical Society and American Institute of Aeronautics and Astronautics, Astrodynamics Specialist Conference; Jun 25, 1979 - Jun 27, 1979; Provincetown, MA
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    NASA TECHNICAL REPORTS
  • 4
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    A preliminary estimate of the EUVE cumulative distribution of exposure time on the unit sphere (1984)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: A preliminary study of an all-sky coverage of the EUVE mission is given. Algorithms are provided to compute the exposure of the celestial sphere under the spinning telescopes, taking into account that during part of the exposure time the telescopes are blocked by the earth. The algorithms are used to give an estimate of exposure time at different ecliptic latitudes as a function of the angle of field of view of the telescope. Sample coverage patterns are also given for a 6-month mission.
    Keywords: ASTRODYNAMICS
    Type: AIAA PAPER 84-2017
    Format: text
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  • 5
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    Aerobraking techniques for planetary missions (1981)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: Aerobraking techniques are applied to trajectory design for the VOIR (Venus Orbiting Imaging Radar) mission to enhance overall mission performance. The approach can be used for other planetary missions such as a Titan orbiter or a Mars sample return to earth. The orientation of the elliptical insertion orbit must be chosen in such a way that the combined effects of solar gravity, central-body harmonics, and drag on periapsis altitude are minimized to avoid frequent maneuvers. The selection of the base and width of a corridor for periapsis altitude depends upon the temperature and integrated heat of the aerobrake shield, the atmospheric density profile, the duration of aerobraking, the number of maneuvers, the time available between maneuvers, and uncertainties in the central-body gravity field and atmospheric density. Flight path profiles for an aerodynamically stable spacecraft and an inertially fixed spacecraft passing through the free molecular flow regime, as well as the transitional flow regime, are illustrated
    Keywords: ASTRODYNAMICS
    Type: AAS PAPER 81-132 , Astrodynamics Specialist Conference; Aug 03, 1981 - Aug 05, 1981; Lake Tahoe, NV; US
    Format: text
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