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1	Design And Analysis Of Flexible Beam Platform As Vibration Isolator For Space Applications Kamesh, D 02 1900 (has links) (PDF) Spacecrafts are generally equipped with high precision optical and other sensor payloads. The structures of most of the spacecrafts are light-weight, flexible and have low damping. Vibrations are often induced in the spacecraft body due to the presence of many disturbance sources such as momentum/reaction wheels, control thrusters used for attitude control and cryocoolers etc. Low damping leads to long decay time for vibrations hence during this period the spacecraft sensors cannot be used effectively. One possible solution is to isolate the precision sensor from the rest of the satellite and this strategy has been used for spaceborne telescopes and interferometers that have extremely precise positional and vibratory tolerances imposed on them in order to achieve scientific goals. Another strategy is to isolate the vibration source itself from the spacecraft body. This thesis deals with modelling, analysis and experimentation of a novel low frequency flexible space platform designed to serve as a mount for the disturbance source in order to insulate the source generated vibrations reaching critical areas of the structure. The novel space platform consisting of folded continuous beams, is light-weight and is capable of isolating vibration generated by sources such as reaction/momentum wheels. Finite element analysis of the platform is carried out for static and dynamic load cases. Simulation studies are carried out on flexible beam platform in order to firm up the design for passive vibration isolation. Modal analyses is done to simulate the response of each mode. Active control has been studied by embedding the platform’s beam elements with piezo actuators and sensors. The simulation results show that the space platform can effectively attenuate vibration and further improvement in vibration attenuation is possible with active control. Based on the analysis, a prototype low frequency platform has been designed and fabricated. An experimental validation has been done to test the usefulness of the low frequency platform to act as a mount for reaction wheels and to mitigate the vibration disturbances/effects transmitted from the reaction wheel assembly to structure. Measurements and tests have been conducted at varying wheel speeds to quantify and characterize the amount of isolation to the reaction wheel generated vibrations. The time and frequency domain analysis of test data clearly show that level of isolation is significant and an average of 13 dB of isolation is seen. The level of isolation is different for different isolators and it depends upon the isolator design and wheel speed. Forces and moments measured at the base for wheel with isolator and wheel without isolator clearly demonstrate and confirm a reduction in the disturbance levels of atleast one order. These isolators are further tested successfully for launch dynamic loads in order to confirm the design adequacy to sustain such loads. Results indicate that the flexible mounts of the type discussed in this thesis can be used for effective passive vibration isolation in spacecrafts with reaction/momentum wheels. Flexible Beam Platforms Vibration Isolator Spacecrafts Flexible Folded Beam Space Platforms Four Beam Platforms Reaction Wheel Disturbances Space Frame Platforms Passive Vibration Isolators Flexible Space Platform Vibration Isolation Vibration Isolators Astromechanics
2	Characterization of Quasi-Periodic Orbits for Applications in the Sun-Earth and Earth-Moon Systems Brian P. McCarthy (5930747) 17 January 2019 (has links) <div>As destinations of missions in both human and robotic spaceﬂight become more exotic, a foundational understanding the dynamical structures in the gravitational environments enable more informed mission trajectory designs. One particular type of structure, quasi-periodic orbits, are examined in this investigation. Speciﬁcally, eﬃcient computation of quasi-periodic orbits and leveraging quasi-periodic orbits as trajectory design alternatives in the Earth-Moon and Sun-Earth systems. First, periodic orbits and their associated center manifold are discussed to provide the background for the existence of quasi-periodic motion on n-dimensional invariant tori, where n corresponds to the number of fundamental frequencies that deﬁne the motion. Single and multiple shooting diﬀerential corrections strategies are summarized to compute families 2-dimensional tori in the Circular Restricted Three-Body Problem (CR3BP) using a stroboscopic mapping technique, originally developed by Howell and Olikara. Three types of quasi-periodic orbit families are presented: constant energy, constant frequency ratio, and constant mapping time families. Stability of quasi-periodic orbits is summarized and characterized with a single stability index quantity. For unstable quasi-periodic orbits, hyperbolic manifolds are computed from the diﬀerential of a discretized invariant curve. The use of quasi-periodic orbits is also demonstrated for destination orbits and transfer trajectories. Quasi-DROs are examined in the CR3BP and the Sun-Earth-Moon ephemeris model to achieve constant line of sight with Earth and avoid lunar eclipsing by exploiting orbital resonance. Arcs from quasi-periodic orbits are leveraged to provide an initial guess for transfer trajectory design between a planar Lyapunov orbit and an unstable halo orbit in the Earth-Moon system. Additionally, quasi-periodic trajectory arcs are exploited for transfer trajectory initial guesses between nearly stable periodic orbits in the Earth-Moon system. Lastly, stable hyperbolic manifolds from a Sun-Earth L<sub>1</sub> quasi-vertical orbit are employed to design maneuver-free transfer from the LEO vicinity to a quasi-vertical orbit.</div> Aerospace Engineering aerospace engineering trajectory trajectory design astrodynamics three body three body mechanics circular restricted three body problem CR3BP quasi-periodic orbits periodic orbits orbit mechanics celestial mechanics dynamical systems dynamical systems theory dynamics applications orbits astronautics differential corrections invariance invariant circle invariant torus torus invariant tori stroboscopic mapping manifolds Sun-Earth system Earth-Moon system stability multiple shooting stroboscopic map eclipse avoidance vertical orbit halo orbit distant retrograde orbit quasi-halo quasi-vertical lissajous quasi-DRO DRO NRHO quasi-NRHO synodic resonance trajectory arcs mission design astromechanics spaceflight mechanics dynamical structures

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Design And Analysis Of Flexible Beam Platform As Vibration Isolator For Space Applications

Characterization of Quasi-Periodic Orbits for Applications in the Sun-Earth and Earth-Moon Systems