Multibody Dynamics and Control of Tethered Spacecraft Systems

This doctoral research conducts high-fidelity multiphysics modeling for tethered spacecraft systems, such as electrodynamic tether systems, electric solar wind sail systems, and tether transportation systems with climbers. Two models are developed based on nodal position finite element method. The first model deals with the tethered spacecraft system with fixed length tether, while the second model deals with the tethered spacecraft system with variable tether length using an arbitrary Lagrangian-Eulerian description. First, the nodal position finite element method is applied to model the orbital motion of tethered spacecraft systems with fixed tether length over a prolonged period. A Symplectic integration scheme is employed to attenuate the accumulation of error in the numerical analysis due to the long-term integration for tethered spacecraft systems, such as the space debris deorbit by electrodynamic tethers. A high fidelity multiphysics model is developed for electrodynamic tether systems by considering elastic, thermal, and electrical coupling effects of the tether. Most importantly, the calculation of electron collection by the electrodynamic tether is coupled with the tether libration and flexible deformation, where the orbital motion limited theory for electron collection is discretized simultaneously by the same finite element mesh used for the elastodynamic analysis of tether. The model is then used to investigate dynamics and libration stability of bare electrodynamic tethers in deorbiting end-of-mission spacecraft. Second, the model of tethered spacecraft system with fixed tether length is extended for the modeling of electric solar wind sail systems. The coupling effect of orbital and self-spinning motions of electric solar wind sail systems is investigated together with the interaction between the axial/transverse elastic motion of tether and Coulomb force. A modified throttling control algorithm is implemented in the finite element scheme to control the attitude motion of electric solar wind sail systems through the electric voltage modulation of main tethers. Third, the model of tethered spacecraft with variable tether length is applied to handle the tether length variation in tether transportation systems. The tether length variation results from the climber moving along tether and deployment and retrieval of tether at end spacecraft. The dynamic behavior of tether transportation systems with single or multiple climbers in characterized and the effectiveness of libration suppression scheme is tested by the high-fidelity model

Codon optimization, constitutive expression and antimicrobial characterization of hen egg white lysozyme (HEWL) in Pichia pastoris

Fusarium oxysporum (F. oxysporum) and Verticillium dahlia (V. dahlia) causes severe cotton disease in China and other cotton-producing countries. Hen egg white lysozyme (HEWL) has antimicrobial properties. In this study, a codon-optimized HEWL gene was synthesized and cloned into the yeast expression vector, pPIC9K, under the control of the Pichia pastoris (P. pastoris) glyceraldehyde-3-phosphate dehydrogenase promoter (pGAP). Results showed that codon-optimized HEWL (oHEWL) was constitutively expressed. Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) indicated that the molecular weight of recombinant HEWL (roHEWL) was 14 kDa which corresponds to the standard HEWL. The expression of the roHEWL reached to 54 mg/L. Activity of the roHEWL was 1680 U/mL. The optimum pH for roHEWL was from 6.0 to 7.0, and the optimum temperature was 55°C. In vitro antimicrobial activity assay revealed that roHEWL can lyse cell walls of the gram positive bacteria, Micrococcus lysodeikticus (M. lysodeikticus). In vivo studies showed that it inhibits plant fungi, F. oxysporum and V. dahlia. roHEWL anti-fungal properties might be useful for future genetically engineered cotton plant resistance against pathogenic fungal disease.Keywords: Hen egg white lysozyme (HEWL), antimicrobial activity, glyceraldehyde-3-phosphate dehydrogenase (GAP) promoter, codon optimization, constitutive expressio

Coupled Orbital-Attitude Dynamics Of Flexible Electric Solar Wind Sail

This paper studies the dynamic characteristics of an electric solar wind propulsion system. To study the coupled interaction of the orbital and self-spinning motions of an electric solar wind propulsion system, a high-fidelity model is built up by using the nodal position finite element method, where the axial elastic and transverse dynamic motions of tether with the electric effects are considered. The coupling effects between the orbital and self-spinning motions are identified and explained, its results show that they have a significant impact on the system dynamics

Flight Dynamics and Control Strategy of Electric Solar Wind Sails

This paper studies the flight dynamics and control strategy for electric solar wind sails based on the nodal position finite element method, where the coupling effects between tether dynamics and the electrical field are considered. A modified throttling control strategy is proposed to control the attitude of electric sails by modulating individual tether voltage synchronously with the spinning motion of the sails. The effects of four critical physical parameters (tether numbers, tether length, sail spin rate, and mass of remote units) are investigated. The results show that the effect of the relative velocity of the solar wind has a significant effect on the spin rate of the sail in attitude maneuvering, which in turn affects the attitude dynamics and orbit motion of the sail. Numerical results show that the proposed control strategy work successfully stabilizes the spin rate of sail when the new type sail is adopted

Experimental investigation on expansion characteristics and strength of carbonating reactive magnesia solidified clay

The carbonated reactive magnesia (CRM) method is superior in energy conservation, carbon capture, and rapid solidification in soil improvement. It has been revealed that CRM method could cause apparent volumetric expansion during carbonation, which may cause apparent compaction impact on the surrounding soils when used for soft foundation improvement. However, this expansion hasn't been discussed systematically. In this study, the evolutions of expansion stress (σEx) and volumetric expansion (ΔV) during carbonation process were successfully monitored. Two factors, including reactive MgO content (Cm) and net confining pressure (Pnc), were investigated. The internal relations between σEx, ΔV, and unconfined compressive strength (UCS) together with the influences of Cm and Pnc on them were discussed. Besides, the intrinsic mechanisms were discussed based on density variations, pore structures, and microstructures. According to the findings, σEx and ΔV invariably exhibited a three-stage behavior consisting of stability-rapid increase-stability. By increasing Cm or Pnc, the σEx, UCS, and density were all significantly increased, while the volume increment was obviously reduced. For the completely confined specimen, the σEx and UCS were found to approach 3 MPa and 9 MPa, respectively. The increase in Cm promoted the crystallization of hydrated magnesium carbonates (HMCs), leading to lower porosity in solidified soils. Increasing Pnc also improved the crystallization of HMCs and modified pore structures, causing further increases in σEx and UCS

Spontaneous breaking and re-making of the RS-Au-SR staple in self-assembled ethylthiolate/Au(111) interface

The stability of the self-assembled RS–Au–SR (R = CH₂CH₃)/Au­(111) interface at room temperature has been investigated using scanning tunneling microscopy (STM) in conjunction with density functional theory (DFT) and MD calculations. The RS–Au–SR staple, also known as Au-adatom-dithiolate, assembles into staple rows along the [112̅] direction. STM imaging reveals that while the staple rows are able to maintain a static global structure, individual staples within the row are subjected to constant breaking and remaking of the Au–SR bond. The C₂S–Au–SC₂/Au­(111) interface is under a dynamic equilibrium and it is far from rigid. DFT/MD calculations show that a transient RS–Au–Au–SR complex can be formed when a free Au atom is added to the RS–Au–SR staple. The relatively high reactivity of the RS–Au–SR staple at room temperature could explain the reactivity of thiolate-protected Au nanoclusters, such as their ability to participate in ligand exchange and intercluster reactions