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71.
Variable-geometry truss structures are likely to be used extensively in the future for in-orbit space construction. This paper considers dynamics formulation and vibration control of such structures. The truss system is modelled as a collection of sub-structures consisting of truss booms, prismatic actuator elements, and in some cases a manipulator at the end. Each truss boom is treated as a separate ‘link’ and its flexibility is modelled using the finite element method. Equations of motion for individual sub-structures are obtained which are then assembled. The non-working constraint forces are eliminated to obtain the equations governing the constrained dynamics of the entire system. For vibration control, the singular perturbation method is employed to construct two reduced-order models, for quasi-static motion and for modal co-ordinates, respectively. Computed torque with PD control is applied to maintain the quasi-static motion, while an optimal LQR method is used for vibration control. Typical simulation results are presented for the planar case. 相似文献
72.
Heilbronn L Frankel K Holabird K Zeitlin C McMahan MA Rathbun W Cronqvist M Gong W Madey R Htun M Elaasar M Anderson BD Baldwin AR Jiang J Keane D Scott A Shao Y Watson JW Zhang WM Galonsky A Ronningen R Zecher P Kruse J Wang J Cary R 《Acta Astronautica》1998,42(1-8):363-373
In order to help assess the risk to astronauts due to the long-term exposure to the natural radiation environment in space, an understanding of how the primary radiation field is changed when passing through shielding and tissue materials must be obtained. One important aspect of the change in the primary radiation field after passing through shielding materials is the production of secondary particles from the breakup of the primary. Neutrons are an important component of the secondary particle field due to their relatively high biological weighting factors, and due to their relative abundance, especially behind thick shielding scenarios. Because of the complexity of the problem, the estimation of the risk from exposure to the secondary neutron field must be handled using calculational techniques. However, those calculations will need an extensive set of neutron cross section and thicktarget neutron yield data in order to make an accurate assessment of the risk. In this paper we briefly survey the existing neutron-production data sets that are applicable to the space radiation transport problem, and we point out how neutron production from protons is different than neutron production from heavy ions. We also make comparisons of one the heavy-ion data sets with Boltzmann-Uehling-Uhlenbeck (BUU) calculations. 相似文献
73.
The primary objective of the Laser Interferometer Space Antenna (LISA) mission is to detect and observe gravitational waves from massive black holes and galactic binaries in the frequency range 10−4 to 10−1 Hz. This low-frequency range is inaccessible to ground-based interferometers because of the unshieldable background of local gravitational noise and because ground-based interferometers are limited in length to a few km. LISA is an ESA cornerstone mission and recently had a system study (Ref. 1) carried out by a consortium led by Astrium, which confirmed the basic configuration for the payload with only minor changes, and provided detailed concepts for the spacecraft and mission design. The study confirmed the need for a drag-free technology demonstration mission to develop the inertial sensors for LISA, before embarking on the build of the flight sensors. With a technology demonstration flight in 2005, it would be possible to carry out LISA as a joint ESA-NASA mission with a launch by 2010 subject to the funding programmatics. The baseline for LISA is three disc-like spacecraft each of which consist of a science module which carries the laser interferometer payload (two in each science module) and a propulsion module containing an ion drive and the hydrazine thrusters of the AOCS. The propulsion module is used for the transfer from earth escape trajectory provided by the Delta II launch to the operational orbit. Once there the propulsion module is jettisoned to reduce disturbances on the payload. Detailed analysis of thermal and gravitational disturbances, a model of the drag-free control and of the interferometer operation confirm that the strain sensitivity of the interferometer will be achieved. 相似文献
74.
75.
A. V. Zyuzgin A. I. Ivanov V. I. Polezhaev G. F. Putin E. B. Soboleva 《Cosmic Research》2001,39(2):175-186
The results of processing and interpreting the data of joint Russian–French experiments for studying the heat and mass transfer in near-critical fluids are presented. The experiments were carried out with the ALICE-1 instrument during an orbital flight of the Mirstation from September 30 to October 2, 1995 [1]. For such fluids with a point-like source of heat, when they are placed in the field of uncontrolled inertial accelerations of the spacecraft, the influence of thermovibrational and thermogravitational mechanisms of convection on the propagation of the region of optical irregularity is investigated. It is shown that, near the thermodynamic critical point, local heating of the medium leads to generation of either intense thermogravitational convection or averaged vibroconvective flow, depending on the frequency of variations of the microaccelerations. The structure and characteristics of discovered motions are studied. The results of numerical simulations are presented that confirm the conclusion about a possibility of generation of an averaged convective flow of a vibrational type by the high-frequency component of microaccelerations. 相似文献
76.
A method for approximate analytical optimization of low-thrust transfers, described in [1] and based on linearization of the equations of motion, is considered. A more precise method of reducing the errors due to linearization is suggested. The method of optimization is generalized for the case of a mission to several celestial bodies. 相似文献
77.
A major goal of NASA's In-Space Propulsion Program is to shorten trip times for scientific planetary missions. To meet this challenge arrival speeds will increase, requiring significant braking for orbit insertion, and thus increased deceleration propellant mass that may exceed launch lift capabilities. A technology called aerocapture has been developed to expand the mission potential of exploratory probes destined for planets with suitable atmospheres. Aerocapture inserts a probe into planetary orbit via a single pass through the atmosphere using the probe's aeroshell drag to reduce velocity. The benefit of an aerocapture maneuver is a large reduction in propellant mass that may result in smaller, less costly missions and reduced mission cruise times. The methodology used to design rigid aerocapture aeroshells will be presented with an emphasis on a new systems tool under development. Current methods for fast, efficient evaluations of structural systems for exploratory vehicles to planets and moons within our solar system have been under development within NASA having limited success. Many systems tools that have been attempted applied structural mass estimation techniques based on historical data and curve fitting techniques that are difficult and cumbersome to apply to new vehicle concepts and missions. The resulting vehicle aeroshell mass may be incorrectly estimated or have high margins included to account for uncertainty. This new tool will reduce the guesswork previously found in conceptual aeroshell mass estimations. 相似文献
78.
N. I. Izhovkina I. S. Prutensky S. A. Pulinets A. Kiraga Z. Klos H. Rothkael 《Cosmic Research》2005,43(3):192-198
Measurements of the wave emission of the topside ionosphere made onboard the APEX satellite using the electric component of the wave field in the 0.1–10 MHz frequency band are presented. At middle latitudes a wave intensity decrease was observed in the broad-band spectrum of the electrostatic noise at the electron cyclotron frequency. It is shown that a break in the spectrum of electrostatic modes at the electron cyclotron frequency (the absence of the plasma eigen-frequencies) may be a cause of the observed effect. The increase of the intensity at the electron cyclotron frequency in the ionospheric trough and at latitudes above the trough region as compared to middle latitudes may be explained by the capture by plasma irregularities of the electromagnetic emission of the auroral electron fluxes.__________Translated from Kosmicheskie Issledovaniya, Vol. 43, No. 3, 2005, pp. 201–208.Original Russian Text Copyright © 2005 by Izhovkina, Prutensky, Pulinets, Kiraga, Klos, Rothkael. 相似文献
79.
Remote sensing of agricultural land permits crop classification and mensuration which can lead to improved forecasts of production. This technique is particularly important for nations which do not already have an accurate agricultural reporting system. Better forecasts have important economic effects. International grain traders can make better decisions about when to store, buy and sell. Farmers can make better planting decisions by taking advantage of production estimates for areas out of phase with their own agricultural calendar. World economic benefits will accrue to both buyers and sellers because of increased food supply and price stabilization.
This paper reviews the econometric models used to establish this scenario and estimates the dollar value of benefits for world wheat as 200 million dollars annually for the United States and 300–400 million dollars anually for the rest of the world. 相似文献
80.
Two bed rest analog studies of space flight were performed; one 14 d and the other 28 d in duration. Exercise response was studied in detail during the 28 d study and following both the 14 d and 28 d studies. This paper relates the results of these studies to physiologic changes noted during and following space flight. The most consistent change noted after both bed rest and space flight is an elevated heart rate during exercise. A second consistent finding is a postflight or postbed rest reduction in cardiac stroke volume. Cardiac output changes were variable. The inability to simulate inflight activity levels and personal exercise makes a direct comparison between bed rest and the results from specific space flights difficult. 相似文献