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At least 163 records · Page 9

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NASA NTRS1985

The investigation of tethered satellite system dynamics

A progress report is presented that deals with three major topics related to Tethered Satellite System Dynamics. The SAO rotational dynamics computer code was updated. The program is now suitable to deal with inclined orbits. The output has been also modified in order to show the satellite Euler angles referred to the rotating orbital frame. The three-dimensional high resolution computer program SLACK3 was developed. The code simulates the three-dimensional dynamics of a tether going slack taking into account the effect produced by boom rotations. Preliminary simulations on the three-dimensional dynamics of a recoiling slack tether are shown in this report. A program to evaluate the electric potential around a severed tether is immersed in a plasma. The potential is computed on a three-dimensional grid axially symmetric with respect to the tether longitudinal axis. The electric potential variations due to the plasma are presently under investigation.

Lorenzini, E.↗

NASA NTRS1985

The role of tethers on space station

The results of research and development that addressed the usefulness of tether applications in space, particularly for space station are described. A well organized and structured effort of considerable magnitude involving NASA, industry and academia have defined the engineering and technological requirements of space tethers and their broad range of economic and operational benefits. The work directed by seven NASA Field Centers is consolidated and structured to cover the general and specific roles of tethers in space as they apply to NASA's planned space station. This is followed by a description of tether systems and operations. A summary of NASA's plans for tether applications in space for years to come is given.

Vontiesenhausen, G.↗

NASA NTRS1985

Electrodynamic tether

The electrodynamic tether consists of a satellite deployed to a distance of 20 km by an electrically conducting tether. The space station hardware consists of a 12 meter deployment boom, satellite cradle, tether reel and motor, and other tether support systems. The electrodynamic tether will be used to perform a variety of wave experiments by exciting a wide spectrum of low frequency waves in the ionospheric plasma. The system can also be used to artificially generate and study field aligned currents and associated plasma effects. Hydromagnetic waves generated by the passage of the system through the space plasma are of particular interest in space plasma research.

Roberts, W. T.↗

NASA NTRS1985

The investigation of tethered satellite system dynamics

Progress in tethered satellite system dynamics research is reported. A retrieval rate control law with no angular feedback to investigate the system's dynamic response was studied. The initial conditions for the computer code which simulates the satellite's rotational dynamics were extended to a generic orbit. The model of the satellite thrusters was modified to simulate a pulsed thrust, by making the SKYHOOK integrator suitable for dealing with delta functions without loosing computational efficiency. Tether breaks were simulated with the high resolution computer code SLACK3. Shuttle's maneuvers were tested. The electric potential around a severed conductive tether with insulator, in the case of a tether breakage at 20 km from the Shuttle, was computed. The electrodynamic hazards due to the breakage of the TSS electrodynamic tether in a plasma are evaluated.

Lorenzini, E.↗

NASA NTRS1984

Shuttle tethered satellite program

Features and applications of the Shuttle tethered satellite system (TSS) are outlined. The TSS would be mounted on a normal cargo pallet and used to deploy or retrieve payloads weighing up to 500 kg from the Orbiter by reeling in or unreeling a tether line extended along the gravity gradient vertical. Attaching a conducting wire to the tether would permit electrodynamic measurements of space plasma. An initial mission under development is to deploy an instruments package 20 km above the Orbiter with a conducting wire tether, thereby obtaining data on low frequency hydromagnetic waves generated by tether passage through the space plasma. An aerodynamics package could also be lowered to 130 km, where measurements can be made with magnetometers for geoscience data, atmospheric composition, and cosmic dust can be collected. Details of engineering assignments for the TSS divided up between NASA and the Italian National Research Council are discussed.

Nolan, M. B.↗

NASA NTRS1986

Electrodynamic tether

Roberts, B.↗

NASA NTRS1986

Tether pointing platform and space elevator mechanisms analysis of the key concepts for SATP and scaled SATP

The key concepts for a scaled and full model Science and Applications Tethered Platform (SATP) are analysized. This includes a tether pointing platform and a space elevator. The mechanism concepts and technological solutions are given. The idea of the tether pointing platform mechanism is to control and stabilize the attitude of a platform by means of a movable tether. The idea of the space elevator mechanism for a scaled SATP is to drag the tether gripping it between two rotating wheels.

Turci, E.↗

NASA NTRS1986

The development of optimal control laws for orbiting tethered platform systems

A mathematical model of the open and closed loop in orbit plane dynamics of a space platform-tethered-subsatellite system is developed. The system consists of a rigid platform from which an (assumed massless) tether is deploying (retrieving) a subsatellite from an attachment point which is, in general, offset from the platform's mass center. A Langrangian formulation yields equations describing platform pitch, subsatellite tetherline swing, and varying tether length motions. These equations are linearized about the nominal station keeping motion. Control can be provided by both modulation of the tether tension level and by a momentum type platform-mounted device; system controllability depends on the presence of both control inputs. Stability criteria are developed in terms of the control law gains, the platform inertia ratio, and tether offset parameter. Control law gains are obtained based on linear quadratic regulator techniques. Typical transient responses of both the state and required control effort are presented.

Bainum, P. M.↗

NASA NTRS1986

Roles for tethers on an evolving space station

A scenario was developed for evolution of space station tether capabilities. As tether imposed constaints on station developments and operations were minimized, maximum benefits were derived from a mutually compatible combination of: (1)electrodynamic tethers for power, thrust, and libration control; (2)momentum transfer operations involving the STS or upper stages; (3)aeromaneuvering devices for space station orbital plan change; and (4)tethered constellations and tether/free-flyer combinations. Some concepts determined for advance tether facilities orbiting the moon: (1)stationkeeping deltaVs to stay in precise equatorial or polar orbits; (2)ratio of facility mass to maximum payload mass (surface-orbit-escape); (3)electric-thruster power requirements and maximum rendezvous frequencies; and (4)overall capabilities and major constraints on such facilities.

Carroll, J. A.↗

NASA NTRS1986

Tethered Satellite System (TSS) core equipment

To date, three Tethered Satellite System (TSS) missions of the Italian provided scientific satellite orbiting in the ionosphere connected to U.S. Space Shuttle is foreseen. The first mission will use an electrically conductive tether of 20 km deployed upward from the orbiter flying at 300 km altitude. This mission will allow investigation of the TSS electrodynamic interaction with the ionosphere due to the high voltage induced across the two terminators of the system during its motion throughout the geomagnetic field. The second mission will use a dielectric tether of 100 km deployed downward from the Orbiter flying at 230 km altitude. Tethered-vehicle access to altitude as low as 120 to 150 km from the Orbiter would permit direct long term observation of phenomena in the lower thermosphere and determination of other dynamical physical processes. The third mission would use the same configuration of the first electrodynamic mission with the complete Core Equipment. Study of power generation by tethered systems would be possible by operating the Core Equipment in the inverted current mode. This mode of operation would allow ion current collection upon the TSS satellite by controlling its potential with respect to the ambient ionospheric plasma. The main requirements of the Core Equipment configuration to date foreseen for the first TSS electrodynamic mission is described. Besides the Core Equipment purposes, its hardware and operational sub-modes of operation are described.

Bonifazi, C.↗

NASA NTRS1986

The use of tethered satellites for the collection of cosmic dust and the sampling of man made orbital debris far from the space station

The use of a tethered subsatellite employed downward into the earth's upper atmosphere to an altitude of about 110 km above the earth would eliminate the orbital contamination problem while at the same time affording a measure of atmospheric braking to reduce the velocities of many particles to where they may be captured intact or nearly so with properly designed collectors. The same technique could also be used to monitor the flux of all types of man-made orbital debris out to a distance of more than a hundred kilometers in any direction from the space station. In this way the build up of any debris belt orbiting earth could be determined. The actual collecting elements used for both purposes could be of several different materials and designs so as to optimize the collection of different types of particles with different densities. Stacks of foils, films, plastics, and foams, as well as simple capture cells would be mounted in clusters around the outside of a tethered satellite and protected by iris covers until the tethered had been fully deployed. If the orientation history of the satellite were known the direction of the incoming material could be infered. A chief advantage in deploying such tethered collectors from the Space Station instead of from the shuttle is the ability to maintain deployment of the tether for days instead of hours resulting in much greater yields of intact particles and impact debris.

Corso, G. J.↗

NASA NTRS1986

Development status of first Tethered Satellite System

An MOU between NASA and Italian research organizations has assigned the responsibilities for the development, payload integration, launch and operation of a Tethered Satellite System (TSS). The TSS will include a deployer mounted on a Spacelab pallet, science equipment on a customized mission support structure and a satellite connected to the Orbiter by a tether. The satellite is 1.6 m in diameter and can weight up to 500 kg. The deployer will be insulated from the Orbiter because of the conductive nature of the tether. The first mission is to demonstrate the TSS technology, perform electrodynamic investigations of the interaction of the tether with ambient space plasma, and gather data on atmospheric interactions in the lower thermosphere, geomagnetic phenomena and gravimetric characteristics. The first TSS will be deployed 20 km outward from the Orbiter and the earth on a strand of tin-coated copper wire wrapped in Teflon insulation, braided Kevlar and a Nomek jacket. The first strand is designed to accommodate 53 N tension. The satellite on the end of the tether will be a multimission platform for facile changeout of the scientific package from mission-to-mission.

Sisson, J. M.↗

NASA NTRS1986

Tether propulsion

The principle and applications of tethering are examined. Tethering works by momentum transfer; the center of mass of a system consisting of (for example) a Space Shuttle and a tethered payload such as the Advanced X-ray Astrophysical Facility continues to follow the original orbit. Given a slight outward velocity, the payload begins to lag behind because it has the same linear velocity as the Shuttle but is at a greater distance from the earth. Any displacement from the local vertical causes a restoring force at each end tending to restore the system to a vertical orientation. When vertically above the Shuttle, the payload has the same angular velocity but a greater linear velocity; thus momentum is transferred from the Shuttle to the payload. It is computed that a tether 32 nautical miles long could deploy AXAF into a 320-nautical mile orbit from a lower, elliptical Shuttle orbit, thus saving 5000 pounds of Shuttle propellant. Various types of tether are considered: Kevlar and steel, uniform and tapered. Numerous cases appear to be feasible for boost and deboost as well as momentum transfer, using such reaction masses as the Space Station, a lunar orbiter, the Martian moons Phobos and Deimos, various asteroids, and moons of the major planets.

Bekey, I.↗

NASA NTRS1985

Guidebook for analysis of tether applications

This guidebook is intended as a tool to facilitate initial analyses of proposed tether applications in space. Topics disscussed include: orbit and orbit transfer equations; orbital perturbations; aerodynamic drag; thermal balance; micrometeoroids; gravity gradient effects; tether control strategies; momentum transfer; orbit transfer by tethered release/rendezvous; impact hazards for tethers; electrodynamic tether principles; and electrodynamic libration control issues.

Carroll, J. A.↗

NASA NTRS1987

Electrodynamic tether

Electrodynamic tethers hold promise for a variety of space applications. Electrodynamic tethers depend upon the interactions between a moving insulated conductor and the Earth's magnetic field. An electric field is generated along the tether as in a conductor moving in the magnetic field of a generator. If the circuit is closed to the ambient space plasma via a plasma gun or other equivalent device, a current is enabled to flow in the tether, and electric power is generated at the expense of orbital mechanical energy. The net effect is a decrease in the altitude of the orbiting tethered system. The situation can be reversed by driving current against the electric field via an external power supply such as a photovoltaic array.

Patterson, Michael↗

NASA NTRS1987

The radiation impedance of an electrodynamic tether with end connectors

Electrodynamic tethers are wires deployed across the earth's geomagnetic field through which a current is flowing. The radiation impedance of a tether with end connectors carrying an ac current is computed from classical antenna theory. This simulates the use of a tether on a space structure. It is shown that the current flow pattern at the tether connector is critical to determining the overall radiation impedance. If the tether makes direct electrical contact with the ionosphere then radiation impedances of the order of several thousand Ohms can be expected. If the only electrical contact is through the end connectors then the impedance is only a few Ohms for a dc current rising to several tens of Ohms for an ac current with frequencies in the whistler range.

Hastings, Daniel E.↗

NASA NTRS1987

Analytical investigation of the dynamics of tethered constellations in Earth orbit (phase 2)

Simulation of two short distance crawling maneuvers of the elevator both with and without environmental perturbations acting upon the system is discussed. These simulation runs were performed in order to provide results useful for the interpretation of the data from the tests, on the ground, of a scaled down engineering model of the elevator. In these simulation runs the elevator crawls along the tether in accordance to the developed mirror image motion control law (MIMCL). Results from the simulation of the 4 km long maneuver run were compared to those obtained by adopting the modified hyperbolic tangent control law (MHTCL). A preprocessor was developed for setting up the initial conditions of a tethered system with L platforms, M longitudinal dampers, and N lumped masses (platforms plus tether beads). A short test run of the 4-platform system with 3 longitudinal dampers and 10 lumper mass without any perturbation acting upon the system is illustrated. In support of the Tether Applications Working Group, SAO is preparing a catalog of tether simulations, has prepared a set of simulator test cases, obtained results from SKYHOOK, and solicited results from simulators at other institutions, and prepared a paper on a specific analytic solution.

Lorenzini, Enrico C.↗

NASA NTRS1989

Satellite relocation by tether deployment

Several new uses of satellite tethers are discussed, including: (1) using tether extension to reposition a satellite in orbit without fuel expenditure by extending a mass on the end of a tether; (2) using a tether for energy storage to power the satellite during eclipse; and (3) using a tether for eccentricity pumping to correct perturbations in the orbit and as a means of adding energy to the orbit for boosting and orbital transfer.

Landis, Geoffrey A.↗