Statistics – Applications
Scientific paper
Jan 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002iaf..confe.953w&link_type=abstract
IAF abstracts, 34th COSPAR Scientific Assembly, The Second World Space Congress, held 10-19 October, 2002 in Houston, TX, USA.,
Statistics
Applications
Scientific paper
Several proposed applications of tethers in space, particularly in the area of transportation, have generated a lot of interest in the space community. The most promising applications involve momentum exchange techniques. In particular, spinning tether systems in elliptical orbits can be used to transfer sufficient momentum to a payload to allow it to escape earth orbit and travel to destinations such as the moon and even Mars. It is the ability to provide long moment arms with relatively little material that makes space tether systems such as these so useful. This paper presents a new application for the use of tethers in interplanetary spaceflight. The concept is a natural extension of the payload-raising concept in elliptical orbits. The concept may be described as follows: a payload is deployed along a spinning tether in a hyperbolic orbit to provide a sufficient delta-v such that it is captured in an elliptical orbit at the destination planet. The tether provides the moment arm for the payload, but also provides a means for the system to naturally undergo forced librations as a result of the non-uniform orbital angular velocity of the centre of mass. As a result of the conservation of angular momentum, the mother spacecraft receives a net increase in velocity after the payload is released. This increase in velocity can be used to propel the mother spacecraft to its new destination planet where the maneuver can be repeated. The feasibility of the concept is established in the uncontrolled case using numerical simulations. The required tether mass to sustain the high tensile loads for a particular system configuration during the maneuver is optimised using numerical and iterative techniques. The optimum mass results are validated through mass and force contour plots. The optimum tether length, diameter and mass are determined for each of the major outer planets in the solar system. The results indicate that significant mass savings can be obtained compared to traditional chemical propellant burns. The main conclusion drawn from the uncontrolled case is that it is necessary to have the tether system spinning quite rapidly as it approaches periapsis of the target planet. However, it may prove difficult in practice to achieve the desired spin rates without providing active control to the system. A tension control scheme is introduced to aid in achieving planetary capture. The focus is on achieving capture with a system which is initially radially aligned and non-spinning. The control problems considered in this paper are unconventional in that the final states and final time cannot be clearly defined. To deal with this difficult problem it was necessary to employ innovative nonlinear programming techniques. These techniques provide a more robust solution method than either the conjugate-gradient or shooting techniques, and demonstrate that controlled planetary capture is possible.
Blanksby Chris
Trivailo Pavel
Williams Peter
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