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Static highly elliptical orbits using hybrid low-thrust propulsion

Anderson, Pamela and Macdonald, Malcolm (2012) Static highly elliptical orbits using hybrid low-thrust propulsion. In: 22nd AAS/AIAA Spaceflight Mechanics Meeting, 2012-01-29 - 2012-02-02.

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The use of extended static-highly elliptical orbits, termed Taranis orbits, is considered for continuous observation of high latitude regions. Low-thrust propulsion is used to alter the critical inclination of Molniya-like orbits to any inclination required to optimally fulfill the mission objectives. This paper investigates a constellation of spacecraft at 90deg inclination for observation of latitudes beyond 55deg and 50deg, considering: spatial resolution, radiation environment, number of spacecraft and End of Life debris mitigation measures. A constellation of four spacecraft on a 16-hr Taranis orbit is identified to enable continuous observation 55deg latitude. Neglecting constraints to minimize the radiation allows the number of spacecraft in the constellation to be reduced to three on a 12-hr orbit. Similarly to view continuously to 50deg, seven spacecraft on a 16-hr orbit are required; this is reduced to five neglecting radiation constraints. It is anticipated that it is significantly more cost effective to reduce the number of required launches and employ radiation hardened components. Thus, a constellation of three or five spacecraft on the 12-hr Taranis orbit is considered the most beneficial when observing to latitudes of 55deg and 50deg respectively. Hybrid solar sail / Solar Electric Propulsion systems are considered to enable the Taranis orbits, where the acceleration required is made up partly by the acceleration produced by the solar sail and the remainder supplied by the electric thruster. Order of magnitude mission lifetimes are determined, a strawman mass budget is also developed for two system constraints, firstly spacecraft launch mass is fixed, and secondly the maximum thrust of the thruster is constrained. Fixing mass results in negligible increases in mission lifetimes for all hybrid cases considered, solar sails also require significant technology development. Fixing maximum thrust of the electric thruster increases mission lifetime and solar sails are considered near to mid-term technologies. This distinction highlights an important contribution to the field, illustrating that the addition of a solar sail to an electric propulsion craft can have negligible benefit when mass is the primary system constraint. Technology requirements are also outlined, including sizing of solar arrays, electric thruters, propellant tanks and solar sails.