Static highly elliptical orbits using hybrid low-thrust propulsion

Static highly-elliptical orbits enabled using hybrid solar-sail/solar-electric propulsion are investigated. These newly proposed orbits, termed Taranis orbits, have free selection of ‘critical inclination’ and use low-thrust propulsion to compensate for the drift in argument of perigee caused by Earth’s gravitational field. In this paper, a 12-hr Taranis orbit with an inclination of 90deg is developed to illustrate the principle. The acceleration required to enable this novel orbit is made up partly by the acceleration produced by solar-sails of various characteristic accelerations, 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 maximum thrust increases mission lifetimes, and solar-sails are considered near to mid-term technologies. However, fixing mass results in negligible increases in mission lifetimes for all hybrid cases considered, solar sails also require significant development. This distinction highlights an important contribution to the field, illustrating that 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, propellant tanks and solar sails

Extension of highly elliptical Earth orbits using continuous low-thrust propulsion

The extension of highly elliptical orbits, with free selection of orbit period, using low thrust propulsion is investigated. These newly proposed orbits, termed Taranis orbits, are enabled by existing low-thrust propulsion technology, offering a radically new set of tools for mission design and facilitating new, novel Earth Observation science. One particular example considered herein, using general and special perturbation techniques, is the application of continuous low-thrust to alter the ‘critical inclination’ of an orbit from the natural values of 63.4deg or 116.6deg, to any inclination required to optimally fulfill the mission goals. This continuous acceleration is used to compensate for the drift in argument of perigee caused by Earth’s gravitational field. Pseudo-spectral optimization techniques are applied to the 90deg inclination Taranis orbit, generating fuel optimal low-thrust control profiles, with a fuel saving of ~ 4% from general perturbation results. This orbit provides an alternative solution for high latitude imaging from distances equivalent to geostationary orbits. Analysis shows that the orbit enables continuous, high elevation visibility of frigid and neighboring temperate regions using only three spacecraft, whereas a Molniya orbit would require in excess of fifteen spacecraft, thus enabling high quality imaging which would otherwise be prohibited using conventional orbits. Order of magnitude mission lifetimes for a range of mass fractions and specific impulses are also determined. Finally, a Strawman mass budget is developed, where the mission lifetimes for spacecraft with initial mass of 1000kg, 1500kg, and 2500kg, are found to be limited to 4.3 years, 6 years and 7.4 years respectively

Extension of the Molniya orbit using low-thrust propulsion

Extension of the standard Molniya orbit using low-thrust propulsion is presented. These newly proposed, highly elliptical orbits are enabled by existing low-thrust propulsion technology, enabling new Earth Observation science and offering a new set of tools for mission design. In applying continuous low-thrust propulsion to the conventional Molniya orbit the critical inclination may be altered from the natural value of 63.4deg, to any inclination required to optimally fulfill the mission goals. Analytical expressions, validated using numerical methods, reveal the possibility of enabling a Molniya orbit inclined at 90deg to the equator. Fuel optimal low-thrust control profiles are then generated by the application of pseudo spectral numerical optimization techniques to these so-called Polar-Molniya orbits. These orbits enable continuous, high elevation visibility of the Frigid and Neighboring Temperate regions, using only two spacecraft compared with six spacecraft required for coverage of the same area with a conventional Molniya orbit. This can be achieved using existing ion engines, meaning no development in technology is required to enable these new, novel orbits. Order of magnitude mission lifetimes for a range of mass fractions and specific impulses are also determined, and are found to range from 1.2 years to 9.4 years. Where, beyond 9.4 years the outline mass budget analysis for spacecraft of initial masses of 500kg, 1000kg and 2500kg, illustrated there is no longer a capacity for payload for all initial mass of spacecraft

Sun-synchronous highly elliptical orbits using low-thrust propulsion

Due to restrictions within the current architecture of the global observing system (GOS), space-based remote sensing of Earth suffers from an acute data-deficit over the critical polar-regions. Currently, observation of high-latitude regions is conducted using composite images from spacecraft in geostationary (GEO) and low-Earth orbits (LEOs) [1]. However, the oblique viewing geometry from GEO-based systems to latitudes above around 55 deg [2] and the insufficient temporal resolution of spacecraft in LEO means there is currently no source of continuous imagery for polar-regions obtained with a data refresh rate of less than 15 minutes, as is typically available elsewhere for meteorological observations

A novel design concept for space-based polar remote sensing

Space-based remote sensing of the Earth is conducted from a fleet of spacecraft in two basic orbital positions, near-polar low-Earth orbits and geosynchronous orbits, with each offering its own advantages and disadvantages. Low-Earth orbits provide high-resolution observations at the expense of large-scale contextual information, while geosynchronous orbits provide near-global, continuous coverage at reduced resolutions. However, due to the rapidly decreasing horizontal resolution data-products derived from geosynchronous orbits are of degraded value beyond approximately 55 degrees of latitude. A novel mission design is introduced to enable continuous observation of all longitudes at latitudes between 55 and 90 degrees with an observation zenith angle of less than 60 degrees, without the use of composite images. A single Soyuz launch is used to deliver three spacecraft to 12-hr, highly eccentric true-polar orbits with apogee at 40170 km and electric propulsion is used to maintain the orbit apse-line coincident with the Earth’s poles. It is shown that the science payload mass can be traded against the mission duration, with a payload mass varying between 120 – 90 kg for mission durations between 3 – 5 years, respectively. It is further shown that the payload would have approximately of 2kW of power available during operations as the electric propulsion system is not operated at these times. Whilst the payload mass is less than a typical remote sensing platform in geosynchronous orbit it is considered that the concept would offer an excellent technology demonstrator mission for operational missions, whilst also enabling unique and valuable science