Jeannette Heiligers

Displaced geostationary orbit design using hybrid sail propulsion

Due to an increase in number of geostationary spacecraft and limits imposed by east-west spacing requirements, the geostationary orbit is becoming congested. To increase its capacity, this paper proposes to create new geostationary slots by displacing the geostationary orbit either out of or in the equatorial plane by means of hybrid solar sail and solar electric propulsion. To minimize propellant consumption, optimal steering laws for the solar sail and solar electric propulsion thrust vectors are derived and the performance in terms of mission lifetime is assessed. For comparison, similar analyses are performed for conventional propulsion, including impulsive and pure solar electric propulsion. It is shown that hybrid sails outperform these propulsion techniques and that out-of-plane displacements outperform in-plane displacements. The out-of-plane case is therefore further investigated in a spacecraft mass budget to determine the payload mass capacity. Finally, two transfers that enable a further improvement of the performance of hybrid sails for the out-of-plane case are optimized using a direct pseudo-spectral method: a seasonally transit between orbits displaced above and below the equatorial plane and a transit to a parking orbit when geostationary coverage is not needed. Both transfers are shown to require only a modest propellant budget, outweighing the improvements they can establish.

Trajectory and spacecraft design for a pole-sitter mission

This paper provides a detailed mission analysis and systems design of a pole-sitter mission. It considers a spacecraft that is continuously above either the North or South Pole and, as such, can provide real-time, continuous, and hemispherical coverage of the polar regions. Two different propulsion strategies are proposed, which result in a near-term pole-sitter mission using solar-electric propulsion and a far-term pole-sitter mission, in which the electric thruster is hybridized with a solar sail. For both propulsion strategies, minimum propellant pole-sitter orbits are designed. Optimal transfers from Earth to the pole sitter are designed, assuming Soyuz and Ariane 5 launch options, and a controller is shown to be able to maintain the trajectory under unexpected conditions, such as injection errors. A detailed mass budget analysis allows for a tradeoff between mission lifetime and payload mass capacity, and candidate payloads for a range of applications are investigated. This results in a payload of ab...

Sunjammer: Preliminary end-to-end mission design

This paper provides a preliminary end-to-end mission design for NASA’s Sunjammer solar sail mission, which is scheduled for ground test deployment in 2015 with launch at a later date and targets the sub-L1 region for advanced solar storm warning. The artificial equilibrium points (AEPs) in the sub-L1 region accessible by the Sunjammer sail as well as solar sail Halo orbits are investigated. Subsequently, the fly-out from an Earth GTO into either a selected sub-L1 AEP or Halo orbit is optimized for a trade-off between the AV to be provided at GTO perigee and the time of flight. In addition, interesting, time-optimal extended mission scenarios are presented to underpin future solar sail mission applications, e.g. transferring to an AEP high above the ecliptic plane for high-latitude Earth observation. All analyses are carried out both for an ideal Sunjammer sail performance as well as for a realistic performance derived from a detailed sail structural analysis. A comparison of the results shows that non-ideal sail properties increase the time of flight of the trajectories by 2.4 - 7.9%.

Gossamer Roadmap Technology Reference Study for a Sub-L1 Space Weather Mission

A technology reference study for a displaced Lagrange point space weather mission is presented. The mission builds on previous concepts, but adopts a strong micro-spacecraft philosophy to deliver a low mass platform and payload which can be accommodated on the DLR/ESA Gossamer-3 technology demonstration mission. A direct escape from Geostationary Transfer Orbit is assumed with the sail deployed after the escape burn. The use of a miniaturized, low mass platform and payload then allows the Gossamer-3 solar sail to potentially double the warning time of space weather events. The mission profile and mass budgets will be presented to achieve these ambitious goals.

Distributed reflectivity solar sails for extended mission applications

The dynamics of solar sails with a variable surface reflectivity distribution are investigated. When changing the reflectivity across the sail film, solar radiation pressure forces and torques can be controlled without changing the attitude of the spacecraft relative to the Sun or using attitude control actuators. The reflectivity can in principle be modified using electro-chromic coatings, which are applied here as examples to counteract gravity-gradient torques in Earth orbit and to enable specific shape profiles of a flexible sail film. This ‘optical reconfiguration’ method introduces an adaptive solar sail as a multi-functional platform for novel mission applications.

Inverse problem for shape control of flexible space reflectors using distributed solar pressure

This paper investigates controlled elastic deflection of thin circular space reflectors using an inverse problem approach to non-linear thin membrane theory. When changing the surface reflectivity across the membrane, the distributed loads due to ambient solar radiation pressure can be manipulated optically, thus controlling the surface shape without using mechanical or piezo-electric systems. The surface reflectivity can in principle be modulated using uniformly distributed thin-film electro-chromic coatings. We present an analytic solution to the inverse problem of finding the necessary reflectivity distribution that creates a specific membrane deflection, for example that of a parabolic reflector. Importantly, the reflectivity distribution across the surface is found to be independent of membrane size, thickness and solar distance, enabling engineering of the reflectivity distribution directly during the manufacture of the membrane.

Novel pole-sitter mission concepts for continuous polar remote sensing

The pole-sitter concept is a solution to the poor temporal resolution of polar observations from highly inclined, low Earth orbits and the poor high latitude coverage from geostationary orbit. It considers a spacecraft that is continuously above either the North or South Pole and, as such, can provide real-time, continuous and hemispheric coverage of the polar regions. Despite the significant distance from the Earth, the utility of this platform for Earth observation and telecommunications is clear, and applications include polar weather forecasting and atmospheric science, glaciology and ice pack monitoring, ultraviolet imaging for aurora studies, continuous telecommunication links with polar regions, arctic ship routing and support for future high latitude oil and gas exploration. The paper presents a full mission design, including launch (Ariane 5 and Soyuz vehicles), for two propulsion options (a near-term solar electric propulsion (SEP) system and a more advanced combination of a solar sail with an SEP system). An optional transfer from the North Pole to South Pole and vice-versa allows viewing of both poles in summer. The paper furthermore focuses on payloads that could be used in such a mission concept. In particular, by using instruments designed for past deep space missions (DSCOVR), it is estimated that resolutions up to about 20 km/pixel in the visible wavelengths can be obtained. The mass of these instruments is well within the capabilities of the pole-sitter design, allowing an SEP-only mission lifetime of about 4 years, while the SEP/sail propulsion technology enables missions of up to 7 years.

Shape control of slack space reflectors using modulated solar pressure

The static deflection profile of a large spin-stabilized space reflector because of solar radiation pressure acting on its surface is investigated. Such a spacecraft consists of a thin reflective circular film, which is deployed from a supporting hoop structure in an untensioned, slack manner. This paper investigates the use of a variable reflectivity distribution across the surface to control the solar pressure force and hence the deflected shape. In this first analysis, the film material is modelled as one-dimensional slack radial strings with no resistance to bending or transverse shear, which enables a semi-analytic derivation of the nominal deflection profile. An inverse method is then used to find the reflectivity distribution that generates a specific, for example, parabolic deflection shape of the strings. Applying these results to a parabolic reflector, short focal distances can be obtained when large slack lengths of the film are employed. The development of such optically controlled reflector films enables future key mission applications such as solar power collection, radio-frequency antennae and optical telescopes.

Solar sail heliocentric Earth-following orbits

Solar sail technology development is rapidly gaining momentum after recent successes such as JAXA’s IKAROS mission and NASA’s NanoSail-D2 mission. Research in the field is flourishing and new solar sail initiatives, such as NASA’s Sunjammer mission, are scheduled for the future. Solar sails exploit the radiation pressure generated by solar photons reflecting off a large, highly reflecting sail to produce a continuous thrust force. They are therefore not constrained by propellant mass, which gives them huge potential for long-lifetime and high-energy mission concepts and enables a range of novel applications. One such family of applications are non-Keplerian orbits (NKOs), where the force due to solar radiation pressure on a solar sail is used to displace an orbit away from a natural Keplerian orbit. Different types of NKOs exist, including NKOs in the two-body problem (either Sun-centered or Earth-centered) and NKOs in the well-known circular restricted three-body problem (CR3BP). In the Sun-centered two-body problem, NKOs are determined by considering the solar sail spacecraft dynamics in a rotating frame of reference. By setting the time derivatives of the position vector equal to zero, equilibrium solutions are found in the rotating frame that correspond to displaced circular orbits in an inertial frame. Such Sun-centred NKOs allow a spacecraft to be synchronous with a planet at any heliocentric distance inward from the target planet and/or to displace a solar sail spacecraft out of the ecliptic plane for solar polar observations, interplanetary communication, and astronomical observations. A similar approach can be used to find solar sail NKOs in the Earth two-body problem, creating, for example, orbits on the Earth’s nightside to study its magnetotail and interaction with the solar wind and displaced geostationary orbits to create additional geostationary slots for telecommunication, Earth observation, and weather satellites. Finally, in the CR3BP, solar sails have been demonstrated to extend the five Lagrange points to a continuum of new artificial equilibrium points (AEPs) and can be used to create periodic orbits around these AEPs. The applications of these NKOs are abundant, including one-year periodic orbits high above the ecliptic in the Sun-Earth system for polar observations, solar sail trajectories above the Earth-Moon L2 point to establish an Earth-Moon communication link and solar sail Halo orbits sunward of the Sun-Earth L1 point to increase the warning times for solar storms. Rather than displacing the orbit, the force due to solar radiation pressure on a solar sail can also be used to create an artificially precessing NKO. For example, the GeoSail mission proposed the use of a solar sail to rotate an elliptic geocentric orbit in the ecliptic plane such that apogee remains on the night side of the Earth to enable continuous observations of the geomagnetic tail. In this technical note, the concept of rotating an elliptic orbit by means of a solar sail is considered further by investigating precessing, heliocentric, and Earth-following orbits. The sail orbit’s aphelion follows the Earth’s orbital motion throughout the year and is always directed along the Sun-Earth line, allowing extended observations for space weather forecasting and Near Earth Objects (NEOs) surveillance activities.

New Families of Non-Keplerian Orbits: Solar Sail Motion over Cylinders and Spheres

This chapter presents new families of Sun-centered non-Keplerian orbits (NKOs), where the motion of a high-performance solar sail is confined to either a cylindrical or spherical surface. These orbits are found by investigating the geometrically constrained sail dynamics and imposing further constraints on the angular velocity and lightness number to generate pure solar sail trajectories. By considering the sail motion in the phase space of the problem, families of new NKOs are identified, and by investigating the oscillating behavior of the orbits, true periodic orbits are found. As extension to the well-known families of displaced NKOs, these three-dimensional NKOs generate a wealth of new solar sail orbits and novel sail applications.