Colin McInnes

SOLAR SAIL PARKING IN RESTRICTED THREE-BODY SYSTEMS

Stationary solutions to the restricted three-body problem for solar sail spacecraft in the Earth-sun and Earth-moon systems are investigated. It is found that the usual five Lagrange points are extended to a continuum of new artificial points that form level surfaces parameterized by the sail mass per unit area. Analytic expressions for the sail mass per unit area and the sail attitude required for these stationary solutions are obtained and the stability of the solutions examined. It is found that although in general the solutions are unstable, a simple closed-loop control scheme may be developed to ensure asymptotic stability.

Health, security and foreign policy

McInnes, C., Lee, K. (2006). Health, security and foreign policy. Review of International Studies, 32 (1), 5-23. RAE2008

Survey of highly non-Keplerian orbits with low-thrust propulsion

Celestial mechanics has traditionally been concerned with orbital motion under the action of a conservative gravitational potential. In particular, the inverse square gravitational force due to the potential of a uniform, spherical mass leads to a family of conic section orbits, as determined by Isaac Newton, who showed that Kepler‟s laws were derivable from his theory of gravitation. While orbital motion under the action of a conservative gravitational potential leads to an array of problems with often complex and interesting solutions, the addition of non-conservative forces offers new avenues of investigation. In particular, non-conservative forces lead to a rich diversity of problems associated with the existence, stability and control of families of highly non-Keplerian orbits generated by a gravitational potential and a non-conservative force. Highly non-Keplerian orbits can potentially have a broad range of practical applications across a number of different disciplines. This review aims to summarize the combined wealth of literature concerned with the dynamics, stability and control of highly non-Keplerian orbits for various low thrust propulsion devices, and to demonstrate some of these potential applications.

Autonomous rendezvous using artificial potential function guidance

A novel methodology has been developed for the guidance and control of a maneuvering chase vehicle undergoing terminal rendezvous in the presence of path constraints and multiple obstructions. The method hinges on defining a suitable scalar function which represents an artificial potential field describing the locality of the target vehicle. Using a set of bounded impulses the chase vehicle is guided by the local topology of this potential function. Obstructions and path constraints are introduced by superimposing regions of high potential around these regions. Exact, analytical expressions are then obtained for the required control impulse magnitude, direction and switching times using the second method of Lyapunov. These control impulses ensure that the potential function monotonically decreases so that convergence of the chaser to the target is ensured analytically, without violating the path constraints. Since the components of the potential and control impulses may be represented analytically, the method appears suitable for autonomous, real-time control of complex maneuvers with a minimum of onboard computational power.

GEOSAIL: Exploring the Geomagnetic Tail Using a Small Solar Sail

Conventional geomagnetic tail missions require a spacecraft to be injected into a long elliptical orbit to explore the spatial structure of the geomagnetic tail. However, because the elliptical orbit is inertially fixed and the geomagnetic tail is directed along the sun-Earth line, the apse line of the elliptical orbit is precisely aligned with the geomagnetic tail only once every year. To artificially precess the apse line of the elliptical orbit in a sun-synchronous manner, which would keep the spacecraft in the geomagnetic tail during the entire year, would require continuous low-thrust propulsion or periodic impulses from a high-thrust propulsion system. Both of these options require reaction mass that will ultimately limit the mission lifetime. It is demonstrated that sun-synchronous apse-line precession can be achieved using only a small, low-cost solar sail. Because solar sails do not require reaction mass, a geomagnetic tail mission can be configured that provides a continuous science return by permanently stationing a science payload within the geomagnetic tail.

Solar Sail Halo Orbits Part II - Geocentric Case

The equations of motion of a geocentric orbiting high-performance solar sail are analyzed in a corotating reference frame with the axis of corotation directed along the Sun-Earth line. Stationary solutions are found to the equations of motion in the corotating frame that correspond to Earth-centered halo-type orbits when viewed from an inertial frame. The sail performance requirements are minimized and families of linearly stable trajectories identified. By patching these halo orbits together, it will be shown that complex new trajectories may be formed. Geocentric halo orbits may have useful space science and technology applications.

Safety Constrained Free-Flyer Path Planning at the International Space Station

A path-planning tool is presented to generate safe trajectories from an initial docking release, to a specified observation point, and back to docking for a small free-flying robot camera around the International Space Station. The tool makes use of ellipse of safety trajectories to enforce long-term passive safe requirements in the presence of differential air drag during the fly around phases of the maneuver during transfer between the docking port and observation point. Short-term passive safety (2-3 orbits) is also maintained during all station-keeping and approach maneuvers by checking the safety of the observation point at the initial planning stage, and through the use of precalculated velocities profiles along the r-bar forced motion approaches to the observation point and docking. The observation phase of the mission is enhanced through the use of artificial Laplace potential functions within a constrained volume, to allow for limited maneuvering close to the observation point enabling the available view to be translated and rotated.

Dynamics, Stability, and Control of Displaced Non-Keplerian Orbits

The dynamics, stability, and control of a large family of non-Keplerian orbits are investigated and mission applications are discussed. The orbits are generated by seeking equilibrium solutions to the two-body problem in a rotating frame of reference with an additional thrust-induced acceleration. Viewed from an inertial frame of reference, displaced circular orbits are obtained. Three main families of orbits are presented, and their local stability characteristics are investigated. Although it is found that there are unstable subfamilies of orbits, it is also shown that these orbits are controllable using linear state feedback. Impulse control is also investigated as a means of generating displaced orbits and is compared to continuous thrust control. It is demonstrated that these non-Keplerian orbits can be patched together to provide large additional families of orbits.

Solar polar orbiter: a solar sail technology reference study

An assessment is presented of a Solar Polar Orbiter mission as a Technology Reference Study. The goal is to focus the development of strategically important technologies of potential relevance to future science missions. The technology is solar sailing, and so the use of solar sail propulsion is, thus, defined a priori. The primary mission architecture utilizes maximum Soyuz Fregat 2-1b launch energy, deploying the sail shortly after Fregat separation. The 153 × 153 m square sail then spirals into a circular 0.48-astronomical-unit orbit, where the orbit inclination is raised to 90 deg with respect to the solar equator in just over 5 years. Both the solar sail and spacecraft technology requirements have been addressed. The sail requires advanced boom and new thin-film technology. The spacecraft requirements were found to be minimal because the spacecraft environment is relatively benign in comparison with other currently envisaged missions, such as the Solar Orbiter mission and BepiColombo.

GeoSail: An Elegant Solar Sail Demonstration Mission

In this paper a solar sail magnetotail mission concept was examined. The 43-m square solar sail is used to providethe required propulsion for continuous sun-synchronous apse-line precession. The main driver in this mission was found to be the reduction of launch mass and mission cost while enabling a nominal duration of 2 years within the framework of a demonstration mission. It was found that the mission concept provided an excellent solar sail technology demonstration option. The baseline science objectives and engineering goals were addressed, and mission analysis for solar sail, electric, and chemical propulsion performed. Detailed subsystems were defined for each propulsion system and it was found that the optimum propulsion system is solar sailing. A detailed tradeoff as to the effect of spacecraft and sail technology levels, and requirements, on sail size is presented for the first time. The effect of, for example, data acquisition rate and RF output power on sail size is presented, in which it is found that neither have a significant effect. The key sail technology requirements have been identified through a parametric analysis.