Fast Solar Sailing

Before dealing with any model for realistically computing the trajectories of solar-sail vehicles, this chapter reviews a topic of basic importance for spaceflight: the rocket propulsion. This chapter aims at highlighting features peculiar to in-space rocket vehicles and, consequently, to the related mission designs. In particular, one can see that the exhaust speed not always is the main reference quantity in rocket dynamics, though it is important, of course. In addition, the presence of a gravity field may strongly reduce the performance expected from the simple field-free space equation. The properties of photon rockets and ion propulsion rockets are compared through mission cases. A model of general energy-mass flow in rocket, i.e. independently of the particular engine system, is explained in terms of Special Relativity, and then specialized to: (1) nuclear-based photon rocket, and (2) electric propulsion with system efficiencies including the spreads of the exhausting beam. Focus is on trajectories escaping from the solar system.

In the post-IKAROS/JAXA scenario of space sailing, the main aim of this chapter is to show that one could utilize the radiant energy from the Sun as a very special external-to-spacecraft source of thrust. The chapter begins with a summary of radiometric quantities, and then points out the immense contribution of space-era to the solar physics, especially through the high-precision records of the total and spectral solar irradiances. The time series of such quantities will be important in designing fast solar-sail trajectories. In particular, are highlighted the several principles for realizing a highly versatile in-space propulsion through the utilization of some properties of the solar irradiance. The time fluctuations of the total solar irradiance are emphasized; their influence on sailcraft trajectories will be analyzed in the last chapter of the book.

The concept of sea sailing can be extended to space traveling. The basic idea is to utilize some energy already present in space for avoiding the main drawback of a rocket vehicle: to be forced to carry all necessary reaction mass onboard. The more energetic is the mission transfer the more massive is the spaceship. Future space missions are expected to increase in number, purpose, and energy. It should be clear that any new good method for practically enlarging human exploration and expansion does not mean quitting rocket propulsion. On the other hand, there are a high number of missions that are impossible to rockets, not strictly in mathematical terms, but because of the very large mass and complexity of the involved systems, including space infrastructures. In this chapter, three sailing modes for traveling in space are dealt with; only the third one is a “strict” sail, namely, a two-dimensional object through which an external-to-vehicle momentum flux can be captured and translated into thrust. After a summary of the solar-wind properties, subsequent sections describe concepts regarding generation of thrust via solar wind, but involve large volumes. In principle, this is not a limitation: problems would come from other features, as discussed. All other section/subsections of the chapter describe sailcraft and its main systems, at least as they are conceived today and how presumably may evolve. Solar-wind large fluctuations are emphasized by means of data coming from the NASA’s OMNIWeb interface.

This chapter deals with some aspects of the influence of the solar-wind particles and the ultraviolet/X-soft radiation to photon sail materials. The related sections are preparatory to Chap. 9, where a formal model to be inserted into the trajectory equations is set up. The current chapter consists of three main sections: the first one is devoted to the solar sail immersed in the solar wind, the second one regards the effect of the UV-XUV light onto the sail reflective film, and the third one considers the interaction of solar-wind protons and Helium ions on a sail moving with respect to the wind. As one can see in these sections, there are some simultaneous processes that stem from the particle impact on the solar sail, but ultimately one aims at regarding sail trajectory through the alteration of the optical properties of the sail surface. In addition to references and the author’s personal investigation, plasma data information obtained from NASA’s OMNIWeb interface is utilized again.

Some basic properties of space vehicles endowed with solar-photon sail may be carried out without a detailed model of the thrust induced by the solar radiation pressure. This will be done in the next long devoted chapter. Here, one starts with establishing the main mathematical formalism that will accompany the reader in the course of the subsequent chapters. One should realize that such formalism is not merely an option among many others. As a point of fact, as it is known from many branches of science, some methods can favor investigation considerably. Two sources of acceleration act on the sailcraft as a whole: one is the solar gravity, the other one is the scalar field of solar radiation pressure in space. Perhaps unexpectedly, the second one generates two local vector fields of which one is conservative. The very large potential of sailcraft missions arise from the simultaneous actions of these three fields, namely, two conservative and one non-conservative. In this and the following chapters, one can see how trajectories, unviable to rockets, are possible to sailcraft, thus strongly enlarging the potentials of space exploration and utilization. In the final part of this chapter, the interested reader can find an introduction to thrust maneuvering from an unusual viewpoint. A digression on time and reference frames is included. That has been done for a further reason: much of the scientific software on Astrodynamics, which the student can find in her/his university, generally contains a high number of reference frames and time scales; if left unexplained, these may induce confusion or, worse, some misunderstanding, especially when results from different computer codes are to be compared. Such section may be considered as a short introduction to the subject, which is vast and complicated indeed.

Translating the (scalar) solar radiation pressure into a (vector) acceleration field. Models of radiation-pressure thrust acceleration could contain many and more small effects, therefore resulting in a very sophisticated algorithm to be verified in flight. First-generation sailcraft exhibits very low thrust acceleration, and small effects may result non-measurable: after all, they are beyond the mission’s critical aims. On the other side, sailcraft for advanced missions will request the modeling of many relevant effects. Thus, this chapter contains a detailed model of sail thrust acceleration, which takes into account many factors related to the source of light and the sail. To such aim, the physical nature of a real surface and its related mathematical representation are described here, also considering the various methods of surface manufacturing. A special emphasis is put on the fact that sailcraft thrust, stemming from the interaction between solar photons and sail materials, is driven essentially by the diffraction of light. Here, various scalar and vector scattering theories are considered. Global and local sail’s topography is taken into account in the thrust model. An additional section is devoted to an important generalization of the conventional flat-sail model.

To combine solar gravity and solar radiation pressure efficiently. This chapter is devoted to study the way a sailcraft can achieve so high a cruise speed that the related mission types may allow the exploration of the heliosphere and well beyond in a time interval shorter than the mean human job time, including the design phase. Four main sections and various subsections are devoted to such an aim. Some concepts discussed in Chap. 5 are extended; they use a different view of sailcraft trajectory: the orbital angular momentum reversal via solar radiation pressure. In Chap. 6, how solar irradiance can result in thrust has been detailed. May one utilize both thrust and solar gravity in order to notably increase sailcraft speed, hopefully well higher than the speed obtainable by spiraling about the Sun? The theory of fast sailing is explained for two-dimensional as well as three-dimensional trajectories. The second set is not a mere extension of the first one. Theory relies on some meaningful theorems and propositions. Many numerical cases are discussed with full particulars.

Thrust acceleration models may be extended by taking the fluctuating interplanetary environment into account through different ways. TSI variations change the level of thrust directly, while ultraviolet spectral irradiance may alter the optical properties of the sail’s reflective material. Such modifications may be modeled as a function of the UV-energy absorbed by unit area of this layer as sailcraft moves. One says that UV-light causes an optical degradation. Solar wind ions, though representing a very small perturbation to sailcraft motion from the dynamical pressure viewpoint, however penetrate throughout the reflective layer, and may bring about some degradation if the absorbed dose is high during the flight. This has the twofold effect of lessening thrust and increasing sail temperature, which is an issue in general. Degradation may be induced by UV-light and ion bombardment at the same time. Though a detailed model is explained, however such topics are only hinted here. Further studies—devoted to a wide analysis of quantitative UV/solar-wind impact on sailcraft trajectories/orbits—are needed also in view of new materials for sail making.