Sailcraft

For sailcraft referring to a boat etc., see

• sailboat
• yacht
• dinghy
• ice boat
• land yacht

Sailcraft can also refer to sailing skills

• In the context of spaceflight, Sailcraft is a short name for spacecraft endowed with sail for propulsion purposes. In principle, the sail and its supporting structures, or the sail system, are not restricted to be for solar-sail propulsion, but they could be designed for either laser or microwave propulsion (i.e. beam-driven propulsion) or, as recently proposed, for solar and beam-driven purposes. The key point – from the spacecraft design viewpoint – is the presence of a system with a surface overwhelming those of any other systems the spacecraft is composed of. This affects the geometrical and physical configurations of the spacecraft systems considerably. Nevertheless, the benefits from using a solar sail would be enormous.

• The sail orientation is specified by its axis, or the unit vector, say, n orthogonal to the mean plane of the sail, usually put in the back sail semispace; the fronside of a sail is the side of its reflective layer (e.g. a very thin layer of Aluminium), whereas the backside is the side of either its plastic support layer (e.g. Kapton) or the emissive layer (e.g. an ultra-thin film of Chromium), if any. If the sail is perfectly flat (as often it is assumed to be), n is automatically determined. This axis definition is appropriate for orienting the sail naturally with respect to the direction of the local incident solar light. Thus, a sail orthogonal to the Sun-light and receiving the maximum flux of light has its axis at zero angle with the direction of the incident solar photons. In general, the sail axis is expressed as a function of some pair of angles, say, n(α, δ) that are defined in a suitable frame of reference. These angles generally act as the control angles for orienting the sail.

• Some Design Parameters
  1. a key parameter in designing a sailcraft is the total spacecraft mass on the effective sail area ratio, named the sailcraft sail loading, often expressed in grams per square meter.
  2. together with the thermo-optical properties of the sail material (reflection, diffusion, absorption and emission of light), the sail loading determines the maximum solar-pressure acceleration the spacecraft undergoes at a certain distance from the Sun. This is the value of the thrust acceleration one could get if the sail were orthogonal to its Sun-line and at rest. In this case, the sailcraft acceleration would be totally radial, namely, parallel to the Sun-to-vehicle straight line.
  3. in general, the sailcraft motion should happen by the sail tilted for achieving the mission goals. Thus, the actual vector thrust acceleration can be resolved into three orthogonal components, named the radial, the transversal and the normal ones. The normal component is directed orthogonally to the instantaneous sailcraft trajectory plane.
  4. what matters for the sailcraft trajectory computation is the ratio of such accelerations to the local solar gravitational acceleration. These numbers can change with time mainly because the sail orientation generally varies with respect to the local frame of reference consisting of the Sun-line and its orthogonal plane (where two other orthogonal axes can be defined). The three mentioned scalars altogether constitute a vector function of time, named the lightness vector, say L(t). The magnitude of this vector is the sailcraft's lightness number. The lightness vector is particularly important since the sailcraft orbital energy and angular momentum, and their time rates, are linearly dependent on the L components. These ones, in turn, chiefly depend on α and δ (non-linearly) and the thermo-optical parameters (linearly) of the sail materials. (There are other thrust contributions due to the aberration of light and the physical Sun, which is not a point-like source; however, such effects can be neglected except for some special class of solar-sail missions).
  5. another useful parameter is the (scalar) characteristic acceleration, which is defined as the magnitude of the solar-pressure acceleration vector that a sailcraft would experience at one astronomical unit with the sail orthogonal to the local Sun-line and at rest. Different solar-sail missions could be compared by using the related values of the characteristic acceleration; equivalently, one could use either the maximum lightness number or the sailcraft sail loading and the thrust efficiency at 1 AU. Thrust efficiency accounts mainly for the non-ideal optical parameters and the non-flat geometry of the sail.

• The main goal of the sailcraft mission analyst is to compute the best time history of the sail n, or the vector function n(α(t), δ(t)), for the specific space mission under consideration. The ensuing admissible trajectories have to be connected to the sailcraft technology via an iterative optimization process which shall result in the final optimized design of both the whole mission and sailcraft (quite similarly to the past/current space mission designs).

• Of special importance for many envisaged very deep space solar-sail missions is the design of sailcraft capable to accomplish the fast solar sailing mode.

• References
  1. J. L. Wright, Space Sailing, Gordon and Breach Science Publishers, Amsterdam, 1993
  2. G. Vulpetti, 3D High-Speed Escape Heliocentric Trajectories by All-Metallic-Sail Low-Mass Sailcraft, Acta Astronautica, Vol. 39, pp 161–170, July-August 1996
  3. G. Vulpetti, Sailcraft at High Speed by Orbital Angular Momentum Reversal, Acta Astronautica, Vol. 40, No. 10 pp. 733-758, May 1997
  4. C. R. McInnes, Solar Sailing: Technology, Dynamics, and Mission Applications, Springer-Praxis Publishing Ltd, Chichester, UK, 1999
  5. G. L. Matloff, Deep-Space Probes: to the Outer Solar System and Beyond, 2nd ed., Springer-Praxis Chichester, UK, 2005