Daniel V. Guerrant

Heliogyro Solar Sail Research at NASA

The recent successful flight of the JAXA IKAROS solar sail has renewed interest within NASA in spinning solar sail concepts for high-performance solar sailing. The heliogyro solar sail, in particular, is being re-examined as a potential game-changing architecture for future solar sailing missions. In this paper, we present an overview of ongoing heliogyro technology development and feasibility assessment activities within NASA. In particular, a small-scale heliogyro solar sail technology demonstration concept will be described. We will also discuss ongoing analytical and experimental heliogyro structural dynamics and controls investigations and provide an outline of future heliogyro development work directed toward enabling a low-cost heliogyro technology demonstration mission ca. 2020.

Performance of a Heliogyro Blade Twist Controller with Finite Bandwidth

The heliogyro is a high-performance solar sail design because the sail is divided into long, thin blades stiffened by the centripetal acceleration of the spacecraft’s spin alone, reducing structural mass. A principal concern for heliogyros is the dynamic response of their long, unsupported blades. The linearized dynamics of a single rotating heliogyro blade in twist are studied using finite element analysis, and a root control law is designed using frequency response techniques. Proportional/Derivative/Feed-Forward compensation is implemented at the root to track and damp blade pitch maneuvers. This system is analyzed for performance and stability as a function of controller bandwidth and material damping. Since the material damping of these gossamer sheets is not known, this relationship is critical to blade controller design. Dynamic simulations reveal that a control system bandwidth of 3 cycles/rev (0.017 Hz) is required to meet Low Earth Orbit settling time requirements. These low bandwidths are well within realistic actuator capabilities, making this a promising result for the feasibility of heliogyros.

Recent Advances in Heliogyro Solar Sail Structural Dynamics, Stability, and Control Research

Results from recent NASA sponsored research on the structural dynamics, stability, and control characteristics of heliogyro solar sails are summarized. Specific areas under investigation include coupled nonlinear finite element analysis of heliogyro membrane blade with solar radiation pressure effects, system identification of spinning membrane structures, and solarelastic stability analysis of heliogyro solar sails, including stability during blade deployment. Recent results from terrestrial 1-g blade dynamics and control experiments on “rope ladder” membrane blade analogs, and small-scale in vacuo system identification experiments with hanging and spinning high-aspect ratio membranes will also be presented. A low-cost, rideshare payload heliogyro technology demonstration mission concept is used as a mission context for these heliogyro structural dynamics and solarelasticity investigations, and is also described. Blade torsional dynamic response and control are also shown to be significantly improved through the use of edge stiffening structural features or inclusion of modest tip masses to increase centrifugal stiffening of the blade structure. An output-only system identification procedure suitable for on-orbit blade dynamics investigations is also developed and validated using ground tests of spinning sub-scale heliogyro blade models. Overall, analytical and experimental investigations to date indicate no intractable stability or control issues for the heliogyro solar sail concept.

Heliogyro Attitude Control Moment Authority via Blade Pitch Maneuvers

Heliogyros generate attitude control moments by pitching their sail membrane blades collectively or cyclically, similar to a helicopter. Past work has focused on simple blade pitch profiles with the heliogyro normal to the sun; however, most solar sail missions will require sun angles of at least 35°. Furthermore, combination pitch profiles (e.g. cyclic plus collective) are needed for attitude control during all mission segments. The control moments for such situations vary in an unintuitive, nonlinear fashion. This paper explores heliogyro control moment authority with varying sun angle and combinations of pitch profiles, providing critical insight for future development of heliogyro attitude control schemes. Three strategies for generating control moments using various profile combinations are investigated for three-axis attitude control. These strategies indicate that the heliogyro can generate control moments while edge-on to the sun, allowing for recovery from any orientation. A control algorithm is presented that determines the required blade pitch profile combination to generate the desired attitude control torques. This algorithm could be employed for closed-loop control in attitude dynamics simulations.

Exploring the Heliogyro’s Superior Orbital Control Capabilities for Solar Sail Halo Orbits

Solar sailing is an elegant form of space propulsion that reflects solar photons off a large membrane to produce thrust. Different sail configurations exist, including a traditional fixed polygonal flat sail and a heliogyro, which divides the membrane into a number of long, slender blades. The magnitude and direction of the resulting thrust depends on the sail’s attitude with respect to the sun (cone angle). At each cone angle, a fixed polygonal flat sail can only generate force constrained to a particular magnitude and direction, whereas the heliogyro can arbitrarily reduce the thrust magnitude through the additional control of pitching the blades. This gives the heliogyro more force control authority, which is exploited in this paper for orbital control of solar sail, sun–Earth, sub-L1 halo orbits through a linear-quadratic regulator feedback controller. Two test cases are considered, quantifying either the maximum error in the injection state or the maximum delay in solar sail deployment due to deploym...

Solving the heliogyro’s inverse problem

A heliogyro is a solar sail concept that divides the solar sail membrane into a number of long, slender blades of film extended from a central hub, maintained in a flat state through spin-induced tension. The heliogyro can redirect and scale the solar radiation pressure (SRP) force and can achieve attitude control by twisting the blades, similar to a helicopter rotor. Different pitch profiles exist, including pitching the blades in a collective, cyclic or combined collective and cyclic manner. While the forward mapping, i.e., computing the SRP force and moment generated by the heliogyro for a given pitch profile, is straightforward, the inverse of the problem is much more complex. However, this inverse problem (finding the blades’ pitch that results in a desired SRP force and/or moment) is crucial for heliogyro mission design and operations. This paper therefore solves the inverse problem numerically: first, only for a desired SRP force or SRP moment and subsequently for the fully coupled inverse problem. The developed methods are subsequently applied to track a reference trajectory that corrects for injection errors into a solar sail Sun-Earth sub-L1 halo orbit. I. Introduction