Guidance for LEO Transfers Using Optimized Electric Buffered Propulsion

Paper number

IAC-07-C1.6.05

Author

Prof. Moshe Guelman, Technion, I.I.T., Israel

Coauthor

Dr. Alexander Kogan, Technion, I.I.T., Israel

Coauthor

Mr. Alexander Kazaryan, Israel

Year

2007

Abstract

Solar electric propulsion (SEP) has been used in interplanetary missions and, routinely, for station keeping in Geo-Synchronous Orbit (GSO). SEP was never used as the main propulsion engine for inter-orbital transfers in Low Earth Orbit (LEO). The dynamic environment in LEO differs greatly from that in GSO and, the more so, in the interplanetary space: A) typical thrust-to-weight ratio in LEO is ~ 10e-5, versus ~ 10e-1-10e-2 in interplanetary missions; B) time scale is about 10e2 min, versus ~10e3 min in geosynchronous orbits and ~10e6 min in interplanetary missions; C) a satellite in LEO passes through an eclipse at every revolution increasing the number of on/off cycles and complicating the control algorithms.
Miniaturization poses specific limitations on the satellite design. An important characteristic of small spacecraft is that solar arrays and thrusters are rigidly attached to the satellite body due to weight limitations. A desired thrust direction is obtained by rotating the satellite as a whole. Rotation of the spacecraft around the sun direction does not affect the available power while the thrust vector rotates around this direction along with the spacecraft body. However, rotation around an axis perpendicular to the sun line does affect the available power, due to variations of the of solar rays incident angle. 
Endowing SEP with the capability of energy accumulation is advantageous for several reasons: A) Efficiency of thrust application varies over the orbit. Energy accumulated during the low-efficiency part of the orbital period can be profitably spent at high-efficiency periods. B) A spacecraft equipped with a SEP needs simultaneous pointing the thrust vector along the desired direction and solar panels towards the sun. In general, the two requirements are incompatible. Power sharing between energy accumulation and thrust application removes this obstacle. C) In LEO, power accumulation enables thruster operation during the eclipses. 
In this paper, an autonomous guidance is proposed. The autonomous guidance employs a power distribution policy between the thrusters and the battery, acting as a buffer. The proposed control law is perfectly adapted to Lyapunov-type guidance. Its programmatic implementation is sufficiently compact and quite affordable for a typical onboard computer. The paper presents a comprehensive description of relevant algorithms, as well as numerical simulations of representative examples. The obtained results demonstrate the efficiency of the proposed strategy.

Abstract document

IAC-07-C1.6.05.pdf

Manuscript document

IAC-07-C1.6.05.pdf (🔒 authorized access only).

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