Aerospace and Electronic Systems Magazine July 2017 - 22

Feature Article:

DOI. No. 10.1109/MAES.2017.160104

A Study of Spacecraft Reaction Thruster Configurations
for Attitude Control System
Milad Pasand, Sharif University of Technology, Tehran, Iran;
Ali Hassani, University of Arizona, Tucson, AZ, USA;
Mehrdad Ghorbani, Sharif University of Technology, Tehran, Iran

INTRODUCTION
Reaction thrusters (RTs) are used as an alternative to momentum
exchange devices when disturbance torques exceed the control authority of momentum exchange devices. The reaction control system (RCS) can employ some rocket thrusters to provide attitude
control during the thrusting or coast phase. Within the control loop,
the RCS's target could be either achieving and keeping a certain
attitude or controlling the rate of an attitude change. In the coast
phase, some tasks such as preacceleration, settling of liquid propellant, damping of structural vibrations, providing a velocity increment to separate stages and payloads, and carrying out orbital and
nonorbital maneuvers may be included in its functions. The propulsion perturbation torques, whose size is relatively large, are primarily produced because of the center of mass offset, which itself is
produced as a result of static unbalance, transient gas flow phenomenon, and nozzle cant angle misalignments. Nozzle cant angle misalignments are produced because of manufacturing tolerances and
pressurizations, which should be compensated by the attitude control system. In addition, rocket thruster plume impingement against
surrounding structure or components produces sizable disturbance
torques and cross-coupling effects that degrade the dynamic stability and increase the duty cycle that should be corrected by additional
thrusters to restore vehicle attitude. During each maneuver, there
are always undesirable angular rotations in consequence of some
errors and uncertainties in the various components of a spacecraft.
The attitude control system has to be capable of compensating these
imperfect effects of the mechanical system.
To realize the preceding aims, various types of torque providers are utilized in different space missions. The momentum exchange device, magnetic torque actuator, and solar torque actuator are examples of linear torque providers that produce torque in
ranges of 0.02-1, 10−2-10−3, and 10−5-10−6 Nm, respectively [1].
Authors' current addresses: M. Pasand, M. Ghorbani, Department of Aerospace Engineering, Sharif University of Technology, Azadi Avenue, Tehran, Iran, E-mail: (milad_pasand@
alum.sharif.edu). A. Hassani, Department of Aerospace and
Mechanical Engineering, The University of Arizona, 1130 N.
Mountain Avenue, P.O. Box 210119, Tucson, AZ 85721, USA.
Manuscript received April 26, 2016, revised October 29, 2016,
and ready for publication December 28, 2016.
Review handled by M. Jah.
0885/8985/17/$26.00 © 2017 IEEE
22

The only way to provide the required level of torques for the attitude control system of spacecraft that need a fast attitude maneuver
is to use RTs. RTs are not linear and continuous actuators, and they
operate in a pulsing mode. In other words, the RTs are unidirectional on-off actuators that are capable of providing the desired
amount of torque in any desired direction. Nonlinear attributes of
RTs bring about some difficulties in the attitude control analysis.
RTs produce the force with a constant magnitude; thus, they need
to be commanded through one of modulation methods.
The applications of unidirectional thrusters in the design of an
attitude control system are abundant in underwater vehicles [2]-
[4], space vehicles [5]-[7], and aerial vehicles [8]. A comprehensive study was done in [9] regarding spacecraft failures from 1980
to 2005 that contains 129 different spacecraft. It was shown that almost 50% of attitude and orbit control system (AOCS) failures are
attributed to the combination of the following components: gyroscopes, momentum wheels, and thrusters. Because actuator failure
can lead to complete mission failure, redundancy is a key solution
to having a reliable spacecraft. Along with redundant control configuration, the fault-tolerant control algorithms were developed to
handle thruster failures; in particular, adaptive control allocation
algorithms were applied to overactuated attitude control systems
without changing the main control algorithm [10]-[14].
One of the first researchers who did a comprehensive study regarding the relation between geometry of thruster configurations
and level of redundancy (LR) was Crawford [5], [6]. In this paper,
a necessary and sufficient condition (considering the LR) for judging whether a thruster configuration is capable of achieving an mdimensional task was achieved. Sanchez Pena et al. [15] completed
the previous study and established the necessary and sufficient condition in the presence of bounded thrusts. In [16], the minimum required unilateral thrusters for controllability of a planar rigid body
were investigated. The researcher considered the problem using one,
two, and three thrusters and derived the controllability conditions.
In another study, Chen [17] introduced a definition of thruster configuration performance called control capability (C.C.) or control
authority. The minimum C.C. was defined as forces and torques produce by thrusters in the weakest direction. In [18], a thorough algorithm was presented to verify whether a mission is admissible under
certain conditions by evaluating the minimum distance between the
C.C. boundary and the mission forces and torque requirements.
In the attitude control system based upon RTs, it is necessary
to know which thrusters and when they have to be activated for

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