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TRIBOLOGY OF HIGH SPEED MOVING MECHANICAL SYSTEMS FOR SPACECRAFTS - TRIBOLOGICAL ISSUES
K. Sathyan
Tribology of High Speed Moving Mechanical Systems for Spacecrafts - Tribological Issues
K. Sathyan*
E-mail: krishnan.sathyan@gmail.com
* Department of Mechanical Engineering
Prince Mohammed Bin Fahd University, Po Box:1664, Al-Khobar- 31952, KSA
Tel.: +966 38498532; +966 505181702
ABSTRACT
Spacecraft regardless of size, type and purpose, usually contains a number of moving mechanical systems (MMS). Continual performance of these
systems only can guarantee the intended functions that are essential for successful operation of the spacecraft. Most of the problems encountered with
these moving systems are pertain to tribology. Space tribology is a subset of the lubrication field dealing with the reliable performance of satellites and
spacecraft including the space station. Lubrication of space system is still a challenging task before the tribologists due to the unique factors
encountered in space such as near zero gravity, hard vacuum, weight restriction and attention free operation. Ever since the space exploration, a
number of mission failures reported emanate from bearing system malfunction. A bearing in a moving mechanical assembly can fail due to multiple
reasons such as degradation of lubricant, loss of lubricant from the working zone by surface migration and evaporation, and retainer instability. Unlike
yester years, space missions of today are planned to last for 30 years or more. To achieve such long-term missions, tribologically efficient moving
mechanical systems are essential. This review briefs space tribology and tribological requirements of spacecraft moving mechanical systems.
Keywords: spacecraft, momentum wheel, tribology, lubrication, attitude control
INTRODUCTION
More than 50 years have passed since the beginning of the space
exploration. Still, malfunctioning of spacecraft components have been
observed throughout the world. In many cases, these component
failures lead to partial or total failure of the spacecraft mission. During
these years, tremendous growth has been observed in the electrical,
electronic and electromechanical components through disciplined
design, standardization and quality assurance practices. This progress
has helped in the miniaturization and hybridization of spacecraft
systems and the development of cost effective spacecraft missions.
However, notwithstanding the progresses made in the mechanical
engineering, spacecraft designers are still striving to develop efficient
mechanical systems that can cope with long-term requirements. During
mid-1960’s mission life requirements were 3 to 5 years and by the mid-
1970’s life requirements of 7 to 10 years were common [1]. But today,
attention is focused on the development of subsystems for spacecrafts
with longer mission duration of more than 30 years, a typical case
being the space exploration initiative (SEI) of NASA [2]. These
missions will require mechanical systems that operate for 30 years.
These long life requirements bring a lot of challenges with them,
especially in the area of moving mechanical systems.
Spacecraft incorporate a wide variety of moving mechanical systems
which must operate with total reliability in space environment. These
moving systems can be broadly classified as high speed systems which
include gyroscopes, momentum/reaction wheels etc., and low speed
systems that encompass the hinges, scanners, solar array drive etc. The
moving mechanical systems contain sliding or rolling contacts that are
required to operate with least frictional power loss, in view of limited
power availability on board the spacecraft. Each of these systems is
designed to perform some definite task. For example, gyroscopes are
used in the attitude control system (ACS) as an inertial sensor to detect
the attitude error of the spacecraft with respect to a reference object
(stars, sun, earth etc.). Similarly, momentum/reaction wheels are used
in the attitude control system as actuators to correct the attitude error
and maintain the spacecraft attitude. Thus attitude control can be
defined as the process of achieving and maintaining a desired
orientation of the spacecraft. This is vital in achieving the mission
objectives. Since control and maintenance of spacecraft attitude is a
continuous process, various elements of the attitude control system
have to work continuously from the beginning to end of the mission.
Moreover, high speed mechanical systems involved in this process are
prone to degradation failure. In these systems, failures are mostly
related to tribology. A number of mission failures are reported due to
the tribological malfunction of attitude control systems. Skylab and
Insat-1D are typical examples [3-5] and the most recent is the bearing
failure in the control moment gyro (CMG) of the international space
station on July 2002 [6]. Therefore, the development of high speed
attitude control systems for the future requires advancement of
tribology technology. Hence, by highlighting the tribological issues of
spacecraft attitude control systems here, possible tribological solutions
for the development of attitude control systems for future long-term
applications are elaborated.
SPACE TRIBOLOGY –OVERVIEW
Tribology is defined as the science and technology of interacting
surfaces in relative motion, or in other words, it is the study of friction,
wear and lubrication. It is a truly interdisciplinary field that
encompasses material science, chemistry, physics, mechanics,
thermodynamics etc. The word “tribology” was introduced in 1966 by
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