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184 MEMS and Microstructures in Aerospace Applications

or isothermality across a large area; such requirements can generally be achieved
only by active control techniques.
Designing effective TCS with suitable thermal control method becomes a
challenging task for spacecraft thermal engineers. To develop a successful TCS, it
is necessary to understand the basics of heat transfer in space, the functionality of
a thermal control component, and the operation of an integrated thermal system.
Miniaturization of future spacecraft results in high power densities, lower heat
capacity, and reduced available power. Microelectromechanical systems (MEMS)-
based solutions can provide efficient and miniaturized TCS. As the MEMS know-
ledge base matures, thermal controls are emerging as a viable technology for thermal
engineers. These applications include specialized thermal control coatings, thermal
switches, and filters for instruments. MEMS technology presents both benefits and
challenges for thermal engineers. Lack of in-flight MEMS data is one of the
challenges to using space-based MEMS TCS. As a consequence, in order to design
a MEMS thermal control device and receive the full advantage, it is important for
understanding the potential impact of the space environment on MEMS devices.
The following discussion is intended to provide some insight to these issues, and it
begins with a discussion of basic thermal control design consideration.

9.2 PRINCIPLES OF HEAT TRANSFER

To understand thermal control, one needs to understand the transport of heat in
space. Heat transfer deals with the movement of thermal energy from one quantity
of matter to the other. In the simplest terms, the discipline of heat transfer is
concerned with only two things: temperature and heat flow. Temperature represents
the amount of thermal energy available, whereas heat flow represents the movement
of thermal energy from region to region. Heat is a form of energy transfer. It is
‘‘work’’ on the microscopic scale that is not accounted for at the macroscopic level.
A mass of material may be considered as a thermal energy reservoir, where heat is
manifested as an increase in the internal energy of the mass. A change in internal
energy may be expressed as shown in the following equation:

DE ¼ C p mDT (9:1)

where E: thermal energy change (J)
1
C p : specific heat at constant pressure (J kg 1 K )
M: mass (kg)
T: temperature change (K).

Heat transfer concerns the transport of thermal energy. There are three modes
2
of heat transport, namely, conduction, convection, and radiation. In practice, most
situations involve some combination of these three modes. However, in space, all
heat must ultimately be rejected by radiation.

© 2006 by Taylor & Francis Group, LLC