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Micropropulsion Technologies 233

and electrical) due to a lack of understanding of the flows in such devices. This is due
to the fact that the continuum assumption commonly used in gas and plasma dynam-
ics is no longer valid at smaller densities and characteristic dimensions of flow. The
Knudsen number is defined as the ratio of the mean free path of gas molecules to a
characteristic dimension of flow. As the Knudsen number increases, the collision rate
becomes too low to maintain local thermodynamic equilibrium. Furthermore, the
expansion of a propellant from chamber conditions to vacuum often involves flow
regimes from continuum to transition to free molecular, though the smallest devices
may not have any component in the continuum regime. Therefore, fairly complicated
models are needed for proper evaluation, which goes beyond the scope of this review.
More detailed descriptions of these effects can be found elsewhere. 6,7
All these considerations demonstrate that both chemical and electrical propul-
sion systems need to be included in this chapter and that a decision between either
system has to be made on a case-by-case basis. The emphasis will be put on MEMS
and other low-mass systems (i.e., where the total system dry mass is less than
1000 g). The principle of operation will be discussed for each system, using few
basic equations describing the performance. While simplistic, these basic equations
will nevertheless help to understand the operating characteristics of the various
micropropulsion technologies and calculate rough estimates of their performance.
After describing each system, its key parameters will be discussed and the
performance for each system will be summarized in a table. Technologies discussed
here include (a) chemical propulsion systems, such as hydrogen peroxide thrusters,
cold gas thrusters, solid micro rockets and (b) electric propulsion systems, such as
pulsed plasma thrusters, laser-driven plasma thrusters, field effect thrusters, ion
engines, and resistojets. While many publications about these types of propulsion
systems cite performance specifications of the propulsion device (i.e., the micro-
manufactured emission array or the MEMS-valve), this chapter tries to take a look
at the complete system, thereby providing information that is needed to successfully
design a satellite. Improvements to existing systems and new propulsion technolo-
gies will emerge and may well be superior to those mentioned, which also implies
that the numbers cited here are by no means absolute limitations. In this light, I
would also like to refer to other review articles on micropropulsion, with the most
important and complete one authored by Ju ¨rgen Mu ¨eller from NASA JPL. 8
Regarding the formality of this chapter, I took the liberty of referring to most
publications used in the beginning of each chapter, instead of placing the citations
in the body of the text. By doing so, it became much easier to read, digest, and
summarize. I hope that none of the original authors will take offense even if a
certain thought in the body of the text may have come from a single paper only.

Enjoy!

11.2 ELECTRIC PROPULSION DEVICES
In this review, electric propulsion systems are defined as those where the majority
of the energy needed for operation is electrical energy.

© 2006 by Taylor & Francis Group, LLC