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230 MEMS and Microstructures in Aerospace Applications

11.4.2 System Requirements ................................................................... 264
11.5 Conclusion ................................................................................................. 264
References............................................................................................................. 265

11.1 INTRODUCTION
Development of nanosatellites is presently a strong interest of the USAF as well as
of NASA, DARPA, and MDA. 1–3 Spacecraft designs are tending towards smaller,
less expensive vehicles with distributed functionality. NASA’s future vision is one
of reprogrammable or reconfigurable autonomous systems; small, overlapping
instruments; and small, inexpensive micro-, nano- or even picosatellites. Examples
include the nanosatellite program and the Orion Formation experiment. This new
trend evokes the same advantages that drive computing towards distributed, parallel
systems and the Internet. There are already examples of distributed satellite net-
works, such as the Tracking and Data Relay Satellite System (TDRSS), Intelsat,
GPS, Iridium, Globalstar, and the Space-Based Infrared System (SBIRS). However,
while these are groups of satellites designed to accomplish a common goal, they are
nevertheless ‘‘noncooperating.’’ The new wave of proposed constellations will be
groups of vehicles that interact and cooperate to achieve mission goals. In such
groups, vehicle pointing and positioning will be managed collectively. Fleets will
evolve over time, extending and enhancing the overall capabilities. Also, autono-
mous vehicles will eliminate the need for extensive ground support. From a
programmatic perspective, the concept is to replace multi-instrument observatories
with low-cost, short lead-time spacecraft that would allow adaptation to changing
conditions. This in turn mitigates the risk that not all formation-flying applications
provide full programmatic benefits.

Tomorrow’s Air Force will rely a new generation of smaller, highly capable nano and
picosatellites (having masses of 10 and 1 kg respectively) that will act singly or
4
collaboratively to accomplish various space missions. (M. Birkan, AFOSR )
In order to fulfill the mission requirements for the small spacecraft’s new types
of micro- and nanothrusters are required that offer a wide range of thrust levels from
micronewton (mN) to newton levels at high overall thrust efficiencies and with very
low (<1 kg) total thruster and power processing unit (PPU) mass. This chapter will
try to introduce a variety of technologies that aim to satisfy these goals.
The simplest of all propulsion systems appears to be the cold gas thruster:
a pressurized gas is released to produce thrust, but its exhaust velocity is so small
that it would be necessary to carry a significant amount of propellant for large D-V
missions. Systems like the so-called laser ablation thruster, where mass is energized
by incident laser light to produce a higher exhaust velocity, may carry significant
amounts of overhead mass. Other candidate electric propulsion engines that might
be scaled down include the microcolloid thruster or the field emission electric
propulsion (FEEP) thruster, which produce fairly small (mN) thrust levels and
require high voltages for operation. The vacuum arc thruster as well as the micro-

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