Micropropulsion technology assessment for DARWIN: evaluation
of existing and emerging micropropulsion technologies for
the
DARWIN attitude and position control system

Abstract: The ESA-mission DARWIN will be a formation-flying interferometer,
consisting
of a total of eight spacecraft whereof six will carry telescopes. Launch is
planned for 2015. The primary aim is to detect earth-like planets around
nearby stars, and to determine whether there are any signs of life.

The interferometry requires precise and stable positioning of the
spacecraft
in the constellation, which puts high demands on the attitude and position
control system. The purpose of this report is to evaluate both current and
emerging micropropulsion technologies, resulting in a recommendation and
further investigation of the systems best suited for the DARWIN
micropropulsion system. Field-Emission Electric Propulsion (FEEP), colloid
thrusters, Cold Gas Microthrusters and pulsed-plasma thrusters (PPT) are
among the technologies included in the study. Separate trade-offs are made
for precision-pointing and coarse manoeuvres, taking into account
properties
such as thrust resolution and range, power consumption, mass and volume. In
addition, key mission drivers of DARWIN are closely assessed. Contamination
of the telescopes and other highly sensitive optical hardware is one of the
main issues, together with the extreme demands on resolution and variable
thrust in the micronewton range. The cryogenic payload needs to be kept at
temperatures down to 40 K, which adds thermal restrictions on the
propulsion
system. Furthermore, since the mission lifetime is set to 5 years with a
possible extension to 10 years, reliability and lifetime are other major
factors.

It is found that the emerging concept of micromachined Cold Gas Thrusters
appears to be the most promising choice. Cold Gas Microthrusters with
internal heating have a number of advantages that make them more favourable
than other systems, for example clean propellant, low power consumption and
a wide thrust range. The low specific impulse is the main drawback. With
internal heating however it exceeds 100 s, which seems acceptable for
DARWIN.