• CubeSat
• development
• electric propulsion
• FEEP
• field emission
• indium
• liquid metal
• LMIS
• micropropulsion
• thruster
• wetting

Field Emission Electric Propulsion (FEEP) thrusters are a promising technology in the field of space propulsion, with unparalleled high efficiency, low thrust and noise, which makes them unique micropropulsion candidates for the growing market of CubeSats, apart from drag-free satellites, high-accuracy attitude and orbit control and other space applications.
The present thesis is devoted to the research and development of an indium FEEP thruster. In the past, caesium has been extensively used in FEEP devices due to its desirable wetting and emission characteristics. However, the difficulty and danger of handling it, added to risks of explosion, drive the push towards new, safer FEEP propellants. In this line, indium is selected for this project, given its relatively good properties, such as an atomic mass similar to that of caesium, a higher yet feasible melting temperature, and theoretically a good wettability on metals.
In the first place, an extensive literature study was performed. The physics of FEEPTs are addressed from a general perspective, describing the electrohydrodynamics that govern their behaviour. Furthermore, a detailed emission and performance model was developed and tested in a particular case study (FOTEC's indium capillary emitter, 2011), with good results in typical operating conditions. Building on the work of M. Tajmar in the past years, a novel mass efficiency model was realised.
Besides, despite being customarily neglected, a droplet current model based on the work of Fernandez de la Mora and Loscertales (1994) was developed, proving the latter assumption. However, the model relies on some particular parameters, of which little data is available in the literature, and hence is dependent on test data.
Secondly, considerable experimental activity was carried out at the Space Propulsion Division of Sitael S.p.A. As a first step, the wetting capability of liquid indium was assessed in several tests. Although reported as good in the literature, the wettability of indium proved to be poor on different metals as Inconel and tungsten, even at temperatures up to 700 ºC. It was discovered that indium would wet well nickel already at about 300 ºC, whilst wetting tungsten and tantalum (typical materials
for emitter/reservoir surfaces) at no less than 1000 ºC.
Building on the heritage of Sitael (formerly Alta) on FEEPTs, a slit emitter and a flat-oval accelerator plate were realised, in order to attempt field emission of indium. A two-piece emitter, sputter-deposited with nickel for the sake of wetting,
with a slit height of 8 mum and a "pool" reservoir carved inside, was built. This novel design would greatly simplify the FEEP system, eliminating the need of a feed line and an external tank. After two attempts, field emission was accomplished, but in an undesirable way: at the tip of drops spilling from the edge of the emitter.
The absence of proper emission along the slit is believed to be caused by inaccurate alignment of the half-emitters and excessive flow. Several measures are proposed to resolve said issues and accomplish appropriate emission of the prototype.