Micro-Propulsion

Without a dedicated propulsion system, nano-satellite platforms can never totally realize the potential of replacing their larger counterparts, imposing a limit on the exponential growth that CubeSats launches have shown in recent years. Propulsive capabilities would enable them to engage in a wider range of missions such as those characterized by many satellites flying in formation or in a constellation, possibly even in very low-altitude orbits. The strict mass, volume, and power limitations typically imposed by nano-satellite requirements need unique micro-technologies to help develop a compliant propulsion system. Micro-ElectroMechanical Systems (MEMS) at a micro-scale size and high level integration are very good examples of suitable technologies for this class of satellites.

In the aerospace industry there is currently a growing interest in green, non-toxic propellants. Unfortunately, especially when chemical propulsion concepts are considered, a large portion of the good-performance propellants are apt to be a very active chemical, and most of them are corrosive, flammable, and/or toxic. An alternative is to use electro-thermal propulsion, which allows good performance but also the use of “green” propellants. One of the most typical “green” propellant choices is the use of an inert gas. However, this leads to large storage tanks or, alternatively, to excessively high tank pressures. Ice or liquid water are other potentially promising green propellants, due to their high mass density and low molecular mass.

The Space Engineering Department is well known for its work on the design, development and launch of educational nano-satellites. The study and development of micro-propulsion systems for this class of small satellites is one of the most important short to medium term objectives in our current roadmap. We are presently researching two types of water-propelled micro-resistojets, mostly based on MEMS technologies: a low-pressure free molecular concept, and a more conventional one based on vaporization of liquid water. Both these concepts offer several advantages, including: high integration capability, small volume, light mass, fast response, high thrust-mass ratio, high reliability, and easy applicability to a thruster array. They offer very similar performance but, at the same time, they are complementary to each other in terms of operational conditions (in particular, the allowed propellant storage temperature and pressure). The MEMS components of these thrusters are manufactured through a collaboration with TU Delft’s Else Kooi Laboratory, taking advantage of the wide expertise of this partner in advanced MEMS systems research.

  • The liquid water concept thermally gasifies liquid water to a high temperature vapor for expulsion via a conventionally shaped nozzle, and has a wide potential for in-orbit maneuvers of nano-satellites. The general structure of the thruster is based on a modular design, in which the different parts (inlet section, heating chamber, nozzle) can be interchanged in order to test different combinations of concepts. The resistive micro-heater elements are made of either Silicon Carbide or Molybdenum, based on technologies recently developed at the Else Kooi Laboratory.
  • The free molecular concept, with its low plenum gas pressure of less than 1000 Pa, can provide a thrust level on the order of several mN to a few mN and is suitable for precise attitude control of nano-satellites. In the baseline idea, water is stored in the tank as ice and operates below its triple point, under sublimating conditions: the ice molecules sublimate to maintain the pressure inside the tank equal to the vapour pressure (approximately 600 Pa at 0 °C). The molecules then move from the tank through the feed system into a plenum, and finally flow through one or more micro-channels with high-temperature walls, which in this concept are at the same time the heating elements and the expansion slots.
Parameter Liquid Water,
Resistojet
Free Molecular
Resistojet
[Unit]
Chamber/Channel temperature 550 573 [K]
Chamber/Plenum pressure 5x105 150 [Pa]
Propellant mass flow rate 1.63 1.32 [mg/s]
Power transferred to water 5.25 4.51 [W]
Thrust (vacuum) 1.52 1.14 [mN]
Specific impulse (vacuum) 94.9 88.1 [s]
Total impulse 46.4 43.2 [Ns]
Propellant mass 50 50 [g]
Propellant mass < 360 < 330 [g]

 Table: Target performance of the two micro-propulsion concepts when used in a typical CubeSat mission, under comparable operational conditions with the same amount of propellant.

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