Robotics facilities have a long history in the development of space equipment, since they allow to perform tests on systems like guidance, navigation and control, visual-based navigation and docking mechanisms. Those facilities are based on two manipulators, one representing the a target satellite, the other the chaser satellite which perform a relative motion with respect to the first. This approach has been used in the past to perform tests on docking operations, visual-base navigation system to populate databases. Delft University of Technology recently developed its own robotics facility for GNC and multisatellite systems applications. It hosts two robots on a moving base, which work in synergy to extend the operational space. They operate in a dark environment, where there are lights to simulate the sun disturbance and a beamer that projects the Earth to have a representative background. The purpose of this paper is to describe the laboratory, along with the control architecture of the robots and provide some tests executed to assess the accuracy of them in tracking a given trajectory.

This brief presents a comprehensive approach for the modeling of micropropulsion systems based on the vaporization of a liquid. The model combines the analytical and empirical relations derived from extensive experimental analysis and fundamental physical laws. This allows modeling of key parameters, such as mass flow rate, for the entire system comprising a tank to store the liquid propellant, a valve to control the mass flow, and a microthruster that vaporizes the propellant and accelerates it generating thrust. The model is evaluated by a sensitivity analysis considering the boundaries of the modeling space, and it has been tested in a simulation loop demonstrating the attitude control of a nanosatellite using a set of four thrusters. The results of the simulation are used to test the developed model.

The growing needs of nano- and pico-satellite missions require several enhancements in micropropulsion capabilities to enable the satellites to perform an increasing variety of orbital maneuvers. This paper presents the design and the implementation of a closed-loop control system for thrust magnitude regulation in micro-resistojets. This is achieved by controlling the propellant mass flow in the microvalve of the feeding system which is designed to make extensive use of off-the-shelf components. The Vaporizing Liquid Micro-resistojet (VLM) is one of the micro-thrusters under development at TU Delft and, for this reason, it is selected for performing the tests. In order to develop appropriate controllers, a non-linear state-space model of the microvalve system is developed analytically and integrated with the analytical model of the complete system. The performance of the analytical model is compared to a multi-domain analysis done with finite element method (FEM) and computational fluid dynamics (CFD). The controllers are designed and tested using the models developed. Finally, the closed-loop control system is implemented in the preliminary hardware design of the micropropulsion system, achieving a resolution of 10 μN in the range of 0-800 μN.

Microelectromechanical systems (MEMS) techniques uncovered new opportunities in satisfying the mission requirements of the growing next generation nano- and pico-satellite missions. In particular, micro-propulsion is universally recognized as one of the key enabling technologies to help this class of satellites making the next step and become credible candidates to a wide range of scientific and commercial applications. In this context, TU Delft is developing a miniaturized electro-thermal propulsion system operating with green liquid propellants, for application on a wide range of nano-satellite formats from CubeSats (10x10x10 cm units) to PocketQubes (5x5x5 cm units). A breadboard of the complete micro-propulsion system is under development at TU Delft, including the thruster, propellant tank, the valve and the driving electronics. The design of the system shall be easily adapted to both CubeSat and PocketQube standards, with particular attention to the second one since the system is scheduled for an initial flight demonstration on the first Delfi-PQ satellite. To address this need and to fill an existing gap in the state-of-the-art of micro-propulsion, two kinds of micro-thrusters are considered in this development; a Vaporizing Liquid Micro-resistojet (VLM) and a Low Pressure Micro-resistojet (LPM). A number of test results will be shown in the paper on the electrical, mechanical and functional characterization of the MEMS thrusters, fabricated in the Else Kooi Laboratory at TU Delft, and the other components of the system. Keywords: Micro-resistojet, Microthruster, MEMS, Cubesats, Pocketqubes