Dario Modenini

Systems Design and Performance of Cold Gas Microthruster for Microsatellite Attitude Control

We discuss our approach to the design and manufacturing of a cold gas micropropulsion system, using micro electromechanical systems (MEMS) devices, for an experimental attitude and orbital control system for the ALMASat microsatellite. ALMASat is a small educational spacecraft entirely designed and assembled in the aerospace laboratories of II School of Engineering of the University of Bologna. Its weight is about 12 kg, carried by a cubical bus (side 30 cm), and it is scheduled for launch from Baikonur (Kazahstan) using the DNEPR Launch Vehicle. The core of the micropropulsion system is a De Laval nozzle which accelerates the fluid, molecular nitrogen, from a plenum, through a 40 micron throat at supersonic velocity. Using the nozzle as a simple cold-gas thruster, good Isp efficiencies are demonstrated, although side-wall boundary layer contamination, no continuity effect and the limitations of MEMS fabrication technologies are clear limiting factors. The micropropulsion system consists of a high pressure tank, a pressure regulator and solenoid valves for the open/close thrusters cycles. The extruded nozzle geometry, made on silicon wafer, has been manufactured in collaboration with the Carlo Gavazzi Space (CGS) and IMM section of the Italian National Research Council (CNR) of Bologna, using a Deep Reactive Ion Etching (DRIE) technique and successive bonding.

Heat transfer measurements in a supersonic wind tunnel through inverse temperature data reduction: application to a backward facing step

An inverse heat transfer data reduction method is presented, which allows performing quantitative heat transfer measurements using pre-heated models in a blow-down supersonic wind tunnel. The flow under investigation is a backward-facing step (BFS) at Mach 3. Temperature measurements are performed by means of infrared thermography. The inverse heat transfer procedure is used to calculate the surface heat flux from the measured surface temperature in time by taking into account multi-dimensional and unsteady conduction effects. The robustness and sensitivity of the data reduction algorithm is assessed using synthetic experiments where aspects such as measurement noise, calibration errors, conductivity errors, etc. are addressed. The BFS flow is investigated for laminar, transitional and turbulent-separating boundary layers. It is found that for the laminar and transitional cases, the Stanton number and normalized separation length are influenced by the step height. For turbulent separation, this is only marginally the case; however the Stanton number downstream of reattachment increases with step height. For the transitional separation, large heat transfer peaks are measured at reattachment, of magnitude up to twice the reference values for a developed turbulent boundary layer.

Experimental Verification of a Simple Method for Accurate Center of Gravity Determination of Small Satellite Platforms

We propose a simple and relatively inexpensive method for determining the center of gravity (CoG) of a small spacecraft. This method, which can be ascribed to the class of suspension techniques, is based on dual-axis inclinometer readings. By performing two consecutive suspensions from two different points, the CoG is determined, ideally, as the intersection between two lines which are uniquely defined by the respective rotations. We performed an experimental campaign to verify the method and assess its accuracy. Thanks to a quantitative error budget, we obtained an error distribution with simulations, which we verified through experimental tests. The retrieved experimental error distribution agrees well with the results predicted through simulations, which in turn lead to a CoG error norm smaller than 2 mm with 95% confidence level.