A Hybrid Cold Gas Microthruster System for Spacecraft
12
A hybrid cold gas microthruster system for spacecraft Johan Ko Èhler * , Johan Bejhed, Henrik Kratz, Fredrik Bruhn, Ulf Lindberg, Klas Hjort, Lars Stenmark The A Ê ngstro Èm Laboratory, The A Ê ngstro Èm Space Technology Centre, Uppsala University, Box 534, Uppsala, Sweden Received 25 June 2001; accepted 25 September 2001 Abstract A hybrid cold gas microthruster system suitable for low Dv applications on spacecraft have been developed. Microelectromechanical system (MEMS) components together with ®ne-mechanics form the microthruster units, intergrating four independent thrusters. These are designed to deliver maximum thrusts in the range of 0.1±10 mN. The system includes three different micromachined subsystems: a nozzle unit comprising four nozzles generating supersonic gas velocity, i.e. 455 m/s, four independent piezoelectric proportional valves with leak rates at 10 6 scc/s He, and two particle ®lters. The performances of all these MEMS subsystems have been evaluated. The total system performance has been estimated in two parameters, the system-speci®c impulse and the mass ratio of the propulsion system to the spacecraft mass. These ®gures provide input for spacecraft design and manufacture. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Cold gas micropropulsion; Micronozzle; Piezoelectric valve 1. Introduction The development of microthruster systems, reaching for increasingly smaller thrust levels, strongly depends on the ef®cient utilisation of microelectromechanical system (MEMS) technology [1]. Several concepts have been dis- cussed in [2±7] where main topics include the type of propulsion used and the level of integration aimed at. Here, we investigate a hybrid integration solution, i.e. connecting and mounting of different MEMS units by conventionally machined housing parts. Furthermore, the MEMS units are served by external electronics. Thus, the complete micro- thruster system is assembled as shown in Fig. 2. The system can be designed for maximum thrust levels from 0.1 to 10 mN. Such thrusts can be used for main propulsion of short-term nanoprobe missions, or attitude control and drag compensation on larger spacecraft. This cold gas microthruster system releases minute amounts of gas at supersonic speed through micronozzles, thus, providing precise momentum to the spacecraft. Piezo- electric valvesÐusing feedback from differential pressure sensors straddling each nozzleÐproportionally modulate the gas ¯ow to the nozzles. Heat exchangers with thin ®lm heaters and temperature sensors in the nozzle unit increase the ef®ciency of the thruster. Micromachined ®lters protect these MEMS units from degrading particle contamination due to incoming gas and surrounding space. 2. Cold gas micropropulsion In cold gas micropropulsion, the force to the system is delivered by ejecting mass (i.e. gas) at high speeds. The force is determined by Newton's second law of motion under the assumption of constant exit velocity of the gas, thus, F Q m v e (1) Here, Q m is the mass ¯ow and v e is the exit velocity of the matter. Thus, the mass of the propellant is utilised to its maximum by optimising the exit velocity. In this context, a converging±diverging nozzle is suitable, named a Laval nozzle (Fig. 1). Subsonic gas entering a converging zone increases its velocity, while supersonic gas increases its velocity in a diverging zone. Thus, if the gas reach sonic speed at the nozzle throat, it will continue its velocity increase to super- sonic values beyond the throat in the nozzle expansion area. Sensors and Actuators A 97±98 (2002) 587±598 * Corresponding author. Tel.: 46-1847-17253; fax: 46-1855-5095. E-mail address: [email protected] (J. Ko Èhler). 0924-4247/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0924-4247(01)00805-6
A Hybrid Cold Gas Microthruster System for Spacecraft
