Investigation of the functioning of a liquefied-gas micro-satellite propulsion system

[摘要] ENGLISH ABSTRACT: The focus of this thesis is on the investigation of the functioning of a liquefied-gasthruster. Such a thruster could be used to provide secondary propulsion to a microsatellitein orbit. A general overview of the need for thrusters in micro-satellites is putforward in the introduction. Motivation for deciding to investigate a liquefied-gassystem is presented. Recent developments in the field of micro-satellites arediscussed as well as their relevance to the project undertaken. Fundamentalbackground theory relevant to the engineering problems associated with thedevelopment and analysis of such a system is also presented. Computer programswere written to simulate such a liquefied-gas thruster system. The experimental workcarried out to analyse the system from a practical view-point is documented.Attention is also given to the measurement and calibration techniques used to obtainexperimental data.One-dimensional fully explicit transient mathematical models of the thruster systemwere developed to model the system using both compressed air and butane aspropellants. These models were incorporated into computer programs used tosimulate the transient behaviour of the system. Although it is intended to use butaneas the propellant onboard a satellite, the reason for modelling and simulating a systemusing compressed air is because air is a convenient fluid to work with from both atheoretical and practical point of view.An experimental model of a thruster system was designed, built and tested using airand butane as propellants. Most of the model was built using perspex to allow for theobservation of the two-phase behaviour of the propellant inside the system. Locallypurchased components were used for the solenoid and fill valves. Readily availablebutane lighter fluid was used for butane testing. Self-made heating elements wereused to provide heat input to the propellant. Testing was done at different backpressures ranging from 100 kPa down to 20 kPa in a vacuum chamber.Good comparison between theoretical and experimental results was obtained for air.Theoretical results for peak thrusts tended to over predict experimental results byapproximately 15 % for a system exhausting to a pressure of 100 kPa. Peak thrusts ashigh as 0.2 N were obtained for vacuum tests conducted at an absolute pressure of 20kPa.Peak thrusts of approximately 50 mN were achieved for experimental testing IIIatmospheric conditions using butane with a starting pressure of between 270 and 290kPa. Typical average thrusts of between 20 mN and 30 mN were noted for butanetesting with initial pressure of between 200 to 300 kPa. Peak thrusts of over 0.1 Nwere observed for vacuum testing at an absolute pressure of 20 kPa. An equation tocorrelate the experimentally determined average thrust as a function of pulse durationand starting pressure was developed. This correlated most of the experimental data towithin ±25 %. Theoretical results for butane testing are able to predict peak thrustswithin approximately 20 % for starting pressures in the range of 200 to 300 kPa.Since the project was an exploratory investigation into a liquefied-gas thruster, someadditional aspects relating to such systems were also given attention. The effect ofliquid propellant motion or sloshing was considered and recommendations regardingthe design and placement of the propellant tanks were made. The use of heat pipes asan alternative to electrical heating elements was investigated and some elementarydesign aspects are presented graphically. The management of the liquid propellantusing surface tension devices was examined qualitatively.Recommendations relating to future projects in the field of simple, low-costpropulsion systems for micro-satellites are put forward. More specifically theserecommendations are with regard to: thermo-fluid modelling of the propellant, futureexperimental work to be done, techniques to measure small thrusts and vacuumchamber testing.

[发布机构] Stellenbosch University