Propulsion Systems

As micro and nanosatellites promise revolutionary access to space, several concepts have already been proposed for a standardised architecture. The important functional subsystems are,

• Guidance and navigation

• Attitude determination and control

• Telemetry, tracking and commanding

• Propulsion, power unit and thermal control

• Payload, structure and structural elements

Fig. 9.13 shows a basic diagram of a cold-gas system for control of attitude around one axis of the spacecraft (Manzoni 2001). Among the several subsystems, propulsion and attitude control are the most difficult to be miniaturised, due to the complex interaction between physical principles, technologies and engineering problems. The important micropropulsion approaches are i) Chemical micropropulsion (Liquid-gas propellant (Cold-gas, Monopropellant, Bi-propellant and Turbo-pump fed systems) and Solid propellant, and ii) Electric micropropulsion. Brief descriptions on solid propellant, cold- and warm-gas, monopropellant and bipropellant systems, colloid thrusters and regenerative-pressurisation cycles are presented below.

Fig. 9.13 shows a basic diagram of a cold-gas system for control of attitude around one axis of the spacecraft (Manzoni 2001). Among the several subsystems, propulsion and attitude control are the most difficult to be miniaturised, due to the complex interaction between physical principles, technologies and engineering problems. The important micropropulsion approaches are i) Chemical micropropulsion (Liquid-gas propellant (Cold-gas, Monopropellant, Bi-propellant and Turbo-pump fed systems) and Solid propellant, and ii) Electric micropropulsion. Brief descriptions on solid propellant, cold- and warm-gas, monopropellant and bipropellant systems, colloid thrusters and regenerative-pressurisation cycles are presented below.

Fig. 9.13. Cold Gas main parts [1. Microthruster (Micronozzle, Pressure sensor, Microvalve, Mechanical module); 2. Thrusters group; 3. Fluidic connections; 4. Safety valve; 5. Gas tank (i.e. N2); 6. Mechanical interface; 7. Control unit (Microcontroller (MC), Valve controller (VC), Pressure read circuit (Pread)); 8. Power; 10. Satellite structure; 11. Control Software]

9.3.1 Solid Propellant

In a solid propellant each charge is considered as a single shot. It is possible to obtain an array of charges. Each shot generates a well-quantified impulse and patterns of charges can be fired to generate specific torques as well as thrust vectors. Issues include achieving consistency of propellant cavity filling, reliable ignition, and performance. Initiators as opposed to propellants have been considered; hence analysis of detonation rather than combustion is required. A major question is how the area required for the thrusters array will impact on photovoltaic panel mountings in the smallest satellites, reducing available power levels.

9.3.2 Cold-Gas

Cold-gas is a micro fluidic system. The main development issues are the demonstration of low power and high-pressure valves, the use of high-density impulse propellants (e.g. Xe, Butane, nitrous oxide) and integration of thrust chamber components with ancillary components in a small total package.

9.3.3 Colloid Thrusters

The colloid thrusters are vaporising micro liquid system. Presently there is no activity in Europe but they are studied strongly in USA.

9.3.4 Warm-Gas

Warm-gas improves the performance of a cold gas system by heating the propellant, up to the temperature limits of MST materials. Full integration of all the fluidic devices on the same chip will require a trade-off in terms of temperatures limits, and performance. Deposition of appropriate heaters, heat transfer to the propellant and retention of heat within the chamber are major issues to be resolved.

9.3.5 Monopropellant and Bipropellant Systems

The scaling of a combustion chamber is possible but completely new configurations are expected due to the different thermal behaviour and the underlying scaling effects. Issues such as effectiveness of catalytic decomposition (monopropellant), injection, mixing and combustion (bipropellant) on the microscale are very poorly understood. It requires considerable amount of modelling effort. The main operational challenges to be resolved are operations at high pressure and high temperature. Before development of a bipropellant system considerable research into methods of cooling will need to be carried out.

9.3.6 Regenerative-Pressurisation Cycles

The time needed to efficiently combust propellants is dependent on chemical reaction times, combustion pressure and chamber size. For the small chamber sizes dictated by micropropulsion systems, high-pressure operation is likely to be a prerequisite. Therefore, the selected pressurisation method may mandate pump feed. Pump feed is of particular interest for micropropulsion, as small pumps may compensate for the inability of MEMS valves to seal at high pressures.

9.3.7 ADCS

The purpose of an ADCS is to counteract the effects of disturbances which are present in space. By carrying out the calculation of the disturbances (Manzoni 2000) we obtain that the thrust required to control the attitude of a micro or nanosatellite ranges from few |iN to few mN. It is also very important to evaluate how often the thrusters must "fire" to keep the attitude under the requested range and accuracy (Manzoni 2004). The total number of firing is the ratio between the total mission time and the time necessary to correct the maximum allowed attitude error.

Fig. 9.14 shows how the number of pulses per orbit changes with the spacecraft size and the maximum allowed excursion in order to balance only the effects of the drag. It is clearly recognised that, in the field of chemical microthrusters, which are today the first choice for the fine attitude control of small satellites, the solid propellant thrusters arrays are not useful due to the very limited number of shots, while cold-gas systems are the only reasonable possibility due to the unlimited number of shots.

Fig. 9.14 shows how the number of pulses per orbit changes with the spacecraft size and the maximum allowed excursion in order to balance only the effects of the drag. It is clearly recognised that, in the field of chemical microthrusters, which are today the first choice for the fine attitude control of small satellites, the solid propellant thrusters arrays are not useful due to the very limited number of shots, while cold-gas systems are the only reasonable possibility due to the unlimited number of shots.

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