SMALLSAT PROPULSION Pete Smith, Roland McLellan Marotta UK Ltd, Cheltenham, and Dave Gibbon SSTL, Guildford, UK. ABSTRACT This paper presents an overview of the components, systems and technologies used by Marotta UK and SSTL for smallsat propulsion over the last 5 years, and gives a brief insight into current developments for future smallsat, scientific and other satellite applications. Components which have been developed by Marotta UK have been used extensively in the propulsion systems of the satellites built and flown by SSTL. These include thruster valves, isolation valves, high pressure regulator valves, fill valves and propellant tanks for both liquefied and gaseous propellants. The technologies introduced to the SSTL satellite designs include nitrogen and xenon cold gas, and liquefied nitrous oxide and butane propellants. These technologies have been successfully applied to Rapideye, DMC, Uosat-12, and SNAP-1. In addition, developments with hybrid propellant and water propulsion have been made at SSTL. By building up the capabilities and confidence over an extended period, the cost and risk to any one programme were minimised. Hence, by starting with the low pressure liquefied propellant for Uosat-12, on which satisfactory performance in orbit was demonstrated, it was possible to introduce new components allowing performance improvements on subsequent missions. These have included high pressure gas storage and regulation, and zero-slosh tankage for liquefied propellant applications. This paper describes the enabling components developed by Marotta UK, and the propulsion systems and their performance as established by SSTL. Future component development is aimed at both the improvement of existing components by reducing size, mass and power demand, and the design, testing and qualification of new component and system technologies. With the trend toward smaller, more capable satellites, the size and performance of the propulsion components and propellant volume can become a dominant feature of the vehicle. Developments under investigation aimed at performance improvement include both chemical and electric propulsion components and systems, where electric refers to examples such as low power electrical heating of a resistive element over a period of time prior to thruster operation, and chemical refers to the use of green propellants for monopropellant and bipropellant thrusters. A 200mN bipropellant chemical thruster with a specific impulse of 200s and using green propellants could provide a useful addition to the component range and allow more advanced missions to be contemplated.
The paper also considers some flight performance data and shows how these propulsion systems have been used to enable missions. For example AlSat-1 used its on-board propulsion system to correct a launch injection error of 50km, and a phased constellation of 4 DMC spacecraft was formed, also using their own propulsion systems. SSTL PROPULSION SYSTEMS The various SSTL spacecraft with propulsion systems in orbit, in build or in development are listed below: The paper will briefly describe these propulsion systems. SSTL DEVELOPMENTS Hydrogen peroxide thruster Hybrid thruster (peroxide and PMMA) (Ref 1) Water resistojet (Ref 2) Low power resistojet (Ref 3, 4, 5) Xenon propulsion system Solar thermal engine (Ref 6) These developments will be described in the paper.
MAROTTA UK PRODUCTS Cold gas thruster valve (Ref 7) Several types of cold gas thruster valve are produced by Marotta UK. They are typically designed for low pressure operation, and rely on a solenoid actuator. An example is shown in Figure 1. The inlet pressure to this thruster is controlled by a pressure regulator. Figure 1 Example of Cold Gas Thruster Cold gas high pressure regulator valve A high pressure solenoid valve was produced to act as the active element in an electronic pressure regulator. This valve has an operating inlet pressure of 120bar, and a pressure control loop using a downstream pressure sensor drives open the valve when the downstream pressure falls below a set point. The set point is essentially the thruster valve inlet pressure. Propellant tanks Propellant tanks for use with either liquefied or gaseous propellant have been developed and flown. One of the key aspects of the tank when used with liquefied propellant is the minimisation of propellant slosh by means of a series of baffles. Fill valves The Marotta UK fill valves are a standard product, and are used on many European satellites. The range can be used with high or low pressure gas, and with all types of chemical propellant, fuels and oxidisers. System design, assembly and test (Ref 8) Marotta UK offers a system design, assembly and test capability, as first demonstrated on Uosat-12, and more recently on ESA s Cryosat.
MAROTTA UK DEVELOPMENTS Proportional micronewton thruster (Ref 9) This thruster is capable of producing extremely small thrust levels at high accuracy and stability. The thrust range is from the leakage level, typically 60nN, to 1000µN, and the specific impulse is maintained at a reasonable value of around 70s by means of a nozzle heater. Flow control valve for hydrazine thruster A flow control valve for use with hydrazine thrusters has recently been designed, assembled and tested. Performance is as follows: Parameter Operating Pressure Operating Voltage Electrical Power Pressure Drop Response Time Mass Inlet Filter Value 34.5bar max 24 +/- 4Vdc 14W to open 2W for continuous operation 200mbar at 5g/s water 2.6ms opening at 34.5bar 3.0ms closing at 34.5bar 44gm 20µm nominal 200mN bipropellant thruster The development of a 200mN bipropellant thruster is currently under investigation. Resistojet thruster The resistojet thruster combines the thruster valve technology described above with that of the MicroNewton thruster heated nozzle. The result is a high performance thruster with a specific impulse of up to 150s, depending on the propellant and the heater power. REFERENCES 1. An Alternative Geometry Hybrid Rocket for Spacecraft Orbit Transfer Manoeuvres, G Haag, M Sweeting, G Richardson, IAF-2000-W.2.07. 2. The Design, Development and In-Flight Operation of a Water Resistojet Micropropulsion System, D Gibbon, I Coxhill, AIAA-2004-3798. 3. The Design, Development and I-Flight Performance of a Low Power Resistojet Thruster, D Gibbon, M Baker, D Nicolini, D Robertson, C dye, AIAA-2003-4548. 4. Xenon Resistojet Design and Development, K Chojnacki, R Reinicke, IEPC-99-022.
5. Xenon Resistojets as Secondary Propulsion, G Saccoccia, E Chesta, D Gibbon, M Baker, D Nicolini, D Robertson, IEPC-03-305. 6. Results of a Solar Thermal Engine Ground Test, F Kennedy, P Palmer, M Paul, AIAA- 7. Numerical Simulation and Experimental Validation of Viscous-Dominated Micro-Nozzle Flow in Cold Gas Thrusters, B Thornber et al, ESA Space Propulsion 2004. 8. Cryosat Cold Gas System and Component Development, P Smith, S Edwards, N Solway, AIAA- 2004-9. Design and Performance of a MicroNewton Proportional Thruster, P Smith, N Solway, R McLellan, ESA Space Propulsion 2004.