John K. Ziemer

ST7-DRS Colloid Thruster System Development and Performance Summary

This paper summarizes Buseks five year effort to develop Colloid Micro- Newton Thrusters (CMNT) for the LISA Path Finder (LPF) mission. The LPF is jointly pursued by NASA and ESA as a technology demonstration mission with a planned launch date of 2010. Busek recently delivered two (2) flight CMNT Clusters, each containing four (4) CMNTs, to NASA JPL that sponsored and assisted Busek in the development. Following further integration with additional hardware, JPL will deliver the system to ESA. The LPF and future formation flight/interferometer missions have extraordinary propulsion requirements that proved to be very challenging to meet and to verify by direct measurements. Salient among them is the broadly adjustable thrust from 5 to 30μN that each CMNT must deliver with a step precision better than 0.1mN and thrust noise lower than 0.1mN/root Hertz over a frequency interval of 1m Hz to 4Hz. These specifications required the development of new and pioneering hardware including CMNTs that use novel ionic liquid as a propellant, unique propellant management and storage system with flow controlling microvalves that must regulate propellant flow with nL/min precision, carbon nanotube based field emission neutralizers, and high voltage (0 to 10kV) power processors in compact low mass package. The paper summarizes the system and key component design, the test program, and the salient performance results. Companion papers provide further details.

Colloid micro-Newton thruster development for the ST7-DRS and LISA missions

We present recent progress and development of the Busek Colloid Micro-Newton Thruster (CMNT) for the Space Technology 7 Disturbance Reduction System (ST7-DRS) and Laser Interferometer Space Antenna (LISA) Missions.

ST7-DRS Mission Colloid Thruster Development

To meet the needs of the LISA Pathfinder (LPF) mission, Busek has designed, fabricated, characterized, and delivered two clusters of colloid thrusters, each containing four thrusters. Various tests were performed to demonstrate that the thrusters met mission specifications. These tests included lifetime, environmental extremes, specific impulse, and plume divergence/stability testing. The results showed that the Busek designed thruster delivered 5µN to 30µN with thrust noise levels below 0.1µN/√Hz, over the frequency range of 1mHz to 4Hz. The thruster has been demonstrated for 3462 hours of operation, with no degradation in performance. In particular, the thrust noise remained nearly an order of magnitude below specifications at hour 3400. Furthermore, the thrust vector stability of the ejected plume has been shown to be an order of magnitude below the mission specified value of 2.5mrad/√Hz. Measurements of the thruster plume also showed that the half-angle of the plume divergence is no greater than 24o in any operational condition. This testing demonstrates that the thrusters developed by Busek meets or exceeds the specified requirements of the LPF mission.

Delivery of Colloid Micro-Newton Thrusters for the Space Technology 7 Mission

Two flight-qualified clusters of four Colloid Micro-Newton Thruster (CMNT) systems have been delivered to the Jet Propulsion Laboratory (JPL). The clusters will provide precise spacecraft control for the drag-free technology demonstration mission, Space Technology 7 (ST7). The ST7 mission is sponsored by the NASA New Millennium Program and will demonstrate precision formation flying technologies for future missions such as the Laser Interferometer Space Antenna (LISA) mission. The ST7 disturbance reduction system (DRS) will be on the ESA LISA Pathfinder spacecraft using the European gravitational reference sensor (GRS) as part of the ESA LISA Technology Package (LTP). Developed by Busek Co. Inc., with support from JPL in design and testing, the CMNT has been developed over the last six years into a flight-ready and flight-qualified microthruster system, the first of its kind. Recent flight-unit qualification tests have included vibration and thermal vacuum environmental testing, as well as performance verification and acceptance tests. All tests have been completed successfully prior to delivery to JPL. Delivery of the first flight unit occurred in February of 2008 with the second unit following in May of 2008. Since arrival at JPL, the units have successfully passed through mass distribution, magnetic, and EMI/EMC measurements and tests as part of the integration and test (I&T) activities including the integrated avionics unit (IAU). Flight software sequences have been tested and validated with the full flight DRS instrument successfully to the extent possible in ground testing, including full functional and 72 hour autonomous operations tests. Delivery of the cluster assemblies along with the IAU to ESA for integration into the LISA Pathfinder spacecraft is planned for the summer of 2008 with a planned launch and flight demonstration in late 2010.

Microthruster Propulsion for the Space Technology 7 (ST7) Technology Demonstration Mission

For future applications to precision formation flying missions, NASAs New Millennium Program is scheduled to test colloid micro-Newton thrusters (CMNTs) on the ST7 technology demonstration mission. These CMNTs are part of a disturbance reduction system (DRS) on the ESA SMART-2 Spacecraft or LISA Pathfinder. The goal of the ST7 DRS is to demonstrate technologies necessary to meet the nanometer precision positioning control requirements of the LISA mission. In order to achieve these goals, the CMNTs are required to demonstrate a thrust resolution of less than 0.1 micro-N and a thrust noise of less than 0.1 micro-N/[square root]Hz for thrust levels between 5 and 30 micro-N. Developed by Busek Co. with support from JPL in testing an design, the CMNT has been developed over the last four years into a flight-ready microthrust system. The development, validation testing, and flight unit production of the CMNTs are described. Development tests and analysis include preliminary wear tests, propellant loading process verification, flow testing, and performance verification. Validation and flight unit verification includes thermal and structural analysis, life testing, thermal and dynamic load testing, and performance verification. Final delivery of the units is planned in 2007 with and planned launch and flight demonstration 2009.

Colloid thruster propellant stability after radiation exposure

We present a characterization of the stability characteristics of the ionic fluid EMI-Im to the operation of colloid microthrusters in gamma radiation environments.

Flow Control Micro-valve for the ST7-DRS Colloid Thruster

A piezo-actuated, variable opening micro-valve was developed to meet the control requirements for the ST7-DRS LISA Pathfinder colloid thruster. The micro-valve has demonstrated ~3.5 micro-liters/min maximum liquid flow rate with a 2.5 picoliter/sec resolution. During dynamic thrust operation where commanded thrust varies by ≈5% at 10 Hz the micro-valve is capable of commanding thrust to within 1% error. The thrust error is maintained within 0.01% during steady state thrust operation. The valve has a normal operating range of +10 0 C to +30 0 C, with a max operational range of -5 0 C to +50 0 C, and a non-operational range of -100C to +600C. The micro-valve is fully operational after surviving a 20g quasi-static environment, and is designed to operate in a high voltage (10kV) environment.

Mission enabling and enhancing spacecraft capabilities with microNewton electric propulsion

The capability to significantly improve current spacecraft pointing, precision orbit maintenance and disturbance mitigation were considered using precision, quiescent microNewton electric propulsion systems.1,2 The results of the analyses showed that electric propulsion systems operating in the microNewton to hundreds of microNewtons thrust range can offer significant improvements over state-of-the-art mission capabilities to enable 30 m Earth-fixed orbital tubes, constellation spacecraft position control to within nanometers and exoplanet observatory pointing with 0.1 milliarcsecond precision. Electric propulsion thrust characteristics required to support these capabilities are discussed.

Colloid Thruster Contamination Modeling for the LISA Pathfinder Mission

The LISA Pathfinder (LPF) Mission, a joint ESA-NASA mission, is a demonstration mission of technologies needed for future interfero meter missions designed to detect gravity waves. Among of the technologies to be demonstrated on LPF are micro-propulsion thrusters that are part of the Disturbance Reductio n System (DRS) designed to maintain the spacecraft position relative to the proof mass with in 10 nm. Both cesium Field Emission Electric Propulsion (Cs FEEP) thrusters and Colloid Micro-Newton Thrusters (CMNT) are planned to be flown on LPF. Based on the locations of the thrusters on the spacecraft, neutral cesium flux from the Cs FEEP will impinge on the CMNT. A monolayer of cesium on the CMNT insulator would cause the impedance between electrodes to decrease below the

Colloid Thruster Propellant Conditioning and Storage for the NASA ST7 Mission

An overview of the NASA ST7 Colloid MicroNewton Thruster propellant storage system is presented, detailing some of the unique challenges of the ST7 mission propellant storage and pressurization. Particular focus is upon propellant contribution to the life-limiting mechanism of ion grid contamination during operation. This paper provides a description of the performance effects of propellant impurities and the techniques employed by Busek to overcome them as part of its development efforts for the NASA ST7 mission.