Thomas Randolph

Evaluation of a 4.5 kW Commercial Hall Thruster System for NASA Science Missions

The readiness of commercial Hall thruster technology is evaluated for near-term use on competitively-award, cost-capped science missions like the NASA Discovery program. Scientists on these programs continue to place higher demands on mission performance that must trade against the cost and performance of propulsion system options. Solar electric propulsion (SEP) systems can provide enabling or enhancing capabilities to several missions, but the widespread and routine use of SEP will only be realized through aggressive cost and schedule risk reduction efforts. Significant cost and schedule risk reductions can potentially be realized with systems based on commercial Hall thruster technology. The abundance of commercial suppliers in the United States and abroad provides a sustainable base from which Hall thruster systems can be cost-effectively obtained through procurements from existing product lines. A Hall thruster propulsion system standard architecture for NASA science missions is proposed. The BPT-4000 from Aerojet is identified as a candidate for near-term use. Differences in qualification requirements between commercial and science missions are identified and a plan is presented for a low-cost, low-risk delta qualification effort. Mission analysis for Discovery-class reference missions are discussed comparing the relative cost and performance benefits of a BPT-4000 based system to an NSTAR ion thruster based system. The BPT-4000 system seems best suited to destinations located relatively close to the sun, inside approximately 2 AU. On a reference near Earth asteroid sample return mission, the BPT-4000 offers mass performance competitive with or superior to NSTAR at much lower cost. Additionally, it is found that a low-cost, mid-power commercial Hall thruster system may be a viable alternative to aerobraking for some missions. Suggestions for the near- and far-term implementation of commercial Hall thrusters on NASA science missions are discussed.

Life and Operating Range Extension of the BPT-4000 Qualification Model Hall Thruster

Following completion of the 5,600 hour qualification life test of the BPT-4000 4.5 kW Hall Thruster Propulsion System, NASA and Aerojet have undertaken efforts to extend the qualified operating range and lifetime of the thruster to support a wider range of NASA missions. The system was originally designed for orbit raising and stationkeeping applications on military and commercial geostationary satellites. As such, it was designed to operate over a range of power levels from 3 to 4.5 kW. Studies of robotic exploration applications have shown that the cost savings provided by utilizing commercial technology that can operate over a wider range of power levels provides significant mission benefits. The testing reported on here shows that the 4.5 kW thruster as designed has the capability to operate efficiently down to power levels as low as 1 kW. At the time of writing, the BPT- 4000 qualification thruster and cathode have accumulated over 400 hours of operation between 1 kW-2 kW with an additional 600 hours currently planned. The thruster has demonstrated no issues with longer duration operation at low power.

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) and subsequently delivered to ESA for spacecraft integration. 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) is a payload on the ESA LISA Pathfinder spacecraft along with the European gravitational reference sensor (GRS) as part of the ESA LISA Technology Package (LTP). To achieve the nanometer-level precision spacecraft control requirements, each of eight thruster systems is required to provide thrust between 5 and 30 µN with resolution ≤0.1 µN and thrust noise ≤0.1 µN/vHz. 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. In the summer of 2009 the cluster assemblies were delivered to ESA along with the IAU for integration into the LISA Pathfinder spacecraft. Spacecraft-level testing will include magnetics, acoustic, and thermal vacuum environmental testing with a planned launch and flight demonstration in April 2012. 1 2

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.

Solar Electric Propulsion for Discovery-Class Missions

This paper offers a user-centric consolidation and comparison of the full range of government and commercial solar electric propulsion options available in the near term for primary propulsion on deep-space science missions of the class commonly proposed to NASA’s Discovery program. Unlike previous papers, this work does not emphasize feasibility from a mission-analysis perspective. Rather, it emphasizes requirements uniquely imposed by competitively reviewed cost-capped mission proposals, for which system-level flight heritage and cost credibility can trump sheer performance and mission capture. It describes criteria that mission architects and review boards can use to select and evaluate electric propulsion systems, provides descriptions of viable government and commercial electric propulsion system options, describes the modifications needed to adapt commercial electric propulsion systems to deep space, and discusses appropriate methods for costing commercial-based electric propulsion systems. It concl...

Solar Electric Propulsion Gravity-Assist Tours For Jupiter Missions

Several Hall thrusters (e.g. BPT-4000, BHT-600, SPT-100, etc.) are able that operate with useful thrust at the sub-kilowatt power levels that would be available from solar arrays at Jupiter distance. We have found that a combination of a multi-kilowatt thruster (e.g. the BPT-4000) for the interplanetary trajectory with a sub-kilowatt thruster (e.g. the BHT-600) is sufficient for a Europa flyby mission. A roughly 1200 kg spacecraft using this propulsion approach would be able to launch on a Falcon 9 and reach Jupiter in 4.9 years. We demonstrate the feasibility of a Solar Electric Propulsion (SEP) Jovian tour with an example tour that reaches Europa 1.6 years after Jupiter arrival with a remaining capability of 400 m/s of ΔV for additional flybys.

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.

Power and Propulsion System Design for Near-Earth Object Robotic Exploration

Near-Earth Objects (NEOs) are exciting targets for exploration; they are relatively easy to reach but relatively little is known about them. With solar electric propulsion, a vast number of interesting NEOs can be reached within a few years and with extensive flexibility in launch date. An additional advantage of electric propulsion for these missions is that a spacecraft can be small, enabling a fleet of explorers launched on a single vehicle or as secondary payloads. Commercial, flight-proven Hall thruster systems have great appeal based on their performance and low cost risk, but one issue with these systems is that the power processing units (PPUs) are designed for regulated spacecraft power architectures which are not attractive for small NEO missions. In this study we consider the integrated design of power and propulsion systems that utilize the capabilities of existing PPUs in an unregulated power architecture. Models for solar array and engine performance are combined with low-thrust trajectory analyses to bound spacecraft design parameters for a large class of NEO missions, then detailed array performance models are used to examine the array output voltage and current over a bounded mission set. Operational relationships between the power and electric propulsion systems are discussed, and it is shown that both the SPT-100 and BPT-4000 PPUs can perform missions over a solar range of 0.7 AU to 1.5 AU - encompassing NEOs, Venus, and Mars - within their operable input voltage ranges. A number of design trades to control the array voltage are available, including cell string layout, array offpointing during mission operations, and power draw by the Hall thruster system.

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.