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

Abstract

For future applications to precision formation flying missions, NASA's 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 μN and a thrust noise of less than 0.1 μN/√Hz for thrust levels between 5 and 30 μN. Developed by Busek Co. with support from JPL in testing and 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.

Recent Advances in MEMS-Based Microthrusters

“…Furthermore, an addressable colloid microthruster array was designed by Paine and Gabriel with an expected minimum thrust of 0.1 µN [102], in which the source emitter and extractor were designed in the same wafer. Colloid thrusters also were developed for the ST7-DRS (Space Technology 7 Disturbance Reduction System) and LISA (Laser Interferometer Space Antenna) Missions [103][104][105]. However, further developments were limited to the packaging problems caused by high starting and working voltages ranging from 5 kV to 10 kV [106].…”

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

“…Lower level propulsion performance requirements are derived from the level 1 requirements [1][2][3][4] , as shown in Table 2. The thrust range requirement is determined by the need to counter the solar radiation pressure on the spacecraft and provide full control authority in all axes.…”

“…All the qualification tests and verification of the requirements are discussed more in Section IV. * The LISA thrust range requirement may be lower for the science phase and higher for tip-off recovery † The LISA mission has an operational goal of 8.5 years that will require an additional 3.5 years worth of consumables § By calculation a range of approximately 3-50 µN is possible within the nominal operational constraints of the thruster Environmental temperature requirements for the ST7 CMNT are shown in Table 3 3 . Operational temperature requirements are driven by the viscosity of the propellant at the minimum temperature and the conductivity of the fluid at the maximum temperature to produce the required thrust range.…”

“…The two Colloid Micro-Newton Thruster (CMNT) clusters delivered by Busek Co., Inc. 2,3,7,8,9,[10][11][12][13][14][15] each include four complete and independent thruster subsystems: 4 thruster heads, 4 propellant storage and feed systems (including microvalves), and 4 PPUs, as well as 1 cathode neutralizer and 1 DCIU for each cluster. Thrust is adjustable from 5-30 µN by changing the beam voltage (2-10 kV) and/or the propellant flow rate that determines the beam current (2.25-5.4 µA).…”

“…The Disturbance Reduction System (DRS) for each spacecraft consists of capacitive position sensors in the Gravitational Reference Sensor (GRS), microthrusters as precision position control actuators, and drag-free control laws that maintain the spacecraft position and cancel out the environmental disturbances, mainly due to solar photon pressure. To measure gravitational waves, LISA must operate in a drag-free environment with stringent requirements on both the pointing and the translation of the spacecraft 2,3 .…”