Disturbance reduction system: testing technology for precision formation control

Abstract

The Disturbance Reduction System (DRS) is a space technology demonstration within NASA's New Millennium Program. DRS is designed to validate system-level technology required for future gravity missions, including the planned LISA gravitational-wave observatory, and for formation-flying interferometers. DRS is based on a freelyfloating test mass contained within a spacecraft that shields the test mass from external forces. The spacecraft position will be continuously adjusted to stay centered about the test mass, essentially flying in formation with the test mass. Colloidal microthrusters will be used to control the spacecraft position within a few nanometers, over time scales of tens to thousands of seconds. For testing the level of acceleration noise on the test mass, a second test mass will be used as a reference. The second test mass will also be used as a reference for spacecraft attitude. The spacecraft attitude will be controlled to an accuracy of a few milliarcseconds using the colloidal microthrusters. DRS will consist of an instrument package and a set of microthrusters, which will be attached to the European Space Agency's SMART-2 spacecraft with launch scheduled for August 2006.Keywords: formation flying, colloidal microthrusters, disturbance reduction DISTURBANCE REDUCTION SYSTEM OVERVIEWThe Disturbance Reduction System (DRS) is designed to validate system-level technology required for two types of future missions: measurements of planetary gravity and of cosmic gravitational waves, and precision formation-flying interferometers. The validated technology will feed directly into the LISA gravitational-wave observatory, the MAXIM X-ray interferometer mission, and several other missions within the NASA roadmap. The DRS is based on the concept of a freely floating test mass contained within a spacecraft that shields the test mass from external forces. The test mass will ideally follow a trajectory determined only by the local gravitational field. The spacecraft position must be continuously adjusted to stay centered about the test mass, essentially flying in formation with the test mass. The DRS performance is characterized by the extent to which unwanted accelerations appear on the test mass and the accuracy with which the spacecraft is centered on the test mass. The project goals are to demonstrate acceleration levels below 3×10 -14 m/s 2 /√Hz and position control to 10 nm/√Hz over a frequency range of 1 mHz to 10 mHz. In order to measure the level of accelerations appearing on the test mass, its trajectory must be compared with a reference trajectory. For DRS, the reference is provided by a second identical test mass located within the same instrument assembly. Being located in the same spacecraft, the second test mass must be controlled at frequencies below the measurement bandwidth to keeps its position relative to the primary test mass, while being free of control forces within the measurement bandwidth to provide a reference for acceleration measurements. The position of the second test ma...

Characterization of a Six-Emitter Colloid Thruster Using a Torsional Balance

“…In the absence of a balance, we recommend taking an independent measurement of the mass flow rate (e.g., using an flowmeter in line with the propellant feed system) to verify the accuracy of the value given by Eq. (6), which also serves as a confirmation of the thrust measurement. However, if a more detailed characterization of the thrust is required (e.g., higher measurement accuracy or the determination of thrust noise), a good performing balance is the only option.…”

“…6,8 The objective of this program is to develop and test in space technologies for precision spacecraft position control. The mission is based on two novel technologies: gravity reference sensors (GRS) and colloid thrusters.…”

“…Most applications of this type fall in two groups: drag-free flying and separated spacecraft interferometer missions. Drag-free flying requires the use of thrusters to counterbalance forces such as atmospheric drag in near Earth orbits 5 or solar pressure, 6 so that the spacecraft trajectory is only determined by the gravity field. In the case of separated spacecraft interferometer missions, the position of optics on different spacecrafts must be controlled to a fraction of a wavelength, so that the light collected from different spacecrafts can be combined coherently.…”

The Expansion of Colloid Thruster Beams and its Dependence on Emission Temperature

“…This technology will be tested in the Disturbance Reduction System-Space Technology 7 mission, which is a precursor of the Laser Interferometer Space Antenna mission (LISA). 1 Electrospray thrusters are also characterized by high efficiency at low power (e.g. the propulsive efficiency of an emitter operating at a fraction of a micro-Newton typically exceeds 80%); 2 a very broad thrust range while maintaining high efficiency (emitter multiplexing via microfabrication can produce an electrospray source with an emitter number ranging from one to approximately 1000 emitters per square centimeter, with a typical thrust per emitter of 0.5 N); and the freedom to operate at varying I SP by adjusting the charge to mass ratio of the propellant (rather than by the inefficient reduction of the acceleration voltage).…”

Colloid Micro-Newton Thruster Development for the ST7-DRS and LISA Missions

“…In addition, due to the unique and challenging nature of the measurements, each agency is sponsoring a technology demonstration mission prior to LISA in the near future. Space Technology 7 Disturbance Reduction System (ST7-DRS) is the US technology demonstration mission, sponsored through the NASA New Millennium Program and managed by the Jet Propulsion Laboratory (JPL) 2 . The LISA Test Package (LTP) is the European technology demonstration mission, part of the ESA LISA Program 3 .…”