A small scale system for releasing a test mass into a Geodesic orbit
Abstract/Contents
- Abstract
- Space-based applications in navigation, aeronomy, geodesy, and fundamental physics such as gravitational wave detection and tests of the Equivalence Principle require ultra-sensitive accelerometers or drag-free apparatuses to either measure disturbing forces (such as solar pressure and drag), or cancel them directly. Most solutions utilize a precisely manufactured Test Mass (TM) that is released into a housing (an empty volume) in a satellite in space, and made to follow a geodesic trajectory, with the host spacecraft shielding the TM from external disturbing forces. Depending on its mass, size, shape, and the launch vehicle environment, the TM may be restrained prior to release. Careful attention must be paid to mechanical and electrical forces that may cause the TM to adhere to its restraints or the walls of the cavity. Mechanisms to overcome the force of adhesion (``stiction'') are typically required. In the current state-of-the-art, either the spacecraft has significant propulsive capability to pull the TM from the restraint using inertial force, or the spacecraft uses some compressed gas or mechanical actuator to separate the TM from the housing. The Laser Interferometer Space Antenna (LISA) is the most salient example, as it uses a pure gold coating that is prone to creep and cold welding, and is therefore naturally prone to adhere to itself; its release process removes the caging force in three stages, finishing with a dynamic retraction of two contacting fingers from opposite faces. After release, the system faces a potential critical failure if the TM comes to rest in contact with the housing wall. The host spacecraft must either employ its propulsive capability to accomplish separation, or use some other actuator. Lacking the capability for the former, LISA will use an electrostatic actuator to arrest the resulting motion, since recontact with the housing could cause an unrecoverable condition. Efforts to advance the aforementioned science have been stymied in part by the enormous costs of fielding full-scale drag-free systems. Stanford has previously proposed a gravitational wave observatory called LAser GRavitational-wave ANtenna at GEo-lunar Lagrange Points (LAGRANGE) that would involve a dedicated mission of three spacecraft to the Earth-moon L3, L4 and L5 Lagrange points. Each spacecraft would host a Gravitational Reference Sensor (GRS) comprised of a spherical test mass in an evacuated cavity. The estimated cost for this mission is almost \$1 billion. Stanford has also proposed a Drag Free CubeSat as a pathfinder mission to verify the design of individual components. Placing a GRS in a nanosatellite, such as a standard 3U CubeSat (3 to 4 kg), and sharing a ride to orbit with a large satellite, provides a viable path forward to proving the necessary technologies and building up flagship experiments. This``small scale'' GRS uses a TM of 25.4 mm diameter instead of 70 mm for LAGRANGE that fits completely within a 3U CubeSat, achieving acceleration noise performance of approximately $10^{-12}$ $\frac{m}{s^2}\sqrt{Hz}$ at 1 mHz. A GRS in a 3U CubeSat will present unique challenges that must be solved. In particular, it will have extreme limitations volume (3000 cm\textsuperscript{3}) and propulsive acceleration (as little as $2.5 \times 10^{-5}$ $\frac{m}{s^2}$). The small scale GRS requires a 50 mm diameter volume for the TM to achieve its objective performance, and additional volume for the support mechanisms. The gap size also restricts the effectiveness of electrostatic actuators, so the CubeSat mission cannot use traditional methods to accomplish release of the TM. It is shown in this work that the acceleration authority is sufficient to overcome the force of adhesion (due to van der Waals forces, for example) if the TM comes into contact with the wall of its housing, provided the coating material and injection strategy are carefully selected. Gold coating, although employed by the LISA design, has been shown by others to exhibit unacceptably high adhesion for short durations at light joining loads, compared to the requirements of the Drag Free CubeSat. A new experiment has been performed to measure the adhesion force between the spacecraft interior and TM coated in a promising alternative material for this application: Silicon Carbide. A spacecraft-like environment has been created in the laboratory, and a measurement of the force of adhesion between coated, polished surfaces has been made. The measurement shows that, for short duration contact, a small scale satellite is indeed capable of overcoming the force of adhesion for incidental collision between the spacecraft and the test mass. These data resolve the problem of re-contact after un-caging. A new {\it release strategy} has been developed to resolve these outstanding issues. It has been shown that injection can be accomplished using a mechanism with one moving part and one passive, compliant part. The strategy relies on multiple bounces of the TM against the housing wall, and makes use of the durability and low adhesion properties of SiC coating. A test platform for the mechanism has been developed and demonstrated in a representative microgravity laboratory. The design of the mechanism benefits greatly from recent and concurrent advances in charge management, optical position measurement, and carbide coatings. Together, these technologies enable the performance of science experiments.
Description
| Type of resource | text |
|---|---|
| Form | electronic; electronic resource; remote |
| Extent | 1 online resource. |
| Publication date | 2017 |
| Issuance | monographic |
| Language | English |
Creators/Contributors
| Associated with | Hultgren, Eric L |
|---|---|
| Associated with | Stanford University, Department of Aeronautics and Astronautics. |
| Primary advisor | Rock, Stephen M |
| Thesis advisor | Rock, Stephen M |
| Thesis advisor | Buchman, Saps |
| Thesis advisor | Lipa, John A |
| Advisor | Buchman, Saps |
| Advisor | Lipa, John A |
Subjects
| Genre | Theses |
|---|
Bibliographic information
| Statement of responsibility | Eric L. Hultgren. |
|---|---|
| Note | Submitted to the Department of Aeronautics and Astronautics. |
| Thesis | Thesis (Ph.D.)--Stanford University, 2017. |
| Location | electronic resource |
Access conditions
- Copyright
- © 2017 by Eric Lee Hultgren
- License
- This work is licensed under a Creative Commons Attribution Non Commercial 3.0 Unported license (CC BY-NC).
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