This research was conducted within the framework of the SpaceDrive project funded by the German Aerospace Center to develop propellantless propulsion for interstellar travel. The experiments attempted to measure mass fluctuations predicted by the Mach-effect theory derived from General Relativity and observed through torsion balance measurements by Woodward (2012). The combination of such mass fluctuations with synchronized actuation promises propellantless thrust with a significantly better thrust-to-power ratio than photon sails. Thus, experiments using different electromechanical devices including the piezoelectric Mach-effect thruster as tested by Woodward et al. (2012) were pursued on sensitive thrust balances. The tests were automated, performed in vacuum and included proper electromagnetic shielding, calibrations, and different dummy tests. To obtain appropriate driving conditions for maximum thrust, characterization of the experimental devices involved spectrometry, vibrometry, finite element analysis, and circuit modeling. Driving modes consisted of sweeps, resonance tracking, fixed frequency, and mixed signals. The driving voltage, frequency, stack pre-tension, mounting, and thruster orientation were also varied. Lastly, different amplifier electronics were tested as well, including Woodward’s original equipment.
Experiments on the double-pendulum and torsion balances with a resolution of under 10 nN and an accuracy of 88.1 % revealed the presence of force peaks with a maximum amplitude of 100 nN and a drift of up to 500 nN. The forces mainly consisted of switching transients whose signs depended on the device’s orientation. These force transients were also observed in the zero-thrust configurations. No additional thrust was observed above the balance drift, regardless of the driving conditions or devices tested. In addition, finite element and vibrometry analysis revealed that the vibration from the actuator was transmitted to the balance beam. Moreover, simulations using a simple spring-mass model showed that the slower transient effects observed can be reproduced using small amplitude, high-frequency vibrations. Hence, the forces observed can be explained by vibrational artifacts rather than the predicted Mach-effect thrust.
Then, centrifugal balance experiments measured the mass of a device subjected to rotation and energy fluctuations, with a precision of up to 10 µg and a high time resolution. The measurements relied on piezoelectric- and strain gauges. Their calibration methods presented limitations in the frequency range of interest, resulting in discrepancies of up to 500 %. However, the tests conducted with capacitive and inductive test devices yielded experimental artifacts about three orders of magnitude below the mass fluctuations of several milligrams predicted by the Mach-effect theory. Although the piezoelectric devices presented more artifacts due to nonlinearity and electromagnetic interaction, all rotation experiments did not show the expected dependence on the rotation frequency.
In summary, the search for low thrust and small mass fluctuations consisted of challenging experiments that led to the development of innovative and sensitive instruments, while requiring a careful consideration of experimental artifacts. The results analysis led to the rejection of mass fluctuations and thrusts claimed by Woodward’s Mach-effect theory and experiments. The quest for breakthrough space propulsion must thus continue a different theoretical or experimental path.:List of Figures
List of Tables
List of Abbreviations
List of Variables and Symbols
1. Introduction
1.1 Research Motivation
1.2 Objectives
1.3 Content Overview
1.4 Team Work
2. Literature Review
2.1 Fundamentals of Space Propulsion
2.2 Mach’s Principle
2.3 Woodward’s Mach-effect Theory
2.3.1 Derivation of the Mass Fluctuation Equation
2.3.2 Design of a Mass Fluctuation Thruster
2.4 Woodward-type Experiments
2.5 Force and Transient Mass Measurements
3. Electromechanical Characterization
3.1 Piezoelectric Actuators
3.1.1 Basic Properties
3.1.2 Actuator Design
3.1.3 Mach-effect Thruster Devices
3.1.4 Magnetostrictive Actuator
3.1.5 Numerical Analysis of MET Behavior
3.1.6 Vibrometry Analysis
3.1.7 Impedance Spectroscopy
3.1.8 Circuit Modeling
3.1.9 Predictions
3.2 Electronics
3.2.1 Description
3.2.2 Characterization
3.3 Torsion Balances
3.3.1 Description
3.3.2 Characterization
3.3.3 Simulation
3.4 Double-pendulum Balance
3.4.1 Description
3.4.2 Characterization
3.5 Laboratory Setup
3.5.1 Vacuum Chambers
3.5.2 Software and Test Setup
4. Thrust Balance Experiments
4.1 Torsion Balance I Test Results
4.1.1 Dummy Tests
4.1.2 CU18A
4.1.3 MET03
4.1.4 MET04
4.1.5 Discussion
4.2 Torsion Balance II Test Results
4.2.1 Dummy Tests
4.2.2 MET05
4.2.3 Beam Vibration
4.2.4 Discussion
4.3 Double-pendulum Balance Test Results
4.3.1 Dummy Tests
4.3.2 MET03
4.3.3 Discussion
5. Centrifugal Balance Experiments
5.1 Centrifugal Balance
5.1.1 Description
5.1.2 Centrifugal Devices
5.1.3 Predictions
5.2 Transducer Calibration
5.2.1 Quasi-Static Calibration I
5.2.2 Quasi-Static Calibration II
5.2.3 Dynamic Calibration
5.3 Centrifugal Balance Test Results
5.3.1 Characterization
5.3.2 CD01
5.3.3 CD02
5.3.4 CD03
5.3.5 CD04
5.3.6 CD05
5.4 Discussion & Error Analysis
6 Conclusions
6.1 Research Summary
6.2 Further Research
Appendix A
Appendix B
Bibliograph
