No abstract

With the progression of high-power electric propulsion and high thrust-to-power propulsions system, thrust stand diagnostics require high-fidelity calibration systems that are accurate over a large-range of thrust levels. Multi-mode and variable I(sp) propulsion devices also require that a single stand be capable of measuring thrust from 10's of uNs to 100's of mNs. While the torsional thrust stand mechanic and diagnostics are capable of operating over such a large range, current pulsed calibration schemes are typically limited to a few orders of magnitude of dynamic range. In order to develop a stand with enough dynamic range, two separate calibration methods have been examined and compared to create a combined system. Electrostatic fin (ESF) and piezoelectric impact hammer (PIH) calibration systems were simultaneously tested on a large scale torsional thrust stand system. The use of the these two methods allowed the stand to be calibrated over four orders of magnitude, from 0.01 mNs to 750 mNs. The ESF system produced linear results within 0.52% from 0.01 mNs to 20 mNs, while the PIH system extended this calibration range from 10 mNs to 750 mNs with an error of 0.99%. The two calibration methods agreed within 4.51% over their overlapping range of 10-20 mNs.

Solar thermal propulsion offers a unique combination of high thrust and high specific impulse that can provide competitive advantages relative to traditional satellite propulsion systems. Enhancing the functionality of this technology will require a robust thermal energy storage method that can be combined with a means of thermal-to-electric conversion (i.e. thermophotovoltaic cells). This combination creates a high performance dual mode power and propulsion system that can eliminate the traditional photovoltaic-battery combination on existing satellites. A thermal energy storage system based on the phase change of molten elemental materials is proposed as the enabling technology. Molten boron is identified as the optimal phase change material (PCM), but presents significant engineering challenges. Thus, molten silicon is proposed as a near term, moderate performance storage option. A systems level comparison against existing technologies shows that both thermal storage materials present a performance benefit versus current technological benchmarks, and with optimistic future assumptions, it appears that a boron-based system can provide a ∆V improvement of more than 40% while maintaining rapid satellite maneuverability. An ongoing experimental effort is focused on producing a proof of concept thermal energy storage system. Materials testing has determined the stability of boron nitride in the presence of molten silicon in the short term, and solar furnace testing has resulted in silicon melting for the first time. Testing of the solar furnace using copper as a surrogate PCM has revealed experimental concerns with PCM heat transfer rates and has resulted in a design for a new full scale solar furnace. This furnace will operate at scales that are relevant to spacecraft development.

Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing this collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS. 16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF ABSTRACT 18. NUMBER OF PAGES 19a. NAME OF RESPONSIBLE PERSON

A dual-axis torsional thrust stand was successfully demonstrated at the Air Force Research Laboratory, enabling direct simultaneous thrust and mass loss measurement for the Air Force Electrospray Thruster Series 2 passively fed electrospray thruster. The dual-axis system is effectively two nulled torsional thrust stands sharing a single dual-axis gimbal with a thrust and mass resolution of ±0.2  µN and ±0.04 mg, respectively. The development of this system was inspired by a need for direct efficiency characterization of electrosprays via in situ mass measurements, and performance was compared to thruster masses measured pre- and post-testing using an analytical balance. Mass consumption data captured via the dual-axis stand, which is calibrated to a traceable uncertainty of 1.6%, varied between −5% and 18% as compared to analytical balance measurements throughout a multi-month testing effort highlighting the limitations in pre/post-weighing as a method for capturing propellant consumption due to absorption of atmospheric moisture when thrusters are removed from vacuum. Thrust stand tests were limited to short term operation with a daily available testing window of ∼5 h due to thrust stand drift following the 24 h cyclic temperature variations of the testing facility. A thorough investigation into the root cause of ambient thermal drift suggests that the thermal response of commercial flex-pivot bearings is directly producing spurious torques on the order of 10  μN m/°C. Additionally, unresolved charging effects on thrust stand hardware currently limit thrust stand operation to tests operating with a positive thruster polarity. Further development and long duration test stability require both a targeted investigation into flex-pivot thermal response and minimization of facility effects.