Microwave Ignition of ASCENT via Surface Wave Generator

Kravet, Ethan

Microwave Ignition of ASCENT via Surface Wave Generator

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Author:: Kravet, Ethan
Graduate Program:: Aerospace Engineering
Degree:: Master of Science
Document Type:: Master Thesis
Date of Defense:: November 10, 2023
Committee Members:: Sven G Bilén, Thesis Advisor/Co-Advisor
Amy Ruth Pritchett, Committee Member
Jacob Langelaan, Professor in Charge/Director of Graduate Studies
Sean David Knecht, Committee Member
Jesse Kane Mc Ternan, Special Signatory
Keywords:: ASCENT
AF-M315E
Microwave Ignition
Microwave Smelting
Emergency Power Unit
EPU
Ilmenite
Lunar Regolith
ISRU
In-situ resource gathering
Plasma
Green monopropellant
Microwave resonant cavity
Microwave cavity
Hydrazine
Abstract:: Since the 1960s, hydrazine has had a constant presence in the aerospace industry in spacecraft propulsion and gas generator applications. While hydrazine offers stable and reliable catalytic ignition with good performance, it is highly toxic and difficult to store and handle. These handling characteristics make hydrazine systems costly and less desirable. In the recent decade, significant efforts have been made to transition from hydrazine to a class of propellants called green monopropellants. Green monopropellants, like ASCENT, have significantly improved handling characteristics and offer a 150% increase in volumetric specific impulse. The main drawback of ASCENT is its difficulty igniting. While this quality is what gives it its stable handling qualities, it can become a significant energy consumer aboard air and spacecraft. Typical ASCENT ignition schemes involve preheating a catalyst bed to approximately 400 ºC through resistive heating, which takes on the order of minutes at the cost of several kilojoules of energy. This ignition delay can have consequences when immediate ignition is required, like in the application of emergency power units. The Penn State Space Propulsion and Environments Lab has previously demonstrated the ability to ignite ASCENT with microwave power, which is instantaneous and does not require a catalyst bed. These previous experiments used large acrylic decomposition chambers with bulky magnetron microwave sources, which could not be feasibly used in industry applications. The objective of this research was to develop a small microwave igniter that could be powered by compact solid-state microwave power amplifiers. A surface wave generator, whose applications stem from the field of atomic emission spectroscopy, was developed to ignite ASCENT at an operating frequency of 2.45 GHz. A surface wave generator is a small (approximately 1.5′′ OD × 2.5′′) cylindrical coaxial resonant cavity that forms a strong electric field in a region known as the “gap”. It is in this region where propellant can be subjected to a strong electric field and, thus, ignited. This surface wave generator was able to reliably ignite ASCENT at power levels of 250 W. This is the first known application of liquid ignition via a surface wave generator. Through testing, it was discovered that the surface wave generator is a versatile device that is able to subject a variety of media to the strong electric field (given that it can fit within its propellant tube) without the concern of the media coming into contact with the inner cavity. The surface wave generator was also used for the microwave smelting of lunar regolith simulant. During testing, the surface wave generator successfully interacted with gaseous, liquid, and solid media. The experimental setup used a 550-W solid-state power amplifier, which amplified a 2.45-GHz-wave generated by a signal generator. The surface wave generator first produced a helium plasma at 200 W. It then ignited ASCENT at varying powers and mass flow rates, with the highest mass flow rate being 9.5 mL/min. It was verified that ignition is still possible when the propellant tube is flooded with ASCENT, which was a concern, given that the presence of a liquid in the gap region significantly lowers the absorption of the cavity. Finally, ilmenite (a component of lunar regolith) was placed in the propellant tube and was smelted at a power of less than 250 W as a proof-of-concept test for microwave smelting in a lunar environment.

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