Non-Catalytic Microwave Ignition of Green Hydrazine Replacements

Open Access
Auman, Kerstyn
Graduate Program:
Aerospace Engineering
Master of Science
Document Type:
Master Thesis
Date of Defense:
July 12, 2019
Committee Members:
  • Dr. Michael Micci, Thesis Advisor/Co-Advisor
  • Dr. Amy Pritchett, Committee Member
  • Dr. Sven Bilen, Committee Member
  • AF-M315E
  • hydrazine
  • green
  • microwave
  • ignition
  • catalyst
  • EPU
  • plasma
  • power
  • torch
Hydrazine monopropellant is ubiquitous is in the aerospace industry for low total-impulse maneuvers such as station-keeping, attitude control, and auxiliary/emergency power generation. Its relatively high specific impulse, compared to cold-gas thrusters, and the simplicity of a monopropellant system have been the main contributors to hydrazine’s popularity. The cost of safely handling hydrazine, however, is substantial, as the propellant is toxic, highly flammable, and corrosive. The recent advent of safer, “green” monopropellants over the past couple decades has caused the industry to re-evaluate its use of hydrazine and assess the feasibility of a low-thrust system driven by these alternatives. Of particular merit is the Air Force Research Lab-developed AF-M315E, a HAN-based monopropellant, which offers an approximately 50% increase in density-ISP over hydrazine, but requires a preheated catalyst bed for decomposition initiation. The replacement of the hydrazine-based fuel, H-70 (70% anhydrous hydrazine in aqueous solution) with a diluted variant of AF-M315E (AF-M315EM) in the emergency power unit of the F-16 fighter jet would require near instantaneous ignition of the green monopropellant, which cannot occur catalytically, due to the time-delay associated with catalyst preheat. Therefore, an alternative ignition scheme is necessary. This thesis explores the ignition characteristics of AF-M315EM using a plasma torch–assisted microwave system and assesses the feasibility of its implementation into the F-16 emergency power unit (EPU). Previous research at Penn State has proven the capability of torch-assisted microwave ignition of green monopropellants, including AF-M315E and Swedish Defense Research Organization–developed LMP-103s at atmospheric and sub-atmospheric pressures. The objective of this thesis is to eliminate the current preheat requirement of green monopropellants. Therefore, microwave ignition characteristics at temperatures and pressures indicative of F-16 flight conditions and the ability to sustain the microwave-induced reaction at the required chamber pressure (300 psig) must be assessed. A secondary objective is to evaluate the input power requirements of ignition at various low pressures and AF-M315E formulations. The ultimate goal of this and subsequent research is to create a prototype microwave igniter for implementation into the F-16 EPU. The proposed system will be light-weight, using several solid-state power amplifiers for microwave generation and coaxial cables for EM propagation; while the objectives of this research are tailored to meet the needs of a turbine-driven power generator, as in the EPU, such an ignition scheme would be applicable to low-thrust spacecraft propulsion systems. Several parameters must be assessed to evaluate the merits of a microwave ignition system against other ignition schemes, including: the minimum power needed to ignite the monopropellant, the minimum power needed to sustain the reaction, the optimal flow rate, any ignition delay, the time required to reach operating pressure, and the burn duration capacity. Thus far, AF-M315EM has been ignited and sustained, using 317 and 301 W of microwave power, respectively, at ambient temperature and pressures of 1.9, 4, 6.5, and 14.3 psig, with a 1.0-ml/min optimum flow rate. The power required for HAN-based monopropellant ignition using microwave radiation is independent of fuel composition and pressure (at or below ambient), proving this system is a viable, versatile ignition scheme for ionic, green monopropellants. The torch was capable of sustaining combustion at 327 W, 4 psig for 15 continuous minutes before significant erosion of the molybdenum electrode reduced flame stability. This firing period exceeds the current operational time of the hydrazine-driven F-16 EPU. AF-M315EM can also be ignited at near vacuum pressures at temperatures nearing −40 °C, using approximately 320 W of microwave power and, if it can be brought to operational pressure within 2–3 seconds, a microwave ignition scheme would be verified as a viable alternative to the current hydrazine-driven EPU. Assessing these capabilities is the focus of ongoing research.