The objective of the project was to evaluate whether prior research conducted at The Pennsylvania State University, where various green monopropellants, including AF-M315E, were ignited at atmospheric and subatmospheric pressures using microwave energy, can be transitioned into a robust low-power green monopropellant ignition system that can replace the heated catalyst required by current hydrazine use. Technical questions to be answered by this research are whether the 2.45 GHz magnetron-based microwave source can be replaced by a commercially available solid-state source at a higher frequency, thereby significantly shrinking the physical footprint and increasing the robustness of the microwave igniter. Other technical questions to be answered include quantification of the minimum electrical power required for ignition, the ambient pressure and temperature range over which ignition can be achieved, and validation that the system can accommodate the monopropellant flow rates found in operational systems. Successful results for all these questions have the potential to lead to the design and testing of an operational prototype for use in the substitution of green monopropellants in current hydrazine systems aboard aircraft Emergency Power Units (EPU).

Some aircraft use a form of hydrazine (H-70, 30% diluted by water) to power their EPU. Green monopropellants, such as AF-M315E, offer greater performance, no special handling protection, and are denser than hydrazine. However, AF-M315E and other Hydroxylammonium Nitrate (HAN)-based monopropellants require a heated catalyst to react. The time needed to bring the catalyst up to temperature bars its use in somes EPU. Thus, to implement AF-M315E as the new emergency fuel source aboard those aircraft, an alternate method of ignition was explored in this project. The research objective was to validate the proof of concept for the instantaneous ignition of green monopropellants such as AF-M315E using focused microwave energy to provide a green drop-in replacement for the EPU. 

Previous research led to the development of the Penn State Microwave Plasma Torch (PSMPT), an efficient, single-electrode, plasma torch that concentrates the electromagnetic energy from a waveguide with a gas from a central conducting tube at its tip. The electric field becomes intensified at the torch tip, accelerating the electrons of a gaseous propellant and ultimately igniting a plasma. For this project, the PSMPT was modified to allow the metering of a liquid monopropellant, instead of a gas, through the molybdenum torch via a syringe pump. 

AF-M315EM was successfully ignited instantaneously at low pressures (0.85 psia) and temperatures (−34 °C) using ~330 W of 2.45-GHz microwave power. The microwave torch was capable of instantaneously igniting the HAN-based monopropellants with the same power input, regardless of water concentration up to 60%. At 4 psia and 327 W, AF-M315EM was ignited and the reaction was sustained for 15 continuous minutes before erosion of the injector tip resulted in a less stable plasma flame. When the temperature of the AF-M315EM monopropellant was decreased down to −34 °C it was found that the microwave power required for ignition varied between 311 W to 324 W and was not dependent on the initial temperature.

The benefit of this project is the ability to use a green monopropellant based on AF-M35E to replace the hydrazine. There are significant safety, environmental, and cost issues faced by bases due to the use of onboard hydrazine. Extensive and expensive measures are required for safe handling of hydrazine during transport and fueling. Any firing of EPUs, whether on the ground or in flight, is treated as an emergency by the Hydrazine Response Team and requires considerable risk mitigation. The results of this project provide a means to replace the 70% hydrazine in the H-70 Fuel Tank shown below with a greener but higher performing alternative.