A highly miniaturized, MR-143, green monopropellant thruster was developed for 1N thrust. Testing indicated the initial catalyst bed heater was insufficient. In subsequent development, the thruster was equipped with a more efficient catalyst bed heater. For reliable ignition of the advanced, non-toxic, AF-M315E monopropellant, the catalyst needs to be preheated. This preheat temperature is much higher than what hydrazine thrusters require. Moreover, the combustion temperature of hydroxyl ammonium nitrate (HAN)-based monopropellants is higher than hydrazine, so the catalyst bed heater must be able to withstand repeated soak-back temperatures.
The first catalyst bed heater concept was a coiled heater placed around the injector head. For better heat transfer to the catalyst bed, the coiled heater can be produced with a metal sheath, high-temperature insulator, and conductor in either one or two loops. Usually, this type of heating element is produced by swaging together the sheath, insulator, and conductor to the necessary diameter. Production of the small-diameter heating elements with one or two layers of coils was investigated.
The second concept investigated was a cartridge-type heater with one or two heating elements. The cartridge heater can be produced from metal-sheathed heating elements, heating elements with ceramic insulator(s) and a metal enclosure, or an insulator deposited directly on the combustion chamber body with additional coiled heating element(s) and a metal or ceramic enclosure. Theoretically, the ability to deposit the insulator and wind the heating elements on the chamber is advantageous to minimize interruptions (i.e. thermal resistance) between the elements and the catalyst bed. In practice, the combustion chamber with catalyst bed undergoes several additional fabrication steps such as catalyst bed loading, welding, and brazing that may or may not be compatible with the heater. If the catalyst bed heater is applied to the chamber after all assembly is completed, then the total fabrication schedule is extended (i.e. serial production).
Several ceramics were tested as possible insulators for the high-temperature heater; however, the small diameter of 1N thrusters leads to very compact spacing, and wrapping multiple moving layers is challenging. Plasma-sprayed alumina was selected for further testing because of its ease of manufacturing and the resulting rigidity for the base insulation layers, with the felt and cement added for the final outside layer of the heater.
The first heater design is capable of operational testing in vacuum. The second heater is designed for air and vacuum testing. Both catalyst bed heaters are made as standalone cartridge heaters that fit around the catalyst bed of the thrusters. The cartridge-type heaters can have one or two heating elements. The cartridge heaters were produced using a metal sheath, heating element(s), thermocouple(s), and ceramic insulators.
One novel feature of these heaters is that they are made of materials with operating temperatures exceeding 1700 °C so that the heater is not affected by thermal soak-back from thruster operation. Another novel feature is the construction of the heater, which uses a metallic sheath and heating coils, and plasma-sprayed ceramic insulation.