Tech Briefs

Small satellites, launched as secondary payloads, are increasingly being fielded. Advances in liquid rocket propulsion that enhance the on-orbit maneuverability, increase the on-orbit life, and decrease the time between identified need for and deployment of such spacecraft are of great value. Replacing the nearly ubiquitous yet toxic hydrazine propellant with AF-M315E produces higher specific impulse and density specific impulse, resulting in improved overall velocity change capability and increased on-orbit life.

Ultramet had previously demonstrated nearly 1,000 restarts with repeatable pulse performance and steady-state burn characteristics using AF-M315E mono-propellant and a novel electrically heated foam-based ignition system. The goal of this current NASA work was to scale the igniter and thruster technology to enable sizing of propulsive capability to a level appropriate for secondary payload satellites, i.e., 5-N and 1-N green mono-propellant engines.

A 5-N workhorse thruster was designed and constructed out of Inconel 625 to serve as a testbed for the foam-based ignition system. Refractory silicon carbide foam was fabricated and machined to fit within the combustion chamber. The remainder of the igniter was built out by metallizing with iridium and performing wire bonding. The igniter was integrated with the combustion chamber and injector, and subjected to hot-fire testing with AF-M315E monopropellant.

Scalability of the resistively heated igniter was positively demonstrated at the 5-N level, and no technical hurdles are foreseen with scaling to a size appropriate for a 1-N thruster. Additional work is needed to arrive at a satisfactory solution for the high-temperature-capable, hermetic dielectric power feed-throughs, and additional thermal mass is needed in the catalyst bed to enable sustained propellant reaction.

The ignition system consists of open-cell foam made of refractory materials that can be heated resistively. This approach yields significant electrical power savings compared to traditional bed heaters. With the foam being heated resistively, coupling of the thermal energy directly to the propellant stream can initiate decomposition more efficiently and impart more energy to the propellant than a conventional bed heater. Higher preheat temperatures are possible for the same power used, or a smaller power source can be employed for the same preheat temperature. Even at lower power consumption levels compared to conventional bed heaters, catalyst bed preheat temperatures of well over 1100 °C are obtained in less than 2 seconds. The ability to generate that amount of thermal energy, combined with the short ramp time, translates to reliable fast ignition that may be applicable to rapid response uses, such as liquid divert and attitude control systems, in addition to spacecraft and satellite propulsion. Even on a spacecraft, where rapid response may not be a priority, the resultant short duration of applied power means that overall bed heater energy usage can be quite small. Energy-saving operation is always an asset for spacecraft.

In a more straightforward design approach, the monolithic foam catalyst can be used as a drop-in replacement for granular catalysts in conjunction with a traditional bed heater. The refractory nature of the materials used means that the bed volume will not shrink as it does with the current state-of-the-art AF-M315E catalyst, thereby eliminating dead space, propellant pooling, and detonation.

Ultramet's open-cell foam-based igniter technology is truly crosscutting in that it can be applied to many different programs to enhance the life and ∆V capabilities, and/or reduce the mass of myriad spacecraft. The foam-based ignition system represents an enabling technology that will accelerate the implementation of this high-performance propellant and significantly enhance the capabilities of virtually all future spacecraft using monopropellant propulsion systems.

NASA applications for the thruster-igniter system include orbit transfer, maneuvering, station keeping, and attitude control for satellites and interplanetary spacecraft. It can be used in a number of other commercial and military applications, including attitude control and apogee engines for commercial and military satellites.

This work was done by Matthew J. Wright of Ultramet for Glenn Research Center. NASA is seeking partners to further develop this technology through joint cooperative research and development. For more information about this technology and to explore opportunities, please contact here. LEW-19408-1.

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