Advancing Nitrous Oxide As A Monopropellant Using Inductively Heated Heat-Exchangers: Theory and Experiment

Abstract

Most monopropellant thrusters used for attitude control and station keeping employ hydrazine as their propellant. In recent years, significant effort has been focused on finding an alternative due to its high toxicity. This work focuses on advancing nitrous oxide, a green monopropellant with a strong performance capability, as a replacement for current monopropellant thrusters. A large emphasis is placed on trying to address catalyst degradation experienced in most thrusters due to the high temperatures from decomposition. The approach described here eliminates the dependence for a high catalytic surface area, typically decreased from degradation, and catalysts altogether by using high temperature porous heat exchangers. A 1-D numerical compressible fluid model was created to model a typical decomposition chamber and simulate self-sustained decomposition of nitrous oxide. It implements a preheated, thermally-conductive, metal foam as the heat exchanger. An extensive parameter study was conducted to help understand thermal and fluid effects on steady-state decompositions. Using a copper metal foam, steady-state solutions simulated successful nitrous oxide decomposition, with an exit gas temperature around 1345 K. Simulations were extended to other high temperature metal foams with different thermal conductivities and melting points. Modeling flow rate conditions more representative of current monopropellant thrusters required scaling of the decomposition chamber in order to be self-sustaining. Experiments were conducted using results from the numerical simulations as guidelines. Three different heat exchangers (copper metal foam, copper discs, and stainless-steel discs), all of which have significantly less effective surface area than nominal catalysts used in thrusters, were tested for nitrous oxide decomposition. These heat exchangers were preheated to thermal decomposition temperatures using an inductive heating system and placed in a vacuum bell jar to mitigate heat loss to the environment. Testing with copper metal foam resulted in complete degradation of the heat exchanger due to oxidation from nitrous oxide decomposition. A set of copper discs, uniquely designed to maximize tortuosity of the flow, was implemented in an attempt to address the oxidation issues. While the preliminary test did confirm steady-state decomposition of nitrous oxide within the heat exchanger, further tests resulted in temperatures exceeding the melting point of copper within the discs. The last heat exchanger was a set of stainless-steel discs of the same design. Repeated tests all successfully achieved steady-state decomposition of nitrous oxide within a two-minute interval.