Porous walls are often used for the surfaces of rocket chambers and their experimental simulators. It has been experimentally observed that as the Reynolds number of the flow based on pore size becomes of the order 10 or more, the flow becomes unstable, with large magnitude nonuniformities. These disturbances are observed to be nearly random spatially, but temporally steady a ``pseudo-turbulence'' phenomenon that has not been analytically addressed in the literature. The mechanism is presently believed to be related to the convective drag terms in the equations of motion. Linear instability techniques are being employed to analyze the flow.

A laboratory test facility with high volume pumps and thrust stand is being used to study the performance of pulsed electrothermal thrusters in a simulated space environment. Of particular interest are pulsed thrusters with average power levels of 100 to 200 watts. Emphasis is placed on hydrazine propellants likely to be used in near-Earth space applications.

A nonequilibrium (non-LTE) model of arcjet thrusters is being developed. Accurate predictions of the plasma flow and temperature fields are made possible by allowing for distinct electron and heavy species kinetic temperatures. This kinetic nonequilibrium between electrons and heavy species contributes to thruster losses through both optically thin radiation and inefficient collisional coupling. The effect of nonequilibrium on the plasma flowfield is being investigated. The model will identify regions of severe non-LTE in the plasma and may provide the clues necessary to reduce non-LTE and improve thruster performance.

Experiments are conducted on 1- and 2-kW arcjets using nitrogen/hydrogen-based propellants to evaluate a non equilibrium arcjet plasma model. The results are used to determine model boundary conditions and accuracy. Emission spectroscopy and Langmuir probe diagnostics measure electron temperature, electron number density, and heavy-particle Mach number and kinetic temperature, both inside the arcjet nozzle and the exit. Independent ion velocity measurements, coupled with Langmuir probe techniques, allow comparison of electron and ion temperatures and the extent of thermal nonequilibrium in the plasma.

The goal of the research is to develop high-efficiency pulsed plasma thrusters operating in the few hundred watt range. Both Teflon and hydrazine propellants are being investigated for small satellite applications.

Arcjet thrusters are currently being used as primary propulsion systems for station-keeping on low-Earth orbit satellites. Experiments are conducted on a 1-kW arcjet thruster using nitrogen/hydrogen-based propellants to evaluate a nonequilibrium arcjet plasma model. The results are used to determine model boundary conditions and accuracy. Application of flush-mounted electrostatic probes for internal diagnostics of the nozzle allows direct measurements of plasma parameters in the anode boundary layer. Such measurements allow an understanding of the physics of arc attachment and anode heating, both critical to improving arcjet performance and lifetime.

Low-power propulsion has been identified for orbit-raising applications on small satellites. A pulsed Teflon plasma thruster is being studied, both experimentally and theoretically, for the 10-100 watt power range, which would be capable of both maneuvering and orbit raising missions. The effort is primarily focused on increasing the notoriously low efficiency of this type of thruster.

Research is being conducted to develop an understanding of nonequilibrium phenomena associated with fuels in high-speed ramjets. Modeling of combustion experiments being conducted by the air force is providing new insights into the performance of high-speed ramjets. A nonequilibrium model is being compared to data.

Synthetic mixtures of hydrocarbons are being combusted in air to develop full reaction kinetic models. These models are being simplified for insertion into full CFD descriptions of internal flow systems.

The performance of a high-speed scramjet, burning mixtures of H₂ and C₂H₂, is determined. The system offers a much smaller and lighter vehicle than a conventional hydrogen-fueled scramjet.