Plasma Fueled Engines
Introduction
Sustained hypersonic propulsion is a complex problem due to the high temperatures and velocities associated with these flows. In high-hypersonic flow regimes, combustion is limited by low residence times of the flow and the limited energy density of chemical fuel. The limits of combustion motivate using magnetohydrodynamic (MHD) body forces on an ionized gas for the conversion of power to thrust.
There have been multiple attempts at using MHD augmentation to control hypersonic flows, mostly in wind tunnel applications with various means of ionizing the working gas. An efficient way to ionize the gas for MHD acceleration is through electron beam ionization where high-energy electrons are injected directly into the flow.
Plasma Fueled Engines (PFE) offer a means of MHD acceleration for hypersonic vehicles flying at high altitudes using electron beam ionization to achieve adequate conductivity for significant MHD interaction. In PFE, the electromagnetic fields can be configured to maximize efficiency depending on the flight conditions. For lower altitudes and higher pressures, a crossed-field accelerator can be used with the electric and magnetic fields perpendicular to each other and the bulk velocity. This configuration, known as the Faraday configuration is shown below. As the vehicle climbs altitude, the electric field can tilt to match the Hall configuration seen in many popular space propulsion thrusters.
Method
In an endeavor to understand the underlying physical processes occurring in the PFE, a one-dimensional model has been developed. This model assumes that the low magnetic Reynold’s number is low so that the magnetic field, B, is prescribed throughout the entire domain. Generalized Ohm’s law can be used to calculate the current density, j, based on the electric field, E, and flow velocity, u.
The Lorentz force, jxB, and energy deposition, j∙E, are included in the Euler equations as source terms to the momentum and total energy equations, respectively.
One-Dimensional Results
Solutions from the one-dimensional model show that significant acceleration is possible for a wide range of Mach numbers and altitudes. However, for large inlet Mach numbers, acceleration is preceded by an increase in temperature due to Joule heating. The figure below shows velocity, temperature, and Mach number versus velocity profiles for Mach 14 flow slowed to Mach 5.4 at the inlet of the MHD accelerator portion of the PFE. To achieve acceleration for this inlet velocity and Mach number combination, the electric field in the channel must be between 3.86 kV/m and 9.64 kV/m. Profiles with select electric field strengths in this range are shown.
Therefore, an important task in designing PFE is tailoring the electric and magnetic field strengths to optimize acceleration down the length of the channel while keeping temperatures low.
Future work
The hypersonic CFD code developed by the members of NGPDL, LeMANS, will be used to obtain 3D solutions of the MHD accelerated flow in PFE. Furthermore, the physical models needed to understand the performance of PFE across a wide range of operating conditions will be identified using both the 1D model and LeMANS.
Acknowledgments
Financial support for this project is provided by the Lockheed Martin Corporation.
References
[1] Kava, T. E., Evans, J. A., and Boyd, I. D., "Numerical Simulation of Electron-Beam Powered Plasma Fueled Engines," AIAA 2021-3241. AIAA Propulsion and Energy 2021 Forum. August 2021.