AAWSAP DIRD, MHD Air Breathing Propulsion and Power for Aerospace Applications, November 2010

This Defense Intelligence Reference Document, dated 21 November 2010, was prepared by the Defense Intelligence Agency's Defense Warning Office under its Advanced Aerospace Weapons System Applications (AAWSA) Program. It reviews propulsion concepts that use weakly ionized plasmas and magnetohydrodynamics for hypersonic flight and spacecraft. It concludes that the Ajax MHD energy bypass concept is meaningless below Mach 12. A reverse energy bypass with a Virtual Cowl is judged potentially practical, and electric propulsion is expected to become standard for spacecraft.

From the source: Release of 2026-09-18 Incident: 11/21/10, Las Vegas, Nevada. Released with redactions. This document is a Defense Intelligence Reference Document (DIRD), a technical reference format used by the Defense Intelligence Agency (DIA) to capture baseline knowledge on a specific topic for later analytic use. DIRDs are best understood as reference and synthesis products rather than as original research. It is one of 38 DIRDs produced under the Advanced Aerospace Weapon System Applications Program (AAWSAP) between 2009 and 2011. Because AAWSAP’s scope permitted a broad range of supporting topics, not every DIRD in the series directly concerns aerospace systems or future threat assessment. The following summary reflects the DIRD’s scope and framing at the time of writing and should not be read as implying current validation of the concepts discussed. This DIRD surveys magnetohydrodynamic (MHD) and plasma-based concepts for air-breathing propulsion, onboard power generation, and aerodynamic control, arguing that such systems could in principle extend aerospace performance beyond the limits of conventional chemical propulsion and control surfaces. The report reviews concepts including MHD acceleration, flow control, inlet control, onboard power generation, drag reduction, and plasma-generated “virtual” aerodynamic surfaces, while giving particular attention to hypersonic applications such as scramjet power extraction, reentry vehicles, global-strike gliders, and aero-assisted orbital maneuvers. However, it emphasizes major practical constraints, especially extreme power requirements, system weight and complexity, and the difficulty of achieving useful ionization in colder air at lower hypersonic speeds; on that basis, it argues that Ajax-style MHD bypass concepts, in which energy is extracted from the airflow upstream and reintroduced downstream through an electromagnetic system, are not meaningful below about Mach 12, while treating the “virtual cowl” and related reentry applications as more plausible. The document presents plasma and MHD aerospace systems as a technically serious but highly demanding field whose nearer-term promise lies in specialized hypersonic control, power generation, and reentry applications rather than a fully realized air-breathing propulsion system.

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MHD Air-Breathing Propulsion and Power for
Aerospace Applications
Summary
The paper reviews novel propulsion concepts utilizing plasmas (ionized
gases) and magnetohydrodynamics (MHD). These concepts are shown to
be attractive due to their potential to achieve propulsion and
aerodynamic performance far beyond current conventional technologies.
However, significant difficulties impede the development and application
of these technologies; these include weight, complexity, higher power,
and the need for complex and energy-consuming artificial ionization in
"cold" air (at Mach <12).
A well-publicized Ajax concept of MHD energy bypass has been shown to
be meaningless below at least Mach 12. In contrast, a new "reverse
energy bypass" with Virtual Cowl is potentially practical for air
breathing hypersonic vehicles.
Applications of the Virtual Cowl and other plasma/MHD devices to
reentry, global-strike hypersonic gliders, and aeroassisted orbital
maneuvering are identified as promising in the near future. The ability of
a plasma/MHD system to generate high power onboard and to provide
L/D (lift-to-drag ratio) far beyond that possible conventionally makes
these applications both feasible and desirable for national defense.
However, these applications are also likely to be implemented by
nations such as China, Japan, and Russia.
The outlook for uses and applications of MHD propulsion could increase
dramatically if high-speed (hypersonic) vehicles begin to carry powerful
onboard electricity sources, such as nuclear (fission or fusion) reactors.
For spacecraft, the current trend of replacing chemical rockets with
electric propulsion systems will continue and is likely to become the
standard. Electric systems can provide a much wider range of operation
(e.g., low-thrust fine positioning/pointing, more frequent or
nontraditional maneuvers, and longer times on station) than chemical
systems can.
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