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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forebody vehicle
Flow inlet
Heated ~ owl lip
Deflected region
tream line
Figure 9. Schematic of the Virtual Cowl Concept. Plasma-generated heated region
upstream of the cowl lip deflects the flow and scoops more air into the inlet.
Analysis of the reverse energy bypass involving MHD generator downstream of
the scramjet combustor and an optim ized virtual cowl revealed that, although
enthalpy extraction and entropy production in the MHD generator substantially
reduce the thrust, the combined system with virtual cowl can actually increase
thrust by as much as 20-30% .35, 36
From the system standpoint, this concept, with all its associated weight and
complexity, should be compared with that involving a movable (variable
geometry) cowl that is associated with a heavy and complex electrohydraulic
system and also requires power. An attractive feature of the reverse bypass
concept that might make it a winner is its multifunctional nature. Indeed, MHD
power generation downstream of the combustor can be an attractive power
production option for hypersonic vehicles. Alternatives (batteries, fuel cells, and
the like) are not very competitive for generation of large amounts (hundreds of
kW to 1-10 MW) of power onboard. Thus, if an MHD generator in the propulsion
flowpath is accepted as the power source, its operation in conjunction with a
virtual cowl that significantly increases thrust in off-design conditions, and also
enables aerodynamic control and maneuvering, would become quite practical.
Note also that if another source of high (MW-scale) power, such as a nuclear
reactor, is onboard, its use for virtual cowl, drag reduction, and aerodynamic
control would be straightforward and would greatly increase performance of the
hypersonic vehicle .
MHD APPLICATIONS TO REENTRY AND NEAR- ORBITAL FLIGHT
We now turn to MHD application to reentry and near-orbital flight. Due to the
high velocities and enthalpies involved, the gas temperature in shock and
boundary layers is very high, from several thousand to 10,000-20,000K. At
these temperatures, thermal ionization is very substantial, and at the low end of
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