DOW-UAP-D149, AAWSAP DIRD, MHD Air Breathing Propulsion and Power for Aerospace Applications, November 2010

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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Defense
Intelligence
Reference
Document
Defense Futures
21 November 2010
ICOD: 20 July 2010
DIA-08-1011-006
MHD Air Breathing Propulsion
and Power for Aerospace
Applications
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MHD Air Breathing Propulsion and Power for
Aerospace Applications
The Defense Intelligence Reference Document provides nonsubstantive but auLhoritaLive reference
information related to intelli ence to ics or methodolo ies.
Prepared by:
Technology Warning Division (OWO-4)
Defense Warning Office
Directorate for Analysis
Defense Intelligence Agency
Authors:
AAP Person 86, AAP Person 87
(U) COPYRIGHT WARNING: Further dissemination of the photographs in this publication is not
authorized.
This product is one of a series of advanced technology reports produced in FY 2010
under the Defense Intelligence Agency, Defense Warning Office's Advanced
Aerospace Weapons System Applications (AAWSA) Program, Comments or gue~tions
pertaining to this document should be addressed to._A!_A P_P,.e,,,,r,,s,,,o,,,.n,....1..,,.,..,.,,..,..,,,.__,,,..,,.,.,,..,.,.~J
AAWSA Program Manager, Defense Intelligence Agency, ATTN : JUIAF - DI/DW0-3,
Bldg 6000, Washington D.C. 20340-5100
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Contents
SUMMARY ........................................................................................, . .... iv
Chapter 1: CONCEPT OVERVIEW............................................................1
Weakly Ionized Plasmas for Propulsion Applications ....................... 2
Electric Propulsion Systems ......................................................,......5
Chapter 2: AERONAUTICAL APPLICATIONS .................................. ..., ....11
Basic Principles of Magnetohydrodynamics and Requirements for
MHD Performance .........................................................................11
Nonequilibrium MHD in Cold Air Flows...........................................13
The Ajax Concept: MHD Bypass .................................................,....15
The Reverse Energy Bypass...........................................................18
MHD Applications to Reentry and Near-Orbital Flight ................, . ...20
Chapter 3: SPACE APPLICATIONS........................................................23
Chapter 4: SUMMARY AND PREDICTIONS ............................................25
Chapter 5: ENDNOTES .........................................................................26
Figures
Figure 1. Electrothermal Arcjet Thruster.................................................7
Figure 2. Electrostatic Gridded Ion Thrusters......................................... 8
Figure 3. Field Orientation for Hall Field Systems and PS Hall
Thruster. .................................................................................... 8
, 1, •••••
Figure 4. Electromagnetic Accelerator Field Configuration and Self-
Field Electromagnetic Spacecraft Thrusters.......................................9
Figure 5. Air-Breathing MHD Engine................................................., . ...10
Figure 6. MHD Control of Scramjet Inlet Using E-Beam Ionization........15
Figure 7. Schematic of Ajax Hypersonic Vehicle Concept......................16
Figure 8. The Reverse Energy Bypass Concept...................................... 19
Figure 9. Schematic of the Virtual Cowl Concept...................................20
Figure 10. Reentry Vehicle with Surface-Integrated MHD Device and
Plasma-Enabled Virtual Streamlining and L/D Increase..............., . ...21
Figure 11. Electrothermal Arcjet Thruster on Satellite..........................23
Figure 12. SP-100 Space Nuclear Power System...................................24
Figure 13. Nuclear Electric Propulsion (NEP) Concept Vehicles.........,....24
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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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Chapter 1: Concept Overview
A flight vehicle's speed and altitude limit its available propulsion options.
Traditional air-breathing systems (propeller, turbofan, and turbojet) are typically
limited to altitudes below 80,000 feet. The existing and planned high-altitude
vehicles utilize either high-speed propulsion with ramjet and scramjet engines or
slow-speed systems with large propellers. Chemical rockets can operate at all
altitudes but have limited burn times and require both fuel and oxidizer to be
carried onboard.
High-speed air-breathing propulsion, based on ram/scramjet engines, have well­
known difficulties: external and internal flow compression and shock control;
shock-shock and shock-boundary layer interactions in the propulsion flowpath;
mixing, ignition, and flameholding in the combustor; incomplete combustion and
chemical energy release; and very high temperatures and wall heat fluxes in the
combustor. There are limits to what can be done about these problems with
conventional technologies, which is why the use of plasma (ionized gas) with or
without electric and magnetic fields can offer additional opportunities for control
and propulsion enhancement.
Onboard generation and storage of electric power is one of the main problems
encountered with respect to high-altitude, high-speed flight. Hypersonic vehicles,
both air-breathing and unpowered reentry "gliders," have no rotating
turbomachinery to which an electrical generator could be connected. An
attractive power option can be offered by magnetohydrodynamic (MHD) devices.
For example, placing an MHD generator immediately downstream of a scramjet
combustor can, given the high velocities and temperature of the flow and with
metallic additives to the fuel, provide high power (from tens of kW to several
MW) with no moving parts. For reentry vehicles, both external (i.e., surface­
integrated) and internal-duct MHD generators can generate high power also
without moving parts. Employing additional equipment like electrical generators
imposes a weight penalty that must be optimized with vehicle performance.
The use of electric and magnetic systems can open new potential areas for
aerospace propulsion. While chemical energy sources are limited by the energy
available for particular reactions and are limited to operating conditions that are
conducive to combustion, electromagnetic energy can be added to the flow over
a much wider range of operating conditions. For example, at very high altitudes
(>150 kft), it is difficult to get reliable combustion in hypersonic air-breathing
engines. In an electrothermal system, the combustor would be replaced with an
electrical heating source that can easily and reliably add enthalpy to the flow
even at low pressure. The flow can also be accelerated by manipulating body
forces ( electric and magnetic) on charged particles (ion and electrons) within the
flow.
Outside the atmosphere, we note that operation in space almost always requires
rocket propulsion whether it be chemical, electric, or nuclear. The exceptions
would be sails and tethers. Spacecraft are rapidly transitioning from chemical
rockets to electric (plasma) for most space-based operations.1 The higher
specific impulse (lsp) available for electric systems (2 to 100 times that of
chemical) has a dramatic impact on the vehicle design and operation. Although
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electric (plasma) thrusters have been around for decades, their use in space was
limited by the electric power available onboard the spacecraft.2 The advent of
high-power solar arrays has made systems from a few kW to tens of kW
practical. Chemical systems will probably always be the primary choice for
getting vehicles into space. The thrust levels for electric systems are too low to
be practical for that purpose.
Chemical and electric (or electromagnetic) propulsion systems have intrinsic
differences. For example, chemical propulsion is "energy limited" because the
chemical reactants have a finite amount of energy per unit mass (i.e., their
enthalpy of combustion or reaction), which ultimately limits their achievable
exhaust velocity. However, because the propellants are their own energy source,
the rate at which energy is supplied to the propellant (which is ultimately limited
by the reaction kinetics) is independent of the mass of propellant, so very high
powers and thrust levels can be achieved. By contrast, electric propulsion
systems are typically not energy limited; an arbitrarily large amount of energy
can be delivered (from the external solar, nuclear or chemical power system) to
a given mass of propellant so that the exhaust velocity can be an order-of­
magnitude larger than that of a chemical system. Instead, electric propulsion
systems are "power limited" because the rate at which energy from the external
source is supplied to the propellant is proportional to the mass of the power
system. This has the result of limiting the thrust of the electric propulsion system
for a given vehicle mass. Because of this, electric propulsion vehicles are
typically low thrust-to-weight (T/W) ratio (i.e., low acceleration) vehicles.
WEAKLY IONIZED PLASMAS FOR PROPULSION APPLICATIONS
This review is devoted to a group of emerging technologies centering on weakly
ionized plasmas for propulsion and power.3 Charged particles (ions and
electrons) must be present in the flow so that it can interact with applied electric
and magnetic fields. Space thrusters operate at very low pressures ( < 100 mTorr
or < about 2 psi) with a significant fraction of the working fluid/gas being
partially ionized (from a few percent to nearly 100 percent). In contrast, air­
breathing systems operate at much higher pressures and have low ionization
fractions. The ionization fraction of concern (i.e., the fraction of gas molecules
that are ionized) ranges from as low as 10-s to 10-2, hence the term "weakly
ionized." The gas pressure in the plasmas can take almost any value. In
applications to high-altitude flight, the static pressure is on the order of 10-100
Torr, whereas combustion applications demand near-atmospheric ( ~760 Torr) or
above-atmospheric pressures. The temperature of the gas can be near-ambient
in low-pressure glow discharges, rising to 5,000-10,000K in arc or high-pressure
microwave discharges, or even 20,000-30,000K in laser-generated sparks. The
plasmas can be generated by electric or electromagnetic fields, from DC to RF,
short pulses, microwaves, and optical (laser) beams, or by various combinations
of the above. In general, low pressure plasmas tend to be uniform (diffuse) and
nonequilibrium. The temperature of electrons and internal molecular modes can
be very high, while the gas as a whole stays relatively cold. As the pressure and
power loading increase, plasmas tend to become hotter, getting closer to
thermal equilibrium, and also break into channels (streamers and arcs). The
reality, however, is more complex. In some devices, such as dielectric barrier
discharges, nonequilibrium plasmas are generated even at atmospheric pressure,
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and in devices such as the gliding arc, the plasma evolves from near-equilibrium
to highly nonequilibrium during each of the periodically repeating cycles. In
shock and boundary layers during reentry, the plasma is near thermal
equilibrium while being diffuse. The primary reason for this behavior is that the
ionization in those shock and boundary layers exists without any electric field
and thus is not subject to arcing instabilities.
Plasma Features
What features or properties make weakly ionized plasmas interesting for
propulsion and aerodynamic applications? The most obvious feature is heating- a
consequence of Joule dissipation in an electrically conducting medium placed in
an electric field. As a heating element, plasma has important advantages
compared with conventional heaters. For example, even a surface electric
discharge can effectively heat the gas flow much farther from the wall than a
wall-imbedded conventional heater would. Microwave and laser beams can
create plasmas and heat the gas even far from any surfaces, and the volume and
shape of the heated region can, in principle, be adjusted. Since heated regions
can significantly alter the flow by making the gas flow mostly around them,
plasmas can form switchable, controllable, and tunable virtual bodies or
surfaces. Such virtual surfaces can be deployed on demand for drag reduction,
aerodynamic control (when applied asymmetrically), and optimization of engine
inlet performance, to name a few. It is the localized and transient deployment of
plasma virtual surfaces that results in the most interesting and complex
interactions with gas flows while saving energy compared with large-volume,
steady-state plasma utilization, and thus is especially promising for applications.
Another useful application of plasma heating is ignition. This