Defense Intelligence Reference Document MHD Air Breathing Propulsion And Power For Aerospace Applications

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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.
On board 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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