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

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axis positioning of the plasma region would create steering or pitch/yaw control
moments. 42
An attractive application that utilizes this plasma/MHD-enabled increase in L/D is
orbit inclination changes for space assets. Even a modest (a few degrees) orbit
inclination change requires a very large amount of delta-velocity and energy and
thus a very large amount of fuel to be burned. If the space asset dives into the
upper atmosphere (to altitudes of 200-300 kft), it can use aerodynamic turning
(similar to airplanes), provided the L/D ratio is high enough. Unfortunately,
hypersonic L/D, especially in rarefied air at high altitudes, is not much higher
than 1. Plasma and MHD technologies hold substantial and realistic promise to
achieve hypersonic L/D of 3-10, which would be a game-changer and enable,
among other missions, aerodynamically assisted, on-demand orbital inclination
changes.
We now briefly consider another MHD application: a hybrid chemical/MHD
propulsion. The nozzle exit velocity of chemical systems (air-breathing and
rockets) is limited by the chemical energy available from the fuels/propellants
and the temperature limits of the system materials. One method to increase the
exit velocity of the system is to add an MHD accelerator system to the nozzle.
The flow is first accelerated using a conventional gas dynamic converging-
diverging nozzle and then the MHD system further accelerates the supersonic
flow in the diverging portion of the nozzle. Many ground-based systems have
been developed and tested to accelerate flows using MHD systems. These have
been primarily either proof-of-concept systems or for hypersonic wind tunnels. 43 ,
44 Systems have been proposed for both small in-space systems45 , 46 and for
large engines for launch vehicles. 47 While this concept has great potential and
the accelerator physics are well established, it has several practical limitations.
To be efficient, the energy added to the flow from the MHD system should be on
the order of or greater than the energy added by the chemical stage. This
requires power levels that are not available on either type of vehicles. For
example, for launch vehicles the jet power levels would be in the hundreds of
MW to tens of GW range. The low ionization fractions in the flows also severely
limit the thrust efficiency of the MHD systems to a few percent. This combined
with the large jet powers requires enormous launch-vehicle powers. Similarly for
space systems, a better solution would be to use the available power in a more
efficient electric thruster. The large powers also require large masses for the
MHD system components for reasonable specific mass (kg/kW). To be
comparable to pure electric systems on spacecraft, the MHD augmentation
system specific mass would need to be improved by a factor of 1,000 over state-
of-the-art technologies. 48 One potential solution around the power issue is to
beam the power to the vehicle. 49 Another serious issue is the magnets needed to
provide the 2- to 40-Tesla fields required. In many cases, the weight of the
magnet and magnet power supply would exceed the vehicle mass using existing
technology. The magnet system mass will need to be reduced by several orders
of magnitude to make flight systems practical.
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