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

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entropy increase and thus, as can be easily shown, to higher lsp. The lsp increase
due to the increase in combustor temperature is made smaller by negative
factors such as irreversibilities due to Joule dissipation in both MHD generator
and accelerator and by the thermodynamically "wrong" work extraction before
the combustor.
The lsp increase, however, even in optimal cases, is only several percent. Given
the crude assumptions in the paper28 ( 1D flow, no boundary layer and heat
losses, uniform plasma, no e-beam energy losses, no losses in electric circuitry),
this gain of several percent would turn into a loss of l sp in more realistic analysis.
Additionally, the weight and complexity associated with magnet and e-beam
systems should be kept in mind. Therefore, one can state with certainty that
MHD energy bypass at Mach<12 (where nonequilibrium ionization of air is
required) is not a meaningful technology.
Where the MHD bypass could be useful is at very high Mach numbers
(Mach>12). First, stagnation temperatures at these Mach numbers are high
enough for significant thermal ionization with reasonable amount of alkali seed
(0.01-1% by volume), thus eliminating the need for a heavy, complex, and
entropy-generating nonequilibrium ionization system. Second, at static
temperatures (>2,000K) reached in the combustor at these Mach numbers,
there is no combustion per se, just dissociation of fuel and air molecules followed
by full or partial recombination into other molecules that releases heat into the
flow downstream of the combustor in the expansion nozzle. For such a regime,
the group at NASA Ames showed 29, 30 through modeling that MHD bypass can
indeed increase the lsp. Note, however, that materials and structures, as well as
fuel development, are currently such that air-breathing flight at Mach> 12 is not
realistic. In the future, if air-breathing propulsion at Mach> 12 becomes possible
in principle, reexamination of MHD bypass benefits and flaws will be warranted,
especially if lightweight magnets also become available by that time.
THE REVERSE ENERGY BYPASS
Returning to Mach< 12, one of the authors of this survey, together with his
colleagues, has proposed a very different bypass concept, dubbed the reverse
energy bypass (Figure 8). 31, 32 The energy (in the form of electricity) is extracted
from the flow in an MHD generator placed just downstream of the combustor (or
collocated with the combustor). This at least avoids the need fore-beam
ionization, since the air mixed with combustion products is sufficiently hot right
after the combustor that an acceptable electrical conductivity (on the order of 10
mho/m or higher) can be generated thermally, provided alkali metals are seeded
into the fuel and are thus present in the combustor and downstream of it.
A part of the electrical energy generated downstream of the combustor could be
used upstream of the MHD generator, which is why this is called the reverse
energy bypass (the energy being bypassed is moved in the upstream direction) .
Plasma-assisted combustion (such as ignition, flameholding, and mixing) would
benefit from this electrical energy. Plasma heat addition, in steady or transient
modes, enabled by this electrical power, would be beneficial for control of shock
interaction at the inlet and for drag reduction and/or steering and pitch or yaw
control when used in front of the vehicle's nose.
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