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

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a thermocatalytic "cracking" process that makes a syngas (i.e., CO - H2
gaseous mixture) from the original kerosene-water liquid mixture. The
syngas made onboard is a much better fuel from the lsp standpoint than
liquid hydrocarbons, and with the onboard thermocatalytic conversion there is
no need to carry hydrogen from the takeoff. This part of Ajax is certainly very
meaningful and probably viable.
• MHD energy bypass. This is perhaps the most controversial part of Ajax. An
MHD generator extracts energy from the airflow upstream of the scramjet
combustor; this energy bypasses the combustor and is put back into the flow
via MHD accelerator placed downstream of the combustor. We will discuss
this concept below.
• Plasma for drag reduction. A part of the MHD-generated power can, in
principle, be used to generate a plasma in air upstream of the vehicle nose.
This plasma would weaken the bow shock and reduce the wave drag on the
hypersonic vehicle. Although there were claims by some Russian groups
about 10-15 years ago that weakly ionized plasmas can reduce shock
strength via some unknown physical mechanism, extensive research in the
United States, Europe, and Russia has conclusively shown that the effects are
purely thermal. However, even with purely thermal action, plasma drag
reduction can be quite meaningful and useful for high-speed flight (see
below).
Perhaps the most basic problem with the MHD bypass, as pointed out by D.
Riggins, 25 is that as a propulsion power cycle, it runs in the direction opposite to
that dictated by thermodynamics. Indeed, any thermodynamically correct heat-
into-power conversion cycle has work addition (e.g., compression) prior to heat
addition (e.g., in the form of combustion), and work extraction follows the heat
addition. This is why air is compressed (work added) upstream of the combustor
in all normal propulsion cycles, whether by compressor in a turbojet or a
compression ramp upstream of a scramjet combustor. In this sense, MHD power
(work) extraction before air enters the scramjet combustor, followed by MHD
power addition after the combustor, constitutes a thermodynamically "wrong"
and thus inherently inferior, propulsion system.
However, in criticizing the Ajax power cycle and arguing that the Isp of Ajax is
always less than that of a system without MHD bypass, D. Riggins 26 makes a
significant mistake. In his derivations, he assumes that combustion-generated
heat addition in the combustor occurs at a gas temperature equal to the
stagnation temperature of the flow (i.e., that the flow is fully stagnant in the
combustor). This assumption is in direct contradiction to the very idea of a
scramjet, where combustion occurs in supersonic flow. Heat addition in the
combustor thus occurs at a static, not stagnation, temperature. It is this fact
that at least gives MHD bypass a chance to increase Isp.
Indeed, calculations described in the above-referenced paper27 by the Ajax group
do result, in some conditions and with careful optimization, in an Isp increase.
Our analysis of their calculations shows that increase in static temperature
caused by flow deceleration and Joule dissipation in the MHD generator upstream
of the combustor is the reason for higher Isp. Indeed, since the entropy increase
in the combustor is equal to Q/T, where Q is the heat added and T is the static
temperature at which this heat is added, any increase would lead to lower
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