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

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pressure, Pamb is the ambient/environmental pressure, Pj et is the jet-kinetic
power (thrust power) produced by the engine, and Peiect is the electrical (or other
external source) power supplied to the propulsion system. For air-breathing
systems the mass flow rate of the fuel is very small relative to the mass flow
rate of the air.
Note that an air-breathing system can never fly faster than its exhaust velocity.
Rockets, because they carry an onboard oxidizer, do not have this restriction
and, therefore, have no flight-speed limits. The jet power for high-speed air­
breathing engines is larger than the equivalent jet power for a rocket due to the
inlet and is the difference of two large numbers.
Large power levels are required for aircraft and launch vehicles. For example, an
SR-71 cruising at Mach 3.2 produces a thrust of 24,700 lbf (110 kN) and a jet
power of 104 MW. Climb and maneuver thrust is much higher. Similarly, an RL10
rocket engine produces 15,000 lbf (66.7 kN) thrust at an l sp of 433 seconds, and
has a jet power of 142 MW. By comparison, a Nimitz class nuclear aircraft carrier
propulsion system is 194 MW, and the Hoover
dam produces about 2000 MW. Therefore, any
electric system replacing these applications must
be capable of processing a lot of power.
Spacecraft propulsion systems are typically
hundreds of watts to tens of kW. This, in addition
to powerplant weight issues, is a primary reason
why electric propulsion systems are currently
being used on spacecraft and not on aircraft.
Historically, electric thrusters for spacecraft were
available for flight decades before the power
systems. 12
Figure 1. Electrothermal Arcjet
Thruster. Photograph of a 30-kW
arcjet thruster being tested at the
Jet Propulsion Laboratory .13
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