The pace of development has been accelerating in the past decade, with the first experimental
RDEs running in the U.S. since the 1960s. An RDE
powered turbine at the AFRL has accumulated
more than 20 minutes of operation since 2016.
In August 2017, a team of Japanese researchers
from Nagoya and Keio Universities, JAXA, and
the Muroran Institute of Technology conducted
a test of an ethylene-burning RDE that produced
895 Newtons of thrust. Their aim is to develop a
sounding rocket powered by an RDE.
The promises of increased efficiency, simplicity, and high power density are driving the current
research focus on RDEs. A quiet unannounced
race is ongoing between nations and institutions
to figure out how best to utilize the cycle.
The Detonation Cycle
A gas turbine powered by detonation would
have a detonation wave rotating continuously at
thousands of cycles per second around the inside
of an annular combustion chamber, pressurizing
the products of combustion and producing thrust.
The wave is sustained by a continuous inlet flow
of oxidizer and fuel at one end of the annulus. As
the wave passes over the injectors, the high pressure shuts down the reactant flow. Injection flow
is restarted after the wave passes, creating the
triangular-shaped fill region of unburned gasses
that feed the detonation. No moving parts are
required. The only rotating feature is the wave
The supersonic shock wave within an RDE acts
as a compressor. Combustion starts at a much
higher pressure and temperature than what is
found in an equivalent constant pressure process
at the same initial conditions. As a result, the ideal
detonation cycle produces a higher performance
than the Brayton cycle.
But this means that an RDE uses a different
thermodynamic cycle than the ones familiar to
engineers—the Otto and Diesel cycles found in
automobile engines, the Rankine cycle in steam
turbines, and the Brayton cycle that is the heart
of the gas turbine. Understanding the detonation
cycle is crucial to predicting the amount of
useful energy available for thrust or turbine work
Detonation belongs to a class of cycles called
pressure gain combustion, where a rise in
pressure is produced by the action of combustion
instead of mechanical compression. Tracing the
cycle across the familiar thermodynamic cycle
diagram helps explain why detonation cycles have
sustained such interest.
The detonation cycle will work without pre-compression, although an RDE is usually paired
with a pressurized tank or compressor to boost
efficiency as the first leg of a five-part ideal
cycle. After pre-compression, the detonation
proper starts with shock compression. The
rising temperature and pressure of the shock
compression creates free radicals that initiate
auto-ignition. Heat release further increases
temperature until combustion ends with the
The fourth process of expansion has two parts.
The first expansion produces unrecoverable
energy that is required to power the forward
motion of the leading shock. The second
expansion creates useful work that may be used
for thrust or turbine work extraction. The fifth leg
returns the gas flow to ambient conditions.
Mapped against the Brayton cycle, one sees that
entropy generated by the detonation is less and
the useful work is greater than the Brayton cycle.
For this reason, the detonation cycle has captured
the interest of a world trying to squeeze every
useful joule out of available fuels.
A close examination of the h-s diagram
(opposite) might lead to protests that the peak
enthalpy exceeds that of the heat addition. The
discrepancy is partly due to the fact that a plot
of static enthalpy ignores the inherent kinetic
and rotational energies in the wave. Only then
can the sum of energies be matched against the
BRAYTON AND DETONATION CYCLE ENGINES
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