MECHANICAL ENGINEERING | JULY 2018 | P.37

THERE’S A NEW CYCLE IN TOWN
The rotating detonation combustor promises asimple and efficient engine to transform heat
BY CRAIG A. NORDEEN
directly into work.

AND LEE S. LANGSTON

New technologies appear to come out of nowhere. The longgestation period is often hidden.
Franz Stolze ﬁrst ﬁled a patent application for theturbojet engine in 1873. It was rejected. It wasn’t until the1930s that Hans von Ohain and Frank Whittle designed
operational jet engines. It took another 20 years after that before gas
turbines beat piston engines in thermal efficiency to become the primary propulsion method of modern aviation.
Similarly, Herman Oberth’s writing on electric propulsion waited 60
years for the launch of the ﬁrst ion engine aboard Deep Space I in 1998.
If there is a half-century wait for technologies to develop, then it
is time for the rotating detonation engine (RDE). The RDE was ﬁrst
conceived in the 1950s and the ﬁrst experimental devices were run at
the Lavrentyev Institute in Novosibirsk, Russia, and at the Universityof Michigan. Such an engine would differ from a conventional turbojet
by combusting fuel via detonation rather than deﬂagration.
Deﬂagration and detonation waves differ by structure and characteristic velocity. Deﬂagration—whether the controlled burn inside agas turbine or the rapid burning of gunpowder—features a subsonic
ﬂame front that advances by diffusion of heat and species. In contrast,
detonation is a layered structure of a leading shockwave, subsonic
combustion, and thermal choke followed by a supersonic expansion.
The entire sandwich is driven through the reactants at supersonic oreven hypersonic speeds.
In popular imagination, detonations are short, sharp shocks, but arotating detonation engine will run as long as reactants are supplied,
producing a roar worthy of any NASA rocket.
An engine that uses detonation rather than deﬂagration could havesome key advantages. If harnessed in a gas turbine or rocket, detonation could reduce the need for some expensive hardware, lighten
engine weight and increase power output and efficiency.
Today, variants of the RDE as a combustor for gas turbines, rockets,and scramjets are being explored at the Air Force Research Laboratory(AFRL), Naval Research Laboratory, Naval Postgraduate School, andthe Department of Energy. Similar work is being conducted in Russia,France, Poland, Japan, China, Germany, and several other countries.

A rotating detonating engine
developed in Japan is being
used to evaluate engine thrustand conﬁrm stable operation
under vehicle acceleration.
Nagoya University

MECHANICAL ENGINEERING | JULY 2018 | P.39

The Detonation Cycle
A gas turbine powered by detonation would
have a detonation wave rotating continuously atthousands of cycles per second around the insideof an annular combustion chamber, pressurizing
the products of combustion and producing thrust.
The wave is sustained by a continuous inlet ﬂowof oxidizer and fuel at one end of the annulus. Asthe wave passes over the injectors, the high pressure shuts down the reactant ﬂow. Injection ﬂow
is restarted after the wave passes, creating thetriangular-shaped ﬁll region of unburned gasses
that feed the detonation. No moving parts arerequired. The only rotating feature is the wave
structure.
The supersonic shock wave within an RDE acts
as a compressor. Combustion starts at a much

BRAYTON AND DETONATION CYCLE ENGINES
Conventional twin-spool gas turbine

Low Speed Spool
High Speed Spool

LPC

2.0AHPC

2.5

HPTCombuster

3.0

Detonation gas turbine with an RDE combustor

Low Speed Spool
LPT4.0 4.5

LPC

LPTRDE

5.0

2.0'
B

2.5'

4.5'

3.0' 4.0'

5.0'higher pressure and temperature than what isfound 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 toengineers—the Otto and Diesel cycles found inautomobile engines, the Rankine cycle in steam
turbines, and the Brayton cycle that is the heartof the gas turbine. Understanding the detonation
cycle is crucial to predicting the amount of
useful energy available for thrust or turbine work
extraction.
Detonation belongs to a class of cycles calledpressure gain combustion, where a rise inpressure is produced by the action of combustion
instead of mechanical compression. Tracing thecycle across the familiar thermodynamic cycle
diagram helps explain why detonation cycles havesustained such interest.
The detonation cycle will work without precompression, although an RDE is usually pairedwith 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 shockcompression creates free radicals that initiate
auto-ignition. Heat release further increases
temperature until combustion ends with thethermal choke.
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 thatentropy generated by the detonation is less andthe 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 peakenthalpy exceeds that of the heat addition. The
discrepancy is partly due to the fact that a plotof static enthalpy ignores the inherent kinetic
and rotational energies in the wave. Only thencan the sum of energies be matched against theRDE combustor andnozzle test, showing
shock diamonds inthe supersonic flow.
Photo: AFRL WPAFB

heat addition. Unfortunately, the explanationis lengthy and we must refer the reader to theacademic RDE literature.

Short of Ideal
Ideal cycles tell only half the story.
All ideal cycles become more efficient as thepre-compression increases and asymptoticallyapproaches 100 percent. The ideal Brayton cycle
has zero efficiency with no pre-compression.
With no pre-compression, the ideal detonation
cycle has a signiﬁcant thermal efficiency, about
35 percent because of the leading shock. The
difference between the two cycles narrows withincreasing pre-compression. The ideal detonation
cycle is always more efficient.
The story changes dramatically when realistic
component losses are introduced. Academic
studies often assume compressors and turbines tobe 90 percent efficient to demonstrate impact andtrends. Realistic values might differ considerably,
but the effect of component efficiencies on thetotal system is clear. Instead of continually risingand approaching 100 percent, a maximum system
thermal efficiency is reached at some optimal
point and then drops with an increasing precompression.
The net effect is that perhaps half of the usefulwork of an ideal heat engine is lost. The narrowing
gap between detonation and Brayton cycles is not
immune to this effect. Indeed, there are conﬂicting

conclusions on the utility of a detonation cycle.
Some studies have determined that the Brayton
cycles can actually be better at high pre-compression and the investment in a new cycle is not justiﬁed. Others have concluded that the detonation
cycle will always be more efficient than the Brayton
cycle even with component efficiencies less thanunity. For engines with high pre-compression, evena 0.1 percent increase in efficiency can have a big
effect on fuel consumption over time.
For that reason, the promise of RDEs is signiﬁcant. Many real internal combustion engines areless than 25 percent efficient. The best aeroderiv-IDEAL DETONATION AND BRAYTON CYCLES
D3Detonation
Cycle

Thermal
Choke
B3

D3.5

Enthalpy

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, andthe Muroran Institute of Technology conducteda test of an ethylene-burning RDE that produced
895 Newtons of thrust. Their aim is to develop asounding 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.

MaxPressure

qadd= CpΔT

Brayton
Cycle
B4

D2.5
2

D4

uret PressAmbien1

Entropy

MECHANICAL ENGINEERING | JULY 2018 | P.41

A Long E-Z aircraft
was adapted tooperate with powerfrom a pulsed
detonation engine.
The airplane flew
once over Mohave,
Calif., (inset) andis now on display
at the National
Museum of theU.S. Air Force on
Wright-Patterson
Air Force Base,Ohio.

ative gas turbines, such as the H series turbines
used for electric power generation, might break
40 percent. It is a major challenge for a singlecycle heat engine of any type to achieve 50 percent. Gas turbine and steam combined cycles canapproach 60 percent.
With enough pre-compression and efficientcomponents, however, an RDE gas turbine by itself might top 50 percent. A combined-cycle RDE
plant could possibly reach 70 percent efficiency, agoal of current DOE research.
Whether an RDE could ever reach that goalis an open question. The truth of the matter isburied in the details of any given engine design.
The real component efficiencies of a RDE gas
turbine will never be the same as a conventional
gas turbine.
Simple comparisons based on some equivalency of component efficiency will only show generaltrends. Such a method is not reliable if the difference between the two systems is small and very
sensitive to modeling assumptions.

Photos: Museum of theU.S. Air Force and US
Airforce (inset).

Crack and Roar
The current state of research has passed froma proof of concept to the establishment of aworking theory of operation and the explorationof a range of operating characteristics. The next
step must be actual demonstrations for energy
production or experimental ﬂight.
An allied technology, the pulse detonationengine, has been demonstrated, with the ﬁrst
manned ﬂight of a PDE-powered aircraft occurring in 2008. These engines, which produce
a repeating sharp crack as the shock wave exits
the tube at perhaps 100 times per second, have asmaller power density than an RDE, which emitsa steady 5 kHz roar.
But experimental demonstrations of thethermal efficiency and overall performance of an
RDE are generally lacking. The published results
to date come from rigs that are designed forgeneral study and not speciﬁcally for efficiency.
(It is true, however, that not all results havebeen published, since some of the RDE researchand development is subject to various levels ofrestriction or classiﬁcation.)
In addition to providing efficiency and performance data, the engine must also be operated foran extended period to demonstrate that it can beprotected from the heat and pressure it generates.
To provide cooling for a constantly running

Brayton gas turbine, the compressor-discharge
air is routed around the combustor, driven by
the pressure drop from the compressor to theturbine. The cooling air is ported to oriﬁces in theturbine blades to create a ﬁlm of air to protect theblades from damage. A pressure gain combustor
does not provide this useful drop in pressure, soother means must be used to protect the turbine.
Researchers at the Air Force Research Laboratory
at Wright Patterson Air Force Base are investigating the use of air injectors to provide the necessary cooling, with encouraging results.
Engineers must also perform the exacting workof system integration. No engine cycle exists
without a complex web of factors that tie it to alarger whole. For aircraft and rockets, a heavy
efficient engine may be useless, since weight also

drives the ﬂight range or the deliverable payload.
The opposite is also true; a lightweight engineof mediocre efficiency may bring leverage to reducing structure and other system requirements
throughout the vehicle.
The larger system also includes the time andcost limits of development. For instance, RDEs
may ﬁnd a possible use in heavy lift rocket turbopumps, which represent as much as half of all
rocket development costs. Since the RDE brings
its own shock compression, the turbo pumpsize—and the development cost—may be reduced.
Given the high cost of lifting payloads into Earthorbit, such an effort may be worthwhile, even if
there is no overall performance gain.
The detonation cycle has great promise for

improving performance and reducing the size ofturbomachinery. Until the rotating detonation engine hits its important development milestones,
some skepticism is warranted.
Skeptics should recall the lag between a technological concept’s inception and its realization.
The pace of such development can indeed bemeasured in generations.
We may soon see the day when the RDE transforms from a long-gestating idea to an overnight
success. ME
CRAIG A. NORDEEN is a postdoctoral fellow in the mechanicalengineering department at the University of Connecticut in Storrs. LEE S.
LANGSTON is a professor emeritus there.