RDE combustor and
nozzle test, showing
shock diamonds in
the supersonic flow.
Photo: AFRL WPAFB
heat addition. Unfortunately, the explanation
is lengthy and we must refer the reader to the
academic RDE literature.
Short of Ideal
Ideal cycles tell only half the story.
All ideal cycles become more efficient as the
pre-compression increases and asymptotically
approaches 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 with
increasing 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 to
be 90 percent efficient to demonstrate impact and
trends. Realistic values might differ considerably,
but the effect of component efficiencies on the
total system is clear. Instead of continually rising
and approaching 100 percent, a maximum system
thermal efficiency is reached at some optimal
point and then drops with an increasing pre-compression.
The net effect is that perhaps half of the useful
work 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 than
unity. For engines with high pre-compression, even
a 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 are
less than 25 percent efficient. The best aeroderiv-
IDEAL DETONATION AND BRAYTON CYCLES