ENGINEERING - Royal Australian Navy

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FIGURE 2 THE CANADIAN “DRES BALL”
TYPE OF INFRA-RED EXHAUST
SUPPRESSION SYSTEM.
exhaust plume. The Canadian
design is known as the “DRES
Ball” after the research
organisation that produced it and
its overall shape. The Canadian
stack design includes an external
shield separated from the ducting
to further reduce its temperature
and hence its visibility in the
IR spectrum.
On the Tribal class themselves,
a different solution was chosen,
possibly to reduce weight, and
the rebuilt vessels have heat
absorbent tiles mounted to the
funnel surface to reduce the
IR signature.
US Navy ships of th e
“Ti c o n d e ro ga” and “A rleigh Burke ”
classes use a similar but simp l e r
s ystem of ejector nozzles, with less
s e p a ration of the outer casing.
vessels of the DD963 “Spruance”
class, the DDG993 “Kidd” class
the CG47 “Ticonderoga” class
and the DDG51 “Arleigh Burke”
class are all fitted with eductor
diffuser systems similar to that
illustrated here. This operates
by entraining air through the
diffuser rings, as illustrated, and
cools the exhaust from the outer
edge inward.
This provides a cooler exhaust
plume and considerable
reduction in the detection
probability of the turbine exhaust.
It is effective only to about 70
degrees from the horizon, leaving
a vulnerable area above where
the vessel could be tar geted by a
high flying patrol aircraft, or a
missile programmed to climb
and dive as part of its approach
manoeuvres.
is achieved by injecting seawater
into the exhaust stream. There
are two points of injection, the
first just aft of the turbine itself,
and another in the exit pipe just
before the exhaust reaches the
atmosphere. The second set of
injectors is triggered by
temperature sensors in the
exhaust pipe between the tw o
sets of injectors, and the second
spray is only triggered when
required to maintain the
appropriate temperature at
the exit.
The exhaust is, in this case,
directed sharply downward into
the wake, which should fur ther
reduce any remaining heat and
effectively prevents any “sighting”
of the hot gases in the exhaust
system by an IR seeker or search
and track system.
FIGURE 3 A CANADIAN VERSION OF THE
EDUCTOR/DIFFUSER SYSTEM USED BY THE
USN IN CG AND DDG CLASS SHIPS
The “DRES Ball” diffuser was
developed for the Canadian
Armed Forces, and is used on
the “Halifax” class Frigates. Its
construction can clearly be seen
in the adjacent diagram, from
Davis Engineering of Canada.
A reduction of 95% in the IR
detection of the Gas Turbine
exhaust is claimed for this
pattern of diffuser, and it has
the additional advantage over
conventional eductors of a
reduction in the temperature of
the central core of the exhaust
plume. This results from the
induction of air into the centre
of the stack through the central
“ball” and the hollow struts.
With a conventional eductor,
the core of the exhaust plume
is not cooled, and the ship
can be detected by IR sensors
from above.
The manufacturers, who are also
responsible for the Infra-Red
modelling system NTCS used by
the RAN, claim the DRES Ball to
be the most advanced
suppression system available for
marine gas turbine engines.
Although the USN did not fit any
form of Infra-Red suppression to
their FFG class ships, their larger
The Canadians, much of whose
c o a stline is located towa rd and
b eyond the Arctic Circle, have a
c o n s i d e rable incentive to consider
IR emission ve ry seri o u s ly. Curre n t
te chnical developments in IR
s e e ker head design mean th a t
Navies in wa rmer climates need
to make the same ch a n ges as
h ave the Canadians.
A EUROPEAN SY ST E M
A more complex system of IR
suppression has been developed
by the Norwegian company
MECMAR, for use on the Swedish
“Visby” class frigates and also on
the South African A200 class
frigates.
This involves the use of stainless
steel manifolds and titanium
exhaust ducting, into which salt
water is sprayed to reduce the
exhaust temperature. The
remaining exhaust is ducted t o
the stern and directed at the
wake to further reduce its IR
signature.
The Norwegian manufacturers of
this system, Mecmar, claim a
reduction to 60 degrees Celcius
from the approximate 1000
degrees Celcius expected from a
gas turbine like an LM2500. This
The major part of the exhaust
system is made from titanium,
certainly all of it downstream of
the water injectors. The upstream
section is stainless steel.
It is clear that the some very
corrosive chemicals will be
formed in the cooling process by
reaction with the water at the
primary injector, and while the
titanium ducting will resist these,
they are disposed of through a
drain into the sea. The
manufacturers indicate that they
have had no problems with
corrosion at the drains, which
suggests that the majority of
corrosive products are entrained
in the exhaust plume. This does
suggest to the author that the
temperature reduction at the first
injector is not quite as g reat as
claimed by the manufacturer.
These exhausts are to be
m a n u fa c t u red locally by Mari n e
& Engineering Indust ri e s ,
p ro b a b ly for fa st cata m a ra n
fe rries intended for No rth e rn
E u ropean markets, where th i s
s ystem is used to re d u c e
a t m o s p h e ric pollution fro m
diesel and gas turbine exhaust s ,
ra ther than as an infra - re d
s u p p ression syste m .