Radiation shielding and gas diffusion apparatus

A device for shielding a heated surface from infra-red detection through an opening adjacent to the heated surface. The device includes a gas-conducting member, which is adapted to receive heated gases, and has an exterior surface, an interior surface, an inlet for receipt of heated gases from the opening, and an outlet for the discharge of gases. The gas-conducting member has a configuration which blocks the inlet to the member from line-of-sight view through the outlet to the member. Additionally, the device draws cooling air over the exterior surface of the gas-conducting member. The cooling air may then be mixed with the heated gases within the gas conducting member. This provides cooling of the gas-conducting member to prevent the member from being visible to infra-red detection and may also provide cooling of the gases which are discharged from the outlet of the gas-conducting member by mixing of the heated gases with the cooling air.

BACKGROUND OF THE INVENTION 
Military aircraft are powered by engines which generate heat that produces 
hot metal surfaces within the engine as well as a stream of heated exhaust 
gases. These sources of infra-red radiation, i.e., the hot metal engine 
surfaces and the stream of heated exhaust gas, provide a target source for 
heat-seeking missiles which can lock on the target source and be led to 
the aircraft. 
To provide a military aircraft with a power source which is not easily 
discernible by a heat-seeking missile, a first step would be to insulate 
the surfaces of the aircraft engine. An insulation material which is well 
suited for this purpose is disclosed in U.S. Pat. Nos. 4,037,751 and 
3,948,295. 
The insulation of the aircraft engine surfaces does not, however, prevent 
viewing of the engine by a heat-seeking missile. Even when the aircraft 
engine is well insulated, hot metal surfaces within the engine interior 
may still be viewed by a heat-seeking missile through an opening for 
exhaust gases positioned adjacent to the hot interior surfaces. In 
addition to insulating the exterior surface of the engine, it is, 
therefore, also necessary to block the hot surfaces within the engine 
interior from being viewed through the exhaust gas opening. Additionally, 
it is necessary to reduce infra-red radiation from the hot plume of 
exhaust gases that is emitted by the aircraft engine such that the exhaust 
gases cannot be readily detected by a heat-seeking missile. 
The device disclosed in my prior U.S. Pat. No. 3,930,627 serves to prevent 
the detection of an aircraft by a heat-seeking missile by providing an 
exhaust gas-conducting member that is adapted to receive heated exhaust 
gases from an exhaust opening of an aircraft engine. The device of my 
prior patent provides a configuration which blocks the exhaust opening of 
the engine from line-of-sight view through the outlet of the exhaust 
gas-conducting member. Additionally, the device of my prior patent 
functions to reduce infra-red radiation from exhaust gases emitted by the 
aircraft engine by breaking up the stream of exhaust gas into a plurality 
of smaller streams and mixing cooling air with the heated exhaust gases. 
In accomplishing these beneficial results, the device of my prior U.S. Pat. 
No. 3,930,627 employs cooling air which is received by an air intake which 
leads to the interior of the gas conducting member. The cooling air is 
received by the device of my prior patent as the aircraft moves the device 
through the air. Additionally, airflow may be generated by the propeller 
or rotor that is driven by the aircraft engine, with the airflow being 
received by the air intake and conveyed to the interior of the 
gas-conducting member. The airspeed of a military aircraft, particularly a 
helicopter, will not be constant. As a result, the quantity of cooling air 
received by the gas-conducting member in the device of my prior patent 
will vary during the operation of the aircraft. This variation in the 
quantity of cooling air may produce variations in the heat content of the 
exhaust gases from the exhaust gas member. Also, there may be some degree 
of fluctuation in the temperature of the exhaust gas member in my prior 
device. In a hovering helicopter having an airspeed of zero, the exhaust 
gases discharged from the gas-conducting member in my prior device may 
experience a rapid temperature increase such that the gases become visible 
to infra-red detection. This could be disastrous and result in the loss of 
the aircraft to a heat-seeking missile. 
In view of the possible fluctuations in the operation of the device of my 
prior patent, U.S. Pat. No. 3,930,627, in response to changes in the 
airspeed of the host aircraft, a radiation shielding device would be 
desirable whose operation would be less dependent upon the airspeed of the 
host vehicle. This would permit the radiation shielding device to operate 
efficiently even when the vehicle had an airspeed of zero, as in the case 
of a hovering helicopter. Also, this would permit the use of the radiation 
shielding device on a stationary power source in shielding the power 
source from detection by a heat-seeking missile. 
SUMMARY OF THE INVENTION 
The present invention pertains to an improvement in a radiation shielding 
and gas diffusion apparatus, as disclosed in my prior U.S. Pat. No. 
3,930,627. In the apparatus of the present invention, energy derived from 
the heated exhaust gases is used to provide a relatively constant flow of 
cooling air to cool the radiation shielding device and to cool the heated 
exhaust gases. The device of the invention functions to shield a heated 
surface from infra-red detection through an opening adjacent to the heated 
surface and employs a gas-conducting member which is adapted to receive 
heated gases from the said opening. The gas-conducting member includes an 
exterior surface, an interior surface, an inlet for the receipt of gases 
from the opening, and an outlet for the discharge of gases. The member has 
a configuration which blocks the inlet to the gas-conducting member from 
line-of-sight view through the outlet of the member. Heated engine 
surfaces, therefore, are not visible to line-of-sight view by a 
heat-seeking missile through the outlet of the gas-conducting member. 
Additionally, the present device includes means to draw cooling air over 
the exterior surface of the gas-conducting member with the cooling air 
then being mixed with the heated gases in a mixing region within the 
gas-conducting member. The means to draw cooling air does not depend upon 
an airflow generated by movement of the gas-conducting member through the 
air or upon the movement of an aircraft propeller or rotor. Thus, the 
device of the invention functions when the device is at rest to cool the 
surfaces of the gas-conducting member such that the member is not visible 
to infra-red detection. Also, the device of the invention functions to 
cool the heated exhaust gases through mixing of the heated gases with 
cooling air within the gas-conducting member. 
In drawing cooling air over the exterior surface of the gas-conducting 
member, the cooling air may be drawn to a mixing region where the cooling 
air is mixed with the heated gases, which is positioned adjacent to the 
outlet from the gas-conducting member. Also, the present device may 
function to draw cooling air over the exterior surface of the 
gas-conducting member to a mixing region that is positioned adjacent to 
the inlet to the gas-conducting member. 
The device of the invention preferably includes flow passages formed on the 
exterior surface of the gas-conducting member with the flow passages 
serving to convey cooling air over the exterior surface of the 
gas-conducting member. When cooling air is drawn into a mixing region 
which is adjacent to the outlet from the gas-conducting member, the flow 
of cooling air over the exterior surface is in the same general direction 
as the flow of heated gases within the gas-conducting member, i.e., flow 
of heated gases and the flow of cooling air being cocurrent. However, when 
cooling air is drawn into a mixing region which is positioned adjacent to 
the inlet to the gas-conducting member, the cooling air drawn over the 
exterior surface of the gas-conducting member flows in a direction which 
is counter to the flow of heated gases within the gas-conducting member. 
In this embodiment, the flow of cooling air is countercurrent to the flow 
of heated gases within the gas-conducting member. 
In using the energy of the heated exhaust gases to draw cooling air over 
the exterior surface of the gas-conducting member, an expansion region may 
be provided within the gas-conducting member. The expansion region permits 
the expansion of heated gases which are received through the inlet to the 
gas-conducting member. On expansion of the heated gases, the pressure is 
reduced to generate a partial vacuum which may serve as the driving force 
to draw cooling air over the surface of the gas-conducting member to a 
mixing region within the gas-conducting member where the cooling air is 
mixed with the heated exhaust gases. To provide more efficient cooling of 
the gas-conducting member by the cooling air, means may be employed to 
increase the heat transfer between the cooling air and the gas-conducting 
member. In this manner, the heat transfer through the gas-conducting 
member may be maximized with heat being removed from heated gases within 
the member, and the heat being transferred through the gas-conducting 
member to the cooling air in contact with the member. 
The gas-conducting member may include a plurality of passages therein for 
the receipt of heated exhaust gases and for the discharge of gases. Each 
of the passages may have an exterior surface with the passages being 
separated from each other and with the spaces between the passages forming 
cooling air passages. In this manner, cooling air may be drawn over the 
exterior surface of each of the exhaust gas passages to provide cooling of 
the individual exhaust gas passages within the gas-conducting member. 
In providing a configuration which prevents line-of-sight viewing of the 
inlet to the gas-conducting member through its outlet, the member may have 
a cross-sectional configuration which is elongated and generally 
rectangular or elliptical. The gas-conducting member may then be curved 
such that exhaust gas entering the inlet passes along a curved path before 
being discharged from the outlet. In passing along the curved path, the 
radial dimension across the gas-conducting member may be relatively small. 
This permits making the gas-conducting member more compact than would be 
the case if the gas-conducting member had a circular cross-sectional 
configuration. With a circular cross-sectional configuration, the 
gas-conducting member requires a relatively large curvature to block the 
inlet to the member from line-of-sight viewing through the outlet from the 
member. However, when the gas-conducting member is provided with an 
elongated cross-sectional configuration, the radius of curvature of the 
gas-conducting member may be greatly reduced while preventing 
line-of-sight viewing of the inlet to the member through the outlet from 
the member. 
The shape of the gas-conducting member or the shape of a plurality of 
exhaust gas flow passages within the member in the present device changes 
the shape of the exhaust gas stream discharged from the outlet of the 
gas-conducting member to a shape having a lower emissivity such that heat 
and energy are more readily dissipated from the stream. By providing the 
gas-conducting member with an elongated configuration, such as a generally 
rectangular or elliptical cross-sectional configuration, the exhaust gases 
are more difficult to detect through infra-red radiation than an 
equivalent exhaust gas stream having a circular configuration.

DETAILED DESCRIPTION 
Turning to FIG. 1, a helicopter, generally indicated as 2, includes a body 
4, a main rotor 6, and a tail rotor 8. A power source for the helicopter 2 
is enclosed within an engine compartment 10 with exhaust gases from the 
power source being discharged through infra-red suppressors of the 
invention generally indicated as 12. 
FIG. 2 is a perspective view of an infra-red suppressor 12 with portions 
broken away for ease of illustration. The infra-red suppressor 12 includes 
a cowl 14 having a flange 16 which may be secured to another connector 
flange 18 in any suitable manner in joining the infra-red suppressor to an 
ngine shroud 20. Exhaust conduit 22 having a front flange 24 for securing 
the exhaust conduit to the engine (not shown) is positioned within the 
engine shroud 20. The exhaust conduit 22 terminates in a plurality of 
exhaust nozzles 26 having exhaust openings 28. Exhaust gases are ejected 
through the exhaust openings 28 into exhaust ducts 30 with cooling air 
passages 32 surrounding the exhaust ducts to cool their exterior surfaces. 
The exhaust ducts 30 have a larger cross-sectional area than the openings 
28 such that the discharge of exhaust gases from the openings into the 
ducts causes a reduction in pressure within the ducts to provide a first 
stage ejection region 33. Cooling fins 34 may be positioned within the 
passages 32 on the exterior surfaces of the exhaust ducts 30 to promote 
transfer of heat from the heated gases within the exhaust ducts to cooling 
air within the cooling air passages. 
A main body portion 35 of the suppressor 12 forms the exterior wall of the 
suppressor with the main body portion merging rearwardly into a skirt 
portion 36. The exhaust ducts 30, cooling air passages 32, and cooling 
fins 34 are supported in spaced relation in any convenient manner relative 
to the body portion 35 while separators 38 positioned across the skirt 
portion 36 divide the skirt portion into a plurality of second stage 
ejection regions 39. The cross-sectional areas of the second stage 
ejection regions 39 are larger than the areas of the exhaust ducts 30 such 
that there is a reduction in pressure within the ejection regions as gases 
pass from the ducts into the ejection regions. Cooling air passing through 
cooling air passages 32 is mixed with exhaust gases passing through the 
exhaust ducts 30 within the second stage ejection regions 39. The energy 
of exhaust gases passing from the exhaust ducts 30 into the ejection 
regions 39 produces the reduction in pressure within the ejection regions 
which provides a driving force that draws cooling air through the cooling 
air passages 32 into the ejection regions. Separators 38 include inward 
extensions 38a that extend into air passages 32 such that cooling air flow 
within the cooling air passages is divided by the extensions with a 
portion of the air flow being directed to one ejection region 39 on one 
side of the extension while the remainder of the air flow is directed to 
the ejection region on the opposite side of the extension. A plurality of 
air intake openings 40 extend through the main body portion 35 to admit 
cooling air to the cooling air passages 32 which surround the exhaust gas 
ducts 30. 
Turning to FIG. 3, which is a top sectional view through the suppressor 12 
of FIG. 2, air intake openings 42 pass through the wall of the engine 
shroud 20 to the plenum 43 between the engine shroud and the exhaust 
conduit 22. The primary exhaust stream designated by the arrow A within 
the exhaust conduit 22 is broken up into a plurality of secondary exhaust 
streams B within the exhaust nozzles 26. Cooling air streams indicated by 
the arrows C are drawn through the air intake openings 40 and 42, as 
described, by the reduction in pressure within the first stage ejection 
regions 33 and the second stage ejection regions 39. The cooling air C 
introduced through intake openings 42 is admixed with the secondary 
exhaust streams B in the first stage ejection regions 33 while cooling air 
introduced through intake openints 40 passes through the cooling air 
passages 32 to the second stage ejection regions 39. Cooling air C may be 
drawn to ejection regions 33 from any opening into the plenum 43. Thus, 
for example, the upstream end of the plenum 43 may receive cooling air C 
from any location on the helicopter 2, such as the passenger compartment 
(not shown). 
Within the ejection regions 33 and 39, the cooling air streams C mix with 
the gases received from the exhaust openings 28. This provides cooling of 
the exhaust gases to produce diluted exhaust streams indicated by the 
arrows D which are discharged from the infra-red suppressor 12. The 
diluted exhaust streams D are difficult to detect by an infra-red detector 
because the temperature of the streams D is reduced because of the mixing 
with the cooling air C. Additionally, the streams D may have an elongated, 
generally-rectangular configuration to reduce the emissivity of the 
streams as compared, for example, with streams having a circular 
configuration. The use of elongated, generally rectangular ducts 30 also 
assists in shielding the hot openings 28 from line-of-sight view through 
the downstream end of the infra-red suppressor 12, while permitting a 
reduction in the size and degree of curvature of the suppressor that is 
required for line-of-sight shielding. For example, if the suppressor 12, 
as viewed in FIG. 3, employed only a single duct to replace the ducts 30, 
the exhaust openings 28 would be readily visible to line-of-sight view 
through the downstream end of the infra-red suppressor 12. To shield the 
exhaust openings 38 from line-of-sight view from the downstream end of the 
suppressor 12 under these circumstances, it would then be necessary to 
make the suppressor much larger so as to give the suppressor a much 
greater curvature than in FIG. 3. This would make the suppressor heavier 
and would make it more difficult to incorporate into an airframe structure 
because of its increased size. 
In sizing the component parts of a suppressor 12, as illustrated in FIG. 3, 
it is desirable to have an equal pressure reduction in each of the first 
stage ejection regions 33 and an equal pressure reduction in each of the 
second stage ejection regions 39. Thus, the area ratio of each nozzle 
opening 28 with respect to the specific duct 30 into which the opening 
exhausts gas stream B may be maintained relatively constant, i.e., if the 
particular nozzle opening is larger or smaller than another of the nozzle 
openings, the duct into which the nozzle exhausts is also proportionately 
larger or smaller to maintain the area ratio constant. 
Similarly, the area ratio of each duct 30 with respect to the specific 
second stage ejection region 39 into which the particular duct discharges 
is preferably maintained relatively constant. By maintaining the pressure 
reduction in each of the first stage ejection regions 33 relatively 
constant, and the pressure reduction in each of the second stage ejection 
regions 39 relatively constant, the flow of the cooling air streams C is 
more uniform throughout the suppressor 12. This provides uniformity in the 
cooling of the exterior surfaces of the ducts 30 and uniformity in the 
mixing of the cooling air streams C with the exhaust gas streams B to 
provide discharge streams D having a uniform temperature. 
FIG. 4 is a sectional view taken along line 4--4 of FIG. 3. As indicated, 
the separators 38 are joined to the skirt portion 36 with the inward 
separator extensions 38a extending between the exterior surfaces of 
adjacent exhaust ducts 30 to divide the cooling air passages 32 (see FIG. 
3). The cooling fins 34, as illustrated, extend longitudinally along the 
outer surfaces of the exhaust ducts 30. Due to the generally cylindrical 
cross-sectional configuration of the main body portion 35 (see FIG. 3) the 
outer two exhaust ducts 30a are smaller and have a different configuration 
than the inner two exhaust ducts 30b. The outer ducts 30a have a generally 
trapezoidal cross-sectional configuration, while the inner ducts 30b have 
a generally rectangular configuration. As stated, the fins 34 extend along 
the outside surfaces of the exhaust ducts 30. However, there are lines of 
separation 44 between the fins 34 which extend along corners of the walls 
of ducts 30. The corners of the walls of ducts 30 along the lines of 
separation 44 may be thought of as lines having only length and no 
area--thereby not requiring cooling. 
FIG. 5 is a sectional view taken along the line 5--5 of FIG. 3. As 
indicated, the difference in the size and configuration of outer exhaust 
ducts 30a as compared with the inner exhaust ducts 30b is matched by outer 
nozzles 26a which are smaller than inner nozzles 26b. The outer nozzles 
26a terminate in exhaust openings 28a which are likewise smaller than the 
exhaust openings 28b which are formed at the ends of inner nozzles 26b. 
FIG. 6 is a detail view looking inward at the outer surface of exhaust duct 
30a along the line 6--6 as shown in FIG. 4. As shown in FIG. 6, the 
cooling fins 34 are preferably straight in their configuration and extend 
longitudinally along the surfaces of the exhaust ducts 30. 
FIG. 7 is a top sectional view, similar to FIG. 3, of another embodiment of 
my invention. An infra-red suppressor 46 includes a cowl 48 having a 
flange 50 or similar connector through which the cowl may be secured to a 
structure such as an aircraft body. A plurality of exhaust gas ducts 52 
may be formed within the suppressor 46 with the ducts each having an 
elongated configuration, as described with respect to the suppressor 12 of 
FIG. 3. Cooling air passages 54 surround the exhaust ducts 52 to cool the 
exterior surfaces of the ducts. A primary exhaust gas stream, indicated by 
the arrows E, may pass through the exhaust conduit 22 with the primary 
stream being broken up into a plurality of secondary streams F within a 
plurality of exhaust nozzles 26. Cooling air streams represented by the 
arrows G may enter the suppressor 46 from a region forward of the 
suppressor along the exterior of exhaust conduit 22. The region around the 
exterior of exhaust conduit 22 may form an air plenum similar to the 
plenum 43 formed as described in regard to FIG. 3. 
The cooling air streams G may enter directly into the exhaust ducts 52 
while other cooling air streams indicated by arrows H may first pass over 
the exterior surfaces of the ducts 52 through cooling air passages 54 
before entering the ducts. The cross-sectional area of the openings 28 is 
less than that of the ducts 52 with discharge of the gas streams F into 
the ducts 52 producing ejection regions 55 having a reduced pressure which 
is the driving force in drawing cooling air streams G and H into the 
ejection regions. Cooling air from the streams G may encounter flow 
separators 56 which divert the cooling air into various of the exhaust 
ducts 52. Additionally, flow reverser surfaces 58 are positioned to 
encounter cooling air streams H in reversing the flow direction of the 
streams in flowing from the cooling air passages 54 into the exhaust ducts 
52. 
An end plate 60 may be positioned against the end of the suppressor 46 to 
close the ends of the cooling air passages 54. As will be described, holes 
are provided in the end plate 60 for the passage of diluted exhaust 
streams I from the ducts 52. The cowl 48 may terminate in a cowl end 
surface 62 to provide inlet openings 64, 66, and 68 for the entrance of 
cooling air into the cooling air passages 54. 
Turning to FIG. 7a, taken along the lines 7a--7a of FIG. 7, the end plate 
60 may be secured to the end surfaces of the exhaust gas ducts 52 with 
outer ducts 52a having a generally trapezoidal cross-sectional 
configuration, and inner ducts 52b having a generally rectangular 
cross-sectional configuration. In addition to the air inlets 64, 66 and 68 
(see FIG. 7) air inlets 70 may also be formed on either side of the cowl 
48 as shown in FIG. 7a. 
FIG. 7b, which is a sectional view taken along the line 7b--7b of FIG. 7a, 
illustrates air passing into a cooling air conduit 54 through air intake 
openings 64, 66, and 68 (see FIG. 7) with the air being distributed across 
the width of passage 54 by diverter members 72 and 74. The diverter 
members 72 extend further into the cooling air passages 54 than members 
74. A stream of air H.sub.1 entering the inlet opening 68 is diverted to 
the left as viewed in FIG. 7b by encountering diverter member 74 while a 
stream of air H.sub.2 entering inlet opening 66 is likewise directed to 
the left or rearwardly by diverter member 72. The stream of air H.sub.3 
does not encounter the diverter members 72 and 74 and, thus, passes to the 
center of the cooling air passage 54 before flowing in a rearward 
direction. The effect of diverter members 72, 74, as described, is to 
spread the flow of cooling air across the width of a cooling air passage 
54 to achieve more uniform cooling by the total air flow. Moreover, by 
separating the flow of cooling air streams H.sub.1, H.sub.2 and H.sub.3 
from the passage of diluted exhaust streams I through the end plate 60 
(see FIG. 7), the tendency for the gases in the exhaust streams I to be 
drawn into openings 64, 66 or 68 is minimized. 
FIG. 8 is a side view of a circular engine exhaust conduit 22 and an 
exhaust nozzle such as one of the nozzles 26 (FIG. 2 and FIG. 7). As 
illustrated, the configuration of the stream of exhaust gases may be 
changed as the exhaust gases pass from conduit 22 into one of the nozzles 
26 to provide the stream with an elongated, generally rectangular 
sectional configuration as opposed to a circular configuration. This 
reduces the emissivity of the stream, as described, which makes the stream 
more difficult to detect by an infra-red radiation detector. 
FIG. 9 is an end view of equally sized exhaust nozzles 75 emanating from an 
exhaust conduit 22 and terminating in exhaust openings 76. As the exhaust 
gas stream in conduit 22 passes into the nozzles 75, the stream is broken 
up into a plurality of smaller streams having an elongated, generally 
rectangular cross-sectional configuration. This assists in reducing the 
emissivity of the exhaust gas streams. In the embodiments of FIGS. 3 and 
7, the exhaust nozzles 26 are not equally sized. However, in other 
respects, the nozzles 26 of FIGS. 3 and 7 function in the same manner as 
the nozzles 75 of FIG. 9 in changing the shape and emissivity of the 
exhaust gas streams. The use of equally sized nozzles 75, as in FIG. 9, is 
simply determined by the outer configuration of the infra-red suppressor 
which may have a rectangular cross-sectional configuration with equally 
sized exhaust ducts receiving exhaust gases from equally sized nozzles 75. 
This is in contrast to the use of unequally sized nozzles 26 and unequally 
sized exhaust ducts 30 and 52 when the suppressor 12 or 46 has a circular 
cross-sectional configuration (FIGS. 3 and 7). 
FIG. 10 is a top sectional view, similar to FIG. 7, showing a further 
embodiment of the invention in which cooling air flows in a countercurrent 
direction to the flow of exhaust gases in cooling the exterior surfaces of 
ducts which carry the exhaust gases. An engine housing 77 may include a 
pair of engine exhaust conduits 78 mounted therein with each conduit 
terminating in a pair of exhaust nozzles 80 and 81. 
The outer exhaust nozzles 80 and inner exhaust nozzles 81 are formed at the 
distal ends of each of the exhaust conduits 78 with the outer exhaust 
nozzles being somewhat smaller and positioned further to the rear than the 
inner exhaust nozzles 81. The engine housing 77 may terminate at an outer 
surface 82 and an infra-red suppressor, generally indicated as 84, may 
include a connecting flange 86 or a similar connecting member which is 
joined in any suitable manner to the outer surface 82. 
Exhaust gas streams indicated by the arrows designated J may be discharged 
from the exhaust nozzles 80 and 81 into discharge members 88, 90, 92, and 
94 with cooling air streams designated by the arrows K flowing through a 
plenum 96 surrounding the exhaust conduits 78. The discharge member 88 
includes a cooling air passage 98 formed about the exterior surface of an 
exhaust duct 100 while the member 90 includes a cooling air passage 102 
formed about the exhaust duct 104. The member 92 includes a cooling air 
passage 106 formed about an exhaust duct 108 and the member 94 includes a 
cooling air passage 110 formed about the exterior surface of an exhaust 
duct 112. The cooling air passages 98, 102, 106, and 110 are open at their 
distal ends with cooling air streams designated by the arrows L flowing 
into the open ends of the cooling air passages to remove heat from the 
exterior surfaces of the exhaust ducts 100, 104, 108 and 112. Gas diverter 
members 115 and 117 direct cooling air streams L into the ducts 100, 104, 
108 and 112 and also assist in directing streams J and K to individual 
ducts. The openings from the exhaust nozzles 80 and 81 have 
cross-sectional areas that are smaller than the cross-sectional areas of 
the exhaust ducts 100, 104, 108 and 112 that receive the exhaust gas 
streams J from the openings such that ejection regions 113 are produced 
within the ducts which have a reduced pressure. As indicated, the ducts 
104 and 108 are larger than ducts 100 and 112 just as the inner nozzles 81 
are larger than the outer nozzles 80. However, the ratio of the areas of 
nozzles 81 with respect to ducts 104 and 108 is preferably the same as the 
ratio of the areas of nozzles 80 with respect to the areas of ducts 100 
and 112 to provide essentially the same pressure reduction in each of the 
ejection regions 113. The flow of the cooling air stream L is 
countercurrent to the flow of exhaust gases J through the ducts 100, 104, 
108 and 112 because of the reduced pressure within the ejection regions 
113. Also, the cooling air streams K are drawn to the ejection regions 113 
with the cooling air streams K and L being admixed with the exhaust gas 
streams J in the ejection regions to produce diluted exhaust streams M 
which are discharged from the ducts 100, 104, 108 and 112. 
As indicated, the discharge members 80, 90, 92 and 94 are positioned to 
separate the streams M from each other. The direction of diluted exhaust 
streams M is indicated by a center line 114 for each of the streams with 
discharge boundaries for the streams being indicated by dotted line 116. 
By arranging the discharge members 88, 90, 92 and 94 as indicated in FIG. 
10 to separate the gas streams M, the emissivity of an individual stream 
does not reinforce the emissivity of another stream to provide a higher 
emissivity which would be more readily visible to infra-red detection. 
Also, as indicated, the streams M are arranged such that there is a void 
region 118 between the streams discharging in the same general direction 
and a void region 119 between the streams discharging in a generally 
opposite direction. The void regions 118 and 119 permit the flow of the 
cooling air streams L to the cooling air passages 98, 102, 106, 110 
without pulling gas from the exhaust streams M into the cooling air 
passages. This eliminates the need for an end plate or equivalent 
structure, such as the end plate 60 referred to in FIG. 7, in separating 
the entering cooling air streams L from the exhaust streams M being 
discharged. 
The ducts 100, 104, 108 and 112 may be supported by Z-supports generally 
indicated as 120. As illustrated in FIG. 11, which is a sectional view 
taken along the line 11--11 of FIG. 10, the Z-supports 120 may include a 
center leg 122 and side legs 124, 126. The center leg 122 may act as a 
strut while the side legs 124 or 126 contact the ducts 100, 104, 108, 112 
and support the ducts within a support shell generally indicated as 128. 
As indicated in FIG. 10, the Z-supports 120 may be positioned along the 
lengths of the ducts 100, 104, 108, 112 with the number of supports per 
duct being varied in relation to the length of the particular duct. For 
example, three Z-supports 120 may support the inner curved surfaces of 
ducts 100 and 112 while five Z-supports 120 may support the outer curved 
surface of these ducts. Similarly, seven Z-supports 120 may support the 
inner curved surfaces of ducts 104 and 108 while nine Z-supports may 
support the outer curved surfaces of these ducts. Due to the configuration 
of the Z-supports 120, the supports do not interfere unduly with the flow 
of cooling air streams L through the cooling air passages 98, 102, 106, 
110 where the supports are positioned. 
FIG. 12 is a detail section view taken along the line 12--12 of FIG. 11 to 
illustrate a specific form of the support shell 128. As indicated, the 
support shell 128 may include an outer sheet 130, an inner sheet 132, and 
a plurality of honeycomb separators 134 positioned between the inner and 
outer sheets. For example, inner and outer sheets 130 and 132 may be made 
of a material such as fiberglass, while the honeycomb separators 134 may 
be made of nylon. As indicated in FIG. 11, the support shell 128 not only 
passes around the exterior of the infra-red suppressor 84, but also may 
include cross members 128a, 128b, and 128c, to which the Z-supports 120 
may be connected in supporting the ducts 100, 104, 108, 112. The ducts 
100, 104, 108, 112 may be formed of aluminum while the regions between the 
aluminum ducts and the shell 128 define the air cooling passages 98, 102, 
106, and 110.