Aircraft cooling method

A method for supplying cooling air on vehicles such as high speed aircraft includes diverting high pressure air from the compressor section of a gas turbine engine, cooling this air in a heat exchanger, and expanding the diverted high pressure air through an auxiliary turbine. Coolant in the heat exchanger may be inlet ram air, fan air, or aircraft fuel (which may be endothermic), and the coolant is ultimately introduced into the engine downstream of the compressor section. An auxiliary compressor may be used to further compress the diverted high pressure air or coolant prior to the flowing thereof through the heat exchanger, and one or more auxiliary turbines may be used to power the auxiliary compressor, or mechanical accessories on the vehicle.

FIELD OF THE INVENTION 
This invention relates to a method for cooling aircraft components and 
exhaust systems of gas turbine engines. 
BACKGROUND OF THE INVENTION 
Survivability and structural requirements in advanced aircraft require 
cooling and thermal management of aircraft and propulsion structures. 
Conventional methods for propulsion system cooling in current aircraft 
engines typically employ either engine fuel, or air from one of the 
various sources in the propulsion system as a coolant. Among the 
traditional sources of cooling air are 1) ram air from the inlet, 2) air 
from the fan (in turbofan engines), or 3) air from the high compressor. 
These sources for cooling air have generally been adequate for cooling 
aircraft components up to this time, the cooling air being primarily used 
for maintaining structural integrity of engine components. Although 
cooling air diverted from the aforementioned sources impacts overall 
engine performance, the cooling requirements have heretofore been achieved 
with only minimal impact on engine performance. However, as the amount of 
electronic and other heat generating equipment carried on aircraft has 
increased, the requirement for cooling system capability has 
correspondingly increased. In addition, as aircraft speeds and 
capabilities increase beyond about Mach 3, the demands on the cooling 
systems of aircraft increase as well. These increased speeds and 
capabilities require cooling of aircraft components such as leading edges 
of the airframe, and certain parts of the engine exposed to high 
temperature combustion products. The increasingly stringent requirements 
for future vehicle/engine systems will require improved sources of low 
temperature coolants. What is needed is a method of providing greater 
cooling capability for aircraft components and engines without 
substantially increasing the amount of cooling air diverted from the 
traditional sources of cooling air. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a method for 
increasing the cooling capability for vehicle components, and engine 
components of the vehicle. 
Another object of the present invention is to provide a method for 
increasing the cooling capability for aircraft components and engines 
without substantially increasing the amount of cooling air diverted from 
the traditional sources of cooling air. 
Another object of the present invention is to provide a method for 
producing cooling air on aircraft that fly at speeds in excess of Mach 3. 
According to the present invention a method is disclosed that provides a 
supply of cooling air which can be used for cooling vehicle components and 
engine components of the vehicle, especially vehicles such as high speed 
aircraft. The method includes diverting high pressure air from the 
compressor section of a gas turbine engine, cooling this air in a heat 
exchanger, and expanding the diverted high pressure air through an 
auxiliary turbine. The coolant in the heat exchanger may be inlet ram air, 
fan air, or aircraft fuel which may be endothermic. The coolant is 
ultimately introduced into the engine downstream of the compressor 
section, thereby recovering the heat energy that was absorbed from the 
diverted high pressure air. In some embodiments of the present invention, 
power from the auxiliary turbine is used to drive mechanical accessories 
on the vehicle. Some other embodiments of the present invention use the 
power from the auxiliary turbine to drive an auxiliary compressor which 
further compresses the diverted high pressure air, and one embodiment uses 
the power from the auxiliary turbine to drive an auxiliary compressor 
which further compresses the ram air. One embodiment includes expanding 
the fuel through a second auxiliary turbine and using the work extracted 
therefrom to provide additional power for the auxiliary compressor. 
The foregoing and other features and advantages of the present invention 
will become more apparent from the following description and accompanying 
drawings.

DETAILED DESCRIPTION OF THE INVENTION 
The method of the present invention is for generating cooled air for 
cooling components of aircraft vehicles that incorporate large amounts of 
electronic or other heat generating equipment on board, or for cooling 
components of a high speed vehicle 10, such as an aircraft that flies at 
supersonic speeds in excess of Mach 3. Practicing this invention requires 
at least one gas turbine engine 12 in the vehicle, such as the engine 
shown in FIG. 1. 
This engine, which may be a turbojet 12, includes, in serial flow 
arrangement, an engine inlet section 14 for receiving ambient air 16 and 
delivering the ambient air 16 to the compressor section 18, and aft of the 
inlet section 14 is the compressor section 16 for compressing the ambient 
air 16 thereby producing compressed air 22. Aft of the compressor section 
18 is a combustor section 24 for mixing fuel with the compressed air 22 
and igniting the fuel and compressed air 22 to produce combustion products 
26. Aft of the combustion section 24 is a turbine section 28 for expanding 
the combustion products 26 and driving the compressor section 18 via an 
engine shaft 30, and aft of the turbine section 28 is an exhaust section 
32 for conveying the combustion products 26 from the turbine section 28, 
through the nozzle 20, and out of the aft end of the gas turbine engine 
12. 
As shown in FIG. 1, the vehicle 10 also includes a heat exchanger 34 and an 
auxiliary unit 36. The heat exchanger 34 has first 38 and second 40 flow 
paths extending therethrough, and each flow path 38, 40 has an inlet 42, 
44 and an outlet 46, 48. The auxiliary unit 36 includes an auxiliary 
compressor 50 and an auxiliary turbine 52, and the auxiliary turbine 52 is 
connected to the auxiliary compressor 50 by an auxiliary shaft 54 to 
provide power thereto. The auxiliary compressor 50 has an inlet 56 and an 
outlet 58, and the auxiliary turbine likewise has an inlet 60 and an 
outlet 62. 
The inlet 56 of the auxiliary compressor 50 is connected by a conduit 64 to 
the inlet section 14 of the engine 12 to receive ram air 16 at ambient 
conditions therefrom. The outlet of the auxiliary compressor 58 is 
connected by a second conduit 66 to the inlet 42 of the first flow path. 
The outlet 48 of the first flow path is connected by a third conduit 68 to 
the exhaust section 32 of the engine 12 to deliver the air exiting the 
first flow path 38 to the exhaust section 32. 
The inlet 44 of the second flow path is connected by a fourth conduit 70 to 
the compressor section 18 to receive compressed air therefrom, and the 
outlet 46 of the second flow path is connected by a fifth conduit 72 to 
the inlet 60 of the auxiliary turbine 52 to deliver compressed air exiting 
the second flow path 40 thereto. The outlet 62 of the auxiliary turbine 52 
is connected to a sixth conduit 74 which routes the compressed air exiting 
the outlet 62 of the second flow path to the components 76 of the vehicle 
or engine which need to be cooled. 
In operation, a first portion of the ambient ram air 16 from the engine 
inlet section 14 is diverted therefrom through the first conduit 64 and 
delivered to the inlet 56 of the auxiliary compressor. The first portion 
of ambient ram air 16 is then compressed in the auxiliary compressor 50, 
thereby increasing the pressure and temperature of the first portion of 
ambient ram air exiting the outlet 58 of the auxiliary compressor. A 
second portion of air, this being compressed air 22, is diverted from the 
compressor section 18 of the engine 12 through a compressor bleed 78. This 
second portion of compressed air is cooled, and the first portion is 
heated, as follows. 
The first portion flowing from the auxiliary compressor 56 is directed to 
the inlet 42 of the first flow path of the heat exchanger 34 and flows 
through the first flow path 38 thereof, and the second portion flowing 
from the compressor section 18 through the fourth conduit 70 is delivered 
to the inlet 44 of the second flow path of the heat exchanger 34 and flows 
through the second flow path 40 thereof. Within the heat exchanger 34, the 
second portion is cooled simultaneously with the heating of the first 
portion through the transfer of heat energy from said second portion to 
the first portion. The first portion then exits the first flow path 38 
through the outlet thereof 48, and the second portion then exits the 
second flow path 40 through the outlet 46 thereof. 
The second portion exiting the heat exchanger 34 is delivered to the 
auxiliary turbine 52 through the fifth conduit 72 and expanded through the 
auxiliary turbine 52, thereby reducing the temperature of the second 
portion and producing work to drive the auxiliary compressor 50 via the 
auxiliary shaft 54. The second portion exiting the auxiliary turbine 52 is 
then routed through the sixth conduit 74 to the components 76 of the 
vehicle or engine which require cooling, and used to cool those components 
76. The first portion exiting the heat exchanger 34 is delivered through 
the third conduit 68 to the exhaust section 32 and discharged into the 
combustion products 76. 
As those skilled in the art will readily appreciate, compression of the ram 
air 16 prior to its use in the heat exchanger 34 as a coolant allows 
introduction of the heated ram air into the engine exhaust section, 
thereby improving engine performance. The compression requirement for the 
ambient ram air must be at least equal to the engine pressure ratio (the 
pressure of the combustion products in the exhaust section divided by the 
pressure of the ambient ram air in the inlet section) plus the pressure 
loss across the heat exchanger 34. The balance between available turbine 
power and ram compressor power determines the mass flow of ram air flow 
available for cooling. 
FIG. 2 illustrates a second embodiment of the method of the present 
invention modified to accommodate use on vehicles which use turbofan gas 
turbine engines. As compared to the turbojet, the compressor section 18 of 
the turbofan 80 has, in serial flow arrangement, a low pressure 
compressor, or "fan" 82, and a high pressure compressor 84. Likewise the 
turbine section 28 of the turbofan 80 has in serial flow arrangement, a 
high pressure turbine 86 and a low pressure turbine 88. A bypass duct 200 
connects the exhaust section 32 to the outlet of the low compressor 82 to 
permit fan air 114 to bypass the high compressor 84, combustion section 
24, and turbine section 28. The low pressure turbine 88 drives the fan 82 
via the low shaft 90 which connects the low pressure turbine 88 to the fan 
82, and the high pressure turbine 86 drives the high compressor 84 via the 
high shaft 92 which connects the high pressure turbine 86 to the high 
compressor 84. Otherwise, the elements of the turbofan 80 are the same as 
those shown for the turbojet in FIG. 1, except for the conduits. 
As in the first embodiment, the vehicle 10 includes a heat exchanger 34 
which is likewise similar to the heat exchanger 34 of the first 
embodiment. An auxiliary turbine 52 having an inlet 60 and an outlet 62 is 
likewise provided, and the auxiliary turbine 52 is connected to a power 
take-off shaft 94 to provide mechanical energy to power accessories on the 
vehicle 10 as desired. 
A first conduit 100 is connected at one end to a fan bleed 102 and at the 
other end to the inlet 42 of the first flow path. The outlet 48 of the 
first flow path 38 is connected by a second conduit 104 to the exhaust 
section 32 of the engine to deliver the air exiting the first flow path 38 
to the exhaust section 32. 
The inlet 44 of the second flow path is connected by a third conduit 106 to 
a high pressure compressor bleed 108 to receive compressed air therefrom, 
and the outlet 46 of the second flow path 40 is connected by a fourth 
conduit 110 to the inlet 60 of the auxiliary turbine 52 to deliver 
compressed air exiting the second flow path 40 thereto. The outlet 62 of 
the auxiliary turbine 52 is connected to a fifth conduit 112 which routes 
the compressed air exiting the outlet 62 of the auxiliary turbine to the 
components 76 of the vehicle or engine which need to be cooled. 
In operation, a first portion of the fan air 114 from the low pressure 
compressor 82 is diverted therefrom through the fan bleed 102 and the 
first conduit 100, and delivered to the inlet 42 of the first flow path 38 
of the heat exchanger and flows through the first flow path 38 thereof. A 
second portion of air, this being high compressor air 22, is diverted from 
the high pressure compressor 84 through a high compressor bleed 108. The 
second portion flowing from the high compressor 84 through the third 
conduit 106 is delivered to the inlet 44 of the second flow path of the 
heat exchanger 34 and flows through the second flow path 40 thereof. 
Within the heat exchanger 34, the second portion is cooled simultaneously 
with the heating of the first portion as described above. The first 
portion then exits the first flow path 38 through the outlet 48 thereof, 
and the second portion then exits the second flow path 40 through the 
outlet 46 thereof. 
The second portion exiting the heat exchanger 34 is delivered to the 
auxiliary turbine 52 through the fourth conduit 110 and expanded through 
the auxiliary turbine 52, thereby reducing the temperature of the second 
portion and producing work to power accessories 116 on the vehicle via the 
power take-off shaft 94. The first portion exiting the heat exchanger 34 
is delivered through the second conduit 104 to the exhaust section 32 and 
discharged into the combustion products 26, or used to cool components 
with noncritical requirements. The second portion exiting the auxiliary 
turbine 52 is then routed through the fifth conduit 112 to the components 
76 of the vehicle or engine which require cooling, and used to cool those 
components. 
As those skilled in the art will readily appreciate, the heated fan air, 
which is mixed into the exhaust stream 26, produces a net increase in 
thrust as compared to merely bleeding high compressor air and cooling it 
with some other source. Alternatively, the heated fan air may be used to 
cool components with noncritical requirements. The cooled high pressure 
air that has been expanded through the auxiliary turbine 52 to the lowest 
pressure level allowed by the system achieves a substantially reduced 
temperature. This cold expanded air is then available for critical 
component 76 cooling, while the power extracted by the auxiliary turbine 
52 via the power take-off shaft 94 can be used to satisfy aircraft 10 
power requirements, instead of using mechanical power extraction from the 
engine 80. 
FIG. 3 illustrates a third embodiment of the method of the present 
invention, also for use on vehicles which use turbofan gas turbine 
engines. As FIG. 3 shows, the elements of the turbofan 80 are the same as 
those shown for the turbofan in FIG. 2, and therefore the reference 
numerals are the same. As in the second embodiment, the vehicle includes a 
heat exchanger 34 which is likewise similar to the heat exchanger 34 of 
the second embodiment. An auxiliary unit 36 is also provided, including an 
auxiliary compressor 50 and an auxiliary turbine 52, and the auxiliary 
turbine 52 is connected to the auxiliary compressor 50 by an auxiliary 
shaft 54 to provide power thereto. The auxiliary compressor 50 has an 
inlet 56 and an outlet 58, and the auxiliary turbine 52 likewise has an 
inlet 60 and an outlet 62. 
A first conduit 120 is connected at one end to the fan bleed 102 and at the 
other end to the inlet 42 of the first flow path. The outlet 48 of the 
first flow path is connected by a second conduit 122 to the exhaust 
section 32 of the engine 80 to deliver the air exiting the first flow path 
38 to the exhaust section 32. The inlet 56 of the auxiliary compressor 50 
is connected by a third conduit 124 to the high compressor bleed 108 to 
receive high pressure compressed air therefrom. The outlet 58 of the 
auxiliary compressor 50 is connected by a fourth conduit 126 to the inlet 
44 of the second flow path 40. The outlet 46 of the second flow path 40 is 
connected by a fifth conduit 128 to the inlet 60 of the auxiliary turbine 
52 to deliver compressed air exiting the second flow path 40 thereto. The 
outlet 62 of the auxiliary turbine 52 is connected to a sixth conduit 130 
which routes the compressed air exiting the outlet 62 of the auxiliary 
turbine 52 to the components 76 of the vehicle or engine which need to be 
cooled. 
In operation, a first portion of the fan air 114 from the low pressure 
compressor 82 is diverted therefrom through a fan bleed 102 and the first 
conduit 120, and delivered to the inlet 42 of the first flow path of the 
heat exchanger 34 and flows through the first flow path 38 thereof. A 
second portion of air, this being high compressor air, is diverted from 
the high pressure compressor 84 through a high compressor bleed 108. The 
second portion flowing from the high compressor 84 through the third 
conduit 124 is delivered to the inlet 56 of the auxiliary compressor. The 
second portion is then compressed in the auxiliary compressor 50, thereby 
increasing the pressure and temperature of the second portion exiting the 
outlet 58 of the auxiliary compressor. The second portion flowing from the 
auxiliary compressor 50 through the fourth conduit 126 is delivered to the 
inlet 44 of the second flow path of the heat exchanger 34, and flows 
through the second flow path 40 thereof. Within the heat exchanger 34, the 
second portion is cooled simultaneously with the heating of the first 
portion as described above. The first portion exits the first flow path 38 
through the outlet 48 thereof, and the second portion exits the second 
flow path 40 through the outlet 46 thereof. 
The second portion exiting the heat exchanger 34 is delivered to the 
auxiliary turbine 52 through the fifth conduit 128 and expanded through 
the auxiliary turbine 52, thereby reducing the temperature of the second 
portion and producing work to drive the auxiliary compressor 50 via the 
auxiliary shaft 54. The second portion exiting the auxiliary turbine 52 is 
then routed through the sixth conduit 130 to the components 76 of the 
vehicle or engine which require cooling, and used to cool those 
components. The first portion exiting the heat exchanger 34 is delivered 
through the second conduit 122 to the exhaust section 32 via the bypass 
duct 200 and discharged into the combustion products 26. 
The auxiliary compressor 50 is used to supercharge the high compressor 
bleed air 22, thus heating it to higher temperatures prior to cooling it 
with low pressure fan bleed air 114. This allows more heat extraction from 
the compressed air diverted from the high pressure compressor bleed, thus 
yielding lower temperatures when the high pressure compressor bleed air is 
finally expanded to low pressure through the auxiliary turbine 52. The 
compression ratio of the auxiliary compressor 50 is preferably 
significantly less than the expansion ratio of the turbine 52, so that all 
of the power required for compressing the air diverted from the high 
pressure compressor can be supplied by the auxiliary turbine 52. 
FIG. 4 illustrates a fourth embodiment of the method of the present 
invention for use on a vehicle 10 which uses either turbojet or turbofan 
gas turbine engines. Although the fourth embodiment is shown and described 
in terms of a turbofan, those skilled in the art will readily appreciate 
that this method is also applicable to a turbojet, since neither ram air 
nor fan air is used as the coolant in this fourth embodiment. Again, the 
elements of the turbofan 80 are the same as those shown for the turbofan 
80 in FIG. 2, and the vehicle 10 includes a heat exchanger 34 which is 
similar to the heat exchanger 34 of the second embodiment. The elements of 
the auxiliary unit 36 likewise are identified by the same reference 
numerals used to identify similar elements in FIG. 3. 
In addition to the elements of the first three embodiments, the fourth 
embodiment of the method of the present invention uses a fuel source 132 
connected to a fuel pump 134, such as the type typically used for 
supplying fuel to the combustion section of gas turbine engines. A first 
conduit 140 is connected at one end to the fuel pump outlet 142 and at the 
other end to the inlet 42 of the first flow path 38. The outlet 48 of the 
first flow path is connected by a second conduit 144 to the combustion 
section 24 of the engine 80 to deliver the fuel exiting the first flow 
path 38 to the combustion section 24. 
The inlet 56 of the auxiliary compressor 50 is connected by a third conduit 
146 to the high compressor bleed 108 to receive high pressure compressed 
air therefrom. The outlet 58 of the auxiliary compressor 50 is connected 
by a fourth conduit 148 to the inlet 44 of the second flow path, and the 
outlet 46 of the second flow path is connected by a fifth conduit 150 to 
the inlet 60 of the auxiliary turbine 52 to deliver compressed air exiting 
the second flow path 40 thereto. The outlet 62 of the auxiliary turbine 52 
is connected to a sixth conduit 152 which routes the compressed air 
exiting the outlet 62 of the auxiliary turbine 52 to the components 76 of 
the vehicle or engine which need to be cooled. 
In operation, fuel from the fuel source 132 is pumped by the fuel pump 134 
through the first conduit 140, delivered to the inlet 42 of the first flow 
path of the heat exchanger 34, and flows through the first flow path 38 
thereof. A second portion, that being compressed air from the high 
compressor 84, is diverted from the high pressure compressor 84 through 
the high compressor bleed 108. The second portion flowing from the high 
compressor 84 through the third conduit 146 is delivered to the inlet 56 
of the auxiliary compressor. The second portion is then compressed in the 
auxiliary compressor 50, thereby increasing the pressure and temperature 
of the second portion exiting the outlet 58 of the auxiliary compressor. 
Within the heat exchanger 34, the second portion is cooled simultaneously 
with the heating of the fuel flowing through the first path 38. The fuel 
then exits the first flow path 38 through the outlet 48 thereof, and the 
second portion then exits the second flow path 40 through the outlet 46 
thereof. 
The second portion exiting the heat exchanger 34 is delivered to the 
auxiliary turbine 52 through the fifth conduit 150 and expanded through 
the auxiliary turbine 52, thereby reducing the temperature of the second 
portion and producing work to drive the auxiliary compressor via the 
auxiliary shaft 54. The second portion exiting the auxiliary turbine 52 is 
then routed through the sixth conduit 152 to the components 76 of the 
vehicle or engine which require cooling, and used to cool those 
components. The fuel exiting the heat exchanger 34 is delivered through 
the second conduit 144 to the combustion section 24 where it is mixed with 
compressed air 22 exiting the high compressor 84, ignited, and combusted. 
The fourth embodiment of the method of the present invention uses the heat 
sink capability of the fuel to cool the highly compressed air bled from 
the high pressure compressor 84 prior to expanding the air through the 
auxiliary turbine 52. This further expands the capability for generating 
low temperature air, yielding either lower temperature cooling air, or a 
larger volume of cooling air at higher temperatures. As gas turbine engine 
hydrocarbon fuels are developed which have greater thermal stability, the 
capacity of the fuel to be used as a heat sink improves as well. 
In particular, the advent of endothermic fuels allows an improvement to the 
fourth embodiment, as shown in FIG. 5, the fifth embodiment, in which the 
fuel source 154 contains an endothermic fuel, such as methylcyclohexane 
(MCH), and the heat exchanger is also a catalytic thermal reactor. The 
method of the fifth embodiment is the same as the fourth embodiment, 
except that the fuel exiting the heat exchanger/reactor 35 is in the form 
of high pressure gaseous hydrocarbons, and the heat energy absorbed by the 
fuel in the heat exchanger/reactor is therefore greater, resulting in 
overall greater cooling of the second portion of air bled from the high 
compressor 84. The heat exchanger/reactor 35 is a combined air-fuel heat 
exchanger and catalytic converter. The catalyst, specifically selected for 
the chosen fuel, is coated or packed within the heat exchanger/reactor 35 
in a manner such that the fuel is in intimate contact with the catalyst 
during heating. The net result, well known in the art and demonstrated in 
chemical and fuel technology, is that the fuel decomposes into new 
chemical structures with a large attendant absorption of heat. Most 
endothermic fuels thus far identified as potential aircraft fuels, which 
undergo this heat absorption at elevated temperatures 
(600.degree.-1200.degree. F.), are compatible with the fifth embodiment of 
the present invention. 
A sixth embodiment of the method of the present invention is shown in FIG. 
6. In addition to the elements shown in FIG. 5, the sixth embodiment 
includes a second auxiliary turbine 156 connected to the auxiliary 
compressor 52 by the auxiliary shaft 54 to provide power thereto. The 
second auxiliary turbine 156 has an inlet 158 and an outlet 160, and the 
outlet 48 of the first flow path is connected by a second conduit 162 to 
the inlet 158 of the second auxiliary turbine 156 to deliver the fuel 
exiting the first flow path 38 thereto. The outlet 160 of the second 
auxiliary turbine is connected to a seventh conduit 164 which routes the 
fuel exiting the outlet 160 of the second auxiliary turbine 156 to the 
combustion section 24 of the engine. 
In operation, fuel from the fuel source 154 is pumped by the fuel pump 134 
through the first conduit 140, delivered to the inlet 42 of the first flow 
path of the heat exchanger/reactor 35, and flows through the first flow 
path 38 thereof. A second portion, that being compressed air from the high 
compressor 84, is diverted from the high pressure compressor 84 through 
the high compressor bleed 108. The second portion flowing from the high 
compressor 84 through the third conduit 146 is delivered to the inlet 56 
of the auxiliary compressor 50. The second portion is then compressed in 
the auxiliary compressor 50, thereby increasing the pressure and 
temperature of the second portion exiting the outlet 58 of the auxiliary 
compressor. The second portion is then delivered to the inlet 44 of the 
second flow path through the fourth conduit 148. Within the heat 
exchanger/reactor 35, the second portion is cooled simultaneously with the 
heating of the fuel flowing through the first path 38. The fuel, heated 
and in contact with a catalyst in the reactor, decomposes in an 
endothermic reaction. The resultant products of the endothermic reaction 
within the heat exchanger/reactor 35 are high pressure gaseous fuels which 
exit the first flow path outlet 48 and are delivered to the inlet 158 of 
the second auxiliary turbine 156 through the second conduit 162. The high 
pressure gaseous fuels are expanded through the second auxiliary turbine 
156 to extract work energy from the high pressure gaseous fuels and drive 
the auxiliary compressor 50 therewith. If all of the power extracted from 
the auxiliary turbine 52 and the second auxiliary turbine 156 is used to 
drive the auxiliary compressor 50, as shown in FIG. 6, then the source of 
the high pressure air may be moved to an interstage bleed on the high 
pressure compressor 84, rather than the compressor exit, thus reducing 
potential performance impact on the engine 12. 
Although this invention has been shown and described with respect to a 
detailed embodiment thereof, it will be understood by those skilled in the 
art that various changes in form and detail thereof may be made without 
departing from the spirit and scope of the claimed invention.