Variable speed boost compressor for gas turbine engine cooling air supply

1. A gas turbine engine comprising:

a compressor section having a low pressure compressor and a high pressure compressor having a downstream most end;

a cooling air system including a tap from a location upstream of said downstream most end, said tap passing air to a boost compressor, and said boost compressor passing the air back to a location to be cooled, said boost compressor being driven by a shaft in said engine through an epicyclic gear system, said shaft being driven by a power takeoff shaft driven by one of a low pressure shaft that drives said low pressure compressor, or a high pressure shaft which drives said high pressure compressor and there being a speed control for changing the relative speed between an input and an output to said epicyclic gear system;

wherein said speed control incorporates a system for selectively providing a resistance to one of the gears in said epicyclic gear system in order to achieve a desired speed for same boost compressor; and

wherein said one of said gears is a ring gear.

2. The gas turbine engine as set forth in claim 1 , wherein said epicyclic gear system is a planetary gear system.

3. The gas turbine engine as set forth in claim 2 , wherein said shaft rotates with said high pressure compressor to drive said power takeoff to drive said epicyclic gear system.

4. The gas turbine engine as set forth in claim 2 , wherein said shaft rotates with said low pressure compressor to drive said power takeoff to drive said epicyclic gear system.

5. The gas turbine engine as set forth in claim 1 , wherein a throttle valve is positioned downstream of said hydraulic pump and is controlled to restrict the flow from said hydraulic pump to in turn control the resistance on rotation of said ring gear.

6. The gas turbine engine as set forth in claim 1 , wherein a resistance control selectively controls the resistance such that said boost compressor rotates at higher speeds during higher power operation of the gas turbine engine, and is rotating at slower speeds during lower power operation.

7. The gas turbine engine as set forth in claim 1 , wherein a friction brake provides said resistance to rotation of said ring gear.

8. The gas turbine engine as set forth in claim 1 , wherein a friction brake provides said resistance to rotation of said one of said gears.

9. The gas turbine engine as set forth in claim 8 , wherein a resistance control selectively controls the resistance such that said boost compressor rotates at higher speeds during higher power operation of the gas turbine engine, and is rotating at slower speeds during lower power operation.

10. The gas turbine engine as set forth in claim 2 , wherein said speed control changing the relative speeds between said input and said outlet to achieve a desired pressure in the air passing to said location to be cooled.

11. The gas turbine engine as set forth in claim 1 , wherein said shaft rotates with a lower pressure compressor to drive a power takeoff to drive said epicyclic gear system.

12. The gas turbine engine as set forth in claim 1 , wherein said speed control changing the relative speeds between said input and said outlet to achieve a desired pressure in the air passing to said location to be cooled.

13. The gas turbine engine as set forth in claim 1 , wherein said speed control incorporates a system for selectively providing a resistance to one of the gears in said epicyclic gear system.

14. The gas turbine engine as set forth in claim 13 , wherein said one of said gears is a ring gear.

15. The gas turbine engine as set forth in claim 13 , wherein a hydraulic pump rotates with one of said gears, and an output of said hydraulic pump is controlled to control the resistance on rotation of said one of said gears.