Variable speed boost compressor for gas turbine engine cooling air supply

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.