Gearbox for boost spool turbine engine

1. A turbine engine comprising:

a core engine including a first spool and a second spool rotatable about a main engine longitudinal axis;

a boost spool powered by a secondary drive system;

an accessory gearbox coupled to the core engine and the boost spool;

a tower shaft is coupled to one of the first spool or the second spool;

a differential gear system coupled between the core engine, the boost spool and the accessory gearbox for distributing power between the boost spool, the core engine and the accessory gearbox, the differential gear system including a first differential gear and a second differential gear both coupled to a differential shaft, the core engine is coupled to the first differential gear and the accessory gearbox is coupled to the second differential gear, the first differential gear and the second differential gear are supported in a rotatable differential carrier and a boost spool input gear is coupled through a ring gear to drive rotation of the rotatable differential carrier, wherein a first gear ratio between a tower shaft gear driven by the tower shaft and the first differential gear is different than a second gear ratio between an accessory drive gear and the second differential gear.

2. The turbine engine as recited in claim 1 , including a boost spool clutch for selectively coupling the boost spool to the ring gear, a locking clutch between the tower shaft gear and the accessory gear box for directly coupling the tower shaft to drive the accessory gear box and a brake coupled to a first input shaft driven by the tower shaft gear for selectively preventing rotation of the tower shaft.

3. The turbine engine as recited in claim 2 , including a boost gear system including a plurality of intermediate gears supported in a selectively rotatable boost carrier, an inner gear engaged to the intermediate gears and an outer gear coupled to the boost spool, wherein power is transmitted from the outer gear through the intermediate gears to the inner gear when the carrier is fixed and power is not transmitted between the outer gear to the inner gear when the carrier is allowed to rotate.

4. The turbine engine as recited in claim 3 , wherein in a first operating configuration, the brake is engaged to prevent rotation of the first input shaft and the boost spool drives the accessory gearbox through the ring gear, rotatable differential carrier and a coupling between the second differential gear and the accessory drive gear.

5. The turbine engine as recited in claim 4 , wherein in a second operating configuration, the tower shaft brake is disengaged, the locking clutch is engaged and the boost spool clutch is disengaged such that the tower shaft directly drives the accessory gearbox and the boost spool is decoupled from the differential gear system.

6. The turbine engine as recited in claim 5 , wherein in a third operating configuration, the tower shaft brake is disengaged, the locking clutch is disengaged and the boost spool clutch is engaged such that the boost spool provides an input torque to both the tower shaft and the accessory gearbox.

7. The turbine engine as recited in claim 1 , where the secondary drive system comprises one of a gas turbine engine or an electric motor-generator.

8. A differential gear system comprising:

a differential carrier rotatable about a longitudinal axis;

a first differential gear and a second differential gear coupled to a common differential shaft, the differential shaft rotatable about a differential axis that is transverse to the longitudinal axis;

a first drive gear coupled to the first differential gear at a first gear ratio;

a second drive gear coupled to the second differential gear at a second gear ratio that is different than the first gear ratio;

a ring gear coupled to the differential carrier; and

an input gear coupled to the ring gear for driving the differential carrier about the longitudinal axis; and

a locking clutch for selectively coupling the first drive gear directly to the second drive gear.

9. The differential gear system as recited in claim 8 , wherein the first drive gear is configured to be coupled to a tower shaft of a turbine engine, the second drive gear is coupled to an accessory gearbox and the input gear is coupled to a boost spool.

10. The differential gear system as recited in claim 9 , including a brake for locking rotation of the tower shaft and a boost spool clutch for selectively coupling the boost spool to drive the ring gear.

11. A method of operating a turbine engine comprising:

coupling a tower shaft of a core engine, a boost spool driven by a secondary drive system and an accessory gear box through a differential gear system; and

inputting torque to both the accessory gearbox and the tower shaft of the core engine from the boost spool driven by the secondary drive system through the differential gear system in one engine operating configuration, wherein the differential gear system includes a brake for preventing rotation of the tower shaft and operating the accessory gearbox according to another engine operating configuration includes engaging the brake to prevent rotation of the tower shaft and driving the accessory gear box with the boost spool through the differential gear system.

12. The method as recited in claim 11 , wherein the differential gear system includes a locking clutch for selectively coupling the tower shaft to directly drive the accessory gear box and a boost spool clutch for decoupling the boost spool from the differential gear system and operating the engine according to a further engine operating configuration includes engaging the locking clutch and disengaging the boost spool clutch such that the tower shaft directly drives the accessory gearbox and the boost spool is decoupled from the differential gear system.

13. The method as recited in claim 12 , including driving the boost spool with one of a secondary gas turbine engine or an electric motor-generator.