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A gas turbine engine arrangement comprises a first gas turbine engine, a second gas turbine engine, a differential gearbox and an electrical generator. The differential gearbox has a first input drive, a second input drive and an output drive. The output drive of the differential gearbox is arranged

A gas turbine engine arrangement comprises a first gas turbine engine, a second gas turbine engine, a differential gearbox and an electrical generator. The differential gearbox has a first input drive, a second input drive and an output drive. The output drive of the differential gearbox is arranged to drive the electrical generator via an external, accessory, gearbox. The external, accessory, gearbox drives other accessories. The first gas turbine is arranged to drive the first input drive of the differential gearbox and the second gas turbine engine is arranged to drive the second input drive of the differential gearbox. The electrical generator and accessories are driven at a constant frequency speed/frequency.

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1. A gas turbine engine arrangement comprising a first gas turbine engine, a power unit, a differential gearbox and at least one electrical generator, the differential gearbox having a first input drive, a second input drive and an output drive,wherein the differential gearbox comprises a sun gear,

1. A gas turbine engine arrangement comprising a first gas turbine engine, a power unit, a differential gearbox and at least one electrical generator, the differential gearbox having a first input drive, a second input drive and an output drive,wherein the differential gearbox comprises a sun gear, a plurality of planet gears, and an annulus gear, wherein the planet gears mesh with the sun gear and the annulus gear and the planet gears are rotatably mounted on a carrier,wherein the first input drive is affixed to one of the sun gear, the annulus gear, or the carrier,wherein the second input drive is affixed to one of either the sun gear or the annulus gear and is not affixed to the same structure as the first input drive,wherein the output is affixed to the carrier or the annulus gear and is not affixed to the same structure as either the first input drive or the second input drivethe first gas turbine engine being an aircraft gas turbine engine,the power unit comprising a second gas turbine engine, the second gas turbine engine being an auxiliary power unit,the output drive of the differential gearbox being arranged to drive the at least one electrical generator,the first gas turbine engine being arranged to supply power to the first input drive of the differential gearbox,the power unit being arranged to supply power to, or remove power from, the second input drive of the differential gearbox,a first brake is provided between the first input drive and the first gas turbine engine, the first brake being operable to prevent rotation of the first input drive if the first gas turbine engine and/or the first input drive fails,a second brake is provided between the second input drive and the second gas turbine engine, the second brake being operable to prevent rotation of the second input drive if the second gas turbine engine and/or the second input drive fails,a clutch or a fluid coupling is operable to allow the second gas turbine engine to start the first gas turbine engine, the clutch or the fluid coupling being provided between the first input drive and the first gas turbine engine or between the second input drive and the second gas turbine engine, andthe output drive has a brake operable to prevent rotation of the output drive to allow the second gas turbine engine to start the first gas turbine engine. 2. A gas turbine engine arrangement as claimed in claim 1 wherein the first gas turbine engine is a turbofan gas turbine engine. 3. A gas turbine engine arrangement as claimed in claim 1 wherein the first gas turbine engine drives the first input drive above a predetermined torque level and below a predetermined speed and the second gas turbine engine drives the second input drive with a torque level less than the predetermined torque level and with a speed above the predetermined speed. 4. A gas turbine engine arrangement as claimed in claim 1 wherein the first gas turbine engine has a power offtake and the power offtake is arranged to drive the first input drive. 5. A gas turbine engine arrangement as claimed in claim 1 wherein there are means to measure the speed of the first input drive and control means to adjust the speed of the second input drive such that the speed of the output drive is maintained substantially constant. 6. A gas turbine engine arrangement as claimed in claim 1 wherein there are means to measure the speed of the output drive and control means to adjust the speed of the second input drive such that the speed of the output drive is maintained substantially constant. 7. A gas turbine engine arrangement as claimed in claim 5 wherein the control means controls the supply of fuel to the second gas turbine engine. 8. A gas turbine engine arrangement as claimed in claim 1 wherein the electrical generator is a motor/generator to allow the electrical generator to start one of the first gas turbine engine and the second gas turbine engine. 9. A gas turbine engine arrangement as claimed in claim 1 wherein the second input drive is affixed to the sun gear, the first input drive is affixed to the annulus gear and the output drive is affixed to the carrier. 10. A gas turbine engine arrangement as claimed in claim 1 wherein the second input drive is affixed to the sun gear, the first input drive is affixed to the carrier and the output drive is affixed to the annulus gear. 11. A gas turbine engine arrangement as claimed in claim 1 wherein the second input drive is affixed to the annulus gear, the first input drive is affixed to the sun gear and the output drive is affixed to the carrier. 12. A gas turbine engine arrangement as claimed in claim 1 wherein the electrical generator is selected from the group comprising a constant frequency electrical generator, a specific frequency electrical generator and a DC electrical generator. 13. A gas turbine engine arrangement as claimed in claim 1 wherein the output drive of the differential gearbox is arranged to drive the electrical generator via an accessory gearbox and the accessory gearbox is arranged to drive at least one accessory. 14. A gas turbine engine arrangement as claimed in claim 1 wherein the first gas turbine is arranged to drive the first input drive of the differential gearbox via an accessory gearbox and the accessory gearbox is arranged to drive at least one accessory. 15. A gas turbine engine arrangement as claimed in claim 14 wherein the output drive of the differential gearbox is arranged to drive the electrical generator via an additional accessory gearbox. 16. A gas turbine engine arrangement as claimed in claim 15 wherein the additional accessory gearbox is arranged to drive at least one accessory. 17. A gas turbine engine arrangement as claimed in claim 13 wherein the at least one accessory is selected from the group comprising at least one fuel pump, at least one oil pump and at least one oil breather. 18. The gas turbine engine arrangement as claimed in claim 1 wherein the power unit further comprises a responsive electrical machine. 19. A gas turbine engine arrangement as claimed in claim 18 wherein the responsive electrical machine is selected from the group comprising an electrical machine arranged to supply an electrical load and a variable speed electrical motor. 20. A gas turbine engine arrangement comprising a first gas turbine engine, a power unit, a differential gearbox and at least one electrical generator, the differential gearbox having a first input drive, a second input drive and an output drive,wherein the differential gearbox comprises a sun gear, a plurality of planet gears, and an annulus gear, wherein the planet gears mesh with the sun gear and the annulus gear and the planet gears are rotatably mounted on a carrier,wherein the first input drive is affixed to one of the sun gear, the annulus gear, or the carrier,wherein the second input drive is affixed to one of either the sun gear or the annulus gear and is not affixed to the same structure as the first input drive,wherein the output is affixed to the carrier or the annulus gear and is not affixed to the same structure as either the first input drive or the second input drivethe first gas turbine engine being an aircraft gas turbine engine,the power unit comprising a second gas turbine engine, the second gas turbine engine being an auxiliary power unit,the output drive of the differential gearbox being arranged to drive the at least one electrical generator,the first gas turbine engine being arranged to supply power to the first input drive of the differential gearbox and the power unit being arranged to supply power to, or remove power from, the second input drive of the differential gearbox,a first brake is provided between the first input drive and the first gas turbine engine, the first brake being operable to prevent rotation of the first input drive if the first gas turbine engine and/or the first input drive fails,a second brake is provided between the second input drive and the second gas turbine engine, the second brake being operable to prevent rotation of the second input drive if the second gas turbine engine and/or the second input drive fails,a clutch or a fluid coupling is operable to allow the second gas turbine engine to start the first gas turbine engine, the clutch or the fluid coupling being provided between the first input drive and the first gas turbine engine or between the second input drive and the second gas turbine engine,the output drive has a brake operable to prevent rotation of the output drive to allow the second gas turbine engine to start the first gas turbine enginethe output drive of the differential gearbox being arranged to drive the electrical generator via an accessory gearbox and the accessory gearbox being arranged to drive at least one accessory,a sensor, the sensor being arranged to measure one of the group consisting of the speed of the first input drive and the speed of the output drive, anda controller being arranged to adjust the speed of the second input drive such that the speed of the output drive is maintained substantially constant. 21. A gas turbine engine arrangement comprising a first gas turbine engine, a power unit, a differential gearbox and at least one electrical generator, the differential gearbox having a first input drive, a second input drive and an output drive,wherein the differential gearbox comprises a sun gear, a plurality of planet gears, and an annulus gear, wherein the planet gears mesh with the sun gear and the annulus gear and the planet gears are rotatably mounted on a carrier,wherein the first input drive is affixed to one of the sun gear, the annulus gear, or the carrier,wherein the second input drive is affixed to one of either the sun gear or the annulus gear and is not affixed to the same structure as the first input drive,wherein the output is affixed to the carrier or the annulus gear and is not affixed to the same structure as either the first input drive or the second input drivethe first gas turbine engine being an aircraft gas turbine engine,the power unit comprising a second gas turbine engine, the second gas turbine engine being an auxiliary power unit,the output drive of the differential gearbox being arranged to drive the at least one electrical generator,the first gas turbine engine being arranged to supply power to the first input drive of the differential gearbox and the power unit being arranged to supply power to, or remove power from, the second input drive of the differential gearbox,a first brake is provided between the first input drive and the first gas turbine engine, the first brake being operable to prevent rotation of the first input drive if the first gas turbine engine and/or the first input drive fails,a second brake is provided between the second input drive and the second gas turbine engine, the second brake being operable to prevent rotation of the second input drive if the second gas turbine engine and/or the second input drive fails,a clutch or a fluid coupling is operable to allow the second gas turbine engine to start the first gas turbine engine, the clutch or the fluid coupling being provided between the first input drive and the first gas turbine engine or between the second input drive and the second gas turbine engine,the output drive has a brake operable to prevent rotation of the output drive to allow the second gas turbine engine to start the first gas turbine enginea sensor, the sensor being arranged to measure one of the group consisting of the speed of the first input drive and the speed of the output drive, anda controller being arranged to adjust the speed of the second input drive such that the speed of the output drive is maintained substantially constant.