802,264. Shafts. ROLLS-ROYCE. Ltd. Feb. 1, 1956 [Feb. 18, 1955], No. 4998/5 5. Class 80 (2). [Also in Group XXVI] A propeller driving gas turbine engine has the turbine rotor connected to drive both a compressor rotor and the propeller through shaft means including first and second coaxial nested shafting arranged so that in the event of failure in the first or second shafting the turbine rotor remains drivingly connected to the compressor rotor or the propeller or both. The gas turbine engine shown, Fig. 1, comprises a two-stage centrifugal compressor 10 supplying air to combustion equipment 11 feeding a two-stage turbine 12. A propeller (not shown) is driven by the engine through reduction gear 13. The two-stage turbine comprises a pair of rotor discs 14, 15, which are torsionally interconnected near their centres and at a greater radius by Z-section rings 18, 19. The rotor disc 14 has a series of bosses 20 through which pass bolts 23 which secure the turbine shaft 22 to the disc. The shaft 22 is supported in stationary structure by a ball-thrust bearing 24. The upstream end of the shaft 22 is connected to a high pressure compressor shaft 25 which is supported by a ball-thrust bearing 27, by a coupling 26 which permits slight angular malalignment and relative axial movement. The impeller 28 is in splined engagement with the shaft 25 which is formed with external splines 29 at its upstream end which engage internal splines 30 of a low pressure compressor shaft 31 which has the impeller 32 splined to it. The shaft 31 is supported by a ball thrust bearing 33 and a roller bearing 34. A bush 34a mounted on a quill shaft 35 extends into the bore of the shaft 31. The shaft 35 forms the input shaft of the propeller reduction gear 13. The engine fuel pump is normally driven from the reduction gear 13, e.g. from the layshaft 39 or from an auxiliary gear-box driven from a compressor shaft of the engine. The quill shaft 35 is driven by the turbine by an additional shaft 50 which extends coaxially through the shafts 22, 25, 31. This shaft is formed with a flange 51 which is secured to the low pressure turbine disc 15 by bolts 52. The shaft 50 is supported along its length from the shafts 22, 25, 31 by sleeves 53, 54, 55 which engage bushes 56, 57, 58 on the shaft 50. The bushes may be of rubber or similar material. The shaft 50 is substantially unrestrained torsionally with respect to the shafts 22, 25, 31 and has external splines 59 formed at its forward end which engage splines 60 in the quill shaft 35. The arrangement is such that if the shaft 50 fails, the turbine will be relieved of the propeller load and will drive the compressor at a higher speed, acting as a simple jet propulsion engine. If one of the shafts 22, 25, 31 fails, the turbine will continue to drive the propeller. If the failure is between the compressor and the turbine, the air supply to the combustion equipment will fall and the power output of the turbine will also fall. Under certain circumstances the propeller will windmill and drive the turbine but not the section of the compressor on the side of the shaft failure remote from the turbine. In a modification of the above arrangement, Fig. 3 (not shown), the shaft 50 is formed in two parts which are connected together by splines and the shaft 50 is bolted to a flange formed on the front face of the turbine disc 14. In a further modification, Fig. 2, the shaft 50 is replaced by a stand-by shaft 150 and a drive shaft 145. The compressor shaft 25 is formed internally with splines 146 at its forward end which engage external splines 147 on the drive shaft 145. The drive shaft 145 also has splines 148 which engage the internal splines 60 of the quill shaft 35. The quill shaft 35 is thus normally driven by the turbine through shaft 22, shaft 25, splines 146, 147 drive shaft 145 and splines 148, 60 and the low pressure compressor shaft 31 is driven from the shaft 25 through splines 29, 30. The stand-by shaft 150 is preferably secured to the low pressure turbine disc 15 as described for the shaft 50 in Fig. 1, but in an alternative arrangement, the shaft 150 may be provided with a flange 151 which is secured between the flange 21 on the turbine shaft and the bosses 20 on the high pressure turbine disc 14. In this arrangement, the turbine discs 14, 15 are torsionally interconnected at the inner radius by a large diameter bolt and nut 143, 144. The bolt may be splined to the low pressure disc and the discs 14, 15 may be in spigot engagement with one another. At its forward end the stand-by shaft 150 is formed with internal and external splines 163, 161 which co-operate with splines 164 and 162 respectively. The two sets of splines 161, 162 and 163, 164 are arranged to have a circumferential clearance in normal operation so that no drive is transmitted through them. The extension 165 of the drive shaft 145 is engaged with the stand-by shaft 150 by means of a nut member 166 which is threaded on the extension 165 and abuts an axial face within the stand-by shaft 150 through a ring 167. The nut member is locked against rotation with respect to the extension 165 as described in Specification 802,263, [Group XXXIII], by means of a locking sleeve 168 which is serrated at 169, 170 to engage the nut member 166 and extension 165. The sleeve is prevented from moving axially by a spring ring 171. In this arrangement if the turbine shaft 22 or coupling 26 should fail, the clearance between the splines 161, 162 will be taken up and a drive transmitted from the turbine to the high pressure compressor shaft through the stand-by shaft 150. If the shaft 25 should fail forward of the compressor impeller 28, the clearance in the splines 163, 164 will be taken up and the low pressure compressor will be driven through the shaft 150, splines 163, 164, extension 165, splines 146, 147 and splines 29, 30. The quill shaft 35 will also be driven through the extension 165, drive shaft 145 and splines 148, 60.