Ceramic-to-metal turbine shaft attachment

An engine, comprising:

• a plurality of turbo-compressor spool assemblies, each turbo-compressor spool assembly comprising a compressor and a turbine attached by a common shaft and a first of the turbo-compressor spool assemblies is in fluid communication with a second of the turbo-compressor spool assemblies;
• a free power turbine driven by a gas flow output by at least one of the turbo-compressor assemblies;
• and
• a combustor operable to combust a fuel and a gas output by one of the plurality of turbo-compressor spool assemblies, wherein a selected turbo-compressor spool assembly comprises a metallic compressor rotor and a ceramic turbine rotor connected by a metallic-to-ceramic attachment joint, wherein a first and second bearings are located along a common shaft of the selected turbo-compressor spool assembly, and wherein at least one of the following is true:
• (i) a turbine rotor of a selected turbo-compressor spool assembly operates in a no-failure regime of the ceramic material and the metallic-to-ceramic attachment joint is located to be in a no-failure regime of the ceramic material;
• (ii) the metallic-to-ceramic attachment joint is located between the first and second bearings;
• (iii) a ceramic portion of the common shaft has a length of at least about 40% of a length of the shaft;
• and
• (iv) respective diameters of the ceramic portion and a metallic portion of the common shaft are substantially the same in the vicinity of the metallic-to-ceramic attachment joint.