An attempt is made to show some of the key mechanical difficulties inherent in designing shaft, bearing, and seal systems for three desirable types of small gas turbine engines. This is accomplished by showing the bearing, shaft, and seal system of a family of large turbine engines which has demonstrated its high reliability, high efficiency, and low cost per unit of power as the thermodynamic, material and structural component technology improves and as the design power requirement is reduced.Three different solutions to the problems posed above (for radically different applications) are offered. The first and second solutions are for sophisticated, efficient engines for weapon systems consideration (Types I & II). Solution three describes a potential industrial, low cost, high efficiency, moderately heavy engine. (Type III.)A summary is made of the key shaft, bearing, or seal problems which must be overcome for each of the three types of engines described.