For turbines and turbos, look to consumption rate and provision for engine lubricants.  Public info suggests 50-100 hours with the customary 15% margin.  Rotary will be substantially lower because they denature the properties of lubricants at a faster rate in comparision and tend to carry less.  Hope that's worth something.

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Y then is it that we don't see this configuration today?  Because the deadweight and drag penalties suggest a higher efficiency approach.  Even with push-pull config (you avoid some drag due to assymetry) the hull is occupied by an extra system that sees perhaps 50% use per mission.  You're better off eliminating the extra and carrying more oil instead.  I really wish I could post cahrts here.  Cheers.

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Interesting British design.  I believe some of them literally shook apart due to the asymmetric torque placed on the the power transfer mechanism when operated on a single engine. 

Newer engines are tested in static environments for thousands of hours continuous operation with unlimited fuel and lubricant.  In the air, oil capacity is the limiter provided you have aerial refueling. 

Which begs an interesting question I suspect the Brits can resolve.  Why not combine aerial refueling with replenishment of lubricant?  You'd have to heat the pipe to achieve uniform flow but that's about the only real issue with the idea.  Or so I claim ;>)

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