Can a turbine engine be switched from providing torque (like a helicopter) to the rotor to transfer power to provide exhaust thrust (like a jet plane)?
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I suppose the F135-PW-400 powerplant on the F-35B does just that.
A turbine engine is used to create high enthalpy gas which can either pass through a turbine to produce mechanical work or pass through a diffuser to accelerate it and create a reaction impulse. General Electric has designed power generation gas turbines which use the gas core from the GE F404 fighter engine changing from a thrust application to a power application.
As to doing this on the fly, other than the F-35B application I don’t know of a gas turbine which can dynamically change between the two. High bypass turbofans do get a small portion of their total thrust from a hot diffuser for turbine exhaust gases as do turbopropeller engines. But a total dynamic shift from one to the other? No I’m not aware of that.
The main way to convert a gas turbine to provide thrust rather than shaft horsepower is to attach a large fan or propeller in place of the previous load. For example, the GE CF6, which powers aircraft such as the 747, 767, and A330, has a gas turbine variant called the LM6000 that provides around 50 MW of power.
If you want to provide thrust from the exhaust, you're talking about a turbojet, which will require a lot of changes. A gas turbine like that in the CF6 is designed to extract most of the power out of the exhaust gas to drive a load such as a fan or a generator. Just removing that load is going to cause the engine to be unstable. Each stage in a turbine engine is dependent on the stages around it for smooth operation and even small changes can cause problems. You might be able to salvage the high speed spool but the rest will have to be redesigned to provide a stable compressor section and provide most of the thrust from exhaust.
Turbojets are very loud and only start to be an efficient choice at high speeds. Even modern fighter jets are powered by low-bypass turbofans.
Most turbine engines designed to drive some sort of mechanical system such as a hydraulic pump and electrical generators are not designed to provide thrust because their fuel controller can only run it at one speed. In other words, there is no throttle. Also, the internal components may not be designed for that type of application. So while it is in theory possible, it can be expensive and can result in some interesting failure modes that will entertain people standing far away.