Andrew Sarangan writes:

I have heard that argument many times, but I have never seen that
thermodynamic argument presented. I just borrowed the book on Aircraft
Gas Turbine Engines from the library and plan to read it to find out
what the real story is. My suspicion is that the limitation is in the
materials, not thermodynamics.

Measure the heat of a gas turbine exhaust; the difference between that and
ambient inlet temperature is wasted energy. An ideal turbine would extract so
much energy from the heat of combustion that the exhaust would barely be warm,
but we're a long way from a turbine like that.

A small
turbine may sound far fetched now, but I am sure GPS also sounded far
fetched 20 years ago, but became commonplace after heavy military
investment.

Actually, the principles behind GPS were known and accepted half a century
ago. It just took a long time to get a working system perfected--just as
improvements in jet engines tend to be gradual.

Having said that, I know of at least two companies working on small
turbines. One is Innodyn, and the other one is M-dot. The latter one I
believe has some DoD contracts to be build turbines for UAVs. I doubt
these companies would even exist if the basic physics is flawed.

Low efficiency can be compensated by other advantages.