[GZG] Gas turbine efficiency. (Was: Scale Creep)

Gas turbines are notoriously fuel-inefficient, but that's not the
only consideration in the choice of power-plant. Gas turbines are
mechanically simple, can be brought quickly up to full power from
cold, and typically have high power-to-weight and power-to-size
ratios. Presumably it was those features that attracted the designers
of the M-1 Abrams.

 From a fuel-economy point of view, something like the turbo-compound
Napier Nomad diesel turbine would be ideal as a tank engine, but
keeping so complex a power-plant running in the field might present a
difficult challenge. An ordinary turbo-diesel might well be a better
compromise.
http://en.wikipedia.org/wiki/Turbo-compound_engine
http://en.wikipedia.org/wiki/Napier_Nomad

Essentially the crucial problem with gas-turbine engines, from an
efficiency point of view, is that they throw too much energy away in
their hot exhaust (creating incidentally a large infra-red
signature). "Combined Cycle" gas turbine systems use heat exchangers
to capture some of this wasted energy to drive steam-turbines, and
can achieve very high efficiencies. However, such systems are bulky,
complex, and have so far only really been used in fixed electricity-
generation applications.
http://en.wikipedia.org/wiki/Combined_cycle