Early turbojets achieved overall efficiencies of about 0.2, lowbypass-
ratioenginesmade available in the early 1960s offeredoverall
efé cienciesof about 0.25,whereas early 1990shigh-bypass-ratio
turbofans have achieved substantial improvements in both thermal
and propulsive efficiencies and offer about 0.35 overall efé ciencies.
Even higher overall efficiencies, perhaps in the region of 0.50 may
be possible after 2010 with ultra-high-bypass-ratio turbofans, such
as propfans and advanced ducted propulsion systems
For an increase of
g from 0.3 to 0.5, the threshold formation
temperature of contrails for kerosene-driven aircraft increases by
4.2–4.9 K (for 0–100% ambient humidity), implying 650–760 m
lower altitude in the standard atmosphere (Fig. 1); the altitude difference
increases with RH. The present global mean cover of the
Earth by contrails is about 0.1%. If
g grows from 0.3 to 0.5 in a
future fleet of aircraft, contrail cover is expected to grow by about
20% of its value for otherwise fixed conditions.1
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