Environmental Engineering Reference
In-Depth Information
brought total displacement of reciprocating engines in
fighters and bombers and, starting in 1958, on transoce-
anic passenger flights. (The British Comet, introduced in
1952, did not succeed owing to structural defects rather
than to the inadequacy of jet engines.) This transforma-
tion reached a temporary plateau with the introduction
of hundreds of jumbo airplanes during the 1970s (the
first flight of a Boeing 747 was in 1969). These airplanes
were powered by large turbofan engines capable of nearly
250 kN of thrust at the sea level and delivering about 65
MW during the flight at the cruising altitude. A low
weight/power ratio and a high ratio of thrust per frontal
area characterized the evolution of these increasingly
powerful aircraft gas turbines.
At the beginning of the twenty-first century the best
commercial turbofan engines were rated above 500 kN
(0.06-0.07 g/W), had a thrust/weight ratio above 6
(8.5 for military engines), and had high bypass ratios
(90% of the compressed air bypasses the combustion
chamber, thereby lowering specific fuel consumption
and reducing engine noise.) Their high reliability made
it possible to deploy two-engine aircraft not only on
trans-Atlantic crossings but even on trans-Pacific ones.
Other notable niches conquered by gas turbines have
been in driving centrifugal compressors for natural gas
pipelines, starting in the late 1940s, with current capaci-
ties up to 15-30 MW; oil fields and oil refineries;
chemical syntheses, most notably in ammonia production
since the early 1960s; steel mills; powering fast trains,
hydrofoils, and military and cargo ships with engines of
up to about 80 MW; and driving electric generators
(15-80 MW to 150 MW) used for emergencies, peaking
service, and base load (Islas 1999). The best efficiencies
in these applications, just above 40%, match the perfor-
mance of the best steam turbines, and the weight/power
ratios of the largest stationary gas turbines are around
2g/W.
The only prime movers that outperform gas turbines
in terms of weight/power ratio are liquid- or solid-
propellant rocket engines for missiles and space vehicles.
These large-thrust jet propulsion engines are used to ac-
celerate loads to high velocities, often in stages, in short
periods of time. Intensive engineering development of
this ancient idea started only during the first decades
of the twentieth century, but the founders of modern
rocket science, Konstantin Tsiolkovsky and Hermann
Oberth, correctly envisaged rapid advances (von Braun
and Ordway 1975). In 1942 the 13.8-m-long ethanol-
powered German V-2 missile had a range of 340 km
and a destructive payload of 1 t. Its 931-kg engine had a
sea-level thrust of 249 kN, imparting the maximum
speed of 1.7 km/s during its 68-s burn. This translates
to a maximum power rating of about 6.2 MW and an en-
gine weight/power ratio of 0.15 g/W.
In contrast, during the 150-s firing, the liquid-fuel
(kerosene and hydrogen) engines of the 109-m-tall
Saturn C 5 rocket, which sent Apollo 11 on its journey
to the Moon on July 16, 1969, had to impart an escape
velocity of 11.1 km/s to a mass of 43 t by providing a
combined thrust of nearly 36 MN, or an equivalent of
about 2.6 GW. Even when including all the fuel in the
weight of the three booster rockets with 11 engines,
their weight/power ratio would be just 0.001 g/W.
The thrust/weight ratio of individual rocket engines is
as high as 150, 1 OM above the best military jets. This
rapid transformation of chemical energy in fuels to ki-
netic energy of rising rockets is the most dramatic dem-
onstration of powerful conversions mastered since the
end of WW II. But the most important, although cer-
tainly much less spectacular, energy transformation that
