combatant (PG), and the landing craft, air
that may contain substances or objects that can
cushion (LCAC). These crafts are capable of high
harm it. Also, the large amount of air used by
speed, can carry formidable weapons systems, and
a GTE requires large intake and exhaust ducting,
are being seen in increasing numbers in our fleet.
which takes up much valuable space on a small
In civilian versions, hydrofoils have served for
ship. This adds to the complexity of the
many years to transport people on many of the
installation and makes access for maintenance
world's waterways. Hovercraft are being used
more difficult.
more and more as carriers of people. They are
From a tactical standpoint, the GTE has two
capable of speeds up to 100 knots. If beach
major drawbacks. The first is the large amount
gradients are not too steep, they can reach points
of exhaust heat produced by the engines. Most
current antiship missiles are heat-seekers.
inland over virtually any type of terrain.
Therefore, the infrared (IR) signature of a GTE
is an easy target. Countermeasures, such as
GAS TURBINE ENGINE THEORY
exhaust gas cooling and IR decoys, have been
developed to reduce this problem.
The second tactical disadvantage is the require-
Two elements are required for proper
operation of a GTE. One is expressed by
ment for depot maintenance and repair of major
Newton's third law (action/reaction). The other
casualties. On the whole, the turbines are too
is the convergent-divergent process (or Bernoulli's
complex to overhaul in place. They must be
principle). Convergent means coming closer
removed and replaced by rebuilt engines if any
together, as the inner walls of a tube that is
major casualties occur. However, this problem is
narrowing. Divergent means moving away from
reduced by the design of the system. A GTE on
each other, as the inner walls of a tube that flares
a frigate, cruiser, or destroyer can be changed out
outward. The venturi of an automobile carburetor
in about 4 days if crane service and a replacement
is a common example of Bernoulli's principle and
engine are available. A GTE on a hovercraft can
the convergent-divergent process. Before we
be changed out in 8 hours. Gas turbine ships can
discuss GTE construction and design, we will
operate or be repaired to the same standards as
discuss a little more on cycles and theory.
their steam- or diesel-driven counterparts.
THEORETICAL CYCLES
FUTURE TRENDS
A cycle is a process that begins with certain
As improved materials and designs permit
conditions, progresses through a series of
operation at higher combustion temperatures and
additional conditions, and returns to the original
pressures, GTE efficiency will increase. Even now,
conditions. The basic GTE cycle is named for the
GTE main propulsion plants offer fuel economy
Boston engineer, George Brayton, who proposed
and installation costs comparable to diesel
it in the late nineteenth century.
engines. Initial costs are lower than equivalent
The Brayton cycle is one in which combustion
steam plants that burn distillate fuels. These
occurs at constant pressure. In GTEs, specific
improvements have made GTEs the best choice
components are designed to perform each
for nonnuclear propulsion of naval ships up to,
function of the cycle separately. These functions
and including, an underway replenishment ship
are intake, compression, combustion, expansion,
in size.
and exhaust. Refer to figure 1-6 as we explain the
At present, marine GTEs use derivatives of
Brayton cycle graphically.
aircraft jet engines for GGs. These are slightly
modified for use in a marine environment,
Intake--At point A, air enters the inlet at
particularly in respect to corrosion resistance. As
atmospheric pressure and ambient temperature.
marine GTEs become more widely used, specific
designs for ships may evolve. These compressors
Compression--As the air passes through
may be heavier and bulkier than aircraft engines
the compressor, it increases in pressure and
and take advantage of regenerators to permit
temperature and decreases in volume (line A-B).
greater efficiency.
The high power-to-weight ratios of GTEs
Combustion--At point B, combustion
permit the development of high-performance
occurs at constant pressure while the addition of
craft, such as the hovercraft and the hydrofoil,
heat causes a sharp increase in volume (line B-C).
t h e patrol combatant missile the patrol
1-4
