Propulsion is a means of creating force leading to movement. The term is derived
from two Latin words: pro, meaning before or forward; and pellere, meaning
to drive. A propulsion system consists of a source of mechanical power, and a
propulsor. A technological system uses an engine or
motor as the power source, and wheels and axles, propellers, or a propulsive
nozzle to generate the force. Components such as clutches or gearboxes may be
needed to connect the motor to axles, wheels, or propellors.
Biological propulsion systems use an animal's muscles as the power source,
and limbs such as wings, fins or legs as the propulsors.
Vehicular propulsion = Air propulsion=
An aircraft propulsion system generally consists of an aircraft engine and some
means to generate thrust, such as a propeller or a propulsive nozzle.
An aircraft propulsion system must achieve two things. First, the thrust
from the propulsion system must balance the drag of the airplane when the
airplane is cruising. And second, the thrust from the propulsion system must
exceed the drag of the airplane for the airplane to accelerate. In fact, the
greater the difference between the thrust and the drag, called the excess
thrust, the faster the airplane will accelerate.
Some aircraft, like airliners and cargo planes, spend most of their life in a
cruise condition. For these airplanes, excess thrust is not as important as
high engine efficiency and low fuel usage. Since thrust depends on both the
amount of gas moved and the velocity, we can generate high thrust by accelerating
a large mass of gas by a small amount, or by accelerating a small mass of gas
by a large amount. Because of the aerodynamic efficiency of propellers and
fans, it is more fuel efficient to accelerate a large mass by a small
amount. That is why we find high bypass fans and turboprops on cargo planes and
airliners. Some aircraft, like fighter planes or
experimental high speed aircraft, require very high excess thrust to
accelerate quickly and to overcome the high drag associated with high speeds.
For these airplanes, engine efficiency is not as important as very high thrust.
Modern military aircraft typically employ afterburners on a low bypass
turbofan core. Future hypersonic aircraft will employ some type of ramjet
or rocket propulsion. = Ground=
Ground propulsion is any mechanism for propelling solid bodies along the
ground, usually for the purposes of transportation. The propulsion system
often consists of a combination of an engine or motor, a gearbox and wheel and
axles in standard applications. = Maglev=
Maglev is a system of transportation that uses magnetic levitation to
suspend, guide and propel vehicles with magnets rather than using mechanical
methods, such as wheels, axles and bearings. With maglev a vehicle is
levitated a short distance away from a guide way using magnets to create both
lift and thrust. Maglev vehicles are claimed to move more smoothly and
quietly and to require less maintenance than wheeled mass transit system error
= Marine= Marine propulsion is the mechanism or
system used to generate thrust to move a ship or boat across water. While paddles
and sails are still used on some smaller boats, most modern ships are propelled
by mechanical systems consisting a motor or engine turning a propeller, or less
frequently, in jet drives, an impeller. Marine engineering is the discipline
concerned with the design of marine propulsion systems.
Steam engines were the first mechanical engines used in marine propulsion, but
have mostly been replaced by two-stroke or four-stroke diesel engines, outboard
motors, and gas turbine engines on faster ships.Nuclear reactors producing
steam are used to propel warships and icebreakers, and there have been
attempts to utilize them to power commercial vessels.Electric motors have
been used on submarines and electric boats and have been proposed for
energy-efficient propulsion. Recent development in liquified natural gas
fueled engines are gaining recognition for their low emissions and cost
advantages. = Space=
Spacecraft propulsion is any method used to accelerate spacecraft and artificial
satellites. There are many different methods. Each method has drawbacks and
advantages, and spacecraft propulsion is an active area of research. However,
most spacecraft today are propelled by forcing a gas from the back/rear of the
vehicle at very high speed through a supersonic de Laval nozzle. This sort of
engine is called a rocket engine. All current spacecraft use chemical
rockets for launch, though some have used air-breathing engines on their
first stage. Most satellites have simple reliable chemical thrusters or
resistojet rockets for orbital station-keeping and some use momentum
wheels for attitude control. Soviet bloc satellites have used electric propulsion
for decades, and newer Western geo-orbiting spacecraft are starting to
use them for north-south stationkeeping and orbit raising. Interplanetary
vehicles mostly use chemical rockets as well, although a few have used ion
thrusters and Hall effect thrusters to great success.
= Cable= A cable car is any of a variety of
transportation systems relying on cables to pull vehicles along or lower them at
a steady rate. The terminology also refers to the vehicles on these systems.
The cable car vehicles are motor-less and engine-less and they are pulled by a
cable that is rotated by a motor off-board.
Animal Animal locomotion, which is the act of
self-propulsion by an animal, has many manifestations, including running,
swimming, jumping and flying. Animals move for a variety of reasons, such as
to find food, a mate, or a suitable microhabitat, and to escape predators.
For many animals the ability to move is essential to survival and, as a result,
selective pressures have shaped the locomotion methods and mechanisms
employed by moving organisms. For example, migratory animals that travel
vast distances typically have a locomotion mechanism that costs very
little energy per unit distance, whereas non-migratory animals that must
frequently move quickly to escape predators are likely to have costly but
very fast locomotion. The study of animal locomotion is typically
considered to be a sub-field of biomechanics.
Locomotion requires energy to overcome friction, drag, inertia, and gravity,
though in many circumstances some of these factors are negligible. In
terrestrial environments gravity must be overcome, though the drag of air is much
less of an issue. In aqueous environments however, friction becomes
the major challenge, with gravity being less of a concern. Although animals with
natural buoyancy need not expend much energy maintaining vertical position,
some will naturally sink and must expend energy to remain afloat. Drag may also
present a problem in flight, and the aerodynamically efficient body shapes of
birds highlight this point. Flight presents a different problem from
movement in water however, as there is no way for a living organism to have
lower density than air. Limbless organisms moving on land must often
contend with surface friction, but do not usually need to expend significant
energy to counteract gravity. Newton's third law of motion is widely
used in the study of animal locomotion: if at rest, to move forwards an animal
must push something backwards. Terrestrial animals must push the solid
ground, swimming and flying animals must push against a fluid or gas. The effect
of forces during locomotion on the design of the skeletal system is also
important, as is the interaction between locomotion and muscle physiology, in
determining how the structures and effectors of locomotion enable or limit
animal movement. See also
Transport References
External links Media related to Propulsion at Wikimedia
Commons Pickering, Steve. "Propulsion
Efficiency". Sixty Symbols. Brady Haran from the University of Nottingham.