Jet propulsion is thrust produced by passing a jet of air or water in the opposite direction to the direction of motion. By Newton's third law, the moving body is propelled in the opposite direction to the jet.

A number of animals, including cephalopods, sea hares, arthropods, and fish have convergently evolved jet propulsion mechanisms. This is most commonly used in the jet engine, but is also the means of propulsion utilized by NASA to power various space craft.

Jet propulsion is most effective when the Reynolds number is high - that is, the object being propelled is relatively large and passing through a low-viscosity medium.

In biology, the most efficient jets are pulsed, rather than continuous: at least when the Reynolds number is greater than 6. (In fluid mechanics, the Reynolds number (Re) is a dimensionless quantity that is used to help predict similar flow patterns in different fluid flow situations.)

Jet propulsion in cephalopods is produced by water being exhaled through a siphon, which typically narrows to a small opening to produce the maximum exhalent velocity. The water passes through the gills prior to exhalation, fulfilling the dual purpose of respiration and locomotion. Sea hares (gastropod molluscs) employ a similar means of jet propulsion, but without the sophisticated neurological machinery of cephalopods they navigate somewhat more clumsily.

Jet propulsion is a method of aquatic locomotion where animals fill a muscular cavity and squirt out water to propel them in the opposite direction of the squirting water. Most organisms are equipped with one of two designs for jet propulsion; they can draw water from the rear and expel it from the rear, such as jellyfish, or draw water from front and expel it from the rear, such as salps. Filling up the cavity causes an increase in both the mass and drag of the animal. Because of the expanse of the contracting cavity, the animal’s velocity fluctuates as it moves through the water, accelerating while expelling water and decelerating while vacuuming water. Even though these fluctuations in drag and mass can be ignored if the frequency of the jet-propulsion cycles is high enough, jet-propulsion is a relatively inefficient method of aquatic locomotion.

All cephalopods can move by jet propulsion, but this is a very energy-consuming way to travel compared to the tail propulsion used by fish. The relative efficiency of jet propulsion decreases further as animal size increases. Since the Paleozoic, as competition with fish produced an environment where efficient motion was crucial to survival, jet propulsion has taken a back role, with fins and tentacles used to maintain a steady velocity. The stop-start motion provided by the jets, however, continues to be useful for providing bursts of high speed - not least when capturing prey or avoiding predators. Indeed, it makes cephalopods the fastest marine invertebrates,:Preface and they can outaccelerate most fish. Oxygenated water is taken into the mantle cavity to the gills and through muscular contraction of this cavity, the spent water is expelled through the hyponome, created by a fold in the mantle. Motion of the cephalopods is usually backward as water is forced out anteriorly through the hyponome, but direction can be controlled somewhat by pointing it in different directions. Most cephalopods float (i.e. are neutrally buoyant), so do not need to swim to remain afloat. Squid swim more slowly than fish, but use more power to generate their speed. The loss in efficiency is due to the amount of water the squid can accelerate out of its mantle cavity.

AUV (Autonomous Underwater Vehicle) - 150 is an unmanned underwater vehicle (UUV) being developed by Central Mechanical Engineering Research Institute (CMERI) scientists in Durgapur in the Indian state of West Bengal. It is propelled by water-jet propulsion which comprises thrusters for generating motion in different directions to control surge, sway, heave, pitch, and yaw, while preventing the vehicle from rolling. Two arrays of cross-fins have also been fixed at the two ends to provide additional stability to the AUV.

See also:
Jet Propulsion
Aquatic Locomotion and Jet Propulsion

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