JET PROPULSION

MECHANICAL TIPS AND STUDY MATERIAL

Jet propulsion

It is movement of aircraft, ships, rockets and such related bodies by the jet. A orifice is provided in the body through which jet comes out and imposes reaction on the body which allows the body to move in opposite direction of jet. This can be understood with Newton's third law of motion which states that " every action has equal and opposite reaction". Whenever a jet of fluid comes out of nozzle\ orifice, it exerts force of some magnitude on the plate. Let us suppose the magnitude here "F".


As per Newtons law the jet of fluid will exert the same force "F" on the body in opposite direction. This force will help to move the body in forward direction. This is called reaction of jet. This principle of jet propulsion has many application like

1) Jet propulsion of tank.
2) Jet propulsion in ship.

Propulsion in tank

Consider a tank fitted with friction less wheels and orifice at the height "H" from top of water level in the tank. If "F" is the force exerted on the jet of water,

V = Cv(√2gh).

Where V is velocity of jet.
h is the height and g is gravity


The same force will be exerted on the tank but in opposite direction. The horizontal from the center line of orifice, the tank will experience force. This will give tank some momentum (say U) and it will start moving forward. The velocity at the orifice exit will now be equal to relative velocity of jet with respect to tank.

Vr = velocity of jet with respect to tank
U = velocity of tank
V = velocity of jet

Vr = V - U

But U is in opposite direction of V, hence we take negative sign for U.
Therefore     Vr= V - ( -U ). = V +U



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