Jet Engine Question Homework: Find Thrust w/ 100% Efficiency

[didinyc]

Homework Statement

The major difference between a jet engine and a rocket engine is that a rocket carries its
own oxidizer, usually in the form of liquid oxygen (LOX). A jet takes in air from the
atmosphere as it moves, uses it to burn the fuel, and then ejects all the gases out the
back.
At a certain instant, a jet engine is moving 210 m/s. The engine takes in air at a rate of 74.2
kg/s, combines it with 3.20 kg/s of fuel, and ejects the products of combustion at constant
speed 502 m/s relative to the jet. Assuming 100% efficiency: find the numerical value of the
thrust of this engine.
HINT: For a rocket engine: Thrust rel= v(relative)* (dm/dt)

How must this expression be modified for a jet
engine?

The Attempt at a Solution

I tried to read the book to solve it but cannot start anything if i get an idea of how to start, I will try to solve it! Thanks for the help!

 

[Percussim]

Thrust is a force and Force = mass x acceleration. The amount of burnt fuel gas in the air flowing through an engine is so small as to be negligible and is not included in calculations for thrust. Thus it is the mass of cold air in (which must come out) times the increase in velocity, viz: Thrust = mass(air) times (V-exhaust minus V free stream)+(Pressure of ambient air minus presure exhaust air) times area of exhaust nozzle

The overall point relating to your question is that the amount of burnt fuel gas in a straight rocket exhaust is considerable compared to that in an air breathing jet engine.