How rockets work?

Thrust is a measurement of the force of a rocket, or the amount of “push” exerted backward to move a [[KW]] rocket [[/KW]] forward. Thrusts vary greatly from rocket to rocket. Engineers measure thrust in units of weight or force (newtons [N] in the metric system and pounds [lb] in English measurements).
Specific impulse measures the efficiency and power of [[KW]] rocket engines [[/KW]] and propellants. Specific impulse (Isp) is the thrust produced per kilogram or pound of propellant per second. Measuring Isp is similar to measuring the efficiency of [[KW]] cars [[/KW]] in kilometers per liter or miles per gallon. Modern solid propellants have specific impulses of about 3,400 to 3,900 N per kg per sec (about 350 to 400 lb per lb per sec) and advanced liquid propellants typically have Isps of about 4,200 to 4,400 N/kg/sec (about 425 to 450 lb/lb/sec).
Exhaust velocity, or the speed at which exhaust leaves the rocket, is another way to measure rocket performance. The higher the exhaust velocity, the greater the thrust. Propellants with higher exhaust velocities also have higher specific impulses. Exhaust velocities can range from 600 to 900 m/sec (2,000 to 3,000 ft/sec) for gunpowder, 2,000 m/sec (8,000 ft/sec) for a mixture of liquid oxygen and gasoline, to 4,000 m/sec (12,000 ft/sec) or more for a mixture of liquid oxygen and liquid hydrogen. Rocket engine performance also depends on the design of the combustion chamber and nozzle and the pressure of the propellant.