Learn about Isp

The efficiency of a rocket motor is measured by a simple formula that compares what goes into the motor with what comes out. The output is thrust, generally measured in pounds. The input is fuel and oxidizer, flowing at a certain rate, and is measured in pounds per second. If outpout is divided by input - that is thrud (pounds) divided by propellant flow (pounds per second) - the pounds cancel each other and we are left with seconds as the unit of merit for the motor. This number is callled specific impulse, of Isp in engineering shorthand. THe higher the Isp the better, as far as propulsive efficiency is concerned. THe old kerosene-oxygen combination produces an Isp of 300 seconds. Hydrogen/oxygen yields 390 seconds, a notable improvement. However other exotic fuels can give 1000 seconds. These numbers are just to give an indication of the eficiency of the motor.