Tavares.Coop@MIT-MULTICS@sri-unix (05/15/82)
Apples and oranges, but both relevant. The specific impulse of a rocket says how much "bang" you get per unit of mass of the propellant. (For example, a stick of dynamite and a long candle may put out the same amount of energy, but the TNT has higher specific impulse.) The horsepower simply says how much total propulsion effort you get out of the rocket. You can double a rocket's horsepower by doubling the mass of fuel in it (very roughly speaking) if the net effect is to double the engine's burn time, all else remaining equal. Horsepower is the same beast as "total impulse" and consists of the same unit dimensionality as Newton-seconds or watts.