REM%IMSSS@SU-AI.ARPA (Robert Elton Maas) (03/10/86)
TS> Date: 6 Mar 86 02:14:03 GMT TS> From: hplabs!sdcrdcf!uscvax!oberon!smeagol!jplgodo!ted@ucbvax.berkeley.edu TS> Subject: Re: Scramjets (specific impulse) TS> Wait a minute, I don't think Mr. Karn *is* right. The best definition of TS> specific impulse (Isp) is "thrust / (weight of propellent mixture used per TS> unit time)". That's exactly the problem. By measuring weight of propellent instead of mass of propellent, you're making it dependent on ambient gravitational force rather than a property of the propellent itself. By your definition, if the spacecraft is in zero gravity, where the weight is zero, the specific impulse is infinity! I don't like that definition at all. Normally the definition is in germs of weight in Earth-normal gravity, rather than just weight period, but that is a very Earth-chauvinistic definition, as bad as if we measured mass as fractions of the Earth's mass, time as fractions of the Earth's revolutional period (sigh, we do!!), temperature as fractions of the difference between melting and boiling point of Earth's most valuable resource (water) measured in Earth-normal atmospheric pressure (sigh, we do in the Centigrade/Celsius scale), ... As we go into space it would be nice to do away with scales that work on Earth but break down elsewhere, and start to use more universal scales. Specific impulse of one second means the rocket could balance itself against Earth-normal gravity for one second. For vertical-launch vehicles that's a dandy measure, but for just about everything else it's a silly unit of measurement. (Opinion of REM; bound to be controversial.)