space@mit-mc (03/14/85)
From: James Parker <Parker.es@XEROX.ARPA> Pavel, in metric units you are quite correct in your reasoning. "specific impulse" is the english engineering term used to obscure the fact you're talking about exhaust velocity. since i don't understand english engineering units, i can't tell you why specific impulse comes out in 1 / sec. i learned the metric system at age 20 and that cleared up lots of questions like yours. if you want to understand physics, think metric. James
karn@petrus.UUCP (03/22/85)
The "true" units of specific impulse is force-time/mass, ie, how much thrust can it produce for how long for a given amount of propellant. In metric units, this is simply newton-seconds/kg, which simplifies to meters/sec (the exhaust velocity). However, in archaic English units this is written as poundsforce-seconds/poundsmass, and the dissimilar pounds units are incorrectly canceled out, leaving so-called "seconds". Because a pound of mass corresponds to a pound of force only in a 1G acceleration field, the earth's surface gravitational acceleration is factored into this so-called "seconds" figure. To convert to exhaust velocity, just multiply by 9.8 m/sec^2 or 32 ft/sec^2. The number you really want to use is exhaust velocity, in the classic rocket equation delta-v = Ve * ln(mass loaded/mass dry) I agree, metric units are the only way to go. I always feel a twinge of embarassment every time I hear the NASA commentator use "knots", "feet per second" and "statute miles" in the same breath during a shuttle launch. The press kits on the Shuttle are even worse. Every measure is given in about three or four sets of units, and many of the conversions are wrong. You don't know which to believe unless there are enough versions to allow majority voting. ESA may use three languages, but at least they have their measurement act together. Phil