John (Steele), I agree with your observation that some parts of NASA are wedded to earth-gravitational ideas.
How does NASA-GRC define the Specific Impulse (Isp) of a Rocket Engine operating in deep space far from earth's gravity field where the value of g and W approach zero? i.e. where W = m g and W also approaches zero? Would they still define Isp as thrust integrated over a short interval of time divided by weight (W)? or would they divide by mass, respecting CGPM resolutions and avoiding the infinity arising from division by zero? Where is the GRC "g" evaluated in W = m g? It is the NASA-GRC "definition" of Specific Impulse that I reject. The GRC equations contradict the CGPM resolution declaring the distinction between mass and weight (defined as a force). I do not agree with your assertion that mass and force are the same thing if...(under any circumstance you select)! It is true that the numerical values of mass and weight can be forced to be equal by introducing a contrived multiplication or division factor, but that *manipulation* does not make the quantities mass and force equal! Only the numerical values can be made equal in a hybrid non-system of units, e.g. where 10 lbf = 10 lbm. There are many other abuses of unit names and unit symbols in the MSFC article on Ares V and in the GRC article on Isp which deviate from standards on SI, too numerous to mention! Eugene Mechtly ________________________________ From: Kilopascal [[email protected]] Sent: Saturday, May 04, 2013 8:15 AM To: U.S. Metric Association Subject: [USMA:52737] RE: NASA mT and K They use the dreaded sec instead of s for seconds. [USMA:52737] RE: NASA mT and K<http://www.mail-archive.com/[email protected]&q=subject:%22%5BUSMA%3A52737%5D+RE%3A+NASA+mT+and+K%22> John M. Steele<http://www.mail-archive.com/[email protected]&q=from:%22John+M.+Steele%22> Sat, 04 May 2013 04:59:16 -0700<http://www.mail-archive.com/[email protected]&q=date:20130504> I think this "derivation" of specific impulse proves NASA is fundamentally wedded to US Customary and gravitational unit systems. Metric is just a glossy conversion at the end to show EO12770 compliance. http://www.grc.nasa.gov/WWW/k-12/airplane/specimp.html At any rate, they mean burn time with the available fuel, assuming constant thrust and fuel flow, it reduces to thrust multiplied by burn time divided by fuel mass, or real specific impulse divided by earth gravity. Mass and force ARE the same thing if acceleration is divided by earth gravities. There are three ways to "fake" coherence of F = ma in Customary, poundals, slugs, or gravities. Poundals are mostly British, and most of us learned slugs in high school but NASA uses gravities. For me, learning metric was slugicide, have killed them on sight ever since.
