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. ________________________________ From: "mechtly, eugene a" <mech...@illinois.edu> To: U.S. Metric Association <usma@colostate.edu> Cc: "mechtly, eugene a" <mech...@illinois.edu> Sent: Fri, May 3, 2013 10:48:59 PM Subject: [USMA:52736] RE: NASA mT and K Patrick, Yes, the PDF file (on the Ares V) by Phil Sumrall needs many corrections of units before it it fit for reprinting!!! For example, for the J-2X Engine Specification, Specific Impulse (Isp) has the same unit as Exhaust Velocity, in meters per second (in proper SI). That is; Force times time divided by mass reduces to a velocity, in m/s. The present error is writing lbf times seconds divided by lbm; equating and canceling lbf and lbm, and claiming that Isp is expressed in seconds. Mass and Force are distinctly different quantities. Weight (lbf) is not relevant in the definition of Specific Impulse!!! Eugene Mechtly ________________________________________ From: owner-u...@colostate.edu [owner-u...@colostate.edu] on behalf of Patrick Moore [pmo...@asnt.org] Sent: Thursday, May 02, 2013 11:17 AM To: U.S. Metric Association Subject: [USMA:52733] NASA mT and K Have a look at the first page of this NASA publication, a free download: <http://www.nasa.gov/pdf/396782main_mf18_sumrall.pdf>. I was given that picture of four launch vehicles, so I then surfed to its online PDF to clarify the symbols. I could guess that mT did not mean millitesla. I would not have guessed metric ton. The NASA editor apparently knew that k means x1000, but look at the goofy way a cap K is appended to numerals. This image's technical information needs a lot of repair before I can can reprint it. [End]
