Online, on sites such as Wikipedia's for Specific Impulse and Thrust-specific fuel consumption, there are these units:

101,972/x g/(kN·s) and 3,600/x lb/(lbf·hr)

Perhaps 3600 comes from the number of seconds in an hour, but.... Otherwise I am confused.... . . . .

EDIT: P.S.: I just realized.... If I divide 101,972 by 1,000 to go from kilonewtons to Newton's.... Then multiply by 4.4482216 to convert from N to lbf ( one lbf is 4.4482216 Newton's...) ... I get 453.59405455, which is almost exactly equal to the number of grams in a pound (453.59237)...Maybe something.....


1 Answer 1


Specific impulse of $1~\text{s}$ is equivalent, in terms of speed, to $g_0\cdot 1~\text{s} = 9.80665~ \text{m/s}$, where $g_0 = 9.80665~ \text{m/s}^2$ is the standard gravitational acceleration. $101972~ \text{g/(kN·s)}$ and $3600~ \text{lb/(lbf·hr)}$ are both the inverse of that value.

We have $$ 1~ \text{g/(kN·s)} = 10^{-6}~ \text{kg/(N·s)} = 10^{-6}~ \text{s/m}, $$ so $$ \frac{1}{g_0\cdot ~1~\text{s}} = \frac{1}{9.80665~ \text{m/s}} = 0.101972~ \text{s/m} = 101972~ \text{g/(kN·s)}. $$ Also, $$ 3,600~\text{lb/(lbf·hr)} = \frac{3600~ \text{lb}}{(1~\text{lb}~\cdot ~g_0)~\cdot~3600~\text{s}} = \frac{1}{g_0\cdot ~1~\text{s}}. $$

  • 1
    $\begingroup$ Just imagine how easy it could be if there where neither customary nor historic units and we just had SI... This post would have only one line... Maybe if we all wish for it for the new year? Btw, thanks for getting the typography (italics / spaces) right! $\endgroup$
    – asdfex
    Dec 30, 2021 at 20:10
  • $\begingroup$ asdfex, that is probably NEVER going to happen..... In the U.S.A., there have been several major attempts, most (in)famously in the mid-to-late-'70s to early 1980s to force us all to change to the metric SI system, and it never quite took..... $\endgroup$
    – Kurt Hikes
    Jan 1 at 1:23
  • $\begingroup$ Have you heard of the Mars Climate Orbiter? NASA lost it because of confusion between metric and old English or Imperial or Customary or 'American' or whatever-you-want-to-call-them-units.... $\endgroup$
    – Kurt Hikes
    Jan 1 at 1:26
  • $\begingroup$ @asdfex it would also only have one line if we only had English units. In fact, it appears to have 2 lines devoted to SI units. $\endgroup$ Jan 1 at 2:50
  • $\begingroup$ @OrganicMarble The second line has a lot of metric, but it's only needed because of the traditional [s] unit for specific impulse which absorbs $g_0$. And in the third line, customary units could be quite clean as well, if not somebody had chosen the hour over the convenient second. That would eliminate all constants from the equations and leave us with plain units. $\endgroup$
    – asdfex
    Jan 1 at 10:01

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