[AR] Re: Microlaunchers Book
- From: Nels Anderson <nels.anderson@xxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Wed, 26 Mar 2014 11:42:31 +0000
On 26/03/14 01:50, Ed LeBouthillier wrote:
> So, the whole issue of Ve = g Isp is, I think a product of the units
> that I decided to express Isp in.
>
>
>
> Is Isp to be expressed in units of seconds or in meters/second? I
> chose units of seconds.
>
I think of it as a choice between weight-specific impulse, with
dimensions of impulse per unit force (weight), i.e., time, and
mass-specific impulse, with dimensions of impulse per unit mass, i.e.,
velocity. To me, specific impulse is, by convention, weight-specific
impulse.
I enjoyed the book, thanks very much. If I may point out one spot where
I think there is a bit of a units issue, it's in the discussion of
pressure-vessel performance factors on page 76. Two versions of the
definition of performance factor have crept in. In the first, it's
defined as p*V/mass, which has dimensions of energy per unit mass and in
the second as p*V/weight, which has dimensions of length. The
numerical examples seem to have been calculated with the weight-based
version of the equation assuming g=10 m/s2, but that factor of g does
not appear in the numerical example. Why not just use the mass-based
version and express PFs in J/kg?
Nels Anderson
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