Re: Units Was: Re: english schminglish

From: Doug Jones (
Date: Mon Mar 06 2000 - 23:39:24 MST

Spike Jones wrote:
> Stirling Westrup wrote:
> > I'm replying to my own reply, since it suddenly occured to me that tensile
> > strength per unit density is exactly the WRONG units to use. This would
> > cause us to use (for example) a 1km thick strand of steel foam in
> > preference to a 1m thick strand of solid steel, because they had similar
> > tensile strengths, but one was much less dense.
> I beg to differ, Stirling. The steel foam would lose tensile strength in
> proportion to its density. Since it doesnt matter the volume of the
> space cable, there is no preference between steel and aluminum
> for instance. Aluminum is approximately a third the density of steel
> but its tensile strength is also about a third. I shall wait to post this
> until I see if Doug Jones has already jumped on this. {8^D

I'm little slow on the uptake tonight, but I looked up the specific
strengths entry in a materials handbook today- to my (not so great)
surprise, it's listed as a characteristic length, due to a factor of g
being thrown in. Listed in kilometers, it's the length of an untapered
filament that could support itself in a mythical one gee field. The
number I recall seeing for graphite was around 100 km.

With v^2 divided by m/s^2, the units drop out to meters. Every bit as
perverse as quoting Isp in seconds... yet another legacy of American
Provincial units.


Doug Jones
Rocket Plumber, XCOR Aerospace

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