[AR] Re: Fw: Hydrogen / oxygen news
- From: Henry Spencer <henry@xxxxxxxxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Thu, 31 Oct 2013 00:07:47 -0400 (EDT)
On Wed, 30 Oct 2013, Keith Henson wrote:
> > Clark calculates the kinetic power of the Saturn V first stage
> > exhaust as about 41 gigawatts... this is on the same scale as
>
> If you google saturn V gw the number 190 GW comes up. I don't know
> what the correct number is.
Both numbers are arguably correct, because it depends on what you're
measuring.
41GW (actually, about 45 -- Clark was using approximate numbers, and both
thrust and Isp were a bit higher than he assumed) was the *useful output*
power -- the kinetic energy per second of a perfectly collimated exhaust
jet with that mass flow producing that thrust. In practice, even the
kinetic-energy power was higher, because the flow had some divergence and
some turbulence. (And it got higher yet at high altitude, because both
thrust and Isp increased as back pressure dropped.)
190GW or so was the *theoretical input* power -- the energy release per
second from burning that mass flow of RP-1 under ideal conditions. In
practice, the conditions were well away from ideal, so even the input
power wasn't nearly that high.
And even after you adjust the former up and the latter down, yes, there's
still a big discrepancy, which is because the conversion of combustion
power to jet kinetic power is not particularly efficient for LOX/RP-1 at
a modest Pc with a low-expansion sea-level nozzle. That's what I was
getting at with my mention of "vaguely ... a factor of two".
There are still other ways of measuring the power input and output, which
yield yet other numbers.
Which method, and thus which number, is "correct" depends on what you want
to use it for. Machines are usually rated on useful output power, but if
you want to replace chemical-energy input with electricity -- which is how
this thread got started -- then if efficiencies were similar (which they
probably wouldn't be), theoretical input power is the interesting number.
The bottom line is that for a rocket engine, no matter which number you
pick, it's big.
Henry Spencer
henry@xxxxxxxxxxxxxxx
(hspencer@xxxxxxxxxxxxx)
(regexpguy@xxxxxxxxx)
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