[AR] Re: [UK OFFICIAL] Moon Express - HTP/kero
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Fri, 14 Jul 2017 17:06:59 -0400 (EDT)
On Fri, 14 Jul 2017, John Dom wrote:
> See the mass spectra in Freeman et al, "Water vapor, whence comest
> thou?"?
Freeman revised his own 1972 Apollo paper in 1991:
https://ntrs.nasa.gov/search.jsp?R=19920030338
Ah, thanks for pointing me to the update. However, while the update
revises his assessment of the cause of the "lunar water event", it doesn't
affect the mass spectra that I was referring to.
But Freeman's "Water vapor, whence comest thou?" title question is odd
because it is simply answered by the above stoichiometry.
Not quite -- his question is about a "water event" that happened a month
after the Apollo 14 crew departed, and was also observed by detectors at
the Apollo 12 site, a couple of hundred km away. This wasn't a direct
observation of LM exhaust, although the source *might* have been adsorbed
exhaust gas in the regolith being released suddenly for some reason.
(More likely, it was something like a descent-stage water tank rupturing.)
Methylated hydrazines (MMH & UDMH or mixtures) would also have CO2 in the
combustion products says Akhmanova et al...
Their assessment is a little oversimplified -- they are thinking of the
products of complete combustion of a stoichiometric mixture, while most
rocket biprop combinations are somewhat fuel-rich, and do not burn
completely because falling density in the nozzle flow largely ends the
reactions while the gas is still quite hot. (This often surprises
non-specialists.) Typically, carbon burns to a mixture of CO and CO2, and
some hydrogen is left unburned while the rest goes to H2O. NTO/hydrazine
exhaust also typically has some NH3 and NO, from incomplete decomposition
of the propellants. (NH3 and NO show up as intermediate products during
the multi-stage decomposition reactions, and both molecules are very
stable and sometimes manage to avoid participating in later steps.)
I remember hydrazine thrusters had to be preheated.
Early ones did. It was the development of cold-start hydrazine catalysts
that caused people to largely abandon HTP... although in fact, I believe
it's still considered preferable to have hydrazine catalysts at least warm
at startup time, because it gives faster and more consistent starts.
Henry
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