[AR] Re: methane to methanol
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Wed, 7 Jun 2017 14:50:09 -0400 (EDT)
On Wed, 7 Jun 2017, John Dom wrote:
NH3 decomposed to H2 (G) results in inert N2 (G) N2 in the mix. I'd
simply go for H2 (L) as the fuel without any ballast N2 (G).
If LH2 wasn't so fluffy that it requires huge heavy insulated tanks
(particularly problematic for small vehicles, where the square-cube law
runs up the surface area per volume), and didn't boil off quite so
enthusiastically even so, and were easier to couple laser energy into, it
would indeed be the preferred choice.
As it is, the N2 makes it much easier to carry and use the H2. And while
it doesn't contribute to combustion energy, it does provide reaction mass
to be propelled by said energy -- because kinetic energy isn't linear with
velocity, you get more impulse per joule by expelling more mass at lower
velocity. NH3 does weigh more, but it can nevertheless come out ahead for
a practical system.
(Isp is about engine performance, but it's *vehicle* performance that
actually delivers payloads, and they are not the same thing. As witness
the fact that LOX/LH2 engines typically get maximum Isp at a mixture ratio
of about 4, but all existing LOX/LH2 stages run at about 6 -- it's not
because their designers were stupid.)
Henry
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