On Sun, 26 Jan 2003, Sean Lynch wrote:
> I think I misread what they were saying and assumed that "elemental
> molecules" meant the atoms...
Right. The zero point for such numbers is arbitrary, so it's taken to be
the most usual form of the pure element, which for hydrogen is H2, not H.
(H might be a more elegant zero point, but it's seldom encountered in
normal chemistry, so the conversion from there to H2 would be a constant
nuisance.)
Also, the minus sign is a giveaway: that means heat is *released* in
formation of the form in question, which most emphatically is not the case
for formation of H from H2. The heat of formation of H is +218kJ/mol
(Hill&Peterson2 p.42), and that's per mole of H (i.e. double it for one
mole of H2 turning into two moles of H).
If (dim) memory serves, expected Isp for pure H -- assuming you can get
Harry Potter to wave a magic wand and stabilize it for you :-) -- is
about 1500s. That's getting up into gas-core-nuclear territory...
An amusing sidelight on this is that in one or two of Heinlein's 1950s
novels, the solid-core nuclear-thermal rockets run on "stabilized
monatomic hydrogen". Well, that does give a nice low molecular weight,
all right. *But*... No matter what trick you're using to stabilize the
stuff, almost certainly it will come unglued at NTR temperatures. That
means the first thing that will happen, when you fire up your NTR, is that
the reactor assembly will melt and wash out the tailpipe (no matter what
it's made of). Which will improve engine performance, since it was just
getting in the way...
Henry Spencer
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