[AR] Re: Nuclear Hydrogen Thruster
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Sat, 16 Jun 2018 15:04:09 -0400 (EDT)
On Thu, 14 Jun 2018, Craig Fink wrote:
If you could elaborate a bit about why solid Duterium was a problem with
Enzmann's starship?
The original design had the starship proper pushing a huge ball of
deuterium ice, that being the fuel. Structurally that just doesn't work,
given the low strength of solid hydrogen. Later revisions reluctantly
accepted the need to enclose the fuel in a tank.
(Hydrogen and helium are sometimes called the "quantum gases"...
because quantum-mechanical issues are quite significant to their
properties, so they don't behave like lighter versions of more
mundane gases.
"Quantum Gases" and quantum-mechanical issues? Could you be more specific
about what your talking about?
The properties of many gases look like scaled versions of each other --
apply the right scaling factor and the graphs look nearly identical. But
hydrogen and helium (and sometimes neon) are oddballs that don't follow
the standard pattern. Helium is the most spectacular example, e.g. at
1_atm it has no freezing point -- it's still liquid even at just a hair
above absolute zero.
The heavier gases have enough nucleons in the nucleus, and enough
electrons orbiting it, that their quantum properties tend to sort of
average out into a more classical blur. Not so with hydrogen and helium,
where quantum effects remain quite visible. For example, the spins of the
two protons in H2 can be in the same direction ("ortho" hydrogen) or
opposite directions ("para" hydrogen), and the two forms have
significantly different properties -- vapor pressure, density, triple
point, specific heat, thermal conductivity, etc. are all noticeably
different. Room-temperature hydrogen at equilibrium ("normal" hydrogen)
is 75% ortho, and if you liquify it you get liquid 75%-ortho, which then
proceeds to boil away because at LH2 temperatures the equilibrium
composition is nearly 100% para, and the ortho->para transition releases
quite a bit of heat. (The fix is to have your liquifier incorporate a
catalyst that speeds up the transition, and take out the released energy
as part of liquifying.)
Henry
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