[AR] Re: Nuclear Hydrogen Thruster
- From: Craig Fink <webegood@xxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Thu, 14 Jun 2018 09:40:41 -0500
On Wed, Jun 13, 2018 at 12:55 PM Henry Spencer <hspencer@xxxxxxxxxxxxx>
wrote:
On Mon, 11 Jun 2018, Craig Fink wrote:
Propellant is shelf stable solid tankless Hydrogen at 3K possibly.
Careful here. Despite what some people have thought in the past -- see
the Enzmann starship design in particular -- solid hydrogen is *not* a
hard strong solid that can reasonably be stored in the open if it's kept
cold. Physically it's more like warm butter than water ice.
Fusion ship, taking the extreme a bit further into the Future than
arc-jets. If you could elaborate a bit about why solid Duterium was a
problem with Enzmann's starship?
Figure 39 on Page 5-42 of the reference I gave has the physical properties
at the temperatures we are discussing. And the yield stress is about 10%
it's tensile strength at 3 K. Just like anything that is solid, if you
want it to maintain it's shape you have to stay below it's yield stress so
that deformations are elastic. So, while nominal yield stress is low, it
will structurally maintain it's shape if not asked to carry loads. With
less than 10% elongation at fracture at 3K maybe "cold" butter would be
more descriptive. The "warm" buttery version at 10K and above won't exist
in a vacuum as the hydrogen will sublime and cool itself down.
(Hydrogen and helium are sometimes called the "quantum gases", with neon a
borderline member of the family, because quantum-mechanical issues are
quite significant to their properties, so they don't behave like lighter
versions of more mundane gases. Liquid hydrogen is sort of halfway to
being a gas -- very low density and viscosity, noticeably compressible --
and solid hydrogen is sort of halfway to being a liquid.)
"Quantum Gases" and quantum-mechanical issues? Could you be more specific
about what your talking about?
The issue about storage without a container really has to do with the
physical property of "vapor pressure". Will the solid hydrogen sublime away
or not. Figure 44 on Page 5-48 of the reference I gave shows vapor pressure
are off the chart on the low side at 3 K for both "Normal" and "Para"
Hydrogen. Solid hydrogen does have a temperature limit where it should be
put back in a bottle to warm up, pick your acceptable vapor pressure and
read the chart.
If an extreme case like Hydrogen can reasonably be stored tankless in
space, then other propellants are even more reasonable to do so. And, may
actually happen after we transition from this one-shot disposable missions
built and launched from Earth to missions that are manufactured and
launched from orbit. The tank is necessary when starting from Earth, but
apparently not Space.
Figure 24 on Page 5-25, the speed of sound of Hydrogen is really quite high
at 3000K, missing the weight of the Neutron. At 3000K with no diverging
nozzle or engine bell, a hydrogen engine with just converging nozzle and
Mach=1 (4 km/s) at the exit will outperform an SSME.
--
Craig Fink
WeBeGood@xxxxxxxxx
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