G. R. L. Cowan wrote:
Robert Clark wrote:

G. R. L. Cowan wrote:
...
Hydrogen is one-ninth the mass of the water it is in ...
but if you want to bring water to somewhere it isn't,
9 kg of it in a 0.5-kg tank beats
1 kg of liquid hydrogen in a 15-to-40-kg tank,
even if oxygen is free at the destination.

(Very big liquid hydrogen tanks able to contain
tens or hundreds of tonnes of it can have more favorable
containment-to-payload mass ratios. 15.3 is the lowest I've
heard of at car scale, however.)



The hydrogen won't be in liquid form otherwise I would have no problem
getting the water to liquify. Perhaps mildly pressurized, 4 bar.


4 bar?! That makes the choice (a) carry 9 kg of ready-made water
in a 0.5-kg tank or (b) carry 1 kg of hydrogen in, like, a 1.8-m-dia
spherical tank, maybe 160 kg if walled with 2-mm steel.

But I suppose if you never let the pressure get down near 1 bar,
it can have a tension wall, and not be rigid. Not quite so heavy.


--- G. R. L. Cowan, former hydrogen fan
Boron: internal combustion, nuclear cachet:
http://www.eagle.ca/~gcowan/Paper_for_11th_CHC.html

(Nice boron article. I've been looking for high energy fuels for space
propulsion apps.)

Specialty ultra high strength steels might be able to contain 4 bar of
hydrogen at light weight - especially when you use the fact that
hydrogen is lighter than air.
The wall thickness to radius ratio of a spherical pressurized tank is
given
by:

h/r = Δp/(2σ) ,

where h is the wall thickness, r the radius of the sphere, Δp the
overpressure, and σ the tensile strength of the material.

This page gives the tensile strength of "Maraging steel" as up to 3.5
GPa = 35,000 bar:

Maraging steel
http://en.wikipedia.org/wiki/Maraging_steel

At a hydrogen pressure of only 4 bar, we can use the ideal gas law to
give its density as .32 kg/m^3 at 300K. For a pressure of 4 bar, the
overpressure is 3 bar, so h/r = 3/(2*35,000) = 1/23,333. Take the
radius of the sphere as 1 meter. The mass of hydrogen is the density
times the volume: .32 *(4/3)*Pi*r^3 = 1.34 kg.
For a thin wall the volume of the wall is 4*h*Pi*r^2 = 4*(1/23,333)*Pi
= 1/1856.8 m^3. At a density of maraging steel of 8100 kg/m^3, the mass
of the wall is 8100*(1/1856.8) = 4.36 kg.
The upward buoyant force is the weight of the displaced fluid, air in
this case. Using the density of air at sea level of 1.2 kg/m^3 the mass
of air that would be contained in a sphere of radius 1 meter is
1.2*(4/3)*Pi = 5.03 kg. So the upward force is (5.03 kg )*(9.8 m/s^2) =
49.3 N.
The total mass of the hydrogen and the steel container is 1.34 + 4.36
= 5.7 kg. This would have a weight of 5.7*9.8 = 55.86 N. Then because
of the buoyant force it would feel like it had a weight of 6.56 N if
you were lifting it, i.e., it would feel like you were lifting a mass
of only 6.56/9.8 = .67 kg.
This compares to the mass of water of 9*1.34 = 12.06 kg that would be
produced.
Of course, for accelerating it you would have to use the true mass of
5.7 kg total mass of the hydrogen and steel.


Bob Clark