James Robinson wrote:

You are correct that the hydrogen itself is very light for each unit of
energy. The problem is the weight of the container necessary to hold
that energy.

Hydrogen molecules are the smallest in nature, and as such, they will
more easily pass through container walls than other types of energy
source. That means any containers have to be more complicated than a
simple tank that holds fuel like avgas. They are typically built with a
number of layers to not only resist impact and rupture, but they also
have to be thick enough to keep hydrogen leakage to a minimum.

If made with steel, they have to be extra thick because hydrogen affects
the physical properties of steel over time, making the steel more
brittle. This hydrogen embrittlement means that any steel vessels have
to be extra thick to retain their ability to resist impacts. Layers of
other materials can help, but not eliminate the problem.

These requirements mean that the containers will necessarily be heavy.
Further, the containers typically have to be cylindrical in shape,
because of the pressures involved. Finding convenient places to put
cylinders in an aircraft would be a challenge.

I was aware of the difficulties in containing the hydrogen, but I was of
the impression that the weight of the container was more than
compensated by the energy to weight ratio of the liquid hydrogen. If
not, why would they use it on the space shuttle? That's not to say that
what works on the space shuttle will work well in an airplane. The
shuttle can be fulled shortly before it is launched, and then burns off
all of it's fuel in just a few minutes. I assume that would allow for
the tank to be lighter than it would have to be if it need to store the
hydrogen for a long period. I also know that things that work on a
large scale often don't work very well on a smaller scale.


--
Chris W
KE5GIX

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