Venik wrote in message

And some comments about the comments in this thread. MiG-25 is not made
of stainless steel but of nickel steel alloy similar in composition to
the nickel alloy used for X-15. The Valkyrie, on the other hand, was
made of predominantly stainless steel.

Yet more comments about steel. Steel can be referred by chemical
composition like nickel steel, chromium-nickel, etc or by it's
intended purpose or property and that is stainless. Any steel
containing 5% nickel or more is considered to be resistant to
atmospheric corrosion in addition most nickel steels contain chromium
in similar quantity as nickel and that is another element that makes
the steel stainless.
On the other hand nickel which is very heavy and expensive is used
only in parts that are subjected to very high temperatures like
turbines of jet engines and alloys in use in aviation do not contain
iron in considerable quantities. North American X-15 was used to test
the effects of re-entry in the atmosphere and was subjected to extreme
heating. The aircraft structure
was built of titanium and stainless steel and nickel alloy InconelX
was used
only for skin panels, with nickel steel used in the areas less
subjected to
heat.
So stainless steel was used widely in the '50 and '60 designs all
around the
world and materials should not be used to prove that one design is
superior over another where flight characteristics, equipment and
mantainability meter
the most.

Nemanja Vukicevic
aeronautical engineering student