In article .com,
wrote:
They required very thin layers of exotic metals
that were difficult to work with and tended to be brittle;

Where's the requirement for "very thin" layers come from? Just weight
requirements alone?

Correct. The high-temperature metals, unfortunately, tend to be really
dense. Things like niobium and molybdenum are nearly as dense as lead;
tantalum and tungsten are substantially *denser* than lead. They have to
be kept thin or the weight just goes out of sight.
--
No, the devil isn't in the details. | Henry Spencer
The devil is in the *assumptions*. |