one-third
6
.
Because of the extreme temperatures generated by the atmosphere fric-
tion, a hypersonic craft will also require complicated cooling measures. One
possibility is using cryogenic fuels, such as liquid hydrogen, as both coolants
7
and propellants. The fuel flowing through the aircraft's skin would cool the sur-
faces as it vaporizes before being injected into combustion chamber.
In addition, specialists in many countries are currently working on new
propeller engines considered much more economical and less noisy than jets.
The only disadvantage is that propeller planes fly slower than jet planes. How-
ever, it has recently been announced that specialists succeeded in
8
solving this
problem. As a result a ventilator engine with a propeller often fibre-glass blades
has been built, each being five meters long. It will be mounted in the experimen-
tal passenger plane.