SUPERSONIC FLIGHTS

Press, Volume XCIV, Issue 28053, 22 August 1956, Page 11

SUPERSONIC FLIGHTS

“Heat The Major

Barrier”

(From a Fcuter Correspondent) BERKELEY, (California) Heat is the major, barrier to supersonic flight to outnosts of the universe, according to Dr. Theodore von Karman, chairman of the Advisory Group for Aeranautical Research a nd Development of the North Atlantic Treaty Organisation. He outlined the problem during an address before an International symposium on high temperatures sponsored by the University of California and Stanford Research Institute. Dr. von Karman said the human pilot still has greater ability to withstand supersonic heat than does the mechanical eauipment which will operate guided missiles and space satel-ites. “One may note with some satisfaction.” he said, “that the discomfort for the electronic equipment begins earlier than for the human pilot.” Speeds of 16 times greater than the speed of sound would generate temparatures as high as 15.700 degrees. This was comparable to the 300 degrees generated at the speed four times the speed of sound “Designers have every reason ’to worry about the effect of aerodynamic heating on the structure, the crew and the passengers, and the equipment carried in the aeroplane or missile.” he said.

Dr. von Karman listed these effects of aerodynamic heating on the structure of the aircraft or missile that could be called the “main sources of danger:” (1) Reduction of the elastic moduli with increasing temperature. The effect reduces considerably the resistance of all structural members against buckling and analogous phenomena of structural instability. (2) Reduction of yield point, or ultimate stress, and especially fatigue stress by high temperatures, causing breakdown of the structure.

(3) Increasing rate of creep, which equally reduces the resistance of structural members exposed to buckling, and may concentrate high loading on members which were not designed for such loads. (4) High thermal stresses caused by uneven thermal extension of various structural members. (5) Loss of stiffness, especially torsional stiffness of wings by uneven temperatures.

(6) In extreme cases, melting away of parts, whose melting point was exceeded by high local temperatures. “From the designers point of view,” Dr von Karman said, “the main problem is to determine the temperatures which can be expected in definite flight conditions.” Continuous flight, comparable to a stationary state, established an equilibrium between the heat input and the heat emission, he said; but before that state could be acheived artificial means were necessary to reduce the heat input or to transfer heat from the aeroplane or missile to the surrounding space He suggested these procedures as “possible” means: Reduction of the input. Some favourable effect could be expected from insulation between the external surface and the inner structural elements. Internal cooling by means of an expendable coolant. In such cases the amount of the necessary coolant could be essentially reduced by insulation. Internal cooling with refrigeration of the coolant. As the amount of coolant was reduced, fuel must be expended for refrigeration. Transpiration or sweat cooling, consisting of pumping of a liquid, gas or vapour through a porous skin.

Mass transfer cooling, consisting of a coating which sublimated or chemically dissociated with increasing temperature, thus keeping the temperature under the allowed limit. Dr. von Karman said that satellites should have little difficulty in maintaining their thermal equilibrium during the period of their two hours’ flight around the globe. Some high altitude missiles probably could get along with compartment cooling which could dissipate the heat produced by their own equipment.