In supersonic flight, the induced high environmental temperature restricts the dissipation of waste heat. Although fuel may be used as a heat sink, its thermal stability properties are limited and significantly higher lubricant temperatures than in subsonic practice are inevitable. In 1959 a provisional British Specification D. Eng. R. D. 2497 was issued requiring a material with a load carrying capacity equal to that of the reference oil to the British subsonic synthetic lubricant specification D. Eng. R. D. 2487 at some 90 F higher temperatures.
Rigs were developed to determine lubricant performance in a bearing at a high lubricant temperature, etc. Over 40 submissions from many manufacturers have now been evaluated. Engine tests have confirmed the validity of the rig test methods and it is confidently expected that the materials approved for engine evaluation will prove adequate for service use.