CO 757 200 From , joined Dec 1969, posts, RR: Posted (11 years 3 months 2 weeks 5 days 22 hours ago) and read 2809 times:
It is my understanding that the Volvo Flygmotor turbofans that power the Saab Draken of the Sweedish Air Force are actually modified Pratt and Whitney JT8D-22 turbofans, the same very turbines that power the 727. Obviously,it had to go through major modifications to make it suitable for the fighter mission since what it has to do in a Draken is way different than it's mission and performance in a commercial airliner such as the 727.
Anyone have any knowledge on the specific modifications done to the turbine to make it suitable for fighter ops?
Broke From United States of America, joined Apr 2002, 1322 posts, RR: 3 Reply 6, posted (11 years 3 months 2 weeks 3 days 3 hours ago) and read 2450 times:
The JT8D uses titanium alloy blades throughout the compressor in its civil configuration. This is not so in the -22. At Mach 2, the compressor inlet temperature is about 250 degrees F, a typical temperature rise, at maximum engine power, for each stage of compression is about 50 degrees F. Titanium loses its strength quickly above 800 degrees F.
800-250 = 550 (temperature rise in the compressor to reach 800 F) 550/50 = 11 (maximum number of compressor stages needed to reach 800 F)
The low pressure compressor has about 8 stages (counting the fan)
The high pressure compressor has about 6 more.
The high compressor blades would have to be fabricated from steel. You would do the whole stage to give you a safety margin and to give you equal thermal expansion throughout the compressor.