Rick50 Posted November 5, 2022 Share Posted November 5, 2022 Turbofan vs turbojet... in an airliner we have seen turbofans were the way to go for efficiency with thrust (dry thrust since no afterburners)... and the A-10 has turbofans too. But in a fighter jet? This makes me curious: did the Brit Spey Phantoms have better fuel efficiency in normal cruise speeds? Or was it pretty much the same? Maybe the aerodynamic changes resulted in cancelling out a slight turbine efficiency... Link to comment Share on other sites More sharing options...
Tengu Posted November 5, 2022 Share Posted November 5, 2022 @exhausted, I read the Wiki article on the RR Spey, but I'm not sure what that means. As you wrote, the comment suggests there was maybe an issue with overtemping the compressor outlet or other equipment in the vicinity? Still, while compressor outlet temperature would have been high (thinking high hundreds / sub-thousand deg-F), combustion chamber and turbine inlet temperatures would undoubtedly have been a few multiples higher, 2x to 4x compressor outlet temp if I had to guess. So it wouldn't have been a material and cooling technology limitation, although material selection issue is possible. Regardless, this feels more an argument for "we could have extracted more thrust from the Spey, but were bottlenecked". But more thrust is more thrust after all, and the RR Spey's already had about 15% more thrust than the GE J79s, even with this compressor outlet temp comment. Maybe it's a semantic argument, but I'm more inclined to go with @r4y30n and his area-rule comment (as to why Spey-powered Phantoms were slower at the top end). The Spey was a physically larger (bypass) engine than the J-79. They drove more mass flow which required which required wider intakes and trunk. The Spey's were also toed in relative to the plan-view centerline to minimize impacts at the mid-fuselage "waist" and their thrust lines were angled even further below the profile waterline than the already noticeable angled J-79s. (See photo below.) Cross-sectional comparisons of a Spey-powered vs J79-powered Phantom would show the RN / RAF fuselage to be wider and/or deeper for the entire flow path from intake to exhaust nozzle. Spey-powered Phantoms had less optimal thrust lines (angled several degrees off their flight vector) and more drag compared than their USN siblings. The bottom of the intake is level with the wing's leading edge. But moving aft, and greatly aided by the white belly paint, we see that roughly a third of the Spey's engine diameter sits below the wing's trailing edge. If you've never checked out his various blog, Tommy Thomason is an excellent resource. A former aerospace engineer, for McDD, he does wonderful scale drawings and contour comparisons of various subjects. "As it happens, my first job after college was working for McDonnell Aircraft as a junior flight test engineer on the F-4K/M Phantom program." For his detailed comments on the Spey Phantoms, see: Tailhook Topics Drafts: The Spey-Powered Phantom Changes 1 1 Link to comment Share on other sites More sharing options...
rkk01 Posted November 5, 2022 Share Posted November 5, 2022 Go on HB - you know you want to… Link to comment Share on other sites More sharing options...
Omega417 Posted November 6, 2022 Share Posted November 6, 2022 http://tailhooktopics.blogspot.com/2012/05/spey-powered-phantom-changes.html Good Blog about the differences between the Spey powered Phantoms and the OGs. Highly recommend reading his other Phantom posts. 1 Link to comment Share on other sites More sharing options...
Recommended Posts