Originally Posted by
Rhino power
Underpowered? No...
Spey - 12,250lbs dry, 20,515lbs reheat
J79 (GE-17) - 11,900lbs dry, 17,900lbs reheat
-RP
My first post on pprune, so please bear with me. Unfortunately, those thrust figures can be misleading, as they are uninstalled, sea level static values. Installed thrust, at the flight condition of interest is what matters. The Spey, by its very nature of bypass, has a lower exhaust velocity than the pure jet J-79, so produces less thrust than the J-79 at higher true airspeeds. But you know that already.
Let me cite another example. In the early days of F-16 development, a one-off lower cost version was tried. In place of the 23,800 lb thrust P&W F100 turbofan, a 17,900 lb thrust J-79 was installed. Like the Spey-powered F-4, it also had a revised inlet to match airflow requirements. Here again, the J-79 proved to have a higher maximum speed, easily reaching 2.05 mach, while the standard airplane struggled to hit 2.0. The J-79 had more available, but prudence and politics held it back.