Why does the TF-30 engines on the A have such weirdly shaped exhaust nozzles?

[WelshZeCorgi]

As title says, why they look like a truncated bell? (when extended)

[WarthogOsl]

It narrows down at lower power to match the ambient air pressure.  Most jet fighter nozzles do this, but they are covered with external "turkey feathers" that mask this appearance.  One exception is the F-15C which doesn't have these covering flaps.  Here's one with the nozzle closed down.

Edited July 23, 2022 by WarthogOsl

[lunaticfringe]

23 minutes ago, WarthogOsl said:

It narrows down at lower power to match the ambient air pressure.  Most jet fighter nozzles do this, but they are covered with external "turkey feathers" that mask this appearance.  One exception is the F-15C which doesn't have these covering flaps.  Here's one with the nozzle closed down.

 

The F-15A and C did, in fact, come with them installed on their F100s originally.  The USAF found them to be a maintenance issue, especially in Europe, and accepted the drag penalty induced by their removal.  You can validate this by looking at line aircraft upon delivery back in the day, as well as the test aircraft, and then soon after having the covers gone- whether by maintenance after delivery, or gone from the factory. 

Conversely, you'll note their installation on 15As, Cs and Es of the IAF in many instances.  The difference, while marginal, is considered worth the effort for some users and applications.

Edited July 23, 2022 by lunaticfringe

[r4y30n]

The nozzle on the A was one of the first examples of a convergent-divergent nozzle, as opposed to the ejector nozzles on earlier turbojets up through the TF-30s on the F-111. Though it may not look it, the A’s nozzle is the intermediary step between something like the F-4’s J79 and modern nozzles like the B’s F110s.

There’s a white paper titled “F-14A Installed Nozzle Performance” that goes into detail about the lower drag and higher thrust versus earlier designs. It also eliminated the need to tune the nozzle in full scale flight testing, as was common previously.

At the end of the day, it functions the same as nozzles on newer fighters. The only difference is how the petals move, sliding forward and backward on tracks as opposed to pivoting on hinges.

[SkateZilla]

Similar Nozzle movement on the F-100D (Pre Nozzle Replacement).