by Tommy H. Thomason

Tuesday, December 18, 2012

J79 Exhaust Nozzles

There are two different J79 afterburner nozzles, "short" and "long." The -8 engine in the F-4B/N, almost all of the Blue Angels F-4Js, A3J-1s, and some RA-5Cs; and the -15 in the F-4C/D have the short nozzle. The -10 engine in the F-4J/S (except for most of the Blue Angels F-4Js) and some RA-5Cs; and the -17 in the F-4E have the long nozzle.

Dave Aungst created a good visual differentiation of the two nozzles from kit parts:

I'll refer to the "non-moving ring" as the "collar." However, it turns out that the ring/collar does move. Also, although his illustration leaves the impression that the -8 engine is notably shorter than the -10, the engines are about the same length.*

Since these two parts are for F-4 Phantom kits, the difference in length of the afterburners shown above is primarily the result of a small difference in the airframe structure if it's represented correctly on the kit: a short versus wide end to the fuselage structure at the opening for the afterburner nozzle. Note that the some of the difference relative to the "step" in the heat-resistant panels in the pictures below may be caused by the angle from which each picture was taken but according to a pretty-good McDonnell lines drawing, the aft end of the -10/17 nozzle does extend 1.28" aft of the -8/15 nozzle's. (The difference in the width of the sheet metal at the forward end of the nozzle is about 3".)

The location of the end of the nozzle relative to the airframe is very inconsistent on static display F-4s and RA-5Cs. This appears to be more common than I would have thought, and even occurs on the same aircraft.

(Picture by John B.)

Some static display airplanes also have the nozzles closed down, which makes the -8/15 nozzle very different in appearance (and much shorter) and the -10/17, a bit shorter. However, the nozzle was only in this position when the engine was running and not in afterburner.

A closed-down -8/15 nozzle is rare (actually, very unlikely) on flightworthy F-4s and RA-5Cs when they are shutdown.
The gray rectangle on the -8/15 drawing is the location of the exposed portion of the -10/17 nozzle. The forward exposed end of the -10/17 nozzle is farther forward as shown in the photos above. Also note the movement of the collar when the -8/15 nozzle closes down for non-afterburning operation. The movement of the collar into and out of the fuselage opening results in some "polishing" of it.
 Craig Kaston Photo

The interior of the two different nozzles is very different (the one on the left is in a museum aircraft and appears to be located much farther forward than it should be, not to mention a screen has been installed to keep birds and critters out):

The -8/15 nozzle has longitudinal ribs on the inside of the petals whereas the -10/17 nozzle has a relatively smooth inner surface. The difference in ejectors is depicted in a Rockwell International maintenance publication:

The presence of the -8 versus the -10 in the RA-5C is much less obvious than shown above in most pictures because the collar is in the shadow of the horizontal stabilizer. As with the F-4, it appears to me that the end of both nozzles is very close to the same location relative to the airframe fairing above the nozzle. (Again, static display airplanes, particularly the RA-5C at the Pima Museum, can't be trusted in this regard.) According to Craig Kaston's research, the -10 was installed in RA-5C BuNos 156608-156643 and the same airframe change (AFC-328) was used to modify nine existing RA-5Cs of early vintage: 146702, 149276, 149287, 149298-9, 149301. 150824, 150831, and 151630. The incorporation of the change is apparent by the difference in the sides of the engine inlet and the extension of the wing inboard leading edge to the front of the inlet.

*A now defunct GE webpage listed the -8 and -10 as having exactly the same length. It may be that the -10 engine was installed slightly farther aft to help resolve a forward cg problem. One stated but perhaps apocryphal reason for the Navy F-4s not getting a nose-mounted gun like the Air Force F-4E is that it would have moved the empty cg too far forward in light of the Navy's requirement for relatively low-speed takeoffs and landings.

Thanks to Craig Kaston for his observation of the -8/15 collar position and photo and to Mark Nankivil for the pretty good McDonnell drawing that I used to depict the nozzle position.


  1. In my opinion regarding the (Picture by John B.) most of the difference in nozzle length is due to the left nozzle being more fully closed and the angle of the photo.

  2. Possibly and it is true that the nozzle will shorten slightly when the nozzle closes. But note that the "bottom" of the pedals is lower as indicated by the red line at the bottom of the picture. I don't think that the hinge line of the pedals moves forward (down in this instance) as the nozzle closes.

  3. The development history of the J79 nozzle is told in SAE680295 "Variable-geometry exhaust nozzles and their effects on airplane performance". It shows the shroud translation on the early nozzle and no translation of the forward hinge on the guided expansion nozzle.
    It also describes the Low base Drag Nozzle on the B-58 and the fully variable (ie independant primary and secondary area control) on the J93 in the XB-70.

    1. I meant also to add that the information in the paper makes it great value for $25. The superb photos in Tommy's topic are a good complement to the descriptions in the paper.

  4. Ref the -10/17 nozzle interior photo, just discernable are slots in the sealing flaps inner surfaces. They show more clearly in this Hellenic Modellers photo

    Of note is that the circumferential slots will still be partially open at cruise whereas the pairs of longitudinal slots are closed completely. Thanks to SAE680295 we know that this 'ventilated seal' arrangement at AB nozzle positions on the -17 nozzle had its origins in similar features to prevent instability in the J79-5 nozzle (B58). The circum slots and 'ventilated seals' also provided the necessary secondary bypass flow - higher at high supersonic flight speeds.
    More details on this instability phenomenon and its ventilated slot solution can be found in this GE Patent

  5. Just a little addition to the comment from above about the closed nozzle position: "However, the nozzle was only in this position when the engine was running and not in afterburner.". The -10's that I worked on in the RA5C had 4 stages of AB, primary core, primary annulus, secondary core and secondary annulus. When you went around the horn(military stop) the engine would go in to the first stage, primary core and the nozzle would still be mostly closed, as you pushed the throttle further forward you would increase thru the stages to full AB, stage 4 and the nozzles would be pretty far open, all controlled by EGT. At least that is what we, the ADJ's would attempt to adjust and trim to. In combat, especially when taking the PBDA (Post Bomb Damage Assessment) photos the pilots would go into stage 1 AB to eliminate the smoke trail which made it harder for the gunners to track them. I still remember the sweet smell of ionized air working aft of the island during the pre-dawn launch when a J-79 went into full AB against the blast deflectors. (at full AB the aircraft would also be burning about 3 gallons of full per second)

  6. HI