Any "new" jet engines with plug nozzles?

[Kryptid]

I can't think of very many jet engines, particularly contemporary ones, that utilize the plug nozzle configuration (in contrast to the ejector or iris configurations). Why is that? Are plug nozzles inherently inferior? What are their advantages and disadvantages versus other types?

I was thinking that a plug nozzle could be good for stealth. The plug could help shield the turbine section from a prying radar in the rear hemisphere. The plug could even be "chined" in the typical low-observable manner to further reduce RCS. Also, since the cross-section of the flow is changed internally by translating the plug fore-and-aft, you could make the external geometry of the nozzle fixed (a fixed nozzle could also be stealthier than a moving nozzle due to fewer edges and discontinuities).

 

[saintkatanalegacy]

The conical plug nozzle shown in figure 4-4(c) is a more recent idea than the divergent ejectors and has not yet been used on a supersonic production engine. The outer boundary of the divergent shroud is replaced by a free streamline that can adjust automatically with changes in nozzle pressure ratio and thus maintains a high thrust efficiency over a wide range of expansion. The ability of the plug nozzle to be "altitude compensating" and to eliminate overexpansion losses has been demonstrated experimentally.
If it is to be used in an afterburning engine, the plug nozzle must include some way to vary the primary area. Various schemes to vary the throat area include the standard iris flap, translation of a fixed flap or plug, and the variable-area centerbody . At high speeds internal expansion occurs in the annular flow passage between the plug and the outer shroud. The internal expansion ratio can be varied by translating the outer shroud. At low speeds the shroud is completely retracted so that the flow is not overexpanded and the primary flap is exposed to the external flow. Because low-angle plugs (half angles of 10°) tend to be long, it may be desirable to truncate some portion of the plug.
One of the main problems of a plug nozzle applied to an afterburning engine is the method of cooling the plug and its support struts, or sting. Severa1 cooling concepts have been proposed, as shown in figure 4-5 , and include convection, film , and transpiration cooling. A completely convection cooled plug (fig. 4-5(a)) may also discharge its cooling flow in the base of a truncated plug to reduce the plug base drag. From an overall performance consideration it may be better to convectively cool part of the plug and to film cool the rest (fig. 4-5(b)). It is desirable to obtain the maximum thrust from the cooling flow. A transpiration-cooled plug is also of interest (fig. 4-5(c)), where the plug would be fabricated from a porous material and the cooling air blown through.
If the cooling problem can be solved, the plug nozzle has some distinct advantages. First, it would not leak as much as a nozzle with flaps and seals, since the length of seal between the movable surfaces (;ould be decreased by an order of magnitude. In addition, the actuation mechanisms appear to be simpler and might be more durable. Finally, some jet noise tests also indicate that annular flow is inherently a little quieter than an equivalent circular jet.

link page 27

 

[Kryptid]

Many thanks!


I was thinking that cooling might be harder for it than for a conventional nozzle, but on the same hand I figured that if the turbine blades could be sufficiently cooled, then so should a plug if it were made of the same materials, etc.