From my vantage point (sharing the test site with the PCB Harrier rig whilst working on more modest Gem engines) I can offer up my (irrelevant?) opinions of PCB in Harrier.
The plenum chamber burning concept actually ran during the mid eighties although I’m sure it was ex BS staff that pushed the original sixties BS concept with the MoD even though it appeared to have limited applications by the time it was tested.
In terms of stealth the Harrier has always had relatively poor IR characteristics, although not burdened with an afterburner, the position of the aft nozzles relative to the airframe results in a large area of (exhaust plume) washed structure that is elevated to reasonably high temperatures increasing the IR signature in terms of level and viewing aspect (certainly more of an issue today when considering 8-14 micron IR systems). This is just common sense and can be deduced purely from the engine/aircraft geometry and a minor grasp of Planck’s Law etc (it is no criticism of a sixties design working in the 21st century). Also, as anyone who has been involved in helicopter exhaust design can tell you, even modest temperature rises can have significant impact on structural life.
The front nozzles of the PCB Harrier were fitted with the reheat system so if ever the design had evolved to a flight demonstrator the material spec for structure aft of the nozzles would have been interesting. And IR stealth characteristics would have gone out the window.
The rig did demonstrate that hot gas recirculation was an issue (although in all honesty that surely should have been flagged as a very high risk at the design phase by having the hot nozzles so close to the intake) and naturally like the std Harrier the JSF aircraft have been designed to have the hot gas flow as far away from the intakes as possible (as featured in the C4 programme). No doubt some RR individual will explain that the rig allowed a useful understanding of hot gas management in the near field but the concept did appear awfully aircraft specific.
The other issue of using reheat as lift comes with ground erosion and the higher the gas velocities the more damage the ground experiences. This can even be a problem with proper runways so imagine the effect on a semi prepared strip and all the potential FOD hazards. (I believe that even the V22 has a potential ground erosion issue when the exhausts are in the vertical position). In short there will be a point at which lift thrust can not be increased by raising the jet velocity but mass flow will have to be increased instead (LM/BAE lift fan for example). I also seem to remember that the far field was also affect in terms of v.high ambient temperatures which I guess would dictate the distance that ground crew could operate in the vicinity of the aircraft.
On an aside – did anyone think that the C4 JSF programmes were “engineering-lite”? Certainly from the flight test aspect there was very little said about the massive telemetry / instrumentation support and how these were probably more critical for monitoring in-flight safety than the chase aircraft. Plus it used my least favourite phrase – “designed by computers” – that’s a bit like saying that the Harrier was designed by an HB pencil - rather than by engineers.
lhb