Gentlemen
The decision to phase out the SHAR AMRAAM capability before there is a replacement available must have been a difficult one given the financial, political and technical issues that had to be considered.
I only feel competent to comment on one facet of the technical issues involved – the weight limit on a VL in hot ambient conditions.
It is a pity that people talk about whether one or two (or more) AMRAAMS can be brought back in high OATs because the technical fact of the matter is that a great deal more load than that can be accommodated without modification to the aircraft.
Allow me to explain – and my apologies to those who already understand these matters because it will not be a short explanation if it is to leave any reader able to think for themselves about this topic.
By the standards of today the Pegasus is a pretty basic low bypass and low tech turbofan engine whose design was determined in the 1960s. It uses turbine blades cooled by pumping ‘cool’ air through internal passages to save them reaching temperatures that would physically damage them. There is a JPT sensor that puts on a red (primary) warning in the cockpit if a certain JPT is exceeded. If this warning is tripped it requires the engine to be removed and the hot end inspected for damage. I seem to remember this warning was set at 760 but it may have been 780.
What I am certain about is that the temperature of this warning could not be reached with the engine running. It could only be reached if the engine surged (which reduced the compressor delivery pressure of the cooling air to the turbine and so the blades were rapidly burnt away) or on a ‘hot’ start where the JPT was allowed to rise out of control before the compressor was able to deliver its cooling air. Goodness me we tried hard enough to cook the backend on more than one occasion in the early days before a JPT limiter was fitted. (Yes that is right the original development batch Harriers did NOT have JPTLs…..).
One way to break the engine however was to overspeed it so that the fan just flew apart - the fan being the weakest component when it came to rotational loads. As a tp in those days I rather naturally developed a close relationship with John Dale the engine chief designer at that time. In fact the Kingston and Dunsold team led by airframe chief designer John Fozard had monthly meetings for many years with their opposite numbers at Bristol led by John Dale and I was privileged to attend most of them.
Be clear - I am asserting that the only way to break the Pegasus while it is running out of surge is to overspeed it. Because the RPM at which the fan would burst was far from certain I can only say that it was more than 110% as I reached that without it happening at a time when the service release allowed 107%. John Dale said to me that I would be OK to 110 but he was not sure after that – and I for one believed him.
So much for the circumstances that will bust the Pegasus.
When it comes to deciding service limits of RPM and JPT we need to do so much more than just stop the donk going bang. We have to agree a service LIFE.
So at last we have come to the SHAR problem of today. The JPT limits for the engine that is in the SHAR were set back in the late 60s and they were set by ministry men who wanted more life for the same purchase price and engine men who did not want a warranty problem. So the short lift dry JPT limit was set at 715. Nobody ever envisaged that this would in any way bother Harrier pilots in 2005 who wanted to go to the hottest parts of the world and commit themselves to a deck VL.
Thirty years ago I used to brief my mates who were worried about being forced into an ‘overweight’ VL to just turn the limiter off, turn on the water switch (whether or not they had water as this reset the RPM governor to 107) and go about their business without looking at the JPT gauge. They did need to make sure they were not running out of RPM but that was all. And yes the JENGO might comment about a few extra counts on the engine life recorder – but really so what?
Today’s operators/ministry men/airworthiness ‘experts’ and military staffs just know what they are told – namely that they should not exceed the JPT limit published in the book. And they believe it because they know no better. But why should they? Most were not born when the trade of life versus performance was agreed.
Pity eh?
PS The engine in the GR7A and GR9A has no JPT limits only RPM ones. But that was agreed after it became clear that a JPT limit was a hell of a bind in some circumstances.
PPS In my view it may be reasonable to phase out the SHAR for many reasons but not because it cannot bring back AMRAAMS. If you really believe that you don’t understand the Pegasus.
PPPS SHAR pilots will appreciate that turning off the limiter before hand is a much better thing to do that going ‘through’ the limiter once it has cut back the thrust and the aircraft is on its way down thus calling for extra thrust to arrest the descent.
JF