Hi Lyman,
At what point does Abernethy's "PARTIAL RAMJET" become a true RJ engine?
I have to quantify partial ramjet in some more concrete terms just for myself first.
I would choose to focus on the rapidly diminishing but still remaining residual contribution of the compressor to the overall installation Pressure ratio.
For the SR-71 at cruise, Intake
PR 38.8, engine
PR 2.9, overall
PR 112. Numbers from Peter Law's presentations on AEHS website.
The engine contribution still remaining is only 2.6% of the overall. It's almost a ramjet,
unless the ramjet had not yet been invented in which case it's a turbojet at very high Mach number.
This definition also applies to the XB-70, for example. It must also be a partial ramjet.
This definition is, I think, intimated by Col Graham "the faster you flew, the more it became a ramjet, utilising the high mach air to augment the thrust of the engines."
However, since the generally accepted term, partial ramjet, applies only to the SR71 I have to, for my own piece of mind, try and understand the definition given by Bob Abernethy. I think it basically revolves around the same criterion, ie diminishing involvement of the compressor. But, instead of a gradual diminution from the compressor with rising flight speed, as in the above definition, we have a sudden diminution when the bleed is opened, but not because the pressure ratio contribution has suddenly dropped. Instead the flow has suddenly increased (see the patent maps) from bypassing some of the compressor. This map shows no significant change in
PR and if it was significant to the story I'm sure it would have been highlighted as have all the other effects.
Using the term partial ramjet does not necessarily add value to the picture.
After all, if the ramjet had not yet been invented we would say "it is an afterburning turbojet with bypass bleed" and it, by definition, behaves like one.
Since the ramjet has been invented and due to it's perceived extreme simplicity, it can be an aid to understanding and, I believe, that's why the term is so widely used. It's taken me a long time to come to this conclusion but at last have piece of mind. I am no longer irritated with the term.
Brian
re the 20 to 40% bleed figure
20% is the right number because Bob Abernethy was quoting all his stuff at the design point. In the definitive memo, which really deserves framing on the wall (well mine anyway), he says "...at both Mach3 and Mach3.5 with 20% bleed." Not wishing to dismiss the 40% I would be interested in more details.
The Flight Manual information,I believe, shows just a 2 position Bleed, closed or open. Thus when open it was a fixed orifice and as such controls to pretty much a fixed percentage irregardless of the actual mass flows.