Hi Brian, 20% appears in the internal P&W memo written by Bob Abernethy when he had solved the challenge of converting the j58 from a Mach 2 engine into Mach3.
http://www.bobabernethy.com/pdfs/Nev...0of%20P&W3.pdf
It also appears in his patent
Patent US3344606 - RECOVER BLEED AIR TURBOJET - Google Patents
The whole story is in the patent. Basically the problem revolved around getting the compressor to behave at M3 cruise, ie 100% mechanical N1 and M3 CIT. This condition for the compressor is similar to starting the engine to idle on the ground, ie same low corrected N1. Just as this engine, like others, needed a start bleed to get to idle so it needed a similar bleed at cruise (because it was the same regime on the compressor map), which turned out to be 20%. The bleed was then available to put into the ab to give additional benefits.
The patent is arguably the best primer I have ever come across anywhere for explaining how jet engine compressors suffer from CIT at high supersonic speeds and what to do about it.
six very large pipes
They may look big for 20% but would have been overlarge to reduce flow losses. The 20% would have been controlled at the compressor case to prevent excess bleed if a pipe cracked.
"turbo ramjet", the term was used by Lockheed and NASA people, the principle behind the term, and what it meant, being well understood
This is a hard nut to crack but only because it is steeped in folklore. With all what was once secret info now available on the web we now know exactly what the engine was and can even deduce it for ourselves by reading the patent for example or the Flight Manual. We no longer have to rely on descriptions from years ago.
Again as spelled out in the patent, a turbo ramjet was one possible solution to get to M3. It was not adopted. The engineer given the task of 'fixing' the engine says in his patent why not. Another would have been variable stators. That's how the XB-70 engines got there.
Few people at Lockheed, NASA or P&W would have been privy to what were secrets back in those days. The facts are all laid bare for us today and we are better informed. They were wrong as stated in the patent. The bleed bypass was a much more cost effective solution due to its simplicity.
BTW Ben Rich called it a bleed bypass turbojet in "F-12 Series Aircraft Aerodynamic and Thermodynamic Design in Retrospect".
With all the fine details available to everyone we basically have to forget all the descriptions from years ago and start again with the facts.
Interesting stuff. Cheers.