Aviation Ancestry - December 2012
From “Gusto” to “Oxcart” – Part III
The Pratt & Whitney J58’s massive after burner; super compressed and heated air from the inlets was routed into these after burners in order to provide thrust for sustained Mach 3 flight. (Scott Schwartz)
By Scott Schwartz
Having been intended for a Navy aircraft that was canceled, the Pratt & Whitney J58 had already been run for roughly seven hundred hours prior to the program’s cancellation. The 26,000 pounds of thrust generated by this engine would have enabled the canceled Navy airplane to hit Mach 3 for a short burst – a few seconds at most. Of course, the Blackbird would require its engines to run with their afterburners on for long periods of time – enabling cruise speeds of Mach 3 at 100,000 feet.
Well, once all of the necessary modifications were made to the J58, the engine was generating 32,500 pounds of thrust. The reader should ponder this for a moment. By the time Skunk Works engineers were through altering the J58 engine, it was the most powerful non-rocket engine ever built. What’s more, the J58 was the first jet engine capable of running its afterburners continuously – burning 8,000 gallons of fuel per hour in the process.
Moving a maximum of 26 inches fore and aft, the inlet cones, or “spikes” regulated the flow of air entering the engine inlets-more or less acting as a throttle. An analog computer moved the spikes based on the aircraft’s speed, attitude, and pitch. (Scott Schwartz)
As powerful as its engines were, by themselves, they generated only 25 percent of the thrust that was needed for the aircraft to cruise at Mach 3. Another issue was the need to slow the Mach 3 air down to sub-sonic speeds so that it could be fed into the engines’ compressors. The latter problem was solved through the use of moveable cones or “spikes” at the fronts of the engine nacelles. Sensing the aircraft’s pitch, roll, and yaw, a computer (of the analog variety) retracted the spikes up to a maximum of 26 inches. Thus, the spikes became throttles – opening and closing, and in so doing, varying the amount of air entering the engines inlets in relation to the aircrafts speed and altitude. So, what did all this have to do with obtaining more thrust from the J58 engines? As the aircraft’s speed increased, the air entering the inlets was compressed and heated as it passed through the narrow openings in the inlets. This super-compressed and super-heated air was directed into the engines’ compressor stages, which, well, compressed the air even further, before it passed through the turbines and into the afterburners. With the inlets providing 64 percent of the engine’s thrust at full throttle, this system rendered the J58 engine a hybrid ramjet/turbojet engine that reached 84 percent propulsion efficiency at Mach 3. At the time, this was 20 percent better than any supersonic propulsion system ever built.
Naturally, the complexities involved with the development of the J58 propulsion system, as well as with the aircraft as a whole, led to delays. In addition, flight tests could not be conducted at just any air base, using just any pilot who happened to be available.
Given the top-secret nature of the program, test-flying the A-12 at Edwards Air Force Base – that bastion of Air Force flight testing – was out of the question. No, the tests would have to be conducted at a place where the prying eyes of civilians and uninvolved military personnel would not get a glimpse of this mysterious aircraft. Yet, the test site would have to be accessible via aircraft and be equipped with sufficient infrastructure to support a lot of personnel. The answer, of course, was to return to the “top secret base in Nevada” that had previously been utilized for the U-2 flight tests.
As for A-12 pilot qualifications; they were simple, really. The applicant only had to:
1. Be between 25- and 40-years old.
2. Be under six-feet tall.
3. Be less than 175 pounds.
4. Be married.
5. Be emotionally stable.
6. Be highly motivated.
7. Have current experience with high performance aircraft (somehow, this writer does not think that Lockheed’s definition of “high performance” was the same as that found in the civilian Federal Aviation Regulations – namely an aircraft whose engine produces more than 230 horse power. The reader should note that A-12 pilots were technically civilians, who would be “borrowed” from military service.)
8. Be experts in the field of air-to-air refueling.
Needless to say, Lockheed’s customer, the CIA, was not happy about the delays – not all of which were caused by the engine development problems and pilot selection process. Un-anticipated problems with the titanium had occurred, because it was harder to work with than expected, and because Lockheed was receiving titanium that contained impurities.
Indeed, when told by Kelly Johnson that there would be a three- or four-month delay, which would push the A-12’s first flight back to Dec. 1, 1961, CIA Director Richard Bissell told Johnson that he was shocked there would be another delay, because he (Bissell) was under the impression that the problems with the titanium had been resolved. Bissell, who was normally a friend and supporter of Lockheed, told Johnson in no uncertain terms that nothing short of an earthquake would be an acceptable excuse for further delays.
But, even without an earthquake, the delays – especially with the development of the J58 engine – continued to the point where Johnson decided to fit the less powerful Pratt & Whitney J75 engine to the A-12 for the initial flight tests.
To be continued…
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