Photo: Patent drawing of an early Pratt & Whitney convergent-divergent nozzle design mated to a core J57 engine. This may be the design for the poorly-documented J57-P-35 engine.
Tuesday, July 15, 2014
This Day in McDonnell Voodoo History: July 13
In 1954, Headquarters USAF gave formal
endorsement to a single-seat interceptor derivative of the F-101A as a backstop
to the advanced Convair F-102B program (later to become the F-106A). Although losing the competition with
McDonnell, the very advanced J57-P-35 engine being developed for the IF-100B
interceptor would go on to become important in the development of the McDonnell
interceptor. One of the first turbojet
engines to incorporate a variable-area convergent-divergent exhaust nozzle, the
new engine would a supersonic exhaust velocity and a significant increase in
engine thrust, particularly at the high operating altitudes for interceptors.
Photo: Patent drawing of an early Pratt & Whitney convergent-divergent nozzle design mated to a core J57 engine. This may be the design for the poorly-documented J57-P-35 engine.
Photo: Patent drawing of an early Pratt & Whitney convergent-divergent nozzle design mated to a core J57 engine. This may be the design for the poorly-documented J57-P-35 engine.
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When did they start using more of an hourglass shape for the exhaust chamber instead of just necking it down??
ReplyDeleteThe short answer is that anything coming out of the back of that necked-down, or "convergent" nozzle is going to immediately expand as it exits and slows to subsonic speed. Thrust is a function of the mass of exhaust gases times velocity. This places an upper limit to the thrust produced by a convergent nozzle, effectively limiting aircraft to a maximum speed of Mach 1.6, which was the top speed of most versions of the Voodoo. In the F-101B interceptor, they were hoping for something closer to Mach 1.85. To get there, the "bell" of the hourglass shape allowed controlled expansion of the exhaust gases and acceleration to some supersonic speed. The design illustrated here could be varied to provide exhaust velocities ranging between Mach 1.6 and Mach 2,0 for a maximum thrust of 17,200 pounds versus 16,000 pounds for the same engine with a conventional nozzle. There is not much documentation of these early test engines, but it appears very close to illustrations that I have seen of the J57-P-35 engine developed for the IF-100B (evolved into the F-107A.) and the original proposed engine for the F-101B after they made the decision to use the new, larger J75 engine in the F-107.
ReplyDeleteA test F-101A with the new engines was able to routinely reach speeds of just under Mach 2, but translating this mechanically-complex variable nozzle into something safe and reliable enough proved problematic. A simpler, fixed geometry design was substituted at the last minute, with an exit velocity of Mach 1.4. This produced 16,900 pounds of thrust and was sufficient to get the production F-101B to a maximum speed of Mach 1.83, not what they wanted but better than anything else at the time and plenty fast for what it needed to do.