Blackbird's thrust question

[Brian Abraham]

Kelly Johnson can call the J58 anything he liked, seems to me. But the classic definition is what it is....

You seem to be having difficulty in understanding the turbo ramjet Lyman.

It is well understood in engineering. The following diagram comes from a NASA paper.



A NASA paper on turbo ramjets. Access forbidden!

It opens by saying "Advanced airbreathing propulsion systems used in Mach 4-6 mission scenarios, usually involve turbo-ramjet configurations. As the engines transition from turbojet to ramjet, there is an operational envelope where both engines operate simultaneously." In the case of the J-58 both are operating simultaneously when above Mach 1.8 - 2.0, below that figure it's just a turbo jet with afterburner as previously mentioned.

It is what is alternatively called a dual cycle engine. Straight turbo jet to start with, becoming part turbo jet and part ramjet once up to the necessary speed for the ramjet function to begin operation.

A further NASA paper Access forbidden!

A few extracts

The engines considered are based on or extrapolated from known performance parameters of rocket-based combined cycle (RBCC) (the Marquardt Corporation ejector ramjet) and turbine-based combined cycle (TBCC) (the Pratt & Whitney J-58 engine used in the Lockheed SR-71 Blackbird).

B. Turbine-Based Combined Cycle Engines: Background
Turbine-based combined cycle engines operate by using a gas turbine propulsion cycle which transitions to a ramjet cycle; they bypass the turbomachinery at high Mach numbers, where pressures, temperatures, and flow velocities make such machinery impractical or redundant or both. Such a configuration by itself is not capable of orbital insertion because at some altitude the ramjet mode will lack the inlet mass flow to sustain thrust; however, TBCC engines hold strong promise for use with carrier vehicles or atmospheric ascent stages. Turbojet propulsion systems are generally limited to Mach 3 due to the rise in inlet temperature present at the compressor face; turbine engines are also in general more limited in altitude with respect to their ramjet counterparts as well. They provide more efficient operation, however, at lower altitudes and Mach numbers relative to ramjet-based engines.14 The limitation to lower Mach numbers and altitudes is not universal, especially if the ramjet mode is effective enough to compensate for the additional weight of the turbomachinery.

Turbine-based combined-cycle engines have seen actual flight time, a qualification which is not shared by RBCC engines. For example, the SR-71 J-58 engine is a turbine-based engine that operates in multiple cycles depending on the flight regime (with known cruise conditions of approximately Mach 3.2 at an altitude of 70,000 ft).

Can the historic configuration of the Pratt & Whitney J-58 with the Lockheed variable-geometry inlet be considered a true TBCC? From an aerospace purist view, the engine is never completely in a pure ramjet mode due to the fact that the first stages of compression are present before the mass flow is bypassed to the afterburner. In the viewpoint of the aerothermodynamist, however, the engine transitions through at least two modes or cycles and, thus, coupled with the turbine, can be classified as a TBCC.

From Colonel Grahams book "this bypass (air to the afterburner) led to the description of the J-58 as being a turbo-ramjet engine." As the Colonel acknowledges "Mr. A. J. "Arnie" Gunderson of Pratt and Whitney, better known as "Mr. J-58" among the crews was instrumental in helping me with specifics of the engine."

I'm somewhat loss Lyman why you can't accept the description provided by the designer, the crews who flew it, the people who instructed on it, and the P & W technical rep.