From: v111qheg@ubvmsb.cc.buffalo.edu (P.VASILION)
Newsgroups: rec.aviation.military
Subject: Re: SR-71 engines
Date: 10 Jan 93 06:04:00 GMT
Organization: University at Buffalo
News-Software: VAX/VMS VNEWS 1.41

In article <greg.726600780@saltydog>, greg@saltydog.dpsi.com (Gregory R. Travis) writes...
>In <C0KsFL.K87@acsu.buffalo.edu> v111qheg@ubvmsb.cc.buffalo.edu (P.VASILION) writes:
> 
>>Well yes and no. Once the SR-71 is positioned downwind and her pilot lights
>>the burners, they never are shut down. As the aircraft accelerates through
>>Mach 2.5 or so, the engine bypass louvers open up and duct air directly from
>>the inlet to the afterburner.
>>	The turbine is NEVER shut down, or idled. If It was idled, it would
>>actually drag on its mounts, and if it were shut down, it would tear off
>>its mounting lugs. 
> 
>Does this mean a flameout (caused by fuel exhaustion, turbulence, a failure
>in the rudder, etc.) implies that the engines will depart the
>airframe?
> 

	No! A flameout would disrupt airflow to the entire engine, not just 
the turbine. Unstarts, common to the SR-71, similar to compressor stalls
would rob the engine of subsonic air (jet engines cant breath super sonic
air), causing the engine to lose power. The only way the engines would
tear off if the turbine blades were to instantly stop while the air/fuel
mixture continued to be burned in the afterburner. 

>	While the turbine is for the most part bypassed, the engine is still
>>not a true ramjet. The turbine provides some 17 percent of total engine
>>thurst while the afterburner provides about 26 percent. It is the INLET 
>>which provides ABOUT 63 PERCENT of the total thrust! A ramjet, by 
>>definition derives its thrust from the combustion of fuel, whereas the SR-71
>>get her thrust from her air intakes.
> 
>I don't follow you here.  By "inlet" do you mean the ramjet?  I have always
>imagined a single SR-71 engine as a "conventional" turbojet buried inside
>a "conventional" ramjet.  The inlet spike varies the mass flow between the
>two - at low speed the air supply to the jacketing ramjet is cut off and
>provided instead to the turbojet.  At high speed the spike moves so that
>most of the mass flow flows around the turbojet and to the ramjet:
> 
>         Fuel in 
>    -----|---------------\       Fuel in
>             RAMJET       \ /====|==
>    /   ============      ---       \
>  <=     |TURBOJET|         AFTERBURNER
>    \   ============      ---       /
>             RAMJET       / \====|==
>    -----|---------------/       Fuel in
>         Fuel in
> 
>(Classified ASCII Graphic.  For your eyes only)
> 
>Can you explain how the "inlet" provides thrust?
> 

	This definitely not the case! The J-58 is 100 percent a turbojet
engine that is maximized for augmented thrust cruise. There never was a
ramjet quality to it except for bypass louvers what dirverted airflow
from the compressor to the final bleed stages of the engine. The spikes
were designed to slow the airflow to subsonic speeds (jet engines dont
run on supersonic air) and to position the shock wave from Mach 3+ air in the
inlet so that it wouldn't enter the compressor. In effect, the inlet, 
comprised of the spike, bleed doors, stator blades and the first compresor
stages, is like a large bernoulli tube. The shock wave in the inlet and the
normal airflow into the engine create a mamoth low pressure zone. This zone
gives the inlet the ability to provide its own thrust component as air
is forced into it. Think of it as a huge vaccuum cleaner sucking its way
through the upper atmosphere. If you dont believe me, read the OPSREY AIR
COMBAT SERIES SR-71 BLACKBIRD, and the SR-71 PILOT's MANUAL.

More to follow,
Peter Vasilion
Newsgroups: rec.aviation.military