From: greg@saltydog.dpsi.com (Gregory R. Travis)
Newsgroups: rec.aviation.military
Subject: Re: SR-71 engines
Date: 10 Jan 93 17:03:47 GMT
Article-I.D.: saltydog.greg.726685427
Organization: Data Parallel Systems, Inc


[My original, classified, pictoral image of the engine was as such:]
>         Fuel in 
>    -----|---------------\       Fuel in
>             RAMJET       \ /====|==
>    /   ============      ---       \
>  <=     |TURBOJET|         AFTERBURNER
>    \   ============      ---       /
>             RAMJET       / \====|==
>    -----|---------------/       Fuel in
>         Fuel in



In <C0MJGn.G1D@acsu.buffalo.edu> v111qheg@ubvmsb.cc.buffalo.edu (P.VASILION) writes:
[About my above picture and description of how I believed the world turns]

>	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.

Well, I violated every USENET principle and went researching this thing
last night.  About the only reference that I could find was in my copy of
_Aircraft Gas Turbine Engine Technology_, Irwin Treager auth.
ISBN 0-07-065158-2

Mr Treager states (about the J58):
	"This engine is a single-shaft, supersonic cruise powerplant equipped
with an afterburner.  It has an eight-stage compressor driven by a two stage
turbine.  Six large tubes are used to bypass a part of the compressor airflow
to the afterburner.  The combustion chamber is a can-annular design with
eight flame tubes.  No RPM, compression ratio, or mass airflow figures are
available.  Specific fuel consumption is .8 to 1.9 lb/lbt/h.  The engine
weighs 6500 lb and produces up to 34,000 lbt with the afterburner in operation.
At the time of this writing, the engine is still classified."


The picture depicts what looks like a fairly typical afterburning turbojet
of 1960s vintage.  The distinguishing features are, of course, the six
large tubes.  The tubes appear to sprout from a collector ring drawing
air from either the engine inlet or just behind the first compressor
stage.  The tubes travel back, in roughly a straight line, to a diffuser
ring immediately aft of the eight burner cans.  My modified ASCII graphic
would have it:


        /-------------------\   Fuel in
       #  Bypass tube(6)     \ /|======
    /  # ===================           \
  <=   |-|||||||-TURBOJET-||  AFTERBURNER
    \  # ===================           /
       #  Bypass tube(6)     / \|=======
        \-------------------/   Fuel in

I wonder if the inlet spike can be moved so as to produce shock waves
forcing the airflow out of the compressor and into the bypass tubes
for supersonic flight?  As the spike is an airframe component and not
an engine component, it does not appear to have the mechanical ability
to vary the engine airflow so it "must" (:-)) do it via shockwave modification.

It is not immediately clear to me how this design differs, in a significant
way, from a standard turboFAN engine.  The obvious answers are that it
employs six distinct bypass tubes instead of the normal can-annular
design and that the fan output is diverted into an afterburner instead of
being discharged directly to the atmosphere.  The latter is what appears to
give the J58 its hybrid afterburning turbojet/ramjet character.

It is interesting to compare the J58 to another supersonic engine
of the same era and power capacity - the Rolls Royce/SNECMA Olympus
engine used on the Concord.  This engine also produces 38,000 ls of thrust
(from 5800 lbs of engine) and is the only commercial engine to use an
afterburner.  It differs from the J58 in two significant details - it is
a two spool design and (of course) lacks the bypass ducts; it is a pure
turbojet and does not bypass any of the compressor flow outside of the
turbine.  It, of course, also does not go as fast the the J58!




>	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.

>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.

I hope someone provides a really good explanation of how this works.  No
engine, regardless of its inlet efficiency, is going to be able to produce
much more than a 14 psi pressure differential across its inlet and this
number is going to decrease rapidly as the aircraft climbs and/or acclerates.
Given the size of the SR-71s inlet it is hard for me to see how even a
perfect vacuum cleaner inside the inlet is going to produce much thrust
ESPECIALLY considering the requirement that inlet airflow be flowed to
subsonic speeds!.  The outside atmospheric pressure combined with the size of
the inlet just don't seem to be sufficient to accelerate a sufficent quantity
of air to budge an SR-71 much less provide 50-60% of its supersonic thrust!

I gotta go, there's a wierd truck parked outside with antennas all over it...

greg













--
Gregory Reed Travis				D  P  S  I
Data Parallel Systems Incorporated   greg@dpsi.com (For MX mailers only!)
Bloomington, IN			     greg@indiana.edu (For the others)