AEROSPACE 410

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AEROSPACE 410 AEROSPACE PROPULSION Lecture
(9/30/2002) TURBO RAMJET ENGINES J-58
PRATTWHITNEY for SR-71 propulsion

Dr. Cengiz Camci
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SR-71 INLET NOSE CONE
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Unlike inlets operating in the Mach 2 and under
regime, the SR-71 inlet must use variable inlet
geometry (see below) in order to manage flow
over the full operating range of the aircraft.
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• There are four requirements
• The engine inlet must meet
• It must match the air flow captured
• by the inlet to the air flow required by
• the engine under all conditions from
• subsonic to Mach 3
• Since all turbojet engines require a
• constant volume of air, they require
• subsonic flow at the inlet to the
• compressor face, it must reduce the
• velocity of flow to about Mach .3 to .5
• as it enters the engine this is no small task

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• While it is reducing the velocity of the air
• at the compressor, it must simultaneously
• retain the greatest possible air pressure
• in order to boost flow to the compressor
• It must minimize the momentary effect upon air
• flow from external perturbations such as
  gusts

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The SR-71 inlet is classified as an axisymmetric
mixed compression inlet. This type was chosen
because it offered higher pressure recovery at
the Compressor face, longer range, and the
desired high-speed cruise performance.
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Mixed compression inlets can provide high
pressure recovery above Mach 2.2 if the shock can
be maintained in such a state that it impinges
just downstream of the inlet throat, even when
the airflow is disturbed. When the shock is
disturbed in any way so that it moves from that
point, the inlet is said to become unstarted.
When this happens, the shock pops out and
stabilizes forward of the inlet lip and the
pressure recovery, airflow to the engine,
and consequently, thrust all drop instantaneously
while drag spikes upward. The nozzle must be
designed to recover from the unstart condition
rapidly to prevent engine damage and, on the
SR-71,to prevent the airplane from yawing too
much toward the unstarted engine.
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Bypass air systems One of the first experiences
Lockheed engineers had with the requirement for
bypass ducts came during the early development of
the P-80 Shooting Star. Pilots reported loud
noises emanating from the intake ducts to the
engine under certain conditions, a
phenomenon they called duct rumble. The cause
was air piling up within the duct along the inner
wall, creating turbulent eddies that produced the
rumble.
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The solution was to provide an overboard exit for
this piled-up air through a system of ducts along
the intakes inner wall. The air entered the duct
and was led to the outside near the top and
bottom of the external skin of the intake.
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Supersonic wind tunnels experienced choking when
air flow was blocked by shock waves that
reflected back into the tunnel. The problem
persisted until slots were incorporated in the
tunnel walls to carry away the air from the
shock waves so they would not be trapped inside
the tunnel. The SR-71s complex series of bypass
doors and ducts are shown in many of the
following diagrams.
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Forward bypass doors (see above) are open when
the gear is down but close when the gear
retracts. They are scheduled to open again at
Mach 1.4 to dump excess flow captured by the
inlet. Beginning at Mach 1.6, the aerospike
begins to retract to the rear, altering the
location of the point at which the shock wave is
formed and moving in proportion to the changing
angle of the shock. The inlet starts at about
Mach 1.7 when the shock finds its way to a point
downstream of the throat. Above Mach 2.2,
bypass doors come into play to help maintain
the shock at its desired location. When an
unstart occurs, both spikes move forward abruptly
and the forward bypass doors are opened to
recycle and obtain a restart. The spikes are
retracted again until the shock returns to the
desired location at the inlet throat.
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On the SR-71, boundary layer (layer closest to
the skin) piles up around the aerospike center
body and is conducted via a porous bleed inlet
(see above) through the center body of the
aerospike to four hollow pylons that conduct the
air out of the aerospike and overboard. Forward
bypass doors match the inlet to the
engines needs, bypassing air overboard.
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Air from the shock trap tubes (see above)
bleeds piled-up air into passages that lead to
the engine for cooling before it exits through
the ejector at the aft end of the engine. At the
rearmost point, the spike has translated
aft about 26 inches. At the same time, the inlets
capture area has increased by 112, and the
throat diameter at the point of minimum
cross-section downstream has been reduced by 54
to maintain the shock in the proper position.
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At Mach 3, the inlet itself produces 54 of
total thrust through pressure recovery, the
engine contributing only 17 and the ejector
system 29. The compression ratio at cruise is 40
to 1.
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