1. J-58 PRATT&WHITNEY for SR-71 propulsion
AEROSPACE 410
AEROSPACE PROPULSION
Lecture (9/30/2002)
TURBO RAMJET ENGINES
J-58 PRATT&WHITNEY for SR-71 propulsion
Dr. Cengiz Camci

23. SR-71 INLET NOSE CONE

24. 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.

27. 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

28. 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

29. 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.

30. 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.

31. 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.

32. 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.

33. 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.

35. 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.

37. 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.

39. 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.

40. 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.