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

SR-71 INLET NOSE CONE

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.

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

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

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.

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.

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.

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.

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.

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.

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.

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.

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.