27 There are four requirements The engine inlet must meet:It must match the air flow capturedby the inlet to the air flow required bythe engine under all conditions fromsubsonic to Mach 3+Since all turbojet engines require aconstant volume of air, they requiresubsonic flow at the inlet to thecompressor face, it must reduce thevelocity of flow to about Mach .3 to .5as it enters the engine; this is no small task
28 While it is reducing the velocity of the air at the compressor, it must simultaneouslyretain the greatest possible air pressurein order to boost flow to the compressorIt must minimize the momentary effect upon airflow 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 theCompressor face, longer range, and the desired high-speedcruise performance.
30 Mixed compression inlets can provide high pressure recovery above Mach 2.2 if the shock can be maintained in such a statethat 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 fromthat point, the inlet is said to become unstarted. When thishappens, the shock pops out and stabilizes forward of the inletlip and the pressure recovery, airflow to the engine, andconsequently, thrust all drop instantaneously while drag spikesupward. The nozzle must be designed to recover from theunstart condition rapidly to prevent engine damage and,on the SR-71,to prevent the airplane from yawing too muchtoward the unstarted engine.
31 Bypass air systems One of the first experiences Lockheed engineers had with the requirement for bypass ducts came duringthe early development of the P-80 Shooting Star. Pilotsreported loud noises emanating from the intake ductsto the engine under certain conditions, a phenomenonthey called duct rumble.The cause was air piling up within the duct alongthe inner wall, creating turbulent eddies thatproduced the rumble.
32 The solution was to provide an overboard exit for this piled-up air through a system of ducts along the intakesinner wall. The air entered the duct and was led to theoutside near the top and bottom of the external skin ofthe intake.
33 Supersonic wind tunnels experienced choking when air flow was blocked by shock wavesthat reflected back into the tunnel.The problem persisted until slots wereincorporated in the tunnel walls tocarry away the air from the shock wavesso they would not be trapped inside the tunnel.The SR-71s complex series of bypass doorsand ducts are shown in many of thefollowing 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 againat 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 formedand moving in proportion to the changing angle of the shock.The inlet starts at about Mach 1.7 when the shock finds its way toa point downstream of the throat.Above Mach 2.2, bypass doors come into play to help maintain theshock at its desired location.When an unstart occurs, both spikes move forward abruptly andthe forward bypass doors are opened to recycle and obtain a restart.The spikes are retracted again until the shock returns to the desiredlocation at the inlet throat.
37 On the SR-71, boundary layer (layer closest to the skin) piles up around theaerospike center body and is conducted via a porousbleed inlet (see above) through the center body ofthe aerospike to four hollow pylons that conductthe air out of the aerospike and overboard.Forward bypass doors match the inlet to the enginesneeds, bypassing air overboard.
39 Air from the shock trap tubes (see above) bleeds piled-up air into passages that lead to the engine forcooling before it exits through the ejector at the aftend of the engine.At the rearmost point, the spike has translated aftabout 26 inches. At the same time, the inlets capturearea has increased by 112%, and the throat diameterat the point of minimum cross-section downstream hasbeen reduced by 54% to maintain the shock in theproper position.
40 At Mach 3, the inlet itself produces 54% of total thrust through pressure recovery, the enginecontributing only 17% and the ejector system 29%.The compression ratio at cruise is 40 to 1.