Lockheed SR-71 / YF-12 Blackbird

Hydroman said:
If you take a look at the cockpit mach indicator for the A-12, YF-12 and SR-71, you will see dashes from m3.5 to m4.0, interesting, just an observation.
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I had the pleasure of working briefly with Mr. Joe Vida (high-time SR-71 RSO) at the B-2 CTF in the 90's, Joe had some neat stories but always left things open for interpretation, could only say so much. Also worked with Bill "Flaps" Flanagan, former SR RSO.
 
One thing to keep in mind that top speed achieved in flight may not be the theoretical top speed reachable by the plane. When traveling at high altitude at very high velocities it is very difficult to brake the aircraft via control surfaces. It is very dangerous to “keep going”, so to speak.
 
siegecrossbow said:
When traveling at high altitude at very high velocities it is very difficult to brake the aircraft via control surfaces. It is very dangerous to “keep going”, so to speak.
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From what I've seen described in a video the SR-71 has to maintain at least zone 1 reheat to maintain speed and if it goes to dry thrust it will rapidly lose speed and altitude.
 
So no straight answer.... The rumoured m3.5+ I is probably it however a short term maximum higher is probably possible.
 
Nigelhg said:
So no straight answer.... The rumoured m3.5+ I is probably it however a short term maximum higher is probably possible.
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I've always assumed that if a Blackbird hit 3.5 that the crew would probably be rather concerned.
 
Nigelhg said:
So no straight answer.... The rumoured m3.5+ I is probably it however a short term maximum higher is probably possible.
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Again, the inlet spikes run out of travel at Mach 3.55 and the inlets unstart. Which causes the airframe to slap the crew upside the head repeatedly until the inlets restart at a lower airspeed.
 
Whether it's true or not...

There's always this "legend" / story:

Somewhere, 13 miles above Arizona, there was a pilot screaming inside his space helmet. Then, I heard it. The click of the mic button from the back seat. That was the very moment that I knew Walter and I had become a crew. Very professionally, and with no emotion, Walter spoke: "Los Angeles Center, Aspen 20, can you give us a ground speed check?" There was no hesitation, and the replay came as if was an everyday request. "Aspen 20, I show you at one thousand eight hundred and forty-two knots, across the ground."

SR-71 Blackbird Speed Check Story

There were a lot of things we couldn't do in an SR-71, but we were the fastest guys on the block and loved reminding our fellow aviators of this...
www.thesr71blackbird.com

That one is right up there with the legend of the mall ninja...

Shrine of the Mall Ninja – LonelyMachines

lonelymachines.org lonelymachines.org
 
Scott Kenny said:
Again, the inlet spikes run out of travel at Mach 3.55 and the inlets unstart. Which causes the airframe to slap the crew upside the head repeatedly until the inlets restart at a lower airspeed.
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I thought that the SR-71 had some sort of analogue yaw stability-augmentation circuitry in the yaw channel to prevent this?
 
NMaude said:
I thought that the SR-71 had some sort of analogue yaw stability-augmentation circuitry in the yaw channel to prevent this?
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Did have some stability-augmentation stuff, but losing 25% of your thrust instantly will still cause a rapid yaw that will smack the crew's helmets with the airframe.
 
AGS-1787 said:
If I understood the sr-71 book I have; the inlets had to be programmed before each flight.
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That ... doesn't seem right. Nothing changed from flight to flight with regard to the Air Inlet Control System. Ranges for Mach Number, Angle of Attack, Angle of Yaw and Load Factor were built into the control system and it adjusted for changes in those variables automatically.

However the NAS-14 Astro-Inertial Navigation System (AINS) computer had to be loaded (programmed from a cassette tape) with a fresh ephemeris (essentially relative star locations) for each flight.
 
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@aim9xray : Density varies considerably in the higher atmosphere with strays out of standard tables that differs considerably from one place of the earth to the other (think convection).
The Blackbird being essentially the faster globe trotter ever, trimming the inlets spike control law to local and timely conditions would have ensured performances where up to the edge.

I have also read similar details.
 
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charleybarley said:
The spike ran out of travel at the vehicle design cruise speed M3.2. The best single source of information on all aspects of the design of the plane has 5 attached files you can read https://authors.library.caltech.edu/records/6s4e6-b2j60
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Also, you do NOT want your inlets to run out of travel at M3.2, you want them to be able to adjust at design cruise for different air temperatures, pressures etc.

They may not have much travel left, but they should not be out of travel. Out of travel means an inlet unstart if they need to retract further.
 
charleybarley said:
The spike ran out of travel at the vehicle design cruise speed M3.2. The best single source of information on all aspects of the design of the plane has 5 attached files you can read https://authors.library.caltech.edu/records/6s4e6-b2j60
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A family friend (BGEN) had his VIP ride in the SR-71B (955) circa 1974. He told me shortly thereafter (January 1975) that Vmax on his flight was Mach 3.225. Did I believe him? Yes. Do I have independent proof of the speed? No. *shrug* Your anecdata mileage may vary.
 
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aim9xray said:
A family friend (BGEN) had his VIP ride in the SR-71B (955) circa 1974. He told me shortly thereafter (January 1975) that Vmax on his flight was Mach 3.225. Did I believe him? Yes. Do I have independent proof of the speed? No. *shrug* Your anecdata mileage may vary.
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That's about what I'd expect, what with the engines being inlet temperature limited. It'd be a strange day to get to M3.55 without overtemping the engines. Like Polar Vortex with a vengeance cold, over Siberia in the winter.
 
I have read that the ambient temperature at 80k doesn’t always behave as you might expect. It often was above standard around the Arctic circle, with some the coldest temperatures encountered in the South Pacific. Your mileage may vary….
 
I wonder how the J58 would've behaved if the pre-cooler technology developed for the SABRE engine had been available back in the late 50s/early 60s?
 
F119Doctor said:
I have read that the ambient temperature at 80k doesn’t always behave as you might expect. It often was above standard around the Arctic circle, with some the coldest temperatures encountered in the South Pacific. Your mileage may vary….
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The thickness of the atmosphere is thinner over the poles. Above 60deg of latitude, at very high FL, you are not always where you think you are in term of atmospheric layer.

iu


earthscience.stackexchange.com

Why is the troposphere 8km higher at the equator than the poles?

Fundamentals of atmospheric modeling show that the height of the tropopause depends on the location, notably the latitude, and that the troposphere is roughly 8km higher at the equator than the poles
earthscience.stackexchange.com earthscience.stackexchange.com
 
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If you get the inlet air to the engine cool enough, you wouldn’t need the bleed bypass system, and the engine contribution to the system thrust would have been higher. Still would have to design the inlet system to accommodate higher Mach numbers.

Don’t forget that P&W built a very high performance heat exchanger for the 304 Suntan engine, used to absorb exhaust heat to vaporize the LH2 fuel and drive the power turbine. The issue with cooling inlet air without a consumable heat sink like LH2 is that you need to dump the heat somewhere, and everywhere else is hotter. You can use a heat pump system to move the heat to a higher temperature area, but this absorbs more power.
 
charleybarley said:
What were these pilots' actual words? It may be that there is no contradiction with the oft-quoted "design" statement which mentions M3.2.
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That the engines are Compressor Inlet Temperature limited to 427degC, the inlet spikes run out of travel at Mach 3.55, and that at Mach 3.55 the nose shockwave will touch the ailerons at the wingtips.

So the pilots basically cruised at whatever speed equalled a 427degC CIT.



F119Doctor said:
If you get the inlet air to the engine cool enough, you wouldn’t need the bleed bypass system, and the engine contribution to the system thrust would have been higher. Still would have to design the inlet system to accommodate higher Mach numbers.
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As I understand it, the bleed bypass system was because the compressor was choking out due to too much mass flow.
 
It would be interesting to see P&W pull a J58 from storage, refurbish and do some ground-tests in conjunction with a pre-cooler adapted from the design used for the SABRE pre-cooler.
 
Scott Kenny said:
That the engines are Compressor Inlet Temperature limited to 427degC, the inlet spikes run out of travel at Mach 3.55, and that at Mach 3.55 the nose shockwave will touch the ailerons at the wingtips.

So the pilots basically cruised at whatever speed equalled a 427degC CIT.




As I understand it, the bleed bypass system was because the compressor was choking out due to too much mass flow.
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You are correct - at 427C inlet temperature, while the indicated engine RPM was approximately 100%, the effective corrected RPM was around 65% - just above idle airflow. At low corrected speed, the back end of the compressor is too small for the front end airflow, resulting in choked flow in the aft stages and excessive blade angle of attack in the front stages, pushing the compressor toward stall and surge. The bleed bypass unloads the front stages at these low corrected RPM conditions and send the excess air to the afterburner where it is available for cooling and burning.

If you somehow cooled the inlet, the corrected RPM would be higher with more air going thru the core, and the bleed bypass would have been unnecessary.
 

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