SR-71 top speed

tank-top · by **tank-top** » 25 Sep 2018, 02:22

I’ve slways been curious as to how the SR-71’s top speed was calculated and published. I’m not asking for the top secret highest attained speed but rather her published speed. The published top speed is Mach 3.3 or about 2,200 mph but using my bad math I believe that equals an altitude of about 30,000 feet. If you adjust for an altitude of 80,000 feet Mach 3.3 is a bit faster than 2,200 mph. Then to throw in a curveball, what if she’s flying with the jet stream and picks up a couple hundred more mph? In other words the published numbers look adjusted for altitude, temp and other factors, can someone with more knowledge than me tell me what the effective ground speed of an SR-71 cruising at 80,000 feet doing Mach 3.3?

wrightwing · by **wrightwing** » 25 Sep 2018, 05:47

There are anecdotes about hitting M3.5 briefly.

sprstdlyscottsmn · by **sprstdlyscottsmn** » 25 Sep 2018, 06:22

I spent some time deconstructing pilot accounts and came up with 3.88.

Gums · by **Gums** » 26 Sep 2018, 02:12

Salute!

I dunno, Spurts, that sounds high.

All the numbers need a common reference. So 2200 miles per hour or 2300 or 2100 do not mean anything until based upon a common reference frame.

Ground speed?
True air speed?
CAS?

My friends that flew the sucker talked about CAS of 300 knots or less, and maybe 3.2 or 3.3 mach. Depends on the temperature for the mach. I also had a friend working in Florida ATC and claimed a track of 40 miles per minute for a tgt one day. So that's 2400 knots, right?

Oh well, the thing was fast. Really fast.

Gums sends...

sferrin · by **sferrin** » 26 Sep 2018, 03:28

sprstdlyscottsmn wrote:I spent some time deconstructing pilot accounts and came up with 3.88.

Sounds high. Highest I'd ever heard in tests (for the much lighter A-12) was Mach 3.6 and 97.000 feet (not on the same flight).

sprstdlyscottsmn · by **sprstdlyscottsmn** » 26 Sep 2018, 14:07

In a discussion about a mission in Lybia, the pilot (forgot name, maybe Schule) talked about putting the throttle to the stops, accelerating through 3.5M and ultimately hitting a Mach number that was "flat out scary" covering a mile every 1.6 seconds. If I recall correctly, I calculated based on ISA at 85 or 90k and no wind. A statute mile only came out to 3.4x so it had to be nautical. 3.88 is what I came up with. He then mentioned pulling the throttle to idle south of Sicily and over running the tanker waiting at Gibraltar.

sferrin · by **sferrin** » 26 Sep 2018, 14:42

sprstdlyscottsmn wrote:In a discussion about a mission in Lybia, the pilot (forgot name, maybe Schule) talked about putting the throttle to the stops, accelerating through 3.5M and ultimately hitting a Mach number that was "flat out scary" covering a mile every 1.6 seconds. If I recall correctly, I calculated based on ISA at 85 or 90k and no wind. A statute mile only came out to 3.4x so it had to be nautical. 3.88 is what I came up with. He then mentioned pulling the throttle to idle south of Sicily and over running the tanker waiting at Gibraltar.

I'd heard the Blackbird wasn't drag limited so I guess it could be "whatever you dare until it comes apart". :shock:

The 3.6 I mentioned was a test point in early development as I recall. "Back in the day" there were a couple good threads on rec.aviation.military (USENET) with Mary Shafer. IIRC she was the head flight dynamics engineer for the Blackbird at Dryden. Of course people asked "what could it really do" and there were some interesting conversations.

sprstdlyscottsmn · by **sprstdlyscottsmn** » 26 Sep 2018, 15:31

My understanding of the speed limit was that it would be thermal, based on the Turbine Inlet Temperature.

Here is the story. The ending bit was put in at the beginning of the page, but here it is.
https://theaviationgeekclub.com/the-sto ... do-canyon/

madrat · by **madrat** » 27 Sep 2018, 04:45

USENET is definitely worth pouring through for quite a few topics. Hard to imagine how much reading material is in the archives, yet the relative file size of the archive is minute because of its nature.

sferrin · by **sferrin** » 27 Sep 2018, 12:36

madrat wrote:USENET is definitely worth pouring through for quite a few topics. Hard to imagine how much reading material is in the archives, yet the relative file size of the archive is minute because of its nature.

Full of interesting tidbits for sure. One that comes to mind is the folding ventral fin on the YF-12A. According to Mary, the one they had at Dryden lost it's central ventral fin on one flight. They recovered it but turns out the YF-12A had sufficient directional stability without it so they left it off.

(Plane in foreground here. You can differentiate the A from the C by the radome up front.)

https://www.nasa.gov/centers/dryden/mul ... -4767.html

https://www.dfrc.nasa.gov/Gallery/Photo ... N-4728.jpg

h-bomb · by **h-bomb** » 28 Sep 2018, 02:07

I was lucky enough to meet Ben Rich a few years before he passed.

He would not say anything about the top speed. He said the speed was limited by the nose cone shock wave, and engine inlet interaction. It could cause a violent unstart and potential loss of the air frame.

He told us that the divots on the side of the nosecone 'held' the shock wave in a stable location. If you can do the math for the shock wave angle to the outer wing/inlet chine you would know the theoretical top speed. Basically the VNE for the SR-71.

He also said the early pilots were also warned, you never wanted to see those mach numbers. You were on the edge of disaster. You could not make the smallest error, do to the risk of an unstart. He rather serious about that point.

I wish we had more then a few hours of his time.

jetblast16 · by **jetblast16** » 28 Sep 2018, 02:26

Not just airframe/shock wave guiding, but speed was more than likely limited to what those J-58s could swallow at the compressor, the CIT.

edpop · by **edpop** » 30 Sep 2018, 03:09

Jay Miller in his Aerofax MiniGraph 1 book on the A-12, YF-12, D-21 written in 1985 has the top speed listed at Mach 3.35 with the following info listed for Mach 3.2.

Cruising speed Mach 3.2
Inlet compressor temp at 800 degrees F
Turbine inlet at 2000 degrees F
Fuel inlet temp 300 degrees F
Oil inlet temp at 550 degree F
Engine thrust to weight is 5.2 to 1
All above info at steady state conditions

duplex · by **duplex** » 30 Sep 2018, 13:50

sprstdlyscottsmn wrote:In a discussion about a mission in Lybia, the pilot (forgot name, maybe Schule) talked about putting the throttle to the stops, accelerating through 3.5M and ultimately hitting a Mach number that was "flat out scary" covering a mile every 1.6 seconds. If I recall correctly, I calculated based on ISA at 85 or 90k and no wind. A statute mile only came out to 3.4x so it had to be nautical. 3.88 is what I came up with. He then mentioned pulling the throttle to idle south of Sicily and over running the tanker waiting at Gibraltar.

His name was Brian Shul

https://theaviationgeekclub.com/the-sto ... do-canyon/

Gums · by **Gums** » 30 Sep 2018, 23:49

Salute!

Yep, it was Shul ( former student of mine and fellow Green Demon of the 356th TFS at The Beach).

PLZ refer to the actual Dash-one and other good stuff here:

https://www.sr-71.org/blackbird/

Many "less than accurate facts" about the engine design and such are out there. The dash-One explicity describes the engines and they were not "turbo ramjets". Ben Rich nailed the speed limits being compressor inlet temp ( like the F-104) and inlet shockwave management ( they are highly related), duhhhh My friends that flew the thing, including Shul, may tell you their "indicated mach", but it would not be more than a few decimal points above the tech order numbers.

For now and until we see the "Aurora" numbers, I'll stick with the Blackbird speed and altitude numbers.

Gums sends...