SR-71 top speed

Unread postPosted: 25 Sep 2018, 02:22
by tank-top
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?

Re: SR-71 top speed

Unread postPosted: 25 Sep 2018, 05:47
by wrightwing
There are anecdotes about hitting M3.5 briefly.

Re: SR-71 top speed

Unread postPosted: 25 Sep 2018, 06:22
by sprstdlyscottsmn
I spent some time deconstructing pilot accounts and came up with 3.88.

Re: SR-71 top speed

Unread postPosted: 26 Sep 2018, 02:12
by Gums
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...

Re: SR-71 top speed

Unread postPosted: 26 Sep 2018, 03:28
by sferrin
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).

Re: SR-71 top speed

Unread postPosted: 26 Sep 2018, 14:07
by sprstdlyscottsmn
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.

Re: SR-71 top speed

Unread postPosted: 26 Sep 2018, 14:42
by sferrin
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.

Re: SR-71 top speed

Unread postPosted: 26 Sep 2018, 15:31
by sprstdlyscottsmn
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/

Re: SR-71 top speed

Unread postPosted: 27 Sep 2018, 04:45
by madrat
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.

Re: SR-71 top speed

Unread postPosted: 27 Sep 2018, 12:36
by sferrin
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

Re: SR-71 top speed

Unread postPosted: 28 Sep 2018, 02:07
by h-bomb
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.

Re: SR-71 top speed

Unread postPosted: 28 Sep 2018, 02:26
by jetblast16
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.

Re: SR-71 top speed

Unread postPosted: 30 Sep 2018, 03:09
by edpop
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

Re: SR-71 top speed

Unread postPosted: 30 Sep 2018, 13:50
by duplex
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/

Re: SR-71 top speed

Unread postPosted: 30 Sep 2018, 23:49
by Gums
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...

Re: SR-71 top speed

Unread postPosted: 10 Oct 2018, 18:24
by foxhound_31
I remember hearing stories about the SR-71 reaching speeds of Mach 4.5 if not straight out Mach 5... :roll:
But on a more serious note, wouldn't it be possible to reach a ground speed of Mach 4, should the plane fly Mach 3.8 while being "carried" by a high speed upper atmosphere wind current?

It would work the same (in the opposite way) as an An-2 or WW1 fighter being able to fly "backwards" compared to the ground while taking advantage of their 35 kph stalling speed and of a 50+kph frontal wind...

Re: SR-71 top speed

Unread postPosted: 10 Oct 2018, 20:16
by jetblast16
The SR-71 was as fast as its limiting factors. Compressor inlet temperature and inlet shock wave management were of primary concern.

Re: SR-71 top speed

Unread postPosted: 10 Oct 2018, 21:05
by sprstdlyscottsmn
jetblast16 wrote:The SR-71 was as fast as its limiting factors. Compressor inlet temperature and inlet shock wave management were of primary concern.

It wasn't "Thrust", that much is for sure. It was a matter of the engine no longer being able to breathe or burning itself up.

Re: SR-71 top speed

Unread postPosted: 13 Oct 2018, 05:59
by Gums
Salute!

Actually, Spurts, I think it WAS the engine.that was the limit, same as the 104.

You can read it yourself in the Dash One. A really neat buncha info here:

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

Gums sends...

Re: SR-71 top speed

Unread postPosted: 13 Oct 2018, 12:21
by sprstdlyscottsmn
Over 1,000 pages? Not something to read on the phone before coffee. I'll have a look a bit later. Thanks Gums.

Re: SR-71 top speed

Unread postPosted: 13 Oct 2018, 23:36
by aaam
Two other things that need to be considered:

First, the ability of the fuel to act as a heat sink for the aircraft skin and second the shock wave off the nose. As the airspeed increases, the angle of the shock wave gets narrower. At a certain point it will start to impinge on the wingtips, with disastrous consequences

Re: SR-71 top speed

Unread postPosted: 17 Oct 2018, 16:10
by Gums
Salute!

c'mon, AAAM, we call them oblique shockwaves, and that's what you get once above the mach. No problems with the oblique things interacting with other parts of the plane. In fact, some planes of the era and maybe today have "diamond" airfoils that exploit the pressure diferential of the oblique shockwaves. Spurts might chip in here.

The "normal" shockwave ( 90 deg from flight path) is found in the intake, so the air is subsonic entering the compressor section. The SR-71 Dash One ref I provided has a good section on how the intake and motor divert some of the incoming air, and that feature gave rise to many folks calling the J-58 a "turbo-ramjet". It wasn't, and our original Prat tech rep at Hill helped me a lot setting up academic sessions on the F100 engine. He had come from the J-58 shop and filled us in on that motor over a few beers.

The motor could have been a dual cycle turbo and ram if they had a bypass like most turbofans - annular, and the compression of the air might have been hot enuf for true ramjet operation using the bypass duct. Back in the day we had Bomarc drones for practice intercepts, and they ran at 3+ mach, so there may have been a way to have a combined cycle motor, but Lockheed and Pratt kept is simple(?, heh heh). Then there were temp and materials to worry about, as AAAM suggested.

Gums sends...

Re: SR-71 top speed

Unread postPosted: 17 Oct 2018, 21:34
by aaam
As explained to me (I don't have the background to work the point out myself), as the Blackbird goes faster, the angle of the shockwave relative to the aircraft grows more acute. At a certain point it would impinge on the widest part of the aircraft itself. I included that bit as a consideration to take into account why the SR couldn't hit those M4-5 airspeeds that some people were tossing out.

Re: SR-71 top speed

Unread postPosted: 17 Oct 2018, 22:54
by sprstdlyscottsmn
You are correct that the shockwave angle relative to the aircraft longitudinal axis decreases as Mach increases. That is now the whole story however. The angle of the shockwave is not a simple trig equation based on speed. The angle or the object itself matters. Think of the "mach angle" as the theoretical minimum "shock angle" that a "thin spike" would make, while a blunt object would make a significantly wider shockwave.

Re: SR-71 top speed

Unread postPosted: 18 Oct 2018, 16:50
by vilters
Simple answer :
The Top speed is "unknown" because nobody ever took the bird to its Top Speed.

Design and test speed was achieved at less then full power.

It was flown faster then designed during certain "escape" ops.

It was never flown at its "true mechanically possible top speed".

Re: SR-71 top speed

Unread postPosted: 18 Oct 2018, 18:25
by sprstdlyscottsmn
vilters wrote:Simple answer :
The Top speed is "unknown" because nobody ever took the bird to its Top Speed.

...

It was never flown at its "true mechanically possible top speed".

That's one way to put it. No pilot seems to have ever stopped accelerating while at full power, for safety reasons.

Re: SR-71 top speed

Unread postPosted: 20 Oct 2018, 00:20
by aaam
sprstdlyscottsmn wrote:You are correct that the shockwave angle relative to the aircraft longitudinal axis decreases as Mach increases. That is now the whole story however. The angle of the shockwave is not a simple trig equation based on speed. The angle or the object itself matters. Think of the "mach angle" as the theoretical minimum "shock angle" that a "thin spike" would make, while a blunt object would make a significantly wider shockwave.


Understood and agree. What I was trying to illustrate with the two considerations I was discussing is that are a number of considerations beyond just thrust vs. drag that determine an aircraft's top speed. I was just giving two more that needed to be taken into account. In this latter case, even if other considerations didn't come into play, the SR was not designed to function with the shock wave impinging on the wingtips. As jet blast16 said: "The SR-71 was as fast as its limiting factors".

Re: SR-71 top speed

Unread postPosted: 12 Jul 2019, 19:54
by piston
Gums wrote:Salute!


Gums sends...


Please, help with the turn radius of that bird, say at 3.3 M and 24 000 km altitude SR-71 was forced to turn as tide as it can....?

Re: SR-71 top speed

Unread postPosted: 12 Jul 2019, 21:39
by sprstdlyscottsmn
Texas.

Re: SR-71 top speed

Unread postPosted: 13 Jul 2019, 15:05
by piston
sprstdlyscottsmn wrote:Texas.


Image

Re: SR-71 top speed

Unread postPosted: 13 Jul 2019, 22:47
by outlaw162
Look at it this way, if in a level turn at FL800, that mach 3.2 turn radius of 50 NM at 44 degrees of bank requires a bone-crushing 1.4 Gs....

....and these guys didn't wear g-suits.

(maybe not all of Texas haha :D , but a sizable portion of I-35, including the never-ending construction....what happens when the sun burns out? They'll have to finish I-35 in the dark)

Re: SR-71 top speed

Unread postPosted: 14 Jul 2019, 03:04
by sferrin
Or as we say in Utah, "there are only two seasons; winter and road construction."