# SR-71 Top Speed Slowly Revealed



## FLYBOYJ (May 21, 2020)

2193.2 mph. SR-71 top speed. That always seemed to be the "party line" when the Black Bird set that speed record in 1976, but it was a known fact that the aircraft could fly a lot faster. If you do the math, 2193.2 mph equates to mach 2.88. It was well reported that the aircraft could push beyond mach 3 but it seems there's been little "official documents" indicating this.

A few weeks ago I caught a You Tube clip talking about the SR-71 and how the ram jet and intake chine worked. Then I noticed something that caught my eye. As the system was explained I recognized a USAF TO shown on the clip. For those of us who worked with TOs, they could be pretty accurate describing system functions. The first thing I noticed was the classified caveat at the top of the page was crossed out. As I paused the clip more revealing information came to light. Well I'll just post some photos...












The lower photo shows engine airflow and blow out doors operating at Mach 3.2 which on my calculator equals 2435.86. I did read that the flight manual says Mach 3.3 (would love to see one).

Has anyone else ever seen anything like this?

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## FLYBOYJ (May 21, 2020)

Well it's been said around here many times, "Google is your friend."

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## Micdrow (May 22, 2020)

Yep on the google part, found this a while back for my son. Hope it helps and is kind of interesting.

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## Joe Broady (May 26, 2020)

[QUOTE="FLYBOYJ, post: 1556649, member: 1128The lower photo shows engine airflow and blow out doors operating at Mach 3.2 which on my calculator equals 2435.86.[/QUOTE]

I see a couple things wrong with the math. First, 2435.86 mph at Mach 3.2 implies Mach 1 = 761 mph. That's true at sea level. In reality, the SR-71 does its thing at, what, 80,000 feet? Plug that altitude into a standard atmosphere calculator (there are many on the web) and get 667 mph. So Mach 3.2 at 80,000 feet equals 2130 mph, not far from the 2193.2 mph "party line" speed quoted earlier.

The second problem is excessive precision in that calculated value of 2435.86 mph, which has six significant digits of precision. On the other hand, the illustration is marked "Mach 3.2." That has two significant digits. You've heard the saying, "garbage in, garbage out." Well, there's no magic way to get output accurate to six digits from input accurate to two digits.

The rounding error in 3.2 could be a few hundredths, and each .01 Mach at 80,000 feet is about 7 mph, so really it makes sense to round mph to the nearest multiple of 10 or the nearest whole number. Of course you can calculate to .01 mph, but the decimal part is just garbage.

I knew a math teacher who had his students compute the diameter of a tree trunk with a tape measure of its circumference. Left to their own devices, most gave the answer to 10 digits because that's what their calculators said. A few with superior instinct for numbers would give the result to the nearest inch. They understood the unavoidable errors in the process made higher precision unrealistic. Garbage in, garbage out.


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## FLYBOYJ (May 27, 2020)

Joe Broady said:


> I see a couple things wrong with the math. First, 2435.86 mph at Mach 3.2 implies Mach 1 = 761 mph. That's true at sea level. In reality, the SR-71 does its thing at, what, 80,000 feet? Plug that altitude into a standard atmosphere calculator (there are many on the web) and get 667 mph. So Mach 3.2 at 80,000 feet equals 2130 mph, not far from the 2193.2 mph "party line" speed quoted earlier.



The "Party Line" is what the aircraft was actually clocked at in 1976, and what was accepted as a world's speed record by the Fédération Aéronautique Internationale (FAI), it's their math, not mine. They only been around since 1905, so maybe you know something they don't.



Joe Broady said:


> The second problem is excessive precision in that calculated value of 2435.86 mph, which has six significant digits of precision. On the other hand, the illustration is marked "Mach 3.2." That has two significant digits. You've heard the saying, "garbage in, garbage out." Well, there's no magic way to get output accurate to six digits from input accurate to two digits.
> 
> The rounding error in 3.2 could be a few hundredths, and each .01 Mach at 80,000 feet is about 7 mph, so really it makes sense to round mph to the nearest multiple of 10 or the nearest whole number. Of course you can calculate to .01 mph, but the decimal part is just garbage.



Again I think you're trying to split hairs - that illustration is from a USAF technical publication probably developed with the assistance of the manufacturer. It was printed to show maintainers how the system worked at projected speeds. So tell you what, for argument's sake let's just round out your calculations to the lowest mach number. BTW, if you think this is sooo wrong there are dozens of world speed records presented this way. Here's a few:






Joe Broady said:


> I knew a math teacher who had his students compute the diameter of a tree trunk with a tape measure of its circumference. Left to their own devices, most gave the answer to 10 digits because that's what their calculators said. A few with superior instinct for numbers would give the result to the nearest inch. They understood the unavoidable errors in the process made higher precision unrealistic. Garbage in, garbage out.


Yea, but your math teacher more than likely didn't design an aircraft that flew more than mach 3 or for that matter any aircraft that established a speed record that was recorded by the FAI - some in "six significant digits of precision."

Bottom line - the SR-71 was capable of speeds in excess of 2193 (decimal points excluded). For the longest time official documentation of it's top speed (MPH or Mach) was never found published in an offical document.

So we'll just address this in mach number - 3.2 at 80,000 feet. 3.3 if authorized by the Commander, just don't let the CIT exceed 427C.

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## Benjdragon (May 28, 2020)

I knew someone who flew in one and said that they hit Mach 3.4 on the flight. He was in the rear seat and just being transported (not a flight crew). The pilot hinted that the plane could go faster.

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## chuter (May 28, 2020)

The pilot I talked to at Beale said that every high speed part of each mission was fully scripted and essentially an operational "test flight".

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## pbehn (May 28, 2020)

In the days when I used to read my brothers "Observers book of Aircraft" like a Bible performance for the SR-71 and Mig 25 were very vague. Now we have Mach 3.2 and 2.8 (I remember Mach 3+ and Mach 2.5 plus) but the Mig 2.5 is an aerodynamic brick in comparison, I still don't believe a word of what is written.


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## Joe Broady (Jun 2, 2020)

FLYBOYJ said:


> The "Party Line" is what the aircraft was actually clocked at in 1976, and what was accepted as a world's speed record by the Fédération Aéronautique Internationale (FAI), it's their math, not mine.



Yes, exactly. That number is within 3% of my conversion from Mach 3.2 to mph, which suggests I didn't go far wrong in my math.



FLYBOYJ said:


> Again I think you're trying to split hairs - that illustration is from a USAF technical publication probably developed with the assistance of the manufacturer. It was printed to show maintainers how the system worked at projected speeds. So tell you what, for argument's sake let's just round out your calculations to the lowest mach number. BTW, if you think this is sooo wrong there are dozens of world speed records presented this way. Here's a few:



I don't see what that has to do with the discussion since nobody ratifies a speed record based on a book figure. The plane has to be timed over a known distance. For example,

1990 SR-71 record flight

See chart #3. The speed course is a set of "timing gates" defined by latitude and longitude. Distance between gates is calculated to 0.01 mile and the times were taken to 0.01 second. From that they calculate speed to 0.01 mph. That's reasonable because the inputs to the computation are so precise. It's _not_ reasonable to believe anything like that can be achieved from a single number in a maintenance manual.

But really, I don't see it as a big deal if a number has an excessive number of decimal places — _as long it was computed correctly_. That's where I think you went wrong getting 2435.86 mph from Mach 3.2. Dividing one by the other gives 761.206 mph = Mach 1. That number is easily verified online, for example,

speed of sound

But there's a catch. That speed is valid at standard sea level conditions, and no SR-71 hit Mach 3.2 at sea level. What we need is the speed of sound at some nominal cruise altitude, say 80,000 feet. No problem. There are many online calculators for standard atmosphere properties. For example:

1976 standard atmosphere calculator

Enter 80,000 feet _geopotentia_l altitude (that's how aircraft altitude is normally expressed) and read 977.822 feet per second as the speed of sound. That equals 666.697 mph, not 761.206.

In that calculation I indulged in excessive decimal places myself. Remember that I picked 80,000 feet. The book illustration doesn't give an altitude, so I just guessed 80,000 would be a reasonable value. But what if the SR-71 is flying at 81,000? Then I get 978.496 fps = 667.156 mph. That's almost half a mile per hour faster. Thus, my three decimal places are excessive since altitude is such a wobbly number.

But let's run with the speed of sound at 81,000. Then Mach 3.2 = 3.2 times 667.156 mph = 2135 mph. That's 200 mph different from your 2435.86! If you still stand by that speed, show how it's calculated. Present a convincing argument, and I'll own up to being wrong!

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## MIflyer (Jun 2, 2020)

Yes, mach number varies greatly with altitude, although in reality it is due mainly to air temperature rather than pressure.

Amazing thing I read about the SR-71 was that at Mach 3 the engines only provided 17% of the required thrust, the rest coming off the airframe.

The SR-71 has a two-step cockpit pressurization approach, which goes to the usual 8,000 ft and then goes to 20,000 ft after it is above 50,000 ft.


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## FLYBOYJ (Jun 2, 2020)

Joe Broady said:


> Yes, exactly. That number is within 3% of my conversion from Mach 3.2 to mph, which suggests I didn't go far wrong in my math.
> 
> 
> 
> ...



Joe - you just made my head hurt, sit back and put the slide rule away. First *THERE IS NO DISCUSSION HERE!* OK my math is wrong, you're right!

*Do you want a lollypop or plastic trophy?!?! *

*The point behind this post is for many years the SR-71's top speed (mach, MPH, knots, kps, whatever) was never found in a technical publication made available to the public!!!!!*

*AND - the aircraft was able to fly faster than what was indicated in the 1976 speed record!!!!*


*Now put it to bed and stop acting like Sheldon Cooper!!!*


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## GrumpyOldCrewChief (Jun 3, 2020)

I had always thought that one of the rules in any online forum was "don't piss off the mods", but maybe not everybody got that memo...

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## gjs238 (Jun 5, 2020)

Benjdragon said:


> I knew someone who flew in one and said that they hit Mach 3.4 on the flight. He was in the rear seat and just being transported (not a flight crew). The pilot hinted that the plane could go faster.



Was that guy's name Jack Ryan?

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## Joe Broady (Jun 6, 2020)

FLYBOYJ said:


> *Do you want a lollypop or plastic trophy?!?! *
> 
> *The point behind this post is for many years the SR-71's top speed (mach, MPH, knots, kps, whatever) was never found in a technical publication made available to the public!!!!!*
> 
> ...



I don't want any trophy, just a civilized discussion. It's not necessary to shout. I would have expected a moderator, of all people, to be a model in that respect. If you goofed the math, OK, it happens. And with a large and technically astute readership, someone will notice your slip. Years ago, in an astronomy discussion group, I posted my own precise calculation of the time when the approaching equinox would occur. It didn't take long before somebody explained that the formal definition of "equinox" was based an a different angle than my computation. In other words, my "right to the second" value was in reality only accurate to a minute or so. Oops.

When that happens, just tell yourself public embarrassment is a small price to pay for getting smarter. Nowadays when the subject comes up I can give precise equinox and solstice times, and they really are accurate to the second, thanks to that guy who corrected my mistake. I can't remember the name, but whoever you were, thanks. You made me a stronger man.

Getting back to the SR-71, the pilot manual was declassified years ago and can be bought in printed form. It's even online. Here's a graph of its limit speed. (The engine airflow illustration in your first post is also in the manual.)

limit speed and altitude envelope


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## FLYBOYJ (Jun 6, 2020)

Joe Broady said:


> I don't want any trophy, just a civilized discussion. It's not necessary to shout. I would have expected a moderator, of all people, to be a model in that respect. If you goofed the math, OK, it happens. And with a large and technically astute readership, someone will notice your slip. Years ago, in an astronomy discussion group, I posted my own precise calculation of the time when the approaching equinox would occur. It didn't take long before somebody explained that the formal definition of "equinox" was based an a different angle than my computation. In other words, my "right to the second" value was in reality only accurate to a minute or so. Oops.
> 
> When that happens, just tell yourself public embarrassment is a small price to pay for getting smarter. Nowadays when the subject comes up I can give precise equinox and solstice times, and they really are accurate to the second, thanks to that guy who corrected my mistake. I can't remember the name, but whoever you were, thanks. You made me a stronger man.
> 
> ...



Ya know, I don't need you to school me and I told you to drop it, read the forum rules, now go sit on the beach.

BTW - after my first post on this I knew my math was wrong, it was based off an calculator that set the pressure altitude at sea level. (go figure that out your self while you sit on the beach!) If you would have simply posted, "hey I think your math is off" I would have said, "Ya know you're right," and pressed on. Instead you tried to babble on and impress us with unwanted information which to me was not impressive. I did not request a tutorial on the significance of 2 or three decimal numbers, discussions about density altitudes or did I want to challenge you to a math debate. I tried cynically to get you to knock it off. A more direct approach didn't seem to work. My dad used to have a saying "so smart but so dumb."

Go back and read "Grumpyoldcrewchief's" post. If you decide to return I will not have this conversation with you again!


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## Aeroweanie (Aug 21, 2021)

Waaay back in the early 1990s, I was told by a friend who worked at *** that the World's Fastest Air Breathing Man and Woman are:

Darryl Greenamyer M=3.6 (I don't know if this was in a SR-71, YF-12 or an A-12)

Marta Bohn-Meyer M=3.2 (she was one of the NASA backseaters)


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## GrauGeist (Aug 21, 2021)

The top two fastest jet aircraft record holders are the YF-12 Archangel (second place) and the SR-71 Blackbird (first place).
However, the absolute undisputed champ is the Shuttle Columbia, STS-2 mission, 14 November 81, with a recorded speed of 17,500 mph (mach 22.8) 😳

Anyway, that was a good catch, Joe - it's been said that the Blackbirds could go much faster, but their actual top-end has always been kept hust-hush.
It was said that back around '85 (as I recall, I may be off on the year), an SR-71 driver put the throttle to the firewall and outran a SAM while overflying Libya. Word has it he pushed over mach 3.5 during the event.

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## FLYBOYJ (Aug 21, 2021)

And....

_"On This Date, Aug. 21, 1967: The modified North American Aviation X-15A-2, 56-6671, made the first of two flights with a heat-protective ablative coating, designed to protect the steel structure of the rocketplane from the extreme heat of flight at high Mach numbers. This was the 186th flight of the X-15 program. After a landing accident which caused significant damage to the Number 2 X-15, it was rebuilt by North American. A 28-inch “plug” was installed in the fuselage forward of the wings to create space for a liquid hydrogen fuel tank which would be used for an experimental “scramjet” engine that would be mounted the the ventral fin. The modified aircraft was also able to carry two external fuel tanks. It was hoped that additional propellant would allow the X-15A-2 to reach much higher speeds. The external tanks were not carried on this flight. With Maj. William J. “Pete” Knight in the cockpit, the X-15A-2 was airdropped from the Boeing NB-52B Stratofortress known as Balls 8, over Hidden Hills Dry Lake, just on the California side of the border with Nevada. This was Knight’s 11th X-15 flight, and the 52nd flight for 56-6671. The launch time was 10:59:16.0 a.m., PDT. Knight fired the 57,000-pound-thrust Reaction Motors XLR99-RM-1 rocket engine and accelerated for 82.2 seconds. The purpose of this flight was to attain a high speed rather than altitude. The X-15A-2 reached Mach 4.94 at 85,000 feet and reached a peak altitude of 91,000 feet. Knight touched down on Rogers Dry Lake at Edwards Air Force Base, Calif., just 7 minutes, 40 seconds after launch."_

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