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SR-71 top speed?

flanker

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Hello, on a different forum there was started a discussion regarding SR-71 top speed. I have always considered it to be around 3.500 km/h. However, a user on that forum said that Brian (SR-71 driver) claimed to have achieved 3.68 mach over Libya in a book called The Untouchables. Mach number sounds fantastic, but somewhat surreal as well. You can read a short version of his Libya story here:

http://gizmodo.com/5511236/the-thrill-of-flying-the-sr+71-blackbird?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+gizmodo%2Ffull+%28Gizmodo%29&utm_content=Google+Reader

Here is the quote from one of my posts, calculating the mach number etc:

"
I Googled "mach 1" and one of the hits was "340.29 m/s". Multiply by 3,600 to get meters/hour, then divide by 1,000 to get km/hr. I didn't delve into it but I assume a standard day at mean sea level. I don't know what computations are involved in determining mach at altitude.

I have googled a bit (because i am interested in this), and found this, read mach92's reply:

http://answers.yahoo.com/question/index?qi...06100930AARLlWb

So at 80 000 feet the mach number should be around:

573.8 x 1.852 = 1062.7 km/h

573.8 x 1.151 = 660.4 miles/h

Even tho i messed up the second with miles, my calculations seems to be still right (please, correct me if i got it wrong, i tripple checked but it might still be wrong), 2250 miles per hour, or 3620 km/h.

To convert 3620 km/h into mach number you just need to use this simple equitation. 3620/1062.7=3.407 So the mach number around that time seems to be 3,41. And to quote:

The plane was flying a mile every 1.6 seconds, well above our Mach 3.2 limit. It was the fastest we would ever fly.

So that kinda contradicts with the mach number given later, 3,5. It is true he says that SR-71 continues to rise in speed, but we dont know how much, maybe just extra 05 mach, or maybe extra 20 mach. :D

To come back to the original statement of 3.68 that turns out to be 3,910 km/h, or 2429.56 miles per hour. I still hard time beliving mach 3.68 to be honest, but i will ask on a forum with a lot of knowledge.

Interesting discussion. :cheers:"

So, i wanted to hear several things:

1 - Are my calculations wrong?
2 - Could anyone confirm if mach 3.68 number was written in that book? Just to double check.
 

DSE

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flanker said:
Hello, on a different forum there was started a discussion regarding SR-71 top speed. I have always considered it to be around 3.500 km/h. However, a user on that forum said that Brian (SR-71 driver) claimed to have achieved 3.68 mach over Libya in a book called The Untouchables. Mach number sounds fantastic, but somewhat surreal as well. You can read a short version of his Libya story here:

http://gizmodo.com/5511236/the-thrill-of-flying-the-sr+71-blackbird?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+gizmodo%2Ffull+%28Gizmodo%29&utm_content=Google+Reader

Stuff snipped for brevity.

1 - Are my calculations wrong?

The crux of this is just what level of precision of atmospheric model do you wish to use for the calculation? To do this as accurately as possible you need the lat/long, date and altitude of the flight to determine the local atmospheric state from a model such as Gramm 95/99/2007 and if possible then use some local measurements if available to tweak the data in the model.

Earth Global Reference Atmospheric Model (GRAM99): Short Course
http://hdl.handle.net/2060/20070039071

The NASA MSFC Earth Global Reference Atmospheric Model-2007 Version
http://hdl.handle.net/2060/20090004463

Then you might consider taking these models back in time to the date of the flight itself. Finally you might add in the effect of local winds.

This is one of the "devils are in the details" when doing trajectory reconstruction for flight tests. For example, see Hyper-X Post-Flight Trajectory Reconstruction, http://hdl.handle.net/2060/20040111310

The details may not be trivial.
 

SOC

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vMax for the SR-71A is a little arbitrary. The jet is limited by the compressor inlet temperature. A maximum of 427 degrees celsius can be tolerated.
 

RyanCrierie

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The problem is, SR-71 top speed is completely arbitrary.

Why?

Because no two SR-71s were alike -- they were all hand built aircraft -- this is why it was so expensive to operate them -- each individual aircraft required it's own spares inventory. You can see this in very limited production planes like the B-2 as well.

The fleetwide safe average speed for SR-71As was Mach 3.2 -- the maximum speed of the slowest aircraft. As you can tell, some aircraft exceeded that by a considerable margin - Mach 3.5+ being one of the examples.

A good example of this kind of arbitrariness is YF-12 60-6934. She was involved in a landing accident in '66 which destroyed the front half of the plane.

But a static test model of a SR-71 was laying around, so they combined the static test model and what was left of YF-12 60-6934 to create SR-71 61-7981.

Old 61-7981 was referred to as "The Bastard" because it never flew quite right; and trimming it was virtually impossible; so the pilot had to fight the plane all the way. This was quite a chore on long flights.
 

Sundog

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According to the book I have on the missions of the Blackbird, the top speed a Blackbird ever saw was M=3.72 IIRC. They could fly from M=3.2 to 3.4 on a mission, but to go faster than 3.5 they needed permission, since it would ruin the engine(s). In the book, the highest mission speed in it was a Mach 3.55 run over Hanoi. The real question is, how fast was the A-12? It was the precursor of the SR-71, for the CIA, was a single seat and was lighter than the SR-71. The design specs called for M=4 @ 120,000 ft., but it hasn't been stated how close the A-12 came to meeting that spec.
 

Abraham Gubler

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flanker said:
I have googled a bit (because i am interested in this), and found this, read mach92's reply:

Calculating air speeds at altitude is a pretty common problem facing aviators. There are many tools available to do it rather than trying to reverse engineer the problem using Google. Most typical aviator tools however don’t go as high as 85,000 feet because its just not needed when you want to plan day tripping in your Cessna. However NASA has an all altitudes calculator (for both Earth and Mars!) online at:

http://exploration.grc.nasa.gov/education/rocket/sound.html

At 85,000 feet Mach 3.68 is a true air speed (or ground speed) of 2,443 mph or 3,931 kph. On Mars it would only be 2,890 kph, though I doubt the SR-71 could fly so high in that thin air.
 

sferrin

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Abraham Gubler said:
flanker said:
I have googled a bit (because i am interested in this), and found this, read mach92's reply:

Calculating air speeds at altitude is a pretty common problem facing aviators. There are many tools available to do it rather than trying to reverse engineer the problem using Google. Most typical aviator tools however don’t go as high as 85,000 feet because its just not needed when you want to plan day tripping in your Cessna. However NASA has an all altitudes calculator (for both Earth and Mars!) online at:

http://exploration.grc.nasa.gov/education/rocket/sound.html

At 85,000 feet Mach 3.68 is a true air speed (or ground speed) of 2,443 mph or 3,931 kph. On Mars it would only be 2,890 kph, though I doubt the SR-71 could fly so high in that thin air.

At least a few SR-71 flights went as high as 85,000 feet as listed in "Beyond the Secret Missions". The A-12 went Mach 3.6 and 90,000ft+ in testing (it was either 92k or 97k). The A-12 info is listed in Airtime Publishing's "Classic Wings" issue on the Blackbird.
 

bobbymike

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Yes I am stating the obvious to all members here but what an incredible aircraft doing this over 50 years ago. I have said before that I cannot believe there is not something else flying that out paces this. I hold out hope for Groom Lake as to think otherwise is quite depressing :-\
 

Abraham Gubler

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bobbymike said:
I have said before that I cannot believe there is not something else flying that out paces this. I hold out hope for Groom Lake as to think otherwise is quite depressing :-\

Even the slowest LEO satellite travels at about 28,000 kph which is at least seven times faster than the SR-71.
 

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AG - Oh how clever of you. :p Of course I am talking about manned aircraft (airbreathers) operating in the atmosphere not satellites. Kinda apples to apples not apples to carrots.
 

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I have published this before and the technique works out pretty well plus
it shows what you can do with simple Aerospace Engineering (AE) 101 equations.

As SOC indicated earlier, the technique uses the CIT (Compressor Inlet Temperature)
max for the J58 and looks up important parameters from the SR-71 dash-1 flight manual.

Per the SR-71 Dash-1 flight manual, pg. 1-18, "Compressor Inlet Temperature (CIT) Gage",
"A dial indicating CIT gage is mounted on the left side of the pilot's instrument panel.
L (left) and R (right) needles indicate the total (ram) air temperature forward of the first
compressor stage in the corresponding engine inlet. ..."

The key word here is "total ... air temperature". This means something very specific to
an AE person. If you look in a basic aero textbook, this is a reference to:
"total temperature" for which there is a very simple algebraic equation:
T0/T = 1 + ((lambda - 1)/2) * M**2.

T0 is the total temperature, or the temperature the air would be at if it was isentropically
(ie:without losses, or modelling a perfect inlet) brought to rest from its freestream speed,
or maximum mach number (what is bing discussed) of the SR-71.
T is the ambient temperature at the flight altitude (say 85,000 ft).
lambda is the ratio of specific heats, or 1.4. As far as we're concerned, a constant.
M is the Mach number.

So why is this important?

Because Bill Brown the head of the Pratt&Whitney J58 program has been quoted in a number of
publications (ex: Lockheed Horizons Winter 1981/82), that CIT design was for 800 deg F
(427 deg C or 700 deg K).

Also, the SR-71 dash-1 flight manual on pg 1-20, Fig 1-11, "Compressor Bleed and IGV Shift Schedule"
on the bottom axis, which is both in Mach Number and CIT deg C, the CIT scale goes up to
400 deg C, again agreeing with what Bill Brown said in Lockheed Horizons (above).

So let's use the simple formula I mentioned, to calculate the Mach number
corresponding to CIT max (427 deg C, 700 deg K, or 800 deg F), at 85,000 ft, with lambda at 1.4.

The original equation (above) can be rearranged to solve for Mach No as follows:
((T0/T) - 1) / ((lambda - 1)/2) = M**2

Now just plug in the numbers:
T0 = 700 deg K (CIT max per Bill Brown and the SR-71 dash-1 Fig 1-11)
T = 219 deg K (ambient temp at roughly 85,000 ft - look this up in an aero engineering book)
Lambda = 1.4
We will solve for Mach number given these input parameters, as follows:

((700/219) - 1) / ((1.4 - 1)/2) = M**2
(3.196 - 1) / (.4/2) = M**2
2.196 / .2 = M**2
10.98 = M**2
3.31 = M

There you have it !!

Max mach number of 3.31 at 85K ft. corresponding to a CIT max of 427 deg C
per the SR-71 dash-1 flight manual. By the way, this completely agrees with
the Mach number scale at the bottom of Fig 1-11 in the SR-71 Dash-1 as
well, but instead of reading it off the axis we calculated it.

Now another tidbit! Evidently Mr. Brown wasn't telling the whole story, evidently.

I noticed in the SR-71 dash-1, that the CIT gage has a max reading.
Previously we referenced a graph (called: "Compressor Bleed and IGV Shift Schedule"),
but here we're looking at the actual CIT instrument or guage which has a "max" reading.
That max reading is 500 deg C (932 deg F, 773 deg K).

So when one plugs and chugs using CIT "max":
((773/219) - 1) / ((1.4 - 1)/2) = M**2
(3.530 - 1) / (.4/2) = M**2
2.530 / .2 = M**2
12.65 = M**2
3.56 = M

So based on CIT "max", M = 3.56 (given our assumptions).
 

flanker

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Abraham Gubler said:
At 85,000 feet Mach 3.68 is a true air speed (or ground speed) of 2,443 mph or 3,931 kph. On Mars it would only be 2,890 kph, though I doubt the SR-71 could fly so high in that thin air.

Ah, very cool. Calculated just 21 km/h wrong. :)

shockonlip said:
I noticed in the SR-71 dash-1, that the CIT gage has a max reading.
Previously we referenced a graph (called: "Compressor Bleed and IGV Shift Schedule"),
but here we're looking at the actual CIT instrument or guage which has a "max" reading.
That max reading is 500 deg C (932 deg F, 773 deg K).

I believe this is the same case we see in the cars: Gauge is maybe showing top speed as 250 km/h, but in reality it can go beyond 220 for example. I am not an expert on SR-71, but 427 number is quoted many different places, so i guess it is the true top temperature, or something close to it.

But very interesting stuff shockonlip, and thanks for everyone else that shared knowledge on SR-71. ;D

I am still hesitant to believe mach number of 3.68 or even above...As i said earlier, it just sounds surreal. Also, a quote:

I don't fully understand all the theorectical math formulas mentioned above, but as a 12 year veteran of mission planning for the SR-71 I am intimately familiar with the planes actual capabilities. It routinely flew at 80,000 feet, plus or minus a couple thousand and a speed of Mach 3.0 to 3.2 while being capable of 3.5 or slightly higher. I have never seen 3.68 mach on a mission recorder tape, but I suppose it is possible under ideal conditions, which Major Schul alluded to several times during that Libya mission.

Darwin
 

sferrin

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shockonlip said:
I have published this before and the technique works out pretty well plus
it shows what you can do with simple Aerospace Engineering (AE) 101 equations.

As SOC indicated earlier, the technique uses the CIT (Compressor Inlet Temperature)
max for the J58 and looks up important parameters from the SR-71 dash-1 flight manual.

Per the SR-71 Dash-1 flight manual, pg. 1-18, "Compressor Inlet Temperature (CIT) Gage",
"A dial indicating CIT gage is mounted on the left side of the pilot's instrument panel.
L (left) and R (right) needles indicate the total (ram) air temperature forward of the first
compressor stage in the corresponding engine inlet. ..."

The key word here is "total ... air temperature". This means something very specific to
an AE person. If you look in a basic aero textbook, this is a reference to:
"total temperature" for which there is a very simple algebraic equation:
T0/T = 1 + ((lambda - 1)/2) * M**2.

T0 is the total temperature, or the temperature the air would be at if it was isentropically
(ie:without losses, or modelling a perfect inlet) brought to rest from its freestream speed,
or maximum mach number (what is bing discussed) of the SR-71.
T is the ambient temperature at the flight altitude (say 85,000 ft).
lambda is the ratio of specific heats, or 1.4. As far as we're concerned, a constant.
M is the Mach number.

So why is this important?

Because Bill Brown the head of the Pratt&Whitney J58 program has been quoted in a number of
publications (ex: Lockheed Horizons Winter 1981/82), that CIT design was for 800 deg F
(427 deg C or 700 deg K).

Also, the SR-71 dash-1 flight manual on pg 1-20, Fig 1-11, "Compressor Bleed and IGV Shift Schedule"
on the bottom axis, which is both in Mach Number and CIT deg C, the CIT scale goes up to
400 deg C, again agreeing with what Bill Brown said in Lockheed Horizons (above).

So let's use the simple formula I mentioned, to calculate the Mach number
corresponding to CIT max (427 deg C, 700 deg K, or 800 deg F), at 85,000 ft, with lambda at 1.4.

The original equation (above) can be rearranged to solve for Mach No as follows:
((T0/T) - 1) / ((lambda - 1)/2) = M**2

Now just plug in the numbers:
T0 = 700 deg K (CIT max per Bill Brown and the SR-71 dash-1 Fig 1-11)
T = 219 deg K (ambient temp at roughly 85,000 ft - look this up in an aero engineering book)
Lambda = 1.4
We will solve for Mach number given these input parameters, as follows:

((700/219) - 1) / ((1.4 - 1)/2) = M**2
(3.196 - 1) / (.4/2) = M**2
2.196 / .2 = M**2
10.98 = M**2
3.31 = M

There you have it !!

Max mach number of 3.31 at 85K ft. corresponding to a CIT max of 427 deg C
per the SR-71 dash-1 flight manual. By the way, this completely agrees with
the Mach number scale at the bottom of Fig 1-11 in the SR-71 Dash-1 as
well, but instead of reading it off the axis we calculated it.

Now another tidbit! Evidently Mr. Brown wasn't telling the whole story, evidently.

I noticed in the SR-71 dash-1, that the CIT gage has a max reading.
Previously we referenced a graph (called: "Compressor Bleed and IGV Shift Schedule"),
but here we're looking at the actual CIT instrument or guage which has a "max" reading.
That max reading is 500 deg C (932 deg F, 773 deg K).

So when one plugs and chugs using CIT "max":
((773/219) - 1) / ((1.4 - 1)/2) = M**2
(3.530 - 1) / (.4/2) = M**2
2.530 / .2 = M**2
12.65 = M**2
3.56 = M

So based on CIT "max", M = 3.56 (given our assumptions).

Consider also that that seems to be for steady state operation. Take into account safety factor and maybe you could get a bit faster for ten or twenty minutes?
 

bob225

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Hello all,

A newbie to posting but love the forum, please be nice!!

Found this site http://www.u2sr71patches.co.uk/sr71performance.htm

No idea if it's right but thought someone with more brains than me would know better.

Have been very fortunate to see this majestic plane fly a number of times, the noise and sight of her was truly amazing, like watching a sci-fi movie for real!
:-\
 

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