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Lockheed A-12 and SR-71 projects


CLEARANCE: Above Top Secret
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Apr 5, 2006
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uk 75

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Sep 27, 2006
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Sweeeeeet Thank you. I must search Fleabay


CLEARANCE: Restricted
Nov 23, 2009
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JimK said:
FighterJock said:
So what was faster in terms of overall speed? A-12 or SR-71A? I have heard many stories over the years that the SR-71A was faster than the A-12, could someone clear this up for me. :-\

From COMIREX-D-12.1/1 (Approved for release Date: Aug 2007)

I would interpret the equality of Mach numbers listed as a propulsion system limitation.

Thanks for that JimK, good information that has gone on to help me with my initial problem. B)
I read an unpublished interview of Bill Park (second pilot to fly the A-12) in which he didn't say the fastest or highest flights. He regarded the question as irrelevant. He said it was hard enough to get the aircraft to fly at the specified speed and altitude, do a 180° turn, and come back, so that Lockheed would have met the terms of the contract and could get paid.


I really should change my personal text
Dec 19, 2011
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The performance characteristics of the Blackbirds have all been fully declassified.

Mach 3.32 was the design cruise speed, but maximum allowable Mach number was dependent on outside air temperature and its effect on compressor inlet temperature (CIT). The pilot was authorized to accelerate to Mach 3.3 as long as CIT remained at or below 427 degrees Centigrade. Speeds exceeding Mach 3.3 were occasionally recorded, but generally the pilot tried to avoid this area of the performance envelope because it placed excessive thermal stress on the airframe.

Some maximum speed milestones:
YF-12A, 1 May 1965, Mach 3.14 (2,070 mph)
A-12, 8 May 1965, Mach 3.29 (2,171 mph)
SR-71A, 28 July 1976, Mach 3.32 (2,193 mph)

The Blackbirds were designed to cruise at 85,000 feet with a useful fuel load and reconnaissance payload. Because the A-12 was 20,000 pounds lighter than the SR-71, it had an altitude capability about 3,000 feet higher than that attained by the SR-71 at any given point in a flight profile for missions of the same range.

Some maximum altitude milestones:
YF-12A, 1 May 1965, 80,257 feet
SR-71A, 1968, 89,650 feet
A-12, 14 August 1965, 90,000 feet

In 1975, Lockheed studied the possibility of expanding the flight envelope of the SR-71 with some modifications. The results of several studies concluded the maximum speed limit could be extended to Mach 3.5 for short periods of time. The only structural limit to speeds above Mach 3.5 was a KEAS (knots equivalent airspeed) limit of 420, set by inlet duct pressures and temperatures that exceeded acceptable values. Limited inlet capture-area and excessive engine CIT also limited operation at higher Mach numbers, even with proposed modifications.

Similar studies addressed the possibility of achieving flights well above 85,000 feet. results indicated the SR-71 could briefly reach an altitude of about 95,000 feet in a zoom-climb profile. The proposed mission could have been accomplished with an airplane having a gross-weight of 85,000 pounds. According to the flight profile, the pilot would accelerate from Mach 3.2 to 3.5 at an altitude of 80,000 feet, then zoom to 95,000 feet as speed decreased to normal cruise mach numbers. The airplane would subsequently settle back down to an altitude of about 84,000 feet. Sustained flight above 85,000 feet was limited by wing surface-area and engine thrust capabilities.


I really should change my personal text
May 13, 2008
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Whisperstream, while what you have written seems the standard response, I can tell you unequivocally that the A-12 flew faster than those "speed" and "altitude" milestones you have recorded here. On the 20 November 1965, during operation Silver Javelin, an A-12 went much faster than 3.2 and flew appreciably higher than 90,000. This is alluded to in the excellent book "Lockheed Blackbird: Beyond the Secret Missions" by Paul Crickmore (revised edition), but without the actual data numbers. What was the exact speed? The aircraft went through a patch of exceptionally cold air, which is a boon to attaining higher speed. It accelerated to (based on what I was told) Mach 3.47 and perhaps an altitude of greater than 93,000 feet. (It may have been as much as 95 or 96K, but the person who was telling me this couldn't remember exactly.) I don't recall the exact duration for this unprecedented speed. Now I have heard rumors that the Mach number I have stated here actually has the two decimal places switched around wrong. But I can't say more, because I don't have any further data. But I thought I would include the rumor also.

I would welcome the actual Mach number and altitude figure for this particular mission. I would like actual facts about this. But this is what was verbally told to me.


I really should change my personal text
Jun 8, 2018
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The aircraft went through a patch of exceptionally cold air, which is a boon to attaining higher speed. It accelerated to (based on what I was told) Mach 3.47 and perhaps an altitude of greater than 93,000 feet. (It may have been as much as 95 or 96K, but the person who was telling me this couldn't remember exactly.)

Mach number is simply the ratio between the speed of the flow or aeroplane and the local speed of sound.

The local speed of sound is proportional to the square root of the absolute static temperature.

Flying through a cold patch of air therefore increases MN without changing the aircraft's speed at all. The flight manual specifically warns about this behaviour. Concorde had similar problems IIRC.

e.g. Mach 3.2 at 80,000' is 1853.5 KTAS on a standard day. At ISA - 20 K, this same TAS corresponds to Mach 3.35.

It would be possible to get 3.47 MN with a temperature change of about 35 K.

The flight envelope diagram is on the next page, followed by an illustration of MN vs OAT and CIT. On a cold day, the MN and hence TAS corresponding to a fixed CIT is increased, because the temperature ratio across the intake is a function of the MN (see e.g. equation 7).

Flying very high is easy for fast aeroplanes because of their prodigious kinetic energy; zooming 10,000' or more really wouldn't be a problem. If the autopilot was in Mach hold and was controlling MN with pitch, it's quite possible that a big temperature shear would naturally produce such a zoom climb. The most likely explanation for the very big numbers is therefore just flying through a big temperature shear with the autopilot in Mach hold mode.

Steady state is another matter. It is likely that a good example could go a bit higher than the book figure at light weights, but the chances are that the handling qualities and risk of inlet unstart were considered unacceptable when set against any increase in survivability (likely to be negligible).

The other thing to consider is altimeter position error, which might well be a function of indicated MN and might have contributed to an exaggerated increase in indicated pressure altitude.

To really understand what was going on, you'd need to compare the pitot static measurements with INS data.

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