What is the REAL top speed of the SR-71 and its derivatives?

The US military LOVES to classify everything about its advanced aircraft. Still today, many people (including me) believe that the SR-71 and its variants are capable of much more than what is released. I have seen claims from as low as Mach 3 all the way up to Mach 14. The average claim is 3.35 Mach. What is your opinion?

The released speed is Mach 3.2. We all know its higher than that, but how much?

The best evidence comes from the accounts of those who built the plane. i believe that an A-12 suffered an airspeed indicator malfunction and went faster than intended (something like Mach 3.41 or so) and upon landing the majority of the wiring in the wings were burned almost to the point of failure.

I suspect that the real speed of the Blackbird is in that neighborhood. However, I am at the mercy of open source literature. Given the structure of the A-12/SR-71 I doubt that it really could go much faster without suffering failure from the heat generated by the flight. Perhaps the engines were capable of going faster, but the airframe was not.

We no longer need to rely on speculation. The performance characteristics of the Blackbirds have all been fully declassified.

Mach 3.32 is 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 package. 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.

Excellent reply Shadowhawk.

It's always been the case that 'heat soak' (the effect of high speed generating sustained very high temperatures and the effect of that on the metals/composites used in airframe construction) was the determining factor.

It's nice to see the engineering reality stated instead of the usual and very silly "I bet it could....." work of vivid imaginations.

The physics, chemistry, metallurgy and aerodynamic engineering reality apply much more rigidly that guessed at political possibilities.

.......of course if they'd built it out of that super-secret pkzxbilxztq material they've been keeping from us and used the rest of the reverse-engineered Rosswell technology they've not told us about then maybe they were really capable of flying at almost light speed to those bases on the moon etc etc and that whole 'cold war titanium spy plane' tale was just a coverstory for the gullible?

You think I was joking with that last bit huh?

........well, you don't think they wore space suits for nothing do you?

Well do you?

Good post Shadowhawk.

sminkey, I believe the super secret material they looked at was called un-obtainium

I must agree 100% with Shadowhawk. Mach 3.5 is the Structural Limit for the airframe according to several newer sources I have. Pushing beyond that would cause the airframe to experience strctural failure and break up. Kelly pushed the limits of science and technology for the day just to get the Blackbird to the Mach 3- 3.5 arena.

Now I have heard altitude claims of 95,000 to 100,000 feet in the Blackbird, however, I have no source to back that up.

Origionally posted by FredT:
sminkey, I believe the super secret material they looked at was called un-obtainium

Good one Fred! Don't ET's also build their flying saucers from Unobtainium?

Tim

Claims of the SR-71 reaching altitudes of 95,000 to 100,000 feet are not accurate. You could get the airplane to 92,000 feet if it had a gross weight of 60,000 pounds. That means you would have to strip it of all mission equipment and limit the fuel load substantially.

This would be useless from an operational standpoint, obviously. Most operational missions were flown between 70,000 and 85,000 feet. The A-12 flight to 90,000 feet and the SR-71 flight to 89,650 feet were undertaken as test missions to evaluate the airplanes' performance envelopes. A great deal of planning is required to identify the proper flight profile and fuel load required for such a flight.

With regards to stress and heat damage. One prominent F-22 pilot has stated that the Raptor runs into the same problem. The engines have more than enough power (and can tolerate a lot more stress) to push the aircraft at higher speed but the airframe cannot take the high temperatures of high speed flight. Funny enough there are no electronic speed restrictions on the F-22 (it's all up to the pilot) so if a driver wanted to he could actually fly the aircraft apart.

In the middle of the sr-71 pilots cockpit there was a card which showed the max airspeed for altitude. If someone wants the max speed... the key is there... or a sr-71 manual.

I'm willing to bet the max speed was Mach 3.40 at 80000+ feet.

[edit on 28/6/07 by JimmyCarterIsSmarter]

The flight manual states it quite clearly:

"Mach 3.2 is the design Mach number. Mach 3.17 is the maximum scheduled cruise speed recommended for normal operations. However, when authorized by the Commander, speeds up to Mach 3.3 may be flown if the limit CIT of 427 degrees C is not exceeded."

Yeah, lots of false speculation that is a lot more exciting about the real reality. 3.4 would have been pushing it. Still plenty fast.






[edit on 28-6-2007 by firepilot]

I guess this just proves things can and will take on a life of their own in the absence of fact

I wonder what sort of speeds the engines would be good for given an airframe which could better cope with the heat/stress..

Really amazing when you think just how long ago these amazing aircraft were designed and built

Kinda makes you wonder just would the boffins of today could come up with given sufficient motivation and budget..

From a very interesting master's thesis on inlet designs for the SR-71 engines to remain self starting in the Mach 6-7 range, it seems that the maximum speed was never higher than Mach 3.29 for inlet reasons:

Interestingly, several websites [42,43] have claimed (through published reports) that the SR-71 flew upwards of Mach 3.5 and that the Skunk Works performed studies to see how fast the SR-71 could actually fly. The websites cited the limiting factors to be the interaction of the nose conical shock with the inlet spike and the heat load on the airframe past Mach 3.5. However, these published reports were never located and the modifications of the inlet (if any) are not known to the author. Additionally, declassified CIA reports indicate that the maximum speed obtained by the SR-71 was actually Mach 3.29 [44]. Note that previously mentioned NASA reports said that the inlet Mach number was generally 3 percent less than the flight Mach number. Three percent less than Mach 3.29 is Mach 3.2.

Perhaps the author of the master's thesis didn't try very hard to find the Lockheed reports. The results of the Lockheed studies are available in a variety of documents from the company and NASA. They are nicely summarized in the SR-71 User's Handbook, Volume II, as well as some Lockheed briefing material from 1991.

Performance extension to Mach 3.4 could have been achieved without significant aircraft modifications. Extension from Mach 3.4 to Mach 3.5 would have required insulation of hydraulic lines, special instrumentation, and possibly addition of inlet actuators.

According to the Handbook, the results of the various studies concluded that "extension to Mach 3.5, for short periods of time (of 10 minutes or more), is feasible." Performance extension beyond Mach 3.5 would have required extensive inlet modifications and yielded only small gains.

The only structural limitation to speeds above Mach 3.5 was a limt of 420 knots equivalent air speed, determined by duct pressure and temperature limitations. Other factors limiting speeds greater than Mach 3.5 included inlet capture area and excessive compressor inlet temperature (normally limited to 427 degrees C).

The aircraft had a maximum speed potential of Mach 3.8 with major inlet geometry changes and the addition of a water injection system. This option was not recommended as the benefits were relatively low considering the costs involved and adverse effects on a number of engine parameters.

Lockheed studies examined, in detail, the characteristics of a variety of propulsion parameters at incremental increases in speed from Mach 3.3 to Mach 3.7. These parameters included inlet duct pressure, recovery, unstart margin, controls, exhaust gas recirculation, and nozzle performance.

At Mach 3.3, all of these parameters were acceptable. At Mach 3.4, recovery and CIT were marginal. At Mach 3.5, only inlet duct pressure remained acceptable while all other parameters were marginal. At Mach 3.6 and 3.7, most parameters (excepting duct pressure) became unacceptable with the standard inlet. A modified inlet made recovery, unstart margin, and control parameters acceptable, but CIT was still unacceptable even with water injection.

Plans to test the proposed modifications were subsequently abandoned.

Nice post Shadowhawk.

I wonder if the max speed is based on level flight? No doubt the craft could go faster in a dive during an emergency, say to evade a missile?

Just wondering.

The Blackbirds weren't designed to take aerobatic evasive action to avoid missiles. The airplane's high-speed climb, turn, and dive characteristics required using a great deal of airspace. Instead they relied on altitude, dash speed, and electronic countermeasures to defeat and avoid hostile fire.

Originally posted by Shadowhawk
Perhaps the author of the master's thesis didn't try very hard to find the Lockheed reports.

Could be, could be not depending on how you interpret the phrasing. The studies were indeed performed but that doesn't mean it could actually fly that fast or the modifications were made. Does any of what you mentioned indicate it really flies significantly faster than Mach 3.29? Just asking.

Even if a Sr-71 could fly Mach 3.5... Do you think it would even make a diferance to survivability? Doubt it.

Just for the record, Everyone is aware of just how fast Mach 3 really is, right?

Folks, we're talking about 2'280 mph. (hint if you live in the US, the fastest posted speed on any interstate is about 65 or 70 mph.) Mach 3 is more than enough speed for about anything.

Tim