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SR-71 Puzzle

Stormbreaker

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Just wondering if anyone knows what the tested maximum altitude and speed of the A-12/SR71 was?

I contacted the USAF recently and they simply stuck to the published figures saying it flew at 85,000ft and could reach Mach 3.3. This doesn't seem to make sense as variants of the F-4, F-104 and F-15 exceeded this altitude, along with several Soviet aircraft.

I have heard reports that the SR-71 was tested to around 110,000ft and briefly reached Mach 3.6.

Does anyone have any idea if this is true - and if so, why the information should be kept secret?
 

Trident

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AFAIK the other aircraft that exceeded this altitude did so in a ballistic zoom climb and could not sustain cruise flight up there - that's the difference. The 85000ft figure is the actual service ceiling of the SR-71 and would be much lower for the other types you mention.

That said, it is quite possible that we don't know the full story about the Blackbird's performance, but I can't see it significantly surpassing the published figures.
 

Archibald

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ok, just a question...

ALL aircrafts have improved variants over the years, including unbuilt variants (what we explore here ;) ).
I suppose the A-12 family was no exception...

so my question is
Did Lockheed drawn improved SR-71/A-12 variants in the 70's /80's ?
Improved J-58s ?
Mach 3.8 / Mach 4 speeds (to the limits of the concept)

I'm curious...
 

Stormbreaker

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Certainly the Streak Eagle, Top flight Phantom, etc were operating well above their normal service ceiling and these flights were momentary peaks that set the records. But it seems to me that if you have an exceptionally high performance aircraft that is capable of cruising above 80,000ft, you are going to push the envelope a few times during trials to see what it will do.

As I recall, the original design spec for this aircraft called for a capability of about 95-100,000 ft? As for maximum speed, it seems to me that there might have been ongoing minor upgrades and improvements to the engines that have never been publicized.
 

sferrin

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I've heard two stories as to why there is a hard edge there on the right. You start to get unacceptable heating at the compressor face and the shockwaves off the nose start to imping on the wingtips (or it could be the shock of the inlet cones I don't recall). Anyone who feels like searching for "SR-71" posted by Mary Shafer on rec.aviation.military (usenet) can get it straight from the horse's mouth so to speak. She was the lead flight engineer or some such for the Blackbird program at Dryden Flight Research Center and has answered lots of questions there. BTW that and sci.military.naval are sources of many interesting "war stories". For example one F-111 pilot said a guy in his squadron had an F111F briefly up to Mach 2.8 in a test flight. That kind of thing. edit: SOC who posts here ocassionaly would be the guy for the answer as he's done a lot of research on the Blackbird.
 

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SOC

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The maximums the A-12 hit in testing were Mach 3.29 and over 90,000 feet if I remember right. I'll see what I can dig up as I unpack over here (I just moved cross-country).
 

SOC

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Check this out, the A-12's flight manual has been released via the CIA's FOIA page.
 

sferrin

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Interesting. The graph I'm looking at has a hard edge at Mach 3.4 and 95,000 feet.
 

Sundog

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The design specs for the A-12 were Mach=4@ 120000 ft. How close it came to that is still classified AFAIK.

As for the SR-71, which had the same wing/propulsion system as the A-12, but was bigger and heavier, the pilots were cleared to take it to M=3.3 It's top speed was reported to be between M=3.55 and 3.65 depending on atmospheric conditions, but there was a possibility of engine damage over M=3.5 so they had to have clearance to go that fast. My understanding is that during one of the operational SR-71 flights over North Vietnam, I think in support of Operation Linebacker (or Linebacker II) an SR-71 made a M=3.55 run over North Vietnam. As for the SR-71's service ceiling I've only been able to find 80,000+ to 85000+ feet with no idea how much the plus is.
 

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I'm sure a "manual" has been released, and I would not be surprised if there was a little bit of factual data listed -- but I"m pretty sure it's filled with a whole bunch of bogus stuff in there in the form of half truths and whole lies.

Ever heard the term "Fudge Factor"?


KJ
 

sferrin

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KJ_Lesnick said:
I'm sure a "manual" has been released, and I would not be surprised if there was a little bit of factual data listed -- but I"m pretty sure it's filled with a whole bunch of bogus stuff in there in the form of half truths and whole lies.

Ever heard the term "Fudge Factor"?


KJ

Never. What is that?
 

F-14D

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You'd be amazed at how much of the SR and A-12, other than sensor and actual mission data, has been released. Back in the '90s, when USAF and the Administration were working hard to kill the program, they declassified a lot of stuff to give the impression that it was "no big deal", and we really need to go on to new shiny programs that the bigwigs for which can get nice promotions and titles.
 

r16

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one poster at Great Planes Forum when it was active said he had talked to a test pilot who had claimed the Blackbird was capable of a Mach 4.25 cruise . Not to challenge what has been posted on this page but just to add something which might just be true . But ı see the web is full of overclaiming and ı might be part of the same trend .
 

KJ_Lesnick

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I'm pretty sure the Blackbird is capable of exceeding Mach 4 for a simple reason: The original J-58 design, sans bleed-bypass pipes, was capable of Mach 4 -- as was it's predecessor the J-91 and its (the J-91) competitor the J-93.

With that said, the modified J-58's used on the Blackbird have almost entirely different metallurgical composition in what would appear to be both the compressor and turbine (with the turbine allegedly able to slightly expand as the turbine casing could expand too -- the engines diameter did change a few inches at high speed) which would bump up the turbine temperatures quite a bit. Air-cooling is drastically increased over the standard J-58 which further bumps up the maximum allowable turbine-inlet temperature. The fact that the bleed bypass system can dump large amounts of airflow around the engine reduces the load on the compressor and keeps the turbine temperatures from becoming excessively high (which further increases the plane's top speed) while simultaneously increasing thrust and fuel-economy. Next, the engine has two systems to reduce the fuel/air ratio in both the combustion-chamber (while maintaining same RPM) and even the afterburner which reduces the turbine inlet temperature and afterburner temperatures (and also increases fuel economy). I should at this point note that both the modified and un-modified J-58's have the moveable IGV which can operate in the axial-position which produces the highest pressure-ratio for low-speed flight, and the cambered position which produces a lower pressure-ratio for high-speed flight. The engine also incorporates active-cooling, using fuel to cool the engine and afterburner down -- the fuel also operates various engine-controls (nozzle, guide-vane) -- this is actually a big one as active cooling can drastically reduce engine temperature, and add significant speed to the design as a result. (For example, MIPCC -- which is based on water-injection, but nonetheless is a simple form of active cooling, fitted to a J-75 could probably squeeze two whole extra mach numbers out of it...)

The front of the engine (and, if not the whole compressor, sizable portions of it) has an increased diameter (54-inches, only 1 inch less than the J-91) than the original J-58 and an increased airflow to go with it, over 400 pounds a second. I would assume the enlargements are to increase thrust and also to provide extra air for cooling purposes.

When you factor all these engine-modifications into the equation you end up with an engine with the potential to easily achieve hypersonic speed. With that said, they would not have modified the engine for this extreme-capability unless the plane was designed to cruise and dash in these speed ranges.


KJ
 

F-14D

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KJ_Lesnick said:
I'm pretty sure the Blackbird is capable of exceeding Mach 4 for a simple reason: The original J-58 design, sans bleed-bypass pipes, was capable of Mach 4 -- as was it's predecessor the J-91 and its (the J-91) competitor the J-93.

With that said, the modified J-58's used on the Blackbird have almost entirely different metallurgical composition in what would appear to be both the compressor and turbine (with the turbine allegedly able to slightly expand as the turbine casing could expand too -- the engines diameter did change a few inches at high speed) which would bump up the turbine temperatures quite a bit. Air-cooling is drastically increased over the standard J-58 which further bumps up the maximum allowable turbine-inlet temperature. The fact that the bleed bypass system can dump large amounts of airflow around the engine reduces the load on the compressor and keeps the turbine temperatures from becoming excessively high (which further increases the plane's top speed) while simultaneously increasing thrust and fuel-economy. Next, the engine has two systems to reduce the fuel/air ratio in both the combustion-chamber (while maintaining same RPM) and even the afterburner which reduces the turbine inlet temperature and afterburner temperatures (and also increases fuel economy). I should at this point note that both the modified and un-modified J-58's have the moveable IGV which can operate in the axial-position which produces the highest pressure-ratio for low-speed flight, and the cambered position which produces a lower pressure-ratio for high-speed flight. The engine also incorporates active-cooling, using fuel to cool the engine and afterburner down -- the fuel also operates various engine-controls (nozzle, guide-vane) -- this is actually a big one as active cooling can drastically reduce engine temperature, and add significant speed to the design as a result. (For example, MIPCC -- which is based on water-injection, but nonetheless is a simple form of active cooling, fitted to a J-75 could probably squeeze two whole extra mach numbers out of it...)

The front of the engine (and, if not the whole compressor, sizable portions of it) has an increased diameter (54-inches, only 1 inch less than the J-91) than the original J-58 and an increased airflow to go with it, over 400 pounds a second. I would assume the enlargements are to increase thrust and also to provide extra air for cooling purposes.

When you factor all these engine-modifications into the equation you end up with an engine with the potential to easily achieve hypersonic speed. With that said, they would not have modified the engine for this extreme-capability unless the plane was designed to cruise and dash in these speed ranges.


KJ

Remember, just because an engine can remain running at a particular speed does not guarantee that an aircraft it's in can go that fast. Consider where the shock wave coming off the nose of an SR would be relative to its wingtips at a TAS of M4.25, let alone hypersonic, along with the fuel requirements. Also, a ground speed approaching M4 does not mean that the aircraft itself is capable of M4. We are talking late '50s-early '60s technology.
 

overscan (PaulMM)

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KJ, please note where you say:

The original J-58 design, sans bleed-bypass pipes, was capable of Mach 4

You actually mean

The original J-58 design, sans bleed-bypass pipes, was claimed by someone to be capable of Mach 4

These are two very different statements. For a start, you would have to look at who claimed the J-58 could be Mach 4 capable, and what they meant by it (capable of withstanding Mach 4 flight for very brief periods, or sustaining Mach 4 flight *if made from different materials*). Even if it was one of the J-58 designers who claimed it, in the mid 1950s when the J-58 was being designed the designers might have thought it would be capable of Mach 4 because, at that stage, they had no experience of Mach 2 or Mach 3 flight and didn't yet know some external limitation that would prevent Mach 4 flight, e.g., impossibility of designing an efficient enough engine intake, airframe limits, or a physical phenomenon that had not anticipated or calculated for.
 

sferrin

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F-14D said:
KJ_Lesnick said:
I'm pretty sure the Blackbird is capable of exceeding Mach 4 for a simple reason: The original J-58 design, sans bleed-bypass pipes, was capable of Mach 4 -- as was it's predecessor the J-91 and its (the J-91) competitor the J-93.

With that said, the modified J-58's used on the Blackbird have almost entirely different metallurgical composition in what would appear to be both the compressor and turbine (with the turbine allegedly able to slightly expand as the turbine casing could expand too -- the engines diameter did change a few inches at high speed) which would bump up the turbine temperatures quite a bit. Air-cooling is drastically increased over the standard J-58 which further bumps up the maximum allowable turbine-inlet temperature. The fact that the bleed bypass system can dump large amounts of airflow around the engine reduces the load on the compressor and keeps the turbine temperatures from becoming excessively high (which further increases the plane's top speed) while simultaneously increasing thrust and fuel-economy. Next, the engine has two systems to reduce the fuel/air ratio in both the combustion-chamber (while maintaining same RPM) and even the afterburner which reduces the turbine inlet temperature and afterburner temperatures (and also increases fuel economy). I should at this point note that both the modified and un-modified J-58's have the moveable IGV which can operate in the axial-position which produces the highest pressure-ratio for low-speed flight, and the cambered position which produces a lower pressure-ratio for high-speed flight. The engine also incorporates active-cooling, using fuel to cool the engine and afterburner down -- the fuel also operates various engine-controls (nozzle, guide-vane) -- this is actually a big one as active cooling can drastically reduce engine temperature, and add significant speed to the design as a result. (For example, MIPCC -- which is based on water-injection, but nonetheless is a simple form of active cooling, fitted to a J-75 could probably squeeze two whole extra mach numbers out of it...)

The front of the engine (and, if not the whole compressor, sizable portions of it) has an increased diameter (54-inches, only 1 inch less than the J-91) than the original J-58 and an increased airflow to go with it, over 400 pounds a second. I would assume the enlargements are to increase thrust and also to provide extra air for cooling purposes.

When you factor all these engine-modifications into the equation you end up with an engine with the potential to easily achieve hypersonic speed. With that said, they would not have modified the engine for this extreme-capability unless the plane was designed to cruise and dash in these speed ranges.


KJ

Remember, just because an engine can remain running at a particular speed does not guarantee that an aircraft it's in can go that fast. Consider where the shock wave coming off the nose of an SR would be relative to its wingtips at a TAS of M4.25, let alone hypersonic, along with the fuel requirements

See the X-15's burnt ventral when they had the scramjet model attached for why you don't want that to happen (shock impingment that is).
 

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Bah! Enough talk!!!


The upper limit to the SR-71s speed can be determined via simple geometry. Look at the graphic below. Two angles are shown... 17 degrees and 14 degrees. At 17 degrees, the shock wave shed from the nose of the SR-71 begins to impinge on the wing. At 14 degrees, it hits the engine. Either one of these would be Bad News for the plane; but the shock wave impinging on the engine would be DISASTROUS. It would completely foul airflow in the engine, as well as nuking the structure of the inlet spike.

Now, simple math. The angle of the Mach wave is determined from:


17 degrees works out to Mach 3.42; 14 degrees works out to Mach 4.13. The SR-71 pilot might be able to stuff his plane past Mach 3.42, but if he was able to land it, it'd need new outer wing panels. And Mach 3.4 seems a terribly WRONG time to start peeling bits of your wing off. Mach 4.13, if it could somehow be reached, would be followed by a popping sound as the engines flame out. Since the engines would probably flame out a few small fractions of a second apart, the next sound you'd hear would be an SR-71 in a flat spin being converted into confetti by the air coming at it *sideways.*

There. Now quit with the "SR-71 could go faster than Mach 4" bullshit.
 

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KJ_Lesnick

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F-14D,

Remember, just because an engine can remain running at a particular speed does not guarantee that an aircraft it's in can go that fast. Consider where the shock wave coming off the nose of an SR would be relative to its wingtips at a TAS of M4.25, let alone hypersonic, along with the fuel requirements. Also, a ground speed approaching M4 does not mean that the aircraft itself is capable of M4. We are talking late '50s-early '60s technology.

As I said, they would not have went to all that effort to modify the engine if the plane they were developing was not to be able to reach that speed. It would make no sense and would be a complete waste of money.


Overscan,

You actually mean

The original J-58 design, sans bleed-bypass pipes, was claimed by someone to be capable of Mach 4

The X-279E (YJ-93) was stated in at least one reliable book about the XB-70A to be rated for Mach 4 performance. I have heard other statements to the same effect -- additionally the XB-70's Chief-Engineer (Walt Spivak - Designer) stated it's inlets were designed for Mach-4 use.

Now considering the J-91 was capable of the same speed as the J-93, and the J-58 was capable of the same speed of the J-91, they would all be Mach 4 capable engines...

These are two very different statements. For a start, you would have to look at who claimed the J-58 could be Mach 4 capable, and what they meant by it (capable of withstanding Mach 4 flight for very brief periods, or sustaining Mach 4 flight *if made from different materials*).

I would assume sustained Mach 4 use as its predecessor, the J-91 was designed for continuous Mach 4 use. Additionally the J-58 was also pitched as a competitor to the J-93 and was kept on even after the J-93 won the competition apparently. That would suggest sustained use as the XB-70 was designed for extremely long range.


Orionblamblam,

Bah! Enough talk!!!


The upper limit to the SR-71s speed can be determined via simple geometry. Look at the graphic below. Two angles are shown... 17 degrees and 14 degrees. At 17 degrees, the shock wave shed from the nose of the SR-71 begins to impinge on the wing. At 14 degrees, it hits the engine. Either one of these would be Bad News for the plane; but the shock wave impinging on the engine would be DISASTROUS. It would completely foul airflow in the engine, as well as nuking the structure of the inlet spike.

You are actually overly simplifying. Technically the shockwave angle is determined by more than just Mach number -- The shape of the aircraft plays a role. For example a thin cone produces a thinner shockwave than a thick cone.

Now, simple math. The angle of the Mach wave is determined from:


17 degrees works out to Mach 3.42; 14 degrees works out to Mach 4.13. The SR-71 pilot might be able to stuff his plane past Mach 3.42, but if he was able to land it, it'd need new outer wing panels. And Mach 3.4 seems a terribly WRONG time to start peeling bits of your wing off. Mach 4.13, if it could somehow be reached, would be followed by a popping sound as the engines flame out. Since the engines would probably flame out a few small fractions of a second apart, the next sound you'd hear would be an SR-71 in a flat spin being converted into confetti by the air coming at it *sideways.*

As I said... the shockwave angle is actually determined by more than just the mach number. A thin cone for example produces a thinner shockwave than a thicker, fatter cone, which produces a shock-wave with a lower sweep to it.

There. Now quit with the "SR-71 could go faster than Mach 4" bullshit.

Your statements/arguments have flaws to them...


KJ Lesnick
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F-14D

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KJ_Lesnick said:
F-14D,

Remember, just because an engine can remain running at a particular speed does not guarantee that an aircraft it's in can go that fast. Consider where the shock wave coming off the nose of an SR would be relative to its wingtips at a TAS of M4.25, let alone hypersonic, along with the fuel requirements. Also, a ground speed approaching M4 does not mean that the aircraft itself is capable of M4. We are talking late '50s-early '60s technology.

As I said, they would not have went to all that effort to modify the engine if the plane they were developing was not to be able to reach that speed. It would make no sense and would be a complete waste of money.





KJ Lesnick
Let's hope I don't get a heart-attack

Maybe, just maybe, since they were gong into unexplored territory, they overdesigned since they would be going into unexplored territory and the way they'd have to get data would be developing with the very systems they were intending to use. Maybe they were building in a reserve for heretofor unexplored conditions. Or, maybe they just needed to do it to get it to work.
 

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KJ, Scott is an engineer who has designed rockets. Therefore I respect his opinion on aerospace matters more than yours. You have repeatedly refused to believe others with more knowledge than yourself and persist with stubbornly asserting a rival theory in the face of all evidence. This is not the correct forum for such behaviour.
 

r16

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challenging basics calls for knowledge of how the situation is different from the point discussed . One might have a valid view , there might be some way of getting around the problem , the basics can be at fault as one particular example of how people believed in the 1820s that no train would need going over 50 miles since people would be dying because lack of air and while all the experience showed it would be hard to breath at "high intake speed" the scientists failed to see that people would be in closed cabins . Yet all ı have is talk , Orionblamblam has math , he has a clear advantage . ı will go on believing in what ı believe (and saying it around with provisions and strictures) but the standarts and the quality of the forum - which ı respect truly and not because of any fear of getting banned - require he is right unless anyone has evidence on the contrary . ı don't have the slighest doubt that Orionblamblam would like to be proven wrong as it would be a new thing ; it is part of the human nature to learn new things but ı , for one , don't have that information or a likely explanation . Hawker Typhoon had locations on its wing that went transsonic or supersonic on dives but it would be a mistake to call the plane faster than sound .
 

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KJ_Lesnick said:
...
Let's hope I don't get a heart-attack


Hey KJ.

Let's learn a relatively simple way of calculating this.
Many years ago I posted a simple calculation on how to compute this on the skunk-works
mail list.

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 (ram) air temperature". This means something very specific to
an AE enthusiast. 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 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.

It's just AMAZING what you can do with simple aero engineering equations.
This stuff is POWERFUL !!

Now another tidbit, again showing the power of physics as embodied in aero
equations and math and what you can do with it.

I nopticed that according to the SR-71 dash-1 flight manual, that the CIT gage
has a max reading. That max reading is 500 deg C (932 deg F, 773 deg K).

Your homework is to calculate the Mach number corresponding to that at
85,000 ft using lambda at 1.4 again.

No I'm not saying the SR-71 would do that. We have simply noticed that the
CIT gages in the cockpit go up to 500 deg C and we are merely using these
powerful AE equations to observe some things !!

Enjoy!!
 

Michel Van

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I have a deep respect for Scott Lowther aka Orionblamblam

he is a experienced aerospace engineers, and wen he say :

Bah! Enough talk!!!
....
The SR-71 pilot might be able to stuff his plane past Mach 3.42, but if he was able to land it, it'd need new outer wing panels. And Mach 3.4 seems a terribly WRONG time to start peeling bits of your wing off. Mach 4.13, if it could somehow be reached, would be followed by a popping sound as the engines flame out. Since the engines would probably flame out a few small fractions of a second apart, the next sound you'd hear would be an SR-71 in a flat spin being converted into confetti by the air coming at it *sideways.*...

Now quit with the "SR-71 could go faster than Mach 4" bullshit.

then its is unpleasant truth
 

Orionblamblam

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r16 said:
Yet all ı have is talk , Orionblamblam has math , he has a clear advantage . ı will go on believing in what ı believe (and saying it around with provisions and strictures) but the standarts and the quality of the forum - which ı respect truly and not because of any fear of getting banned - require he is right unless anyone has evidence on the contrary .

Belief in the face of contrary facts is more-or-less standard human behavior, as every religion on the planet demonstrates. However, "Huh. The facts show that my dearly held belief is wrong. Perhaps I should reconsider" is rare enough that it should be respected.

ı don't have the slighest doubt that Orionblamblam would like to be proven wrong...

Damend straight. As with many (most?) skeptics, I'd *love* for the beliefs that people often have that I know to be built on foundations of wishful thinking to sometimes be true. Aliens, antigravity flying saucers, cars that run on water, [Insert Offensive Political Dig At Socialism Here], disease cures based on hope not science, the USAF actually having technology that's beyond what's obvious, etc.

But *wanting* somethign to be true doesn't make it true.

I went back and re-examined my little graphic. It's true that the shock waves would be slightly different from what I presented; the shape of the nose - specifically, the chines - would effect the shockwave location. With a sharp cone that is swept back sharper than the Mach angle, the shockwave will appear to be attached pretty closely to the nose:



But if the object is blunt, the shockwave is modified.



If you look in my original little sketch, the shockwave appears to "cut" through the chines. This would not be. Instead, you'd end up with the shockwaves not cutting through the chines, but attached to them. A revised sketch is shown below, with revised angles and shock locations. With this, the first interaction - 16.3 degrees - occurs at Mach 3.56. The second, truly devastating interaction - 14.2 degrees, when the shock wave starts to interact with the inlet - occurs at Mach 4.07.

The first interaction might be survivable, but the second... not so much.

Yes, I'm an aerospace engineer. And YES, I've greatly simplified the problem. Shockwaves, like pretty much everythign in the universe, are more complex than a single simple equation. All kinds of things will effect the formation and geometry of shocks. I'm sure that there were some further notable modifications to the shocks based on the geometry of the SR-71. However, a single simple equation generally *can* get you within "good enough" for many if not most engineering purposes.

If I'm wrong, if I've done the math wrong, made wrong assumptions, left out important details... well, it'd hardly be the first time. Engineers, despite our staggering good looks and unquestionable sexual magentism for people of the opposite sex, are not entirely perfect. Jimmy Carter, after all, was a nuclear engineer, and if there's ever been a bigger example of non-perfection, it must've been Clinton. So if I made a mistake... point it out.
 

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F-14D

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Orionblamblam said:
r16 said:
If I'm wrong, if I've done the math wrong, made wrong assumptions, left out important details... well, it'd hardly be the first time. Engineers, despite our staggering good looks and unquestionable sexual magentism for people of the opposite sex, are not entirely perfect. Jimmy Carter, after all, was a nuclear engineer, and if there's ever been a bigger example of non-perfection, it must've been Clinton. So if I made a mistake... point it out.

As an aside, Jimmy Carter was not a nuclear engineer. In fact, as far as I know, his only engineering degree was an honorary one presented to him on Feb 20, 1979. So your humble, modest, self-image can continue unabated. :)

Back to topic. Aside from the shock wave issue I brought up, another limiting factor was how fast the fuel could dissipate the heat load. There's no way it could handle what would be generated at M4+. What it could handle, I suspect, probably tracks quite nicely with where you showed the shock wave issues arrive, just above M3.5.
 

shockonlip

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Orionblamblam said:
... So if I made a mistake... point it out. ...

We agree in the end that the SR can't go Mach 4 or even really close to it.
So no argument there.

But I am uncomfortable that we are using shock_angle = arcsin 1/M to prove this.

This relation is really only valid for Mach waves, or infinitely weak shocks.

Mach waves are caused by continuously curving expansion surfaces in nozzles or isentropic
compression surfaces, or expansion corners.

But the nose of an SR-71 blasting through the atmosphere at the speed of heat is not going
to be causing a Mach wave. It's going to be causing some form of a cone shock which is
stronger than a mach wave and whose shock angle will be standing farther off the vehicle
than a mach wave obeying the arcsin 1/M relation. As you suggest it can probably be simulated
with a simple cone shock, in which case the cone shock tables or Taylor-Maccoll solver is
needed. I have a Taylor-Maccoll solver so if we can agree on a nose cone half-angle, I can
look up the shock angles at various M's.

Anyway, So I believe the best proof is the CIT mechanism I mentioned, or proving that at
Mach 3.2, the inlet is running full (retracted spike shock on cowl lip) with no spill and minimum
wave drag. I think CIT is easier. Of coarse just looking it up in the flight manual is even easier.
But that's no fun!

I appreciate however what you were trying to do.
 

r16

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when ı could afford Air International on subscription back in the early nineties , Roy Braybrook would mention that the easiest way to get Russian representatives in airshows to talk straight was to give them a Bill Gunston book , a truly iconic name . Yet he has a patent or two on electromagnetic propulsion , the stuff flying saucers are supposed to use , he has written books which state that the prospects of antigravity planes are continiously sabotaged by well entrenched interests ; in short he is probably just one step of the abyss where many well meaning people unintentionally fall .And ı am the guy who wrote some 10 years ago he actually doesn't like Harrier the dog and he is the originator of the line that the plane cannot carry a box of matches across a football field . Not many people would accept that . We have every right to have our opinions and keep them even in the face of disagreement , nobody here would try to remove them and ı know this because ı check places time to time where people would be willing to do so .
 

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Regarding the shock angle equation, does it also hold true for instances like the space shuttle re-entering the atmosphere? I am aware that a section of the leading-edge panels becomes discolored as a result of the shock wave (a spot near the wing glove, also near the area where Columbia suffered her fatal foam strike.) I suppose this is less of a problem for the shuttle due to its thicker airfoil and heavier wing structure.

If shock angle is the limiting factor behind the Blackbird's top speed, it puts the Aurora conjecture into a new light. For all the concept art that depicts a triangular planform with leading-edge sweep of 70-75 degrees, the aircraft would be limited to Mach 2.92 or 3.86.
 

sferrin

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Orionblamblam said:
If you look in my original little sketch, the shockwave appears to "cut" through the chines. This would not be. Instead, you'd end up with the shockwaves not cutting through the chines, but attached to them. A revised sketch is shown below, with revised angles and shock locations. With this, the first interaction - 16.3 degrees - occurs at Mach 3.56. The second, truly devastating interaction - 14.2 degrees, when the shock wave starts to interact with the inlet - occurs at Mach 4.07.

The first interaction might be survivable, but the second... not so much.

Yes, I'm an aerospace engineer. And YES, I've greatly simplified the problem. Shockwaves, like pretty much everythign in the universe, are more complex than a single simple equation. All kinds of things will effect the formation and geometry of shocks. I'm sure that there were some further notable modifications to the shocks based on the geometry of the SR-71. However, a single simple equation generally *can* get you within "good enough" for many if not most engineering purposes.

If I'm wrong, if I've done the math wrong, made wrong assumptions, left out important details... well, it'd hardly be the first time. Engineers, despite our staggering good looks and unquestionable sexual magentism for people of the opposite sex, are not entirely perfect. Jimmy Carter, after all, was a nuclear engineer, and if there's ever been a bigger example of non-perfection, it must've been Clinton. So if I made a mistake... point it out.

You're second stab explains the Mach 3.6 figure I've heard as the highest the A-12 hit in flight testing.
 

F-14D

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CFE said:
Regarding the shock angle equation, does it also hold true for instances like the space shuttle re-entering the atmosphere? I am aware that a section of the leading-edge panels becomes discolored as a result of the shock wave (a spot near the wing glove, also near the area where Columbia suffered her fatal foam strike.) I suppose this is less of a problem for the shuttle due to its thicker airfoil and heavier wing structure.

If shock angle is the limiting factor behind the Blackbird's top speed, it puts the Aurora conjecture into a new light. For all the concept art that depicts a triangular planform with leading-edge sweep of 70-75 degrees, the aircraft would be limited to Mach 2.92 or 3.86.

The way the Shuttle "flies" is different from the way from the way conventional airfraft do, including its attitude wherein it is traveling at its highest temperature encountered speeds, which is substantially nose "up", so that the heat load is on the belly.

Shock wave would be one of the limiters on the Blackbird. As mentioned before, its ability to dissipate heat at a given speed at a given altitude at a given temperature is another. The fastest an SR has ever publicly been credited was temporarily M3.5 over Libya and this was acknowledged as not being normal ops.
 

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If shock angle is the limiting factor behind the Blackbird's top speed, it puts the Aurora conjecture into a new light. For all the concept art that depicts a triangular planform with leading-edge sweep of 70-75 degrees, the aircraft would be limited to Mach 2.92 or 3.86.

Wing sweep does not have to stay within the nose or other shock cone. However airfoil, LE radius, TPS must be designed accordingly as in the case of the fabled Aurora. Including type of fuel such as switching from JP to methane to avoid coking issues. Nose and chine shocks impinging on the inlet is another issue altogether. I believe the SR-71 was known for sudden violent yawing inlet unstarts and flameouts. This problem was later resolved with switch from analog to digital spike control. M3.2 was cruise (most efficient speed) and M3.6 was max engine design. Not to say it couldn't go faster in afterburner RJ mode for very brief time perhaps.
 

KJ_Lesnick

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Airrocket,

However airfoil, LE radius, TPS must be designed accordingly as in the case of the fabled Aurora. Including type of fuel such as switching from JP to methane to avoid coking issues.

I could be wrong here, but if you have a good enough TPS, I don't think you don't need to resort to LH2, LCH4 or high temperature fuels. At least from what I remember reading Paul Csysz stated that McDonnell and McDonnell Douglas developed light weight high temperature metallic thermal protection systems that could maintain room temperature inside aircraft doing Mach 12...

Liquid Methane could still be useful on a Mach 6 "Aurora" type design though as it's not as dense as LH2 and cryogenic fuels can be used to drive an Strutjet/RBCC design...


KJ Lesnick
 

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