Hello!Checking the specs for the mighty Pratt & Whitney J-58 turbo-ramjet engine that powered the Lockheed SR-71 Blackbird.Most of what I've read, including Brian Shul's "Sled Driver: Flying the World's Fastest Jet" state that:

At higher Mach numbers the spikes moved aft automatically as the airspeed increased.

Well, this sounds wrong to me asit seems exactly the opposite should be happening, i.e. the as Mach number increases the spikes ought to move forward:1. Dynamic Pressure Increases with Mach NumberAs Mach number increases, dynamic pressure increases (like with the pitot), so more air will be entering the engine, thus the spikes need to move forward to maintain airflow constant. If too much air entered the engine, the engine would flame-out much like a candle goes out when you blow on it. Technically speaking, when the velocity of the air is higher than the velocity at which the flame front can propagate, the flame detaches from the wick (the source of its fuel) and combustion ceases. Similarly when the engine ingests the shock-wave, combustion chamber airspeed increases beyond the fuel's flame front speed, the flame detaches from the fuel injectors and the engine flames-out.2. Shock-Wave Foldback Increases with Mach NumberThe J-58 operated as a RAM-Jet above Mach 1.5. It's range (fuel efficiency) was due to a great breakthrough in the inlet design whence the spike's shock-wave was used to pre-compress the air prior to entering the inlet compressor. The deal with a supersonic schockwave is that the air ahead is supersonic (WRT the aircraft) and behind the schockwave it is subsonic. Now given the law of conservation of energy, if the air slows down (a decrease in kinetic energy), it must commensurately increase in pressure (potential energy) and temperature (thermal energy due to adiabatic heating, meaning no heat is lost to the surrounding air) such that the net change in energy is zero.The SR-71's inlet and spike were thus designed to benefit from this pre-compressed air behind the spike's shock-wave to save on the fuel required to achieve the same compression via mechanical means. To account for varying speeds and temperatures, the spike is mounted on a hydraulic jack that can move the spike forward and back, the idea being that the spike be positioned so the shockwave sits just ahead of the lip of the engine intake and so feeds the compressed air from behind the shock-wave into the engine. Pretty neat! BTW IIRC, at Mach 3+ the frontal aerodynamic load on each spike is 17 tons!Now as speed increases, the shock-wave attached to the spike tip folds back like the waves from a motorboat speeding up (for those interested, the angle of shock-wave foldback is the arctan of the Mach number), and as the shock-wave folds back, the radius of the circular shockwave gets smaller than the radius of the intake lip, so now the high-velocity air ahead of the shock-wave risks entering the engine! So to compensate, as speed increases and the shock-wave folds back, the spike needs to move forward.But wait! Remember Brian Shul said the spikes move back as speed increases!So what's the deal with the spikes? As speed increases, do they move forward or backward?Cheers,- jahman.

They retract.

Yes, I've seen that graphic.Look at the M3.2 schematic at the bottom: If you go faster, the shock-wave folds back further and the spike needs to move forward for the shockwave to remain positioned at the lip of the intake (and "rumours" have it she could do M3.3+). If you go slower (say M3.1), the shockwave "opens-up" and you need to move the spike backwards for the shock-wave to remain positioned.Cheers,- jahman.

Yes, I've seen that graphic.Look at the M3.2 schematic at the bottom: If you go faster, the shock-wave folds back further and the spike needs to move forward for the shockwave to remain positioned at the lip of the intake (and "rumours" have it she could do M3.3+). If you go slower (say M3.1), the shockwave "opens-up" and you need to move the spike backwards for the shock-wave to remain positioned.Cheers,- jahman.

So question answered? They retract, I do know that much. Though I've never looked that in depth at it. Interesting.

There's no guarantee that the spikes were at their maximum end of travel position of 29" compared to fully extended at M3.2, which was supposedly the most economical cruise speed, nor that the illustration itself is totally correct.By the way, I've bought a book called Military Aircraft of the Cold War, which guesstimates the top speed to be M3.5+.

Yes, I've seen that graphic.Look at the M3.2 schematic at the bottom: If you go faster, the shock-wave folds back further and the spike needs to move forward for the shockwave to remain positioned at the lip of the intake (and "rumours" have it she could do M3.3+). If you go slower (say M3.1), the shockwave "opens-up" and you need to move the spike backwards for the shock-wave to remain positioned.Cheers,- jahman.

I find it quite interesting that you question real SR-71 pilots. As the spikes are an extremely vital part of the SR-71 do you really think they don't know exactly in which direction the spikes are moving????FYI, in case of the SR-71 this happens to reduce capture area. Below 30000ft they are fully extended and above that altitude and above M1.6 they start retracting.Fully retracted position is reached at M3.0

There's no guarantee that the spikes were at their maximum end of travel position of 29" compared to fully extended at M3.2, which was supposedly the most economical cruise speed, nor that the illustration itself is totally correct.By the way, I've bought a book called Military Aircraft of the Cold War, which guesstimates the top speed to be M3.5+.

Cool! That's 2,020 knots (standard atmosphere). I've read reports at those speeds fuel milage increased with speed! (Imagine the car-on-a-highway equivalent: But Officer, I had to drive faster so I could make it to the next gas station!) In FSX (great Alphasim release! Hope for backwards compatibility when Flight is released) I get the overspeed warning at M3.43 (1980+KN GS on the GPS).

I find it quite interesting that you question real SR-71 pilots. As the spikes are an extremely vital part of the SR-71 do you really think they don't know exactly in which direction the spikes are moving????

When it comes to physics (and math) I'll question anyone until I get satisfactory answers. The day we all stop asking the right questions is the day we stop making new discoveries. Einstein was a Patent Office clerk when he "questioned" Sir Isaac Newton (of Apple fame). Me, I just want to get to the bottom of something that looks, well, counterintuitive. Galileo was questioning God and look what they wanted to do to him. I'm only questioning what an SR-71 pilot says, so that should not ba a problem, unless you feel SR-71 pilots are a notch above God (might be the case... :-) Of course the whole SR-71 program was under the aegis of the CIA so I'm sure pilots had to sign hefty NDAs befre joining, including perhaps agreeing to feed erroneous information to confuse aerodynamicists working for countries on the "other" side. Recall the first simulator was built behind a grocery store in Brooklyn: They went to extremes to insure the "othersiders" never found out they were actually supplying 90% of the titanium the Blackbird needed to spy on them.

FYI, in case of the SR-71 this happens to reduce capture area. Below 30000ft they are fully extended and above that altitude and above M1.6 they start retracting.Fully retracted position is reached at M3.0

Interesting facts. Please do explain further so we can distill the principles!Cheers,- jahman.

You can probably find it in the airplanes operating manualhttp://www.sr-71.org/blackbird/manual/

You can probably find it in the airplanes operating manualhttp://www.sr-71.org/blackbird/manual/

most of it's still classified :(

Thanks for the link to the Blackbird POH!There is a page with a graph of Spike Rearward Movement vs. Mach that again shows the spike moving back with an increase in speed. Evidently there are aerodynamic choke points inside the intake duct that do the compression, and not just the spike schockwave sitting in the intake lip.This of course assuming the information is correct rather than disinformation ingeniously planted by the s.p.o.o.k.s. (why does the BBboard turn this word into a series of crosshatches?)...Found some other interesting gems while perusing that vade mecum of Blackbirdiana: The recommended pilot manouver when in a spin is to... eject!Or how about this one:Effect of Mach DecreaseA small decrease in Mach ... will cause the aircraft to ... become thrust limited!Turn the preceding around and what you have is thrust increases with speed. What an amazing piece of machinery.Too bad we can't seem to get to the basics of inlet design for shockwave positioning to better understand how this al worked.Cheers,- jahman.

The day we all stop asking the right questions is the day we stop making new discoveriesToo bad we can't seem to get to the basics of inlet design for shockwave positioning to better understand how this al worked.

You are comparing apples and oranges. In this case it's not about making new discoveries but about accusing a professional mixing up facts! There are a few excellent books e.g. from Ray Whitford which do explain this nicely.But this of course might be just to make russion engineers draw wrong conclusions, :(

You are comparing apples and oranges. In this case it's not about making new discoveries but about accusing a professional mixing up facts!

When you don't know the facts there is no difference at all in the process of discovering facts that are new to only you from facts that are new to everybody. An I am not accusing the author of mixing-up facts, rather I am wondering about the possibiity incorrect facts were planted intentionally, which is a whole different ballgame. Imagine if the fellas on the other side had tried to copy the Blackbird (like they tried to copy every other aircraft including the B-70 Valkyrie) and they could never get the jet really screaming because they had the spike movement reversed!

There are a few excellent books e.g. from Ray Whitford which do explain this nicely.But this of course might be just to make russion engineers draw wrong conclusions, :(

OK, so you were never deceived by a spy. I was, the community I belonged to was. The fella planted disinformation for years until until he sheepishly confessed many years later (due to FOIA release of documents) he was in the employ of The Agency, specifically tasked with planting disinformation about the very subject he was an expert in.And if you think planting false information doesn't happen, go see the movie "Fair Game" about how false information was planted at the highest official levels about the sale of uranium yellowcake from Niger to Irak that never happened, misleading even the President himself into believing Irak had weapons of mass destruction. Landed the Vice President's Chief of Staff in jail. So no, I wouldn't rule out The Agency from planting misleading information about the Balckbird either via fake POHs with pseudo-missing pages or via books written by SR-71 pilots.Finally, In Brian Shul's very own words (from the same book):

The reader won't find secrets revealed in this book.

As one who shared intimate secrets with the airplane I feel less inclined to reveal all that she could do.

To betray her confidences would be unthinkabke.

How do you spell "NDA"? And remember, the book is from 1991, that's 20 years ago. The needs for secrecy were quite different back then.Cheers,- jahman.

When you don't know the facts there is no difference at all in the process of discovering facts that are new to only you from facts that are new to everybody. An I am not accusing the author of mixing-up facts, rather I am wondering about the possibiity incorrect facts were planted intentionally, which is a whole different ballgame. Imagine if the fellas on the other side had tried to copy the Blackbird (like they tried to copy every other aircraft including the B-70 Valkyrie) and they could never get the jet really screaming because they had the spike movement reversed!OK, so you were never deceived by a spy. I was, the community I belonged to was. The fella planted disinformation for years until until he sheepishly confessed many years later (due to FOIA release of documents) he was in the employ of The Agency, specifically tasked with planting disinformation about the very subject he was an expert in.And if you think planting false information doesn't happen, go see the movie "Fair Game" about how false information was planted at the highest official levels about the sale of uranium yellowcake from Niger to Irak that never happened, misleading even the President himself into believing Irak had weapons of mass destruction. Landed the Vice President's Chief of Staff in jail. So no, I wouldn't rule out The Agency from planting misleading information about the Balckbird either via fake POHs with pseudo-missing pages or via books written by SR-71 pilots.Finally, In Brian Shul's very own words (from the same book):How do you spell "NDA"? And remember, the book is from 1991, that's 20 years ago. The needs for secrecy were quite different back then.Cheers,- jahman.

From the Patent Office: http://www.google.com/patents?id=yhdpAAAAEBAJ&pg=PA1&dq=3477455&source=gbs_selected_pages&cad=2#v=onepage&q&f=falseDJ

For additional reading, check this out NASA's Contributions to Aeronautics, Volume 2. PDF's pages from 573 onwards contain interesting information. The author has worked as an SR-71 FE as far as I know.

For additional reading, check this out NASA's Contributions to Aeronautics, Volume 2. PDF's pages from 573 onwards contain interesting information. The author has worked as an SR-71 FE as far as I know.

Cool thanks for that. Some good reading there.