Blackbird's thrust question
 

Please can someone give some ideas?
The following extract comes from "F-12 series aircraft propulsion system performance and development" by D. Campbell

BTW Was unable to include Table 1 but it just shows usual falling contribution from the engine mounts with speed.

"In order to understand the importance of each of
the three propulsion system components, it is interesting
to note the proportion of the thrust which is supplied by
each component while operating in maximum afterburner.
Table 1 indicates how the actual thrust forces are distributed
while the engine is inducing flow and heating it
up with maximum afterburner. If the AB is reduced to
minimum AB, the engine would actually be dragging on
the engine mounts at high Mach numbers. Further reduction
of engine thrust below military power will result in no
propulsive thrust on the aircraft."


Q referring specifically to 'is inducing flow and heating it up with maximum afterburner'

I take this to refer to the induced secondary flow which flows over the red-hot afterburner casing and the intimation that this is significant in producing thrust. However due to the high air flow rate (1/3 intake entry flow) and it being a poor design of heat exchanger would the heat transfer really be significant (ie per lb/sec)?
Do I have the wrong end of the stick?
Thank you.

I may be off here, but here's my two cents:
Thrust of a jet engine is caused by a whole lot of mass x acceleration which produces a force. So the goal is to accelerate the air. In supersonic applications the problem is that in order to keep the jet engine going, you first need to slow the air down to below M=0.7 (or thereabouts) in the inlet to avoid surges, stalls etc. The engine can then add energy and a bit of mass (think fuel) to this airflow but if we are talking about high Mach numbers then the resulting exit speed may actually be slower than the aircraft's current speed, and this causes the 'engine to drag on its mounts' as mentioned in your book.

Afterburning can help in this respect as it adds mass, but more importantly heats it up considerably. This causes the air to expand rapidly but as it is moving through a confined tube, the end result is rapid acceleration of this mass of air, and that is what we were after all along. After all it is the increase in speed between entry and exit which causes thrust. The 'inducing flow' then refers to the main engine delivering an airflow to the afterburning section, which is its main task at this point.

There are some side effects which I've overlooked here, such as the pressure rise in the inlet caused by the slowing down of the air mass which also produces thrust, but that's not what your question was about I think.

Hope this helps, or have I just added another wrong end to the stick...

Airflow of J58 as fitted to YF-12A, A-12 and SR-71:

http://i337.photobucket.com/albums/n...8_Airflow1.jpg

Thank you for your replies.
I think I have a good grasp of where all the air goes and what it does having read the patent for both the famous but poorly understood engine bleed bypass and the patent which explains all those arrows on the 5 diagrams and Peter Law's presentations in AEHS website.
Since the author of the extract knew all this stuff as well (he wrote the intake patent) and knew the lingo, when he says induced flow I'm pretty certain he was referring to the secondary flow which bypassed the engine and was induced at the ejector nozzle by the afterburner flow from the primary nozzle (and of course helped by the pressure in the intake).
Hence my question on that particular air which flowed around the engine.

I believe he is saying something beyond all the usual explanations. OTOH I may just be reading too much into one word. Any ideas?

Where is it?
 

If true, that the engine's mounts 'float' at some stage....

First, the idea w/AB is not to pinch the nozzle, to accelerate flow, but to enlarge it, to maximize the AB flow on the cross section of the nozzle outlet: expansion, not restriction, is the goal.

At the point where the mounts float, the gaspath is effectively and functionally part of the airframe.

No distinction need be made to define the gaspath as thrust, or drag, it is neutral.

The AB reacts against the mass of the gaspath, several tons of it. In such a powerfully dynamic status, it becomes easier to get how the intake provides such a large component of Thrust, and the AB drag

It was always easier for me to understand thrust as exquisitely tied to its opposite, and compression also to extremely low pressure.

Don't mind me, I don't make any representation about this other than I think it has merit, maybe.

Lyman :ok:

Salute!

Go to this site and read and learn:

SR-71 Online - The Blackbird Archive

Was privileged to know and fly with a few of the Blackbird pilots, and that sucker was amazing for its time.

The J-58 was a key aspect of the design, although with less-powerful motors the thing still ran well.

Unlike today's "monitoring function" for the crew, the Blackbird required actual piloting skills and decisions and such. My friends that flew the U-2 have similar war stories.

Isn't this case just the plane slowing down so the dragging on the mounts is just an unbalanced phase until new equilibrium?

Isn't this even more slowing down to an even lower speed?

During high-speed flight in the Blackbird, compression of air in the inlets generated most of the vehicle’s thrust. At Mach 2.2 the inlet produced 13 percent of the overall thrust with the engine and exhaust ejector accounting for 73 and 14 percent, respectively. At Mach 3 cruising speeds the inlet provided 54 percent of the thrust and the exhaust ejector 29 percent. At this point the turbojet continued to operate but provided only 17 percent of the total motive force. The inlet had a compression ratio of 40:1 at cruise conditions where each inlet swallowed approximately 100,000 cubic feet of air per second.

Source: Urie, David, “Case Studies in Engineering: The SR-71 Blackbird,” Course Ae107, Presented at the Graduate Aeronautical Laboratories, California Institute of Technology, Pasadena, Calif., April-May 1990.

A significant percentage of air entering the inlet bypassed the engine through ducts and traveled directly to the afterburner. At cruise Mach conditions, fuel burned more advantageously in the afterburner than in the main burner section. Hence, engineers described the powerplant as a turbo-ramjet.

Source: Matranga, Gene, and William J. Fox, “YF-12A Development and Operational Experience,” unpublished paper presented at the Supercruiser Conference, Wright-Patterson AFB, Ohio, February 17-20, 1976, p. 3. NASA Dryden Historical Reference Collection.

Download and read:-

Flying the SR-71 Blackbird

In the cockpit on a secret operational mission

By Col Richard H Graham USAF ( ret )

A good read that has a lot of tables and graphs for thrust settings Perf etc. including checklists and flight plans.

Misunderstandings
 

I've found it's worth treating some Blackbird writings with scepticism even when from seemingly authoritative sources.
No air bypassed the engine and went straight to the ab. Most of the ab air,80%, went through the whole engine. A small amount, 20% which was required to make the compressor function properly at high inlet temps, was bled from the 4th compressor stage and then to the ab.
Sources: US Patent 3,344,606 which is the bleed air patent, F-12 Flight manual Fig 1-1G for example. Also the popular colored airflow diagrams in a previous post which don't show any engine bypass air going to the ab.
The engineers who designed and built the engine(P&W), those who designed and built the installation (Lockheed) and those that serviced and maintained it called it a bleed bypass turbojet with afterburner, eg see Flight manual. This knowledge alone sets the alarm bells off when you see the word ramjet. It was, after all, a turbojet with a bit more compressor bleed than usual.
One of many good sources: 'F-12 series aircraft propulsion system performance and development' by D. Campbell, the Lockheed engineer who also took out US Patent 3,477,455 for the inlet. Incidentally one of the drawings in the patent is the basis for the popular colored airflow diagrams which, in fact, are taken from the flight manual.

I'd be interested where you get those figures from peter. The bleed bypass can be identified on the engine by six very large pipes running from the forth stage to the afterburner.

The bleed air from the forth stage was scheduled by the main fuel control as a function of compressor inlet temperature and engine speed. The transition normally occurred in a compressor inlet temperature range of 85° to 115° C, corresponding to a Mach range of 1.8 to 2.0.

At sea level static conditions military thrust constitutes 70% of maximum thrust, whereas at high altitude it is approx 28% of the maximum available.

The engine will always be producing thrust irrespective of throttle position - assuming it hasn't been shut down that is.While I don't have any evidence that Kelly Johnson or Ben Rich ever referred to the engine as a "turbo ramjet", the term was used by Lockheed and NASA people, the principle behind the term, and what it meant, being well understood.

Hi Brian, 20% appears in the internal P&W memo written by Bob Abernethy when he had solved the challenge of converting the j58 from a Mach 2 engine into Mach3.
http://www.bobabernethy.com/pdfs/Nev...0of%20P&W3.pdf
It also appears in his patent
Patent US3344606 - RECOVER BLEED AIR TURBOJET - Google Patents
The whole story is in the patent. Basically the problem revolved around getting the compressor to behave at M3 cruise, ie 100% mechanical N1 and M3 CIT. This condition for the compressor is similar to starting the engine to idle on the ground, ie same low corrected N1. Just as this engine, like others, needed a start bleed to get to idle so it needed a similar bleed at cruise (because it was the same regime on the compressor map), which turned out to be 20%. The bleed was then available to put into the ab to give additional benefits.

The patent is arguably the best primer I have ever come across anywhere for explaining how jet engine compressors suffer from CIT at high supersonic speeds and what to do about it.

They may look big for 20% but would have been overlarge to reduce flow losses. The 20% would have been controlled at the compressor case to prevent excess bleed if a pipe cracked.

This is a hard nut to crack but only because it is steeped in folklore. With all what was once secret info now available on the web we now know exactly what the engine was and can even deduce it for ourselves by reading the patent for example or the Flight Manual. We no longer have to rely on descriptions from years ago.
Again as spelled out in the patent, a turbo ramjet was one possible solution to get to M3. It was not adopted. The engineer given the task of 'fixing' the engine says in his patent why not. Another would have been variable stators. That's how the XB-70 engines got there.
Few people at Lockheed, NASA or P&W would have been privy to what were secrets back in those days. The facts are all laid bare for us today and we are better informed. They were wrong as stated in the patent. The bleed bypass was a much more cost effective solution due to its simplicity.
BTW Ben Rich called it a bleed bypass turbojet in "F-12 Series Aircraft Aerodynamic and Thermodynamic Design in Retrospect".
With all the fine details available to everyone we basically have to forget all the descriptions from years ago and start again with the facts.
Interesting stuff. Cheers.

Ram means 'to compress'. For "Ram" to apply to the J58, the engine would have to be able to shut down its compressors, and rely on inlet air only to propagate its motive power.

I am guessing that could not be the case with this engine. 'Ram Effect'? All turbine engines experience ram "Effect".

That is the 'semantic' description of "Ramjet". The term 'Ram' and 'jet' are not synonymous. One implies a confined and controlled gaspath (jet), the other, a 'passive' (Ram) supply of airmass whose characteristic energy is created by the velocity of the inlet through the air....

impo....

G'day peter,

You were quite right re the 20%. Found it in some saved files (I'm a real fan of the aircraft).

With respect to the "turbo ramjet", it was interesting to see in your Bob Abernethy link where he says,As to who may have been first to use the term "turbo ramjet" is moot I feel, as that is was what it was, as Bob alludes to.

If increasing mass flow and thrust is the purpose, doesn't water injection do the same? Technically, since bleed air is already mechanically compressed and not "inlet" air, it is not "Ram Air"....

Inlet air that bypasses the mechanicals into the nozzle for combustion, is Ram Air.

1. Inlet air that has migrated through the spike is not ram air, it is "managed"
(slowed).

2. To qualify as a true hybrid, not a 'partial', the J58 needs to derive its power from "unmanaged" air, non 'mechanical' derivation.

3. Question. Can a Ramjet develop thrust (drag "plus") without itself burning fuel?

Being of a left brain I shy away from wooley classifications because they need explanations to go along with them.

I'm guessing that the partial ramjet idea is because at a flight speed typical of some ramjets some proportion of 'engine-compressed' air goes straight to the ab.

This classification for the J58 (if I've got it right) now includes all Mach2+ afterburning bypass engines when they are going at full speed. The F100/TF-30 etc have become partial ramjets?

The turbo ramjet classification is not open to interpretation though as illustrated by Bob himself. In his patent he says re his 3rd possible solution for M3 flight "Such an engine configuration would be called a turbo-ramjet..etc..both large and heavy..etc...combustion efficiency low rel to turbojet" "My solution to the problem is the recover bleed air turbojet (not the turbo-ramjet)".

He's got a lot to answer for. He's making us think for ourselves.

Colonel Richard Graham spent seven years as a pilot, and later, instructor on the aircraft, 1st Strategic Recon Squadron Commander, Director of Program Integration at the Pentagon, and finally, 9th Strategic Recon Wing Commander at Beale.

His words from "SR-71 Revealed", page 50. One would hope the gentleman knew what he was talking about. :E

Ben Rich himself opined that the engine was an ancillary (my interpretation) in the production of thrust in the cruise, when he famously commented on the compressors as "pumps to keep the inlets alive".

Interestingly, in the cruise if the engine is deriched its contribution to thrust drops to 10% approx of the total, vice the normal 17%.

Not at all, they are engines designed and optimised for a completely different mission (fighter), and used in aircraft which are not designed for the sustained Mach 2+ flight necessary.

Just bought my own copy of Col Graham's"Flying the SR-71 Blackbird" only a week ago and have just borrowed from library Paul Crickmore's "Lockheed Blackbird".:ok:

"Meredith effect"?

How about SR71 nacelle secondary flow just because it picks up a little bit of heat from the red hot ab duct.

Here is Kelly Johnson himself uttering that dreaded word - ramjet. :)

Henceforth, the J58 shall be known as "RAMJET"........

No squawks here :ok: