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. |
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 |
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.
A significant percentage of air entering the inlet bypassed the engine through ducts and traveled directly to the afterburner 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. engineers described the powerplant as a turbo-ramjet 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. |
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. Also the popular colored airflow diagrams in a previous post which don't show any engine bypass air going to the ab 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. 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. The engine will always be producing thrust irrespective of throttle position - assuming it hasn't been shut down that is. engineers described the powerplant as a turbo-ramje |
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. six very large pipes "turbo ramjet", the term was used by Lockheed and NASA people, the principle behind the term, and what it meant, being well understood 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, Bypass the bleed air around the compressor at high Mach number into the afterburner and it would solve the surge problem, provide cool air to afterburner and increase the mass flow and thrust significantly. Actually it converted the engine into a partial ramjet with capability above Mach 3. |
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. The J-58 was the first dual cycle engine put into service. At subsonic and transonic speeds it was a standard , single spool turbojet engine, and it essentially transitioned to a ramjet engine around Mach 2. 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%. The F100/TF-30 etc have become partial ramjets |
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:
3. Question. Can a Ramjet develop thrust (drag "plus") without itself burning fuel? 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. :)
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Henceforth, the J58 shall be known as "RAMJET"........
No squawks here :ok: |
Good video.
What I'm looking for is a better understanding of how it all worked, from a pedants viewpoint of course.:8 Not sure if I'm allowed to make any more comments or ask any questions but I'll risk it. One stumbling block was why would it be referred to as a partial ramjet? I've just skimmed through my new book. Fortunately (for me) the author says "the faster you flew the more fuel efficient it became....at Mach 3.0 it used 38,000 lb/hr. Accelerate to Mach 3.15 fuel flow drops to 36,000 lb/hr. The faster you flew the more it became a ramjet utilizing the high Mach air to augment the thrust of the engines." Brian, maybe this explanation was also in the same authors "SR-71 revealed". |
One stumbling block was why would it be referred to as a partial ramjet? Take the afterburner for example. When the engine was deriched its thrust output was reduced, but without affecting the thrust produced by the inlet or afterburner. With the engine deriched, although it is producing less thrust, the rotor RPM remains unchanged, so it's still pumping the same amount of air to the afterburner via the bypass from the 4th stage compressor. The downside was that the exhaust from the engine was now reduced to a temperature where a blowout of the afterburner was possible. The partial ramjet feature is the bypass air from the 4th stage to the afterburner. The bleed valve on the bypass doesn't open until the compressor inlet temperature is in the range 85° to 115° C, which corresponds to Mach 1.8 to 2.0. Up until that temperature range is reached the engine is operating as a normal axial flow turbojet with an afterburner. Once reached, the 4th stage bypass opens, and you have your partial ramjet. Highly compressed air being fed directly to the afterburner, up to 40% of the air consumed. Not sure if I'm allowed to make any more comments or ask any questions but I'll risk it. |
Any Concorde experts, please chime in....
Was the J58 cycle at all like the Olympus 500 while in cruise? GF |
peter
One picture I have of the mechanism. The engine without afterburner peaks at some mach value, say 1.7. After that, the a/c cannot accelerate any further, the drag prevents it. The afterburner lights, and acceleration resumes, but the afterburner is reacting against the gas path, not the engine. The engine mounts are pinned back at the point where the turbosystem peaks, and remain so beyond that velocity. The Afterburner makes its thrust pushing on the gas flow at the start of the AB cabinet cross section. A friend flew the F4 and recounted his experience at M2. It had to be done in a dive. He tells of pulling the engines out of AB, and it felt like he had hit a brick wall. Until he decelerated to some lesser Mach, the extreme deceleration was unnerving, indicating the drag had been there all along. He explained it in a way similar to that above, that the engine mounts were driven back into the frames, not pulling forward on them. Even in full burner at 2+. Sounds counterintuitive. Kelly Johnson can call the J58 anything he liked, seems to me. But the classic definition is what it is.... |
Was the J58 cycle at all like the Olympus 500 while in cruise? |
Kelly Johnson can call the J58 anything he liked, seems to me. But the classic definition is what it is.... It is well understood in engineering. The following diagram comes from a NASA paper. http://upload.wikimedia.org/wikipedi...urboramjet.jpg A NASA paper on turbo ramjets. Access forbidden! It opens by saying "Advanced airbreathing propulsion systems used in Mach 4-6 mission scenarios, usually involve turbo-ramjet configurations. As the engines transition from turbojet to ramjet, there is an operational envelope where both engines operate simultaneously." In the case of the J-58 both are operating simultaneously when above Mach 1.8 - 2.0, below that figure it's just a turbo jet with afterburner as previously mentioned. It is what is alternatively called a dual cycle engine. Straight turbo jet to start with, becoming part turbo jet and part ramjet once up to the necessary speed for the ramjet function to begin operation. A further NASA paper Access forbidden! A few extracts The engines considered are based on or extrapolated from known performance parameters of rocket-based combined cycle (RBCC) (the Marquardt Corporation ejector ramjet) and turbine-based combined cycle (TBCC) (the Pratt & Whitney J-58 engine used in the Lockheed SR-71 Blackbird). B. Turbine-Based Combined Cycle Engines: Background Turbine-based combined cycle engines operate by using a gas turbine propulsion cycle which transitions to a ramjet cycle; they bypass the turbomachinery at high Mach numbers, where pressures, temperatures, and flow velocities make such machinery impractical or redundant or both. Such a configuration by itself is not capable of orbital insertion because at some altitude the ramjet mode will lack the inlet mass flow to sustain thrust; however, TBCC engines hold strong promise for use with carrier vehicles or atmospheric ascent stages. Turbojet propulsion systems are generally limited to Mach 3 due to the rise in inlet temperature present at the compressor face; turbine engines are also in general more limited in altitude with respect to their ramjet counterparts as well. They provide more efficient operation, however, at lower altitudes and Mach numbers relative to ramjet-based engines.14 The limitation to lower Mach numbers and altitudes is not universal, especially if the ramjet mode is effective enough to compensate for the additional weight of the turbomachinery. Turbine-based combined-cycle engines have seen actual flight time, a qualification which is not shared by RBCC engines. For example, the SR-71 J-58 engine is a turbine-based engine that operates in multiple cycles depending on the flight regime (with known cruise conditions of approximately Mach 3.2 at an altitude of 70,000 ft). Can the historic configuration of the Pratt & Whitney J-58 with the Lockheed variable-geometry inlet be considered a true TBCC? From an aerospace purist view, the engine is never completely in a pure ramjet mode due to the fact that the first stages of compression are present before the mass flow is bypassed to the afterburner. In the viewpoint of the aerothermodynamist, however, the engine transitions through at least two modes or cycles and, thus, coupled with the turbine, can be classified as a TBCC. I'm somewhat loss Lyman why you can't accept the description provided by the designer, the crews who flew it, the people who instructed on it, and the P & W technical rep. |
You seem to be having difficulty in understanding the turbo ramjet Lyman. To be honest Lyman writes at great length about so many topics without seeming to understand them that I am starting to wonder whether he is one of those funny 'things' that crop up on PPRuNe from time to time. Hey ho. |
I think communication is the problem, not specifics, or even theory.
If I put a cucumber in brine and vinegar for 48 hours, is it a cucumber or a pickle? I do not deny the J58 satisfies the description of TurboRamjet. That makes it a hybrid, not a Ramjet. A Ramjet engine derives compression and ignition from capturing passive airflow, not in compressing it mechanically. Is that incorrect? If a GE 90 captured some airflow aft of the fan and directed it back into the engine ( it does), is it a partial "Ramjet"? If Einstein called a five a four, is he wrong? |
Doesn't Mach 1 remain a constant TAS above the tropopause? If so, what would be the TAS in knots for this bird doing Mach 3.2 at, say, 65,000?
|
aterpster,
If you go to this site, there are tables that may answer you question. Aerospaceweb.org | Ask Us - Mach vs. Altitude Tables TD |
Lyman, I give up. It's obvious from your last post that you have no understanding of first principles. Should you reread my last you will see where it says,
the engine is never completely in a pure ramjet mode due to the fact that the first stages of compression are present before the mass flow is bypassed to the afterburner |
a couple of questions:
1. At fixed max afterburner throttle angle, for example, when derich is selected what keeps engine speed constant? What happens to nozzle area? 2. Highly compressed air being fed directly to the afterburner, up to 40% of the air consumed 3. What's the explanation for Col Graham's "the faster you flew the more fuel efficient it became....at Mach 3.0 it used 38,000 lb/hr. Accelerate to Mach 3.15 fuel flow drops to 36,000 lb/hr. |
Peter, Brian, Machaca & Gums,
Thanks for the discussions and information on the J58 engine, it has all been enlightening to an old jet engine guy. There have been some posts that have been both confusing and humorous which I intend to address here. There have been four types of jet engines in this discussion: 1. A turbo-ramjet engine, better known as a P&W J58 engine (main topic) 2. A turbojet engine, better know as a GE J79 engine 3. A turbofan engine, better known as a P&W F100 or GE F129 4. A high by-pass turbofan engine, better known as a GE90 All are different and one should not be confused with another. So bear with me here. A turbo-ramjet engine is exactly that, a turbojet working together with a ramjet to power an aircraft to Mach 3+ that could not be achieved independently of one another. To simplify this, A ramjet generates no static thrust and needs a booster to achieve a forward velocity high enough for efficient operation of the intake system. The turbojet is the booster. Ramjets generally give little or no thrust below about half the speed of sound, and they are highly inefficient until the airspeed exceeds 1000 km/h (600 mph) due to low compression ratios. The turbojet is very efficient in this regime. Ramjets work by ingesting relatively low speed air and expelling the air at a higher speed. The difference in speed results in a forward thrust. The burning fuel creates higher pressures inside the engine, causing higher exhaust speeds. But the thrust of the engine depends entirely upon how much air flows through it. No matter how hot the burning air-fuel mixture is, and how high the pressure, if not much air flows into the front of the engine not much thrust is produced. So the trick to improving ramjet efficiency is to increase airflow through the engine. This is accomplished by the spike or obstruction called an innerbody. It is pointed on both ends and thick in the middle and fits inside the intake tube. Air passing into the tube must flow around the innerbody, and the area around it is less than the area of the intake opening. Consequently the air is compressed as it flows around and reaches a maximum pressure in the narrow throat between the innerbody and the intake tube. The same amount of air flows into the engine, but it is raised to a higher pressure. This increases the pressure that the burning gasses must push against, causing the overall pressure inside the tube to increase. Higher internal pressures mean greater amounts of air in the engine, so more fuel can be burned. The result is still higher pressures, increased exhaust gas speed, and greater thrust. But there is a problem that must be dealt with in the turbojet compressor area. When the pressure becomes too high in compressor, the blades tend to flutter, may break, the compressor can stall and the high temperatures can result in mechanical failures. So the P&W designers cleverly bled off air from the compressor to lower the pressure and temperature and fed it back to the burner in the afterburner section, it worked well. 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 do not deny the J58 satisfies the description of TurboRamjet. That makes it a hybrid, not a Ramjet. In a turbojet engine such as the J79, all the air ingested passes through the core of the engine and upon exit may, be mixed with fuel and relighted to obtain additional thrust for acceleration needs in AB mode. In particular, the two engine F-4 phantom jet had the capability to achieve Mach 2.3 at 40,000 feet in level flight, Mach 2.5 minus external stores and fuel tank. It was designed to accelerate rapidly during climb to 40,000 feet and dash even more rapidly to a target. But it did have limitations. A friend flew the F4 and recounted his experience at M2. It had to be done in a dive. He tells of pulling the engines out of AB, and it felt like he had hit a brick wall. Until he decelerated to some lesser Mach, the extreme deceleration was unnerving, indicating the drag had been there all along. He explained it in a way similar to that above, that the engine mounts were driven back into the frames, not pulling forward on them. Even in full burner at 2+. The F100 and F110 engines, being turbofan engines take most of the air through the core but the excess from the fan is funneled around the core engine exterior and dumped back into the afterburner where it is mixed with the core exhaust or where fuel is added and ignited in the AB mode. This is much more efficient than a normal turbojet AB engine system. The GE90 high by-pass turbofan engine develops 60% of its cruise thrust from the fan by-pass. The air is ducted by the nacelle around the engine core and dumped into the air but does not mix with the core exhaust air. If a GE 90 captured some airflow aft of the fan and directed it back into the engine ( it does), is it a partial "Ramjet"? If Einstein called a five a four, is he wrong? If I put a cucumber in brine and vinegar for 48 hours, is it a cucumber or a pickle? TD |
Frankly, the six bleed ducts are superchargers, the pressure is created mechanically, something a ramjet does NOT do. Any additive pressure to the gas path that is created mechanically is not RAM. By definiton.
PW did not "solve" anything by routing bleed to the ejector, neither do they create a ramjet. "Hybrid" predates automotive usage by fifty years.... Turbo Ramjet is fine, not difficult at all. Nomenclature is not a problem, but know this, the standard definition of a Ramjet engine is one that does not rely on mechanical compression of air. The air routes into a combustion chamber directly, fuel is added and ignited, and the exhaust creates thrust. You do a great disservice to Pratt, Whitney (and the ampersand) by denigrating the J58 with terms that imply it is a ramjet. it simply is not. The Ramjet is the simplest concept in aviation, one of. The J58 is a masterpiece of ingenuity and engineering. I used to pass one in the hall on my way to work (it was on a stand), and got to speak to pilots of the Blackbird.... Your usage of all the terms is just fine, to each his own. " In a ramjet, the high pressure is produced by "ramming" external air into the combustor using the forward speed of the vehicle. The external air that is brought into the propulsion system becomes the working fluid, much like a turbojet engine. In a turbojet engine, the high pressure in the combustor is generated by a piece of machinery called a compressor. But there are no compressors in a ramjet. Therefore, ramjets are lighter and simpler than a turbojet. Ramjets produce thrust only when the vehicle is already moving; ramjets cannot produce thrust when the engine is stationary or static. Since a ramjet cannot produce static thrust, some other propulsion system must be used to accelerate the vehicle to a speed where the ramjet begins to produce thrust. The higher the speed of the vehicle, the better a ramjet works until aerodynamic losses become a dominant factor". .....NASA Glenn Research Center bog simple.... great respect...... |
Salute!
I'll stick with the SR-71 manual and other sources that the J-58 never quite got into a "ram jet" mode at all. It simply bypassed air from one of the compressor stages when the "core" engine couldn't use all of it efficiently, but the burner could. Let's face it, the core couldn't handle all the air and then we had temp problems down stream. So the extra air from those bypass ducts in the J-58 allowed the burner to work really well. Lottsa air and somethat cooler that that going straight thru the core. We saw this in the F100 motor from Pratt in the Eagle and Viper. The difference in burner thrust was a much higher percentage than with the older motors that simply added JP-4 to the air that had not burned completely going thru the core and the turbines. The problem with our fan motors was instability in the bypass duct, which was not like those tubes in the J-58. So think another ten years of design and knowledge gained from the J-58. We never had the classic compressor stall effects - loud and physical bangs or chugs. We would have an unpooling motor and "torching" out the burner nozzles, and associated loss of thrust - called it "stall stagnation". Pratt worked on this a lot, and I think the solution was in the nozzle control. BTW, our initial Viper squadron tech rep from P&W had been involved in the J-58 design and testing. The only Blackbird pilot I have discussed this with said they "inlet unstarts", and whoa! Lottsa loss of thrust from one side and it required some good ruddeer and other "pilot things". One of the more intriguing designs would have had the spikes at the inlet and the geometry of the engine close down the core airflow even more than the J-58. An annular bypass and not a half-dozen tubes as the J-58 had. In other words, an F100 type motor with a fancy inlet spike as the SR-71 had. We got by just fine, thank you. That fan really helped with range, and I had flown the SLUF with the TF-41 for a thousand hours and it worked the same. Great discussion here, |
I'll stick with the SR-71 manual and other sources that the J-58 never quite got into a "ram jet" mode at all Not all detail makes it into a flight manual, and that's why you have tech reps who understand what is/isn't going on, as the case may be. A pilot does not need to know the nuts and bolts, or principles behind the engineering. What he needs to know is how to operate the machinery, and sort the problems as they arise. You will find the flight manual goes into immense detail on the operation, both normal and emergency, of the spike, front bleed, rear bleed, nozzle, afterburner, derich, EGT trim, shifting of the IGV, and compressor inlet temperature and pressure. An organ player doesn't have it so tough. What he is doing is managing the operation of the ramjet function and keeping it on song. From, Kerrebrock, Jack L. (1992). Aircraft Engines and Gas Turbines (2nd ed.). Cambridge, MA: The MIT Press. ISBN 978-0-262-11162-1 Heiser, William H.; Pratt, David T. (1994). Hypersonic Airbreathing Propulsion. AIAA Education Series. Washington D.C.: American Institute of Aeronautics and Astronautics. ISBN 1-56347-035-7 The air turboramjet engine is a combined cycle engine that merges aspects of turbojet and ramjet engines. Air passes through an inlet and is then compressed by an axial compressor. That compressor is driven by a turbine, which is powered by hot, high pressure gas from a combustion chamber. The air compressed by the compressor bypasses the combustor and turbine section of the engine, where it is mixed with the turbine exhaust. The turbine exhaust can be designed to be fuel-rich (i.e., the combustor does not burn all the fuel) which, when mixed with the compressed air, creates a hot fuel-air mixture which is ready to burn again. More fuel is injected into this air where it is again combusted. The exhaust is ejected through a propelling nozzle, generating thrust.[ |
Perhaps Lyman can expound on why compressor inlet pressure is of importance.
Hello Brian Our disagreement seems to revolve around the definition of "compression". Bleed air originates in a mechanical (axial) cabinet, ramjet inlet air originates from the mere capture of air in an intake. There is no mechanical enhancement of pressure; the high velocity of the inlet relative to the airstream provides compression sufficient to ennable combustion, after addition of fuel. So it is simply the lack of mechanical enhancement that creates the criterion for the definition. So to me, any "aspect" of ram "effect" would by definition need to have its source of compression separate from the mechanical core of the engine. It seems a workable distinction to me. I can't imagine why it is so difficult. In creating criteria, the format can include or exclude certain aspects that allow for more restrictive or more general conclusions. The easiest thing to do is to agree to disagree. We have different criteria for the discussion, which allows for both of us to be correct. If enough people, especially wise people, use terminology that they agree on, the purpose of understanding is done. You and I use a slighly different set of criteria. It seems important to you that I disagree with your definition. We have no fundamental disagreement, only a slightly different opinion on the background of the mechanical aspect of the origination of the air supplied to the combustor. Consider me wrong, and let's move on? With great respect..... |
Lyman,
PW did not "solve" anything by routing bleed to the ejector, neither do they create a ramjet. You do a great disservice to Pratt, Whitney (and the ampersand) by denigrating the J58 with terms that imply it is a ramjet. it simply is not. Frankly, the six bleed ducts are superchargers, the pressure is created mechanically, something a ramjet does NOT do. Any additive pressure to the gas path that is created mechanically is not RAM. By definiton. There is much information on the internet about this subject, if you would only search and find it. Start by reading this beginning on Pg.2 and continuing to Pg.4: http://www.bobabernethy.com/pdfs/Nev...0of%20P&W3.pdf So if you want to take issue with the P&W guy who recognized the problem, developed the solution idea (Bleed Tubes), and patented it, be my guest. I am sure you will, holding true to form. If you are still having trouble understanding, Then, From the USAF, Hill Air Force Base: The variable-geometry inlets for the engines were quite complex and intricate. The most prominent feature was a hydraulically-actuated conical spike which was automatically moved forward or aft by the Air Inlet Computer as required to keep the supersonic shockwave properly positioned in relation to the inlet throat. Working in conjunction with a series of bypass ducts and doors, the spike prevented supersonic air from entering the inlet and maintained a steady flow of subsonic air for the engine. At Mach 3.2 cruise the inlet system itself actually provided 80 percent of the thrust and the engine only 20 percent, making the J58 in reality a turbo-ramjet engine. From Aircraft Engine Design: Turbojet/Ramjet Combined Cycle Engine The J58 operates as an afterburning turbojet engine until it reaches high Mach when the six large tubes (Fig. 13a) bypass flow to the afterburner. When these tubes are in use, the compressor, burner, and turbine of the turbojet are essentially bypassed and the engine operates as a ramjet with the afterburner acting as the ramjet's burner. From Aerostories Mach numbers the new engine would be unable to cope with the volume of air coming through the air-intakes. This would result in compressor stalling with accompanying loss of efficiency and thrust at high speeds. Pratt & Whitney therefore modified their JT-11 by installing a series of fixed flow-vanes downstream of the 4th compressor stage, which directed the surplus airflow along six longitudinal jet pipes running along the engine casing. The surplus was then carried straight to the afterburner chamber serving to cool the burners, whilst enriching the mixture; so enabling higher combustion temperatures or increased thrust. |
I like this thread. Such deep knowledge displayed here, it's starting to make my teeth hurt.
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I cannot isolate any further the kernel of my position.
If the airflow feeding what you term the Ramjet "section" of the J58 is isolated from the internal machinery of the turbojet in toto, there would (technically) be a Ramjet "component" to the engine's power. If the source of this air is the compressor section, you are completely, and patently, wrong. I do not care who endorses the definition, Ramjets operate from passive air, not mechanically enhanced air. There is bypass air that is directed into the ejector, from forward of the internal part of the Turbojet. It, however, passes through the spike, and also (technically) fails the uninterrupted nature of what has been (classically) the definition of a Ramjet engine. The important part (to me) of this discussion is how you won't address my very basic point, "enhanced airflow disqualifies the definition". My worry is not to be accused of being stupid, but that I seem to have failed completely in receiving even an acknowledgement of this most basic definiton. w/o extraneous data, would you simply say if you understand my position? You do not need to affirm/deny it. |
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