747400sp
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100000 Lb After Bunering Turbo Jet?

Sun Mar 12, 2006 8:01 am

A few month ago I put up a post in military av about a supersonic heavy lift transport. The reply I got back was you can not make an after burning turbo fan. I also been reading about the GE-4/J5P being a giant 60000 lb trust turbo jet. So my question is could a 100000 lb trust after burning turbo jet with super cruise be built. With a engine like this you could build a mach 2 heavy lift transport or who knows, but could it be done.
 
corey07850
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RE: 100000 Lb After Bunering Turbo Jet?

Sun Mar 12, 2006 8:30 am

Quoting 747400sp (Thread starter):
The reply I got back was you can not make an after burning turbo fan

Who said this??? The F414 engines on the F-18 are low bypass turbofans with afterburner
 
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Starlionblue
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RE: 100000 Lb After Bunering Turbo Jet?

Sun Mar 12, 2006 8:48 am

Quoting 747400sp (Thread starter):
The reply I got back was you can not make an after burning turbo fan.

Of course you can. The problem is different. At supersonic speeds that big fan starts becoming a liability. Fans are good at moving lots of air slowly. At supersonic speeds you need to move less air fast, which explains why military jets tend to have low bypass turbofans.

A fictitious multimodal engine "efficient" at both subsonic and supersonic speeds could be a high bypass turbofan that could fold away (or maybe feather) the fan somehow, becoming a turbojet, or just block off the core altogether and become a ramjet. Heck, while we're dreaming why not a scramjet.

The SR-71 came close to this. Its J-58s are actually turboramjets.
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grandtheftaero
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RE: 100000 Lb After Bunering Turbo Jet?

Mon Mar 13, 2006 3:20 am

Actually... theoretically anything is possible if you're just talking about the thermodynamic cycle. In my college vehicle design class I designed a cycle for a modified GE90 with an afterburner. Get this... the thrust with afterburner ~400k lb!!! The computer-based cycle deck said it work would like a charm. The problem was that it needed a two-stage fan and a T41 (turbine inlet temperature) that was just beyond the reach of current materials. The cost and weight of building it would have been astronomical.

Could your notional engine be built? I don't see why not. But when something like is put on the table in industry, you can be sure that multiple trade studies are done to see if such an engine is the best way to go. Such a study could conclude that many smaller, more conventional engines per airframe would be more cost effective.
 
lehpron
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RE: 100000 Lb After Bunering Turbo Jet?

Sat Mar 18, 2006 6:00 pm

Quoting 747400sp (Thread starter):
The reply I got back was you can not make an after burning turbo fan

Don’t be general, the bypass ratio (BPR) is key. BPR is the ratio of how much air flows around the core compared to what goes into it. A turbojet is pure high pressure and high temperature thrust. When higher BPR's are introduced, the pressure and temperature of the exhaust drops. So does the noise and fuel usage for a given thrust output. That is the point.

An afterburner increases the pressure and temperature of the gas leaving the turbine, it does NOT increase the engine’s RPM -- a requirement for turbofans and turboprop systems. Turbofans are finely tuned such that their air pressure leaving the turbine and main fan are pretty close no matter what RPM it is set to. Placing an afterburner on the turbine core would throw that off, you would loose thrust pressure as some will be going sideways.

While it may be easy to imagine placing a GE90-type engine in a B58 hustler-type cowling so the ENTIRE engine is the size of a 735 fuselage hanging under a wing......gawdamn plane must be huge! (I'll make up one and post it here). As others have stated above, there are turbofan afterburners, but their by-pass flow also enters the afterburner section and the ratios are low, most are less than 1.5.

As of placing the afterburner down stream to a higher BPR engine, I can come up with some data based on what I know and learned to show you that supersonic flight is tricky. We'll see, spring break just started; I'll probably get back in less than a week.  Wink

Quoting 747400sp (Thread starter):
also been reading about the GE-4/J5P being a giant 60000 lb trust turbo jet. So my question is could a 100000 lb trust after burning turbo jet with super cruise be built.

We are only limited by our imagination, money and our knowledge of material sciences; even a 1 million lbs thrust engine can be built. An ideal turbojet is a simple system; the thrust is dependent on material temperature max and compression ratio.

Since the military doesn't care for noise or fuel usage (unless range is an issue), I suppose it is a matter of convincing them there is a point to all of this.  Wink
The meaning of life is curiosity; we were put on this planet to explore opportunities.
 
B2707SST
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RE: 100000 Lb After Bunering Turbo Jet?

Sun Mar 19, 2006 2:43 pm

Quoting 747400sp (Thread starter):
A few month ago I put up a post in military av about a supersonic heavy lift transport. The reply I got back was you can not make an after burning turbo fan. I also been reading about the GE-4/J5P being a giant 60000 lb trust turbo jet. So my question is could a 100000 lb trust after burning turbo jet with super cruise be built. With a engine like this you could build a mach 2 heavy lift transport or who knows, but could it be done.

You could, but it wouldn't really be necessary. Even the 740,000 lb. NASA HSCT only needed four low-BPR turbofans in the 70,000 lb. thrust class. For various reasons, I doubt we will ever see a tri- or twin-jet supersonic transport (an SSBJ would probably be a twin, though). An SST would have to approach one million pounds to need four such engines, and those gross weights are not necessary for currently envisioned applications (300 passengers in three classes over 5,000-6,000 nautical miles).

Due to their high fuel consumption per ton-mile, poor payload/weight ratios, and narrow fuselage cross-sections, SSTs would make terribly inefficient freighters, and the added speed is generally of very low value for most cargo applications. The only exceptions might be ultra-high-priority letter and parcel delivery, live exotic animals, time-sensitive medical items like organs and critically ill patients, and other payloads that are likely to be light.

In any case, pure afterburning turbojets are unlikely to be included in any future large civil supersonic aircraft. Although they are the most thermodynamically efficient type of power for supersonic cruise, pure turbojets are much too noisy to meet current regulations and are very inefficient at subsonic speeds. The current consensus seems to favor trading a small amount of efficiency at cruise for much better performance and lower noise at takeoff/landing and subsonic cruise though a low-bypass turbofan. Most post-Concorde/B2707 proposals I've seen feature non-afterburning turbofans in the 1:1 - 3:1 BPR range, with smaller BPRs correlating with faster cruise speeds. A variable bypass turbofan would be the holy grail of SST propulsion, if one could be developed without too much added weight or mechanical complexity.

Additionally, both the Boeing SST (had it been built) and Concorde "B" models (from line number 217) would have dropped the afterburner from the GE4 and Olympus 593, respectively. Both teams decided, apparently independently, that simply upscaling the engine and deleting the afterburner ultimately results in savings on fuel consumption and maintenance costs. Neither program ever made it into production.

By the way, I hope you noticed my post on the your GE4/JTF17A thread. I only stumbled across it a while after the previous last post had been made.

--B2707SST
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Starlionblue
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RE: 100000 Lb After Bunering Turbo Jet?

Mon Mar 20, 2006 3:38 am

Quoting B2707SST (Reply 5):
Most post-Concorde/B2707 proposals I've seen feature non-afterburning turbofans in the 1:1 - 3:1 BPR range, with smaller BPRs correlating with faster cruise speeds

As you say, this is the Concorde-B in many ways.

Quoting B2707SST (Reply 5):
A variable bypass turbofan would be the holy grail of SST propulsion, if one could be developed without too much added weight or mechanical complexity.

I may be needing the foot/mouth surgeon for this, but don't variable geometry intakes (as on many current fighters) in essence result in variable bypass turbofans?

Also, if the fan could change pitch (like a turboprop prop) would this be an advantage? Picture feathering the fan (and the low pressure compressor driving it) at high speeds and making a turbofan into a turbojet. I know the shape of modern fan blades pretty much precludes this, but maybe we could compromise and make them straight like in the 70s.

Just my completely pie in the sky uninformed �0.02.
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B2707SST
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RE: 100000 Lb After Bunering Turbo Jet?

Mon Mar 20, 2006 2:38 pm

Quoting Starlionblue (Reply 6):
I may be needing the foot/mouth surgeon for this, but don't variable geometry intakes (as on many current fighters) in essence result in variable bypass turbofans?

Not directly, but somewhat true in that the purposes of the two systems are related by the fundamental principles of exhaust gas speed and mass flow. Recall that thrust is the product of the volume of air moved by the engine times multiplied by the acceleration of that volume of air. At subsonic speeds, it is more efficient to move a large volume of air slowly, hence high-BPR turbofans like the GE90. At supersonic speeds, the acceleration created by a turbofan is small compared to the aircraft's speed while the large inlet creates enormous drag, so the most efficient propulsion is obtained by greatly accelerating a small amount of air. A variable-bypass turbofan would act to reduce exhaust velocity and increase mass flow at subsonic speeds, while variable-geometry inlets are required to provide effective propulsion at supersonic speeds above about Mach 1.4.

Variable-geometry intakes serve to focus the shock waves present at supersonic speeds to efficiently decelerate the incoming air, since no jet engine can accept supersonic airflow. For example, Concorde's inlets decelerated Mach 2 air to about Mach 0.6 at the engine face. The trick is to do this while minimizing pressure loss and airflow heating, which represents the wasteful conversion of kinetic energy into thermal energy. Moving parts are necessary because the angle of the shock wave changes with Mach number. At speeds above about Mach 1.5, a fixed inlet is unable to deliver efficient airflow.

On Concorde at supersonic cruise, the inlet and exhaust nozzles boost the overall compression ratio from 15.5:1 (engine only) to over 80:1 (total system), making the Olympus the most thermodynamically efficient jet ever built under those conditions. The convergent-divergent exhaust nozzle system handles the equally important task of re-accelerating the exhaust gases to supersonic speeds with minimum energy loss.



It is impossible to overemphasize how important inlet and exhaust nozzle design is to overall propulsion efficiency and how difficult this engineering is. Concorde's intake ramps are controlled by an analogue computer system that was, for its day, extremely sophisticated; in fact, the inlet system was deemed a national security asset by the British goverment due to its potential applications on supersonic bombers.

VG inlet designs include the moveable ramp system found on Concorde, the XB-70, many supersonic fighters, and the B-1A; the axisymmetric or cone-style inlet found on the SR-71 (translating centerbody), B2707 designs (variable-diameter centerbody), and some HSCT designs; and the bifurcated inlet found on the Lockheed L-2000 and other HSCT designs. According to a NASA page I can't seem to find at the moment, axisymmetric inlets are better suited to Mach 2.5+ flight than the ramp system because they are able to create a narrower inlet throat, which is necessary to shock the faster airflow down to subsonic speeds. Concorde's inlets are simpler and more efficient in the Mach 1.8-2.2 speed range.


Boeing 2707/GE4 variable-diameter axisymmetric inlet


Lockheed L-2000 bifurcated inlet

At supersonic cruise, most inlet systems do bypass some air around the engine, simply because more air is available than needed for cruise thrust, since even supersonic inlets usually have to be sized for takeoff airflow. However, air diverted around the engine by the inlet system does not deliver any net thrust; in fact, drag is incurred by slowing and reaccelerating it without any added energy input from the engine. This flow doesn't change the bypass ratio of the engine itself and has no effect at subsonic speeds, where a higher BPR is most needed.

A true variable-bypass turbofan would be able to change the ratio of airflow entering the engine face to airflow bypassing the engine core, with the bypass stream also accelerated by the bypass fan, of course not to the the same extent as the primary stream. The bypass system would have to be carefully integrated with the inlet and exhaust nozzle for maximum bypass at takeoff and subsonic speeds; noise suppression at takeoff and landing; minimum bypass ratio, maximum pressure recovery, and unstart* prevention at supersonic cruise; and minimum weight, drag, and mechanical complexity. In short, it would be a nightmare to design.

Quoting Starlionblue (Reply 6):
Also, if the fan could change pitch (like a turboprop prop) would this be an advantage? Picture feathering the fan (and the low pressure compressor driving it) at high speeds and making a turbofan into a turbojet. I know the shape of modern fan blades pretty much precludes this, but maybe we could compromise and make them straight like in the 70s.

It would work in theory, but as you mention, the intricate shape of current fan blades makes effective feathering nearly impossible. The mechanical complexity -- on top of the rest of the system -- would probably be prohibitive as well.

One proposal for a variable-bypass turbofan was RR's mid-tandem fan, which would use the LP turbine to drive a secondary bypass fan as well as the LP compressor. Bypass doors from the primary inlet and radial auxiliary inlets would open at takeoff and subsonic cruise to allow air into the annular bypass duct. These would close at supersonic speeds, partially sealing off the bypass duct and cutting the bypass ratio in half. It's a fascinating concept that unfortunately hasn't seen much development in the past several years.



 *An unstart occurs when the shock wave is dislodged from its optimal position in the intake throat. The result is a rapid loss in engine thrust, possibly a flame-out, and a terrific jolt for passengers and crew. Intakes must be able to recapture the shock wave as quickly as possible when an unstart happens and also prevent "duct buzz," which occurs when the shock wave pops in and out of the inlet.

I am not aware of any unstart issues on Concorde, so I assume this problem was solved successfully.

--B2707SST

[Edited 2006-03-20 06:41:03]
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lightsaber
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RE: 100000 Lb After Bunering Turbo Jet?

Mon Mar 20, 2006 4:44 pm

Since the thread started mentioned supersonic heavy lift transports, I will note that these aircraft need range and thus would supercruise (no afterburner).

Quoting GrandTheftAero (Reply 3):
I designed a cycle for a modified GE90 with an afterburner. Get this... the thrust with afterburner ~400k lb!!! The computer-based cycle deck said it work would like a charm. The problem was that it needed a two-stage fan and a T41 (turbine inlet temperature) that was just beyond the reach of current materials. The cost and weight of building it would have been astronomical.

Oh boy, good luck avoiding Rayleigh's criterion in that large of a volume. What is Rayleigh's criterion? Basically, afterburners like to "organ pipe." That is, form standing pressure waves (as in a wind musical instrument like an organ pipe) that are then reinforced by the energy released in combustion. If the energy released by the chemical reaction ever gets in phase with the natural frequency of the afterburner... it will rip the engine apart. Almost all augmentor designs have this issue until tweaks are implemented. (e.g., why the F-22 couldn't use all three augmentor stages in the F119 until late in the development program). The larger the diameter of the augmentor, the greater the dynamic issue.

Oh, most augmented engines have 3 stage fans to achieve a high enough augmentor pressure ratio to sustain supersonic flight. Its possible with a two stage fan and even theoretically possible with a super advanced one stage fan (but barely). But, with the desire for supercruise, I suspect 3 stage fans will continue to be the norm.

Also, due to the designs including 3 stage fans, this is another reason for low bypass ratios. The core turbine exit pressure must be higher than the fan delivered pressure or else the engine will reverse flow and stall.

Quoting Starlionblue (Reply 2):
The problem is different. At supersonic speeds that big fan starts becoming a liability. Fans are good at moving lots of air slowly. At supersonic speeds you need to move less air fast, which explains why military jets tend to have low bypass turbofans.

Nice post overall.  Smile

Quoting Lehpron (Reply 4):
An afterburner increases the pressure

Incorrect. An afterburner like any combustion process in a moving flow suffers rayleigh losses. What one gains is a volumetric expansion as well as a temperature increase which increases the engine thrust.

And yes, Rayleigh did a lot of very early combustion theory, so many combustion terms are named after him.

Quoting B2707SST (Reply 7):
It is impossible to overemphasize how important inlet and exhaust nozzle design is to overall propulsion efficiency and how difficult this engineering is.

 checkmark  Very nice post. Well done. I used to work in a department that did nozzles. Interesting and difficult work.

Lightsaber
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Starlionblue
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RE: 100000 Lb After Bunering Turbo Jet?

Mon Mar 20, 2006 10:27 pm

Quoting B2707SST (Reply 7):

I am not aware of any unstart issues on Concorde, so I assume this problem was solved successfully.

Unlike on the SR-71, which experienced unstarts regularly until they redesigned the controlling logic.


Great post B2707SST. Thx for the info and pics!
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lehpron
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RE: 100000 Lb After Bunering Turbo Jet?

Tue Mar 21, 2006 9:13 am

Quoting Lightsaber (Reply 8):
Incorrect. An afterburner like any combustion process in a moving flow suffers rayleigh losses. What one gains is a volumetric expansion as well as a temperature increase which increases the engine thrust.

When aircraft with engines on full engage their afterburners, why do shock diamonds appear afterwards? They were not there before, I interpreted that as sonic flow having to reflect off of the pressure boundies between the exhaust and ambient. So, afterburners increased the temperature and therefore the pressure of the exhaust, in addition to mass flow. What did I miss?
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grandtheftaero
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RE: 100000 Lb After Bunering Turbo Jet?

Tue Mar 21, 2006 11:51 pm

Quoting Lehpron (Reply 10):
When aircraft with engines on full engage their afterburners, why do shock diamonds appear afterwards? They were not there before, I interpreted that as sonic flow having to reflect off of the pressure boundies between the exhaust and ambient. So, afterburners increased the temperature and therefore the pressure of the exhaust, in addition to mass flow. What did I miss?

Shock diamonds appear because the pressure of the exhaust gas is less than that of ambient. See http://www.fluidmech.net/gallery/shocks/sd_phys.htm for discussion.

Jet engines are constant pressure combustion machines. The pressure of the working fluid would never increase as a result of combustion. See any Braton cycle diagram for an illustration. If anything the pressure would drecrease as a result of non-ideal effects.

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