Turbofan

You can't put an afterburner on a high bypass ratio engine. The reason is that the fan pressure ratio isn't high enough to produce a choked nozzle at the exit, so you aren't going to make an afterburner work on that kind of an engine. You could put an afterburner on it, but it wouldn't produce the high thrust that you are thinking it would make, and it won't make the thrust of a low bypass ratio engine with a proper exhaust nozzle.

Low bypass ratio engines have multiple fan stages to increase the pressure ratio, but larger high bypass ratio engines only have one stage and don't have enough fan pressure ratio to use an afterburner. The F100 engine has a bypass ratio of about .4 and even the engines that are using supercruise have similar bypass ratios. A large high bypass ratio engine would become very inefficient at higher than Mach 1 because of the high volume of air entering the engine at those speeds. There is a reason that the F100 and F110, as well as the F135 are all low bypass engines. The reason is that you have to consider overall propulsive efficiency (that nasty term again).

The propulsive efficiency of an air breathing propulsion system is not just the exit velocity ratio dependent term as the speed increases. At low speed inlet velocity is not a consideration, but as the speed increases, you have to consider the mass of air that enters the engine inlet. The reason is that the air has to be slowed down to around .4 Mach at the engine compressor face. So your propulsive efficiency term now includes the mass of the air entering the inlet. That is, you have a drag term related to the mass of air and the speed of the vehicle. Big inlet air volumes result in big drag, so fast aircraft (above Mach 1) don't want engines that use a lot of air, exactly the opposite of low speed engines.

For both of the above reasons the Harrier is limited to speeds near Mach 1.

 

"You must spread some Reputation around before giving it to Yellowjacket again" Damn!
Very good post, just as the rest of your posts in this thread are.

 

It's not my problem that you use your ignorance of duct burning as your sole basis for saying there is no such thing because, oops, THE LITERATURE COMPLETELY SUPPORTS WHAT I SAID.

http://fliu.eng.uci.edu/Publications/J16.pdf

"In a conventional configuration (dashed line in Fig. 1) the HP,
high-temperature gas expands through the turbine, which provides
enough power to drive the compressor and fan and other engine
auxiliaries. To further increase the thrust level, fuel may be injected
and burned in the optional afterburner to increase further the temperature
of the gas before the flow expands through the nozzle to
produce the high-speed jet. For a turbofan engine part of the flow
that comes into the inlet is diverted to the fan bypass. The pressure
of the bypass flow is increased through the fan. The flow state after
the fan is marked as 03f . An optional duct burner behind the fan can
also be used to increase the thrust. The flow then expands through
the bypass nozzle into the atmosphere or mixes with the flow from
the core engine before expanding through a common nozzle."

 

Duct Burning

More on duct burning turbofans

http://www.docstoc.com/docs/5843805...ofan-And-Method-Of-Operation---Patent-7770381

http://www.google.com/patents/US7770381

 

http://www.gizmag.com/ge-aviation-develop-advent-variable-cycle-jet-engine/25556/

"Superjet" variable cycle jet engine could power future fighter aircraft
By Brian Dodson
January 6, 2013

GE Aviation is developing a revolutionary new jet engine that aims to combine the best traits of turbojet and turbofan engines, delivering supersonic speed capability and fuel efficiency in one package.

The new engines are being developed under the USAF ADVENT project, which is seeking 25 percent fuel saving which will in turn lead to an increase in mission capability.

There are two main species of jet engines for aviation: low-bypass turbofans, usually called turbojets, and high-bypass turbofans. Turbojets are optimized for high-performance, pushing fighter jets to above Mach 2 (and the SR-71 "Blackbird" to well over Mach 3), but pay for that performance with terrible fuel efficiency. The performance outcome of a conventional turbojet is dominated by the operation of the high-pressure engine core (compressor, combustion, turbine, and exhaust nozzle).

In contrast, high-bypass turbofans are the heavy lifters of commercial aviation, being optimized for subsonic thrust and fuel efficiency, but performing poorly at supersonic speeds. A conventional turbofan adds lower-pressure airflow from an oversized fan which is driven by the jet turbine. The fan airflow bypasses the combustion chamber, acting like a large propeller.

In an ADVENT (ADaptive VErsitile ENgine Technology) engine, the high-pressure core exhaust and the low-pressure bypass streams of a conventional turbofan are joined by a third, outer flowpath that can be opened and closed in response to flight conditions. For takeoff, the third stream is closed off to reduce the bypass ratio. This sends more of the airflow through the high-pressure core to increase thrust. When cruising, the third bypass stream is opened to increase the bypass ratio and reduce fuel consumption.

The extra bypass duct can be seen running along the top and bottom of the engine. This third duct will be opened or closed as part of a variable cycle to transform it from a strike aircraft engine to a transport-type engine. If the duct is open the bypass ratio will increase, reducing fuel burn, and increasing subsonic range by up to 40 percent, leading to 60 percent longer loiter times on target. If the ducts are closed, additional air is forced through the core and high pressure compressor, enabling thrust and speed to increase and providing world-class supersonic performance.

GE's ADVENT designs are based on new manufacturing technologies like 3-D printing of intricate cooling components and super-strong but lightweight ceramic matrix composites. These allow the manufacture of highly efficient jet engines operating at temperatures above the melting point of steel.

Engineers also designed the new engine to be easy to fly. “We want the engine to take care of itself and let the pilot focus on the mission,” says Abe Levatter, project manager at GE Aviation. “When the pilot says ‘I’m out of danger, I want to cruise home,’ the engine reconfigures itself. We take it upon ourselves to make the engine optimized for whatever the pilot wants.”

GE is now testing the engine’s core components and plans to run a full test in the middle of 2013. The video below provides additional visual description of its operation.

 

Duct Burning

More on duct burning

http://books.google.com/books?id=2W...CAQ#v=onepage&q=turbofan duct burning&f=false

 

Actually...since I was working for Hoverspeed at the time...

...the SRNs were retired because they were at the end of their usefull life, and had only made it that far by the cannibalization of parts, engines, etc from retired craft to keep the last couple operational. Compared to all the new Incat fast catamarans that Hoverspeed was buying, the SRNs were horrendously expensive to operate and maintain and Hoverspeed eventually operated only the catmarans in their cross-channel service. The loss of revenue that resulted from teh formation of the EU (loss of lucrative duty-free sales) and the opening of the "chunnel" eventually caused the catamaran services to largely disappear...later.

As for air-screws propelling hovercraft in the general sense..USN LCACs and all the rest...air screws are the only practical means of propulsion; however poor the propulsive efficiency is.

 

Excuse me but I most certainly am not ignorant in this subject.

I never said there was no such thing as duct burning turbofans. I worked on one back in the days of the SST.

What I said is that you cannot use a afterburner, duct burning or mixed flow burning on a high bypass engine. Please read my post and try to understand what I am saying, not what you think I've said. As I said, a high bypass ratio engine does not have sufficent fan pressure ratio to provide a choked exhaust nozzle, and consequently it can't have a converging/diverging nozzle, and therefore you can't effectively use an afterburner on it. You need a fan pressure ratio of greater than 2.2 to create choked flow and typical fan pressure ratios for high bypass engines are between 1.4 and 1.7. To create a fan pressure ratio that high you need at least two stages of fan, and if you tried to do that with a high bypass engine the work extracted from the turbine would be so large that the core exhaust stream wouldn't have any energy left in it.

There is no such thing as a high bypass ratio augmented turbofan. You can't get there from here. Your statement that it could be applied to an engine like the Pegasus is false, it won't work.

This is getting tiresome. Every time I show you that you do not understand this subject you come up with some other absurd example that either has no relevance, or is lacking in fundamental proplusion knowlege. Then I have to explain it to you in detail as to why you don't know what you are talking about. Then you change the subject to something more innane and absurd. I've grown tired of trying to explain the science of propulsion to you and only hope that in the future others will see your posts and simply discount them because they have learned from this discourse that you simply don't know what you are talking about.

Edit..

And even with variable cycle engines like the ADVENT it is still a relatively low bypass ratio engine, it isn't what anyone in the trade would call high bypass, for the reasons above.

 

Which part of this did you not understand? "I don't give a crap about your COMPLETE inability to demonstrate the LEAST comprehension of the fact efficiency has nothing to do with the capability to generate brute power and the capability to generate brute power has nothing to do with efficiency."

 

Here's another lesson in propulsion system design. Diesel-electric locomotives have never come close to the pulling power of the largest steam locomotives. However, diesel-electric locomotives are much less expensive to maintain. So even though more brute power could be attained with a single steam locomotive, it made more economic sense to switch over to diesel-electric locomotives and use multiple less powerful locomotives when a train needed extra pullling power.

 

In forum vernacular 'please don't feed the troll'.

I am disgusted with this thread. Props to Yellowjacket for keeping the high ground. The old saying goes 'Don't play with a pig, the pig has fun and you get covered in ****.

 

Look, I don't know how old you are, but this way of discussing is showing a mental level of an underage, testosterone-driven child.

You are an extremely arrogant, unpleasant and vulgar person, and honestly it is your luck that this is just a discussion on an internet forum, because in a face to face arguing these manners would earn you a pair of loud slaps over your asinine ears.

You should be ashamed of your behavior, and your parents should be ashamed as well, because they evidently failed to teach you decent and civilized manners.

 

Stuff it. Yellowjacket's complete ignorance of the duct burning concept for turbofans tells me he's never even read a book on jet engine design, let alone worked for General Electric or Pratt & Whitney.

 

Stuff it. I don't suffer ignorant egotistical fools who've never developed the critical thinking skills of a turnip when they think the only criteria for accepting an established fact is whether an "expert" says it.

 

Thank you FR for confirming daiquiri's theory.

I have sent the following to the moderator. If other posters agree let the moderator know. "FranklinRatliff is rude, disruptive and insulting to all who don't agree. Since absolutely no one agrees with this poster, every post is an attack on someone".