Forum: Technology

Whereas the thermal efficiency is often referred to as the internal efficiency of the engine, the propulsive efficiency is referred to as the external efficiency. This latter efficiency explains why the pure jet engine is less efficient than the turbo-prop engine at lower aircraft speeds leading to development of the by-pass principle.

Performance of the jet engine is not only concerned with the thrust produced, but also with the efficient conversion of the heat energy of the fuel into kinetic energy, as represented by the jet velocity, and the best use of this velocity to propel the aircraft forward, i.e. the efficiency of the propulsive system.

It is therefore apparent that at the aircraft lower speed range the pure jet stream wastes considerably more energy than a propeller system and consequently is less efficient over this range. However, this factor changes as aircraft speed increases, because although the jet stream continues to issue at a high velocity from the engine its velocity relative to the surrounding atmosphere is reduced and, in consequence, the waste energy loss is reduced.

To obtain good propulsive efficiencies without the use of a complex propeller system, the by-pass principle is used in various forms. With this principle, some part of the total output is provided by a jet stream other than that which passes through the engine cycle and this is energized by a fan or a varying number of LP compressor stages. This bypass air is used to lower the mean jet temperature and velocity either by exhausting through a separate propelling nozzle, or by mixing with the turbine stream to exhaust through a common nozzle.

This arrangement matches the velocity of the jet nearer to the optimum requirements of the aircraft and results in higher propulsive efficiency, hence lower fuel consumption. For this reason the pure jet engine where all the airflow passes through the full compression cycle is now obsolete for all but the highest speed aircraft.

The thread was not started as a turbofan vs turbojet debate, it was the simple question "is the turbojet dead?"

NO the turbojet is not dead. It lives on as the heart of every modern turbofan/turboprop/turboshaft engine. It is the CORE in every turbine engine from the M1A1 Abrams tank to the F-35 or the A380. Regardless of the engine's type, a turbojet is the "gas generator" deep inside it.

Today's military variety ultra-low-bypass turbofan is more like a turbojet than a turbofan as first introduced in the 1950s. The F100-PW-229, and F119-PW-100 have BPRs lower than any "turbofan" ever used in a military aircraft. Supercruise is more efficient with engines of lower BPR, and this seems to be where the next generation of combat aircraft are headed. Without 'burners a turbojet is more efficient at supercruise (high-speed) than a turbofan. This also translates into a lower BPR engine being better at supercruise than a higher BPR engine roughly the same size/specification.

I'm not saying an augmented turbofan can't do what an afterburning turbojet can do or vice-versa. They can both do each other's job with different degrees of efficiency/economy at given areas of a flight envelope.

Turbojets are still valid propulsion systems for missiles and small high-speed aircraft. For a given mass flow less thrust is produced by the fan engine due to lower exit velocity. To obtain the same thrust, a fan engine must be scaled up to pass a larger total mass airflow than the pure turbojet engine. This is evident in the F100/F110 designs. The F110 needed more airflow (intake area) to make the same (or more) thrust than the F100 did due to it's lower BPR and higher exit velocity. Enlarging an engine diameter or increasing an intake area isn't always possible. (IE the AGM-84 Harpoon/SLAM with Teledyne/CAE J402-CA-400 turbojet engine)

I offer some points concerning propulsive efficiency below. They were taken from Rolls Royce's Book "The Jet Engine" and detail some of the important issues concerning turbojets and some of their various forms. It is interesting to note RR calls a turbofan a "bypass-turbojet".

The highest speed turbine engine being develped today is for RATTLRS (Mach 4) and it's a straight turbojet.

Nobody will say whether there are bleed vents on the spike, but there are pretty obvious bleed vents along the forward quarter of the missile, which Skunk Works deputy program manager Barry Brown confirmed were bleed vents to lower pressure. It’s likely that the airspike’s translation alters how much intake air flows out of those vents, closing them off at lower Mach numbers to create pressure, and opening them in the Mach 3+ regime to keep the air volume from overwhelming the compressor.


Either way, it doesn’t seem like that air is reinjected anywhere else along the line, although Bob Duge of Rolls Royce tells me future versions will employ additional ram air in the combustor. “That will get us closer to hypersonic speeds,” he says.

First off, this is a high supersonic to low transonic missile. You can't compare it to conventional jet engine designs. The thing doesn't even use an afterburner. Exactly how it works is either classified or confidential information. However, if you read the following article you'll get a better idea of why you can't compare this missile's engine to a more conventional low-bypass, afterburning turbofan.

Nobody will say whether there are bleed vents on the spike, but there are pretty obvious bleed vents along the forward quarter of the missile, which Skunk Works deputy program manager Barry Brown confirmed were bleed vents to lower pressure. It’s likely that the airspike’s translation alters how much intake air flows out of those vents, closing them off at lower Mach numbers to create pressure, and opening them in the Mach 3+ regime to keep the air volume from overwhelming the compressor.

Either way, it doesn’t seem like that air is reinjected anywhere else along the line, although Bob Duge of Rolls Royce tells me future versions will employ additional ram air in the combustor. “That will get us closer to hypersonic speeds,” he says.

Well look at that... Mach 4+ speeds require the use of the bypass flow

Ram air... I'm not sure what they mean by that exactly. Do them mean air that has been compressed by the inlet, ducted around the compressor, and piped into the combustor?

Or does the missile/engine have little vents that open up to the - to feed the combustor ram air? I mean... what does additional ram air to the combustor mean? The thing doesn't have an afterburner so you can't pipe the bypass flow into the nozzle section.

The SR-71/J-58 used the bypass flow and afterburner like a ramjet uses its ram air and burner section. If you don't have a burner, where do you inject "ram air" that is not expelled from the engine completely? I guess into the combustor although I haven't heard of that application before. This thing sounds like a completely new type of engine technology. You're comparing a conventional turbojet to some exotic piece of classified/confidential technology. How the thing actually works so well is a secret! How are you going to compare this to a turbojet? It's obviously as much like a conventional turbojet as an SR-71's J-58. By the way, are you still going to tell me that the J-58 is an afterburning turbojet and not a variable bypass afterburning turbofan?

The turbojet is dead in terms of conventional military and civilian aircraft.

The fact that an AB turbofan is basically an AB turbojet if you take away the bypass flow does not mean that the turbojet is alive and well.

There's no such thing as ultra-low-bypass AB turbofans. Mixed flow AB turbofans have low bypass ratios... period. They always have. Exactly what their bypass ratios are is not a function of how efficient the engines are trying to be at supersonic speeds. First off, any bypass ratio below 1.0 is considered low bypass. The difference between high bypass ratio and low bypass ratio is subjective because you now have engines with BPRs of 10+ which makes BPRs of 2.0-5.0 seem pretty low. But anything under 1.0 is definitely low bypass and typically anything under 2.0 is considered low bypass as well. Let's take a look at some various low bypass AB turbofans and their associated bypass ratios, shall well?

Engine(s); BPR; associated airframe(s)
F101-GE-102; 1.91; B-1B
F110-GE-100; 0.87; F-16C
F110-GE-129; 0.76; F-16C
F110-GE-132; 0.87; F-16E (Block 60)
F110-GE-400; 0.87; F-14B/D
F119-PW-100; 0.45; F-22A
F404-GE-400; 0.34; F/A-18A/B/C/D
F404-GE-402; 0.27; F/A-18C/D
F414-GE-400; 0.40; F/A-18E/F
TF30-PW-414(A); 0.88; F-14A

What does all this tell you? Nothing really...

I'm sorry, but a mixed flow AB turbofan is almost always more efficient than an AB turbojet. Using a turbojet as opposed to a non-afterburning turbofan (generally high bypass) on a commercial or military transport aircraft is unthinkable.

What small, high speed aircraft are you talking about? As for cruise missiles and such... not even a valid comparison.

I know what book you're talking about and it's simply not appropriate for these types of highly technical/scientific debates. This is not a textbook that one would use in an aerospace propulsion undergrad or graduate course. It's not something engineers would reference when trying to design or analyze a gas turbine's performance.

The SR-71 makes extensive use of bypass flow to obtain its Mach 3+ speeds.

Well look at that... Mach 4+ speeds require the use of the bypass flow principle. Sounds like the SR-71 all over again, except with more advanced technology. In any case, bypass flow seems to assist in going faster. You can use a pure turbojet up to a point, but eventually you need to make use of bypass flow to go faster.

So this quote from the Rolls Royce book does nothing to support your argument that the lower the bypass ratio, the more efficient the engine is at high Mach.

And for the umpteenth time, you can't use anecdotal evidence like this to make blanket statements like "turbojets are more efficient than turbofans at high Mach." Why? Because in general they're not. In general, turbofans are more efficient than turbojets. Just because you can find some special case where an aircraft goes really fast with a pure turbojet doesn't mean that turbojets are more efficient at high speed.

Exhaust velocity has nothing to do with it either because 1 pound of thrust is 1 pound of thrust regardless of the exhaust velocity -- you can throw less molecules at a higher speed or more molecules at a lower speed to arrive at 1 pound-force, but 1 pound-force is 1 pound force.

What's the difference between a "fan" stage and a compressor stage? They both compress air. Some of the air from the fan gets further compressed by the core compressor stages and the rest is bypassed around the entire core. The difference is that the SR-71 used some of its compressor stages as fan stages when it turned into a "bypass turbofan".

As for the RATTLRS, if you agree that it's going to by getting to Mach 4+ speeds by utilizing ram air flow directly from the inlet and bypassed around the compressor into the combustor, that's like a... umm... a turboramjet?

In fact, I'm sure this engine's operation is far from a normal, conventional turbojet's. I don't think they just "run it hot".

If it was so simple, the details wouldn't be classified.

The turbojet is dead... and even when it's not, the turbofan concept of bypass flow always seems to come into play at really really high speeds.