Jet engines and compressors

[Cheif Operations Director]

How is a jet engine any better off by using blades to suck in air instead of just using an air intake that leads directly into the combustion chamber?

Edited July 28, 2018 by Cheif Operations Director

[Cunjo Carl]

Here it's all about pressure. The layers of turbine blades (and stators) will boost the air pressure higher and higher until its driven into a very high pressure reaction chamber to burn with the jet fuel. The advantage of high pressure is that the fuel burns faster, more completely, and yields more mechanically available energy. It's an identical story to the pistons in a car by the way! They compress the gas+air before igniting them.

[Cheif Operations Director]

Just now, Cunjo Carl said:

Here it's all about pressure. The layers of turbine blades (and stators) will boost the air pressure higher and higher until its driven into a very high pressure reaction chamber to burn with the jet fuel. The advantage of high pressure is that the fuel burns faster, more completely, and yields more mechanically available energy. It's an identical story to the pistons in a car by the way! They compress the gas+air before igniting them.

How do these blades actually compress the air

[ARS]

2 minutes ago, Cheif Operations Director said:

How do these blades actually compress the air

By spinning the blades, it sucks the air into compression chamber, where the volume of air entering the chamber is higher than the air exiting the chamber, thus increasing the air pressure. The layers of turbine ensures that the airspeed entering the chamber is high enough to prevent pressure drop

[Cheif Operations Director]

Thanks

[Cunjo Carl]

A mixture of direct pushing and a little centrifugal force. Same as a ceiling fan!

Think about sticking your hand out the car window to use it as a 'wing'. If you tilt your hand up, it catches air and pushes it down. It's the same with the turbine blades, except they spin in circles and push the air toward the combustion chamber. If you want to keep boosting the pressure higher and higher though, you need to put things called stators between them, which keep the air going straight back (and pressurizing) rather than just spinning in circles.

*sniped*

Edited July 28, 2018 by Cunjo Carl

[Cheif Operations Director]

Just now, Cunjo Carl said:

A mixture of direct pushing and a little centrifugal force. Same as a ceiling fan!

Think about sticking your hand out the car window to use it as a 'wing'. If you tilt your hand up, it catches air and pushes it down. It's the same with the turbine blades, except they spin in circles and push the air toward the combustion chamber. If you want to keep boosting the pressure higher and higher though, you need to put things called stators between them, which keep the air going straight back (and pressurizing) rather than just spinning in circles.

Ohh!

Question

Let me type

Does this work by having your fan/turbine directly against a flat circular plate with holes will keep it straight?

[Cunjo Carl]

6 minutes ago, Cheif Operations Director said:

Does this work by having your fan/turbine directly against a flat circular plate with holes will keep it straight?

Your idea would work as a stator, for sure! Actual ones in turbines are apparently just a bunch of metal slats all pointing in to the center like an asterisk * . This said, I've never seen one in real life besides on pumps, so I only know what I've heard here. If someone know more specifically, feel free to chime in!

Edited July 28, 2018 by Cunjo Carl

[Cheif Operations Director]

Just now, Cunjo Carl said:

Your idea would work as a stator, for sure! Actual ones in turbines are apparently just a bunch of metal slats all pointing in to the center like an asterisk * . This said, I've never seen one in real life besides on pumps, so I only know what I've heard here. If someone know more specifically, feel free to chime in!

Thanks

@Cunjo Carl

What if you use a blade turning the other direction as a stator and compressor simplifying the process

[Reactordrone]

1 hour ago, Cheif Operations Director said:

How is a jet engine any better off by using blades to suck in air instead of just using an air intake that leads directly into the combustion chamber?

Ramjet engines do exist that just have the air going through a narrowing intake to compress the air before reaching the combustion chamber. The disadvantage is that they need high speed intake air in order to be efficient so they're not great for take off.

[Cheif Operations Director]

44 minutes ago, Reactordrone said:

Ramjet engines do exist that just have the air going through a narrowing intake to compress the air before reaching the combustion chamber. The disadvantage is that they need high speed intake air in order to be efficient so they're not great for take off.

But you could combine the system?

[Cunjo Carl]

I'm a bit out of my depths here, but I believe the unholy union of a turbojet and a ramjet into a single engine is simply called a turboramjet. I think.... I think Sabre technically counts as one (the rocket that KSP's Rapier is based on.) It also has closed-cycle functionality where it turns off its airbreathing abilities, and turns into a rocket engine. It's basically the coolest possible rocket plane engine.

As for turboramjets, I don't think they've ever heard of one in practical application, it's mostly a fun idea. There's a few prototypes around though. Likewise, ramjets don't really appear on airplanes except as extra engines on a normal plane for use in research and novelty, though I have heard of a ramjet missile once. Anyone correct me if I'm wrong though, has anyone heard of a production ramjet plane?

Also along these lines are air augmented rockets, which don't bother with any of that silly Oxygen stuff in the air and just use the stream of air coming into the engine for added reaction mass = Isp. They also haven't seen practical use, but also sounds totally awesome!

[Cheif Operations Director]

Just now, Cunjo Carl said:

I'm a bit out of my depths here, but I believe the unholy union of a turbojet and a ramjet into a single engine is simply called a turboramjet. I think.... I think Sabre technically counts as one (the rocket that KSP's Rapier is based on.) It also has closed-cycle functionality where it turns off its airbreathing abilities, and turns into a rocket engine. It's basically the coolest possible rocket plane engine.

As for turboramjets, I don't think they've ever heard of one in practical application, it's mostly a fun idea. There's a few prototypes around though. Likewise, ramjets don't really appear on airplanes except as extra engines on a normal plane for use in research and novelty, though I have heard of a ramjet missile once. Anyone correct me if I'm wrong though, has anyone heard of a production ramjet plane?

Also along these lines are air augmented rockets, which don't bother with any of that silly Oxygen stuff in the air and just use the stream of air coming into the engine for added reaction mass = Isp. They also haven't seen practical use, but also sounds totally awesome!

Final question, I think,  

How does the impeller play into this

[Cunjo Carl]

Not really sure. I only know the term from other industries, but it has a pretty standard meaning.

The impeller (or impellers in this context?) should just be another word for the compressor fans which boost the pressure of the incoming air to make the combustion work better. They're something that turbojets have and ramjets don't. The word impeller could also be used to refer to part of the pump which pressurizes the fuel in order to squirt it into the combustion chamber, but I think in this context they're talking about the air compressor fans.

EDIT:

@Cheif Operations Director

Using a blade turning the other direction to act as a stator and also the next stage of the compressor/turbine is a great idea, and I seem to recall seeing a system that worked like that, but it was definitely a terrestrial application. The main reason it's not done more is probably the mechanical issues involved... counter rotating fan blades can be surprisingly hard to make, because that means their shafts need to be concentric (one inside the other) and the bearings to hold all this in place suddenly become much more touchy and expensive. It would however theoretically boost thrust to mass on a plane... Ah! Here's a paper of people doing exactly that. http://softinway.com/wp-content/uploads/2013/10/Comparison-of-Counter-rotating-and-Traditional-Axial-Aircraft-Low-pressure-Turbines-Integral-and-Detailed-Performances.pdf

They have some ideas to make it work with a parallel shaft rather than a concentric one... interesting. Anyways, good idea!

Edited July 28, 2018 by Cunjo Carl

[Cheif Operations Director]

Just now, Cunjo Carl said:

Not really sure. I only know the term from other industries, but it has a pretty standard meaning.

The impeller (or impellers in this context?) should just be another word for the compressor fans which boost the pressure of the incoming air to make the combustion work better. They're something that turbojets have and ramjets don't. The word impeller could also be used to refer to part of the pump which pressurizes the fuel in order to squirt it into the combustion chamber, but I think in this context they're talking about the air compressor fans.

Thanks for the clarification 

[wumpus]

My understanding is that these "compressor fans" really do most of the work in modern passenger "jets".  The actual term is "high bypass turbofan" and they appear to act largely similar to a (shrouded) turboprop + jet (compressed air + fuel) engine.  Just don't discount the way the bypassed compressed air (previously called "turboprop" action) adds to the jet engine similar to an air augmented rocket (or the other way around with high bypass turbofans).  The result is extremely efficient up to transsonic speeds.

It should also be obvious that you can't get a ramjet to work at all at slow speed (this is said to work out to ~mach .9 for ramjets designed for supersonic flight), and the efficiency improves as speed increases.

[magnemoe]

17 minutes ago, wumpus said:

My understanding is that these "compressor fans" really do most of the work in modern passenger "jets".  The actual term is "high bypass turbofan" and they appear to act largely similar to a (shrouded) turboprop + jet (compressed air + fuel) engine.  Just don't discount the way the bypassed compressed air (previously called "turboprop" action) adds to the jet engine similar to an air augmented rocket (or the other way around with high bypass turbofans).  The result is extremely efficient up to transsonic speeds.

It should also be obvious that you can't get a ramjet to work at all at slow speed (this is said to work out to ~mach .9 for ramjets designed for supersonic flight), and the efficiency improves as speed increases.

A bit different, high bypass turbofans produces most of it trust by the fans this give good fuel economy and low noise levels but are subsonic. 

An high speed engine like the one used in the SR 71 has bypass air who goes directly from the intake to the afterburner who produces most of the trust at high speed. 
Not sure if this is used on other high speed planes like the F22 but I assume so but less extreme because lower maximum speed. 

[wumpus]

The SR-71 is basically a ramjet with a compressor to get it started and to operate at slow speeds (such as when being fueled by a tanker).  By the time it gets moving, there's enough air bypassing the compressor to fly the plane and I'm not even sure the compressor can survive the blast from the intake.

There's also the issue of the fuel: JP-7, which is said to cost as much as "a fine scotch".  No idea if this is a tech writer's, engineer's, or a general's idea of "a fine scotch", but it sounds too expensive for a fuel used in fleetwide aircraft in any case.  I really don't think the engines used on the blackbird have been used anywhere else, which is doubly tragic as they are said to have "consumed most of the career" of the engineer who designed them.

[ARS]

5 hours ago, wumpus said:

There's also the issue of the fuel: JP-7, which is said to cost as much as "a fine scotch".  No idea if this is a tech writer's, engineer's, or a general's idea of "a fine scotch", but it sounds too expensive for a fuel used in fleetwide aircraft in any case.  I really don't think the engines used on the blackbird have been used anywhere else, which is doubly tragic as they are said to have "consumed most of the career" of the engineer who designed them.

The JP-7 fuel for SR-71 is an exotic fuel which is also doubles as coolant. When SR-71 travels at high speed, the body heats up (So much that according to the pilot's experience, they heat their rations by sticking it on the window) that the airframe is increased in length by one meter. JP-7 is pumped and circulated across the airframe to act as a coolant. JP-7 is a compound mixture composed primarily of hydrocarbons; including alkanes, cycloalkanes, alkylbenzenes, indanes/tetralins, and naphthalenes; with addition of fluorocarbons to increase its lubricant properties, an oxidizing agent to make it burn more efficiently, and a caesium-containing compound known as A-50, which is to aid in disguising the radar and infrared signatures of the exhaust plume. The SR-71 Blackbirds used approximately 36,000–44,000 pounds (16,000–20,000 kg) of fuel per hour of flight. JP-7 is unusual, in that it is not a conventional distillate fuel, but is created from special blending stocks in order to have very low (<3%) concentration of highly volatile components like benzene or toluene, and almost no sulfur, oxygen, and nitrogen impurities. It has a low vapor pressure, and high thermal oxidation stability. The fuel must operate across a wide range of temperatures: from near freezing at high altitude, to the high temperatures of the airframe and engine parts that are being cooled by it at high speed. Its volatility must be low enough to make it flash-resistant at these high temperatures. The very low volatility, and relative unwillingness of JP-7 to be ignited, required triethylborane (TEB) to be injected into the engine in order to initiate combustion, and allow afterburner operation in flight. The SR-71 had a limited capacity for TEB, and therefore had a limited number of available 'shots' of TEB (usually 16) for re-starts, and those had to be managed carefully on long-duration flights with multiple stages of relatively low-altitude air refueling and normal high-altitude cruise flight. It's not just the fuel cost that ends the SR-71 service, but the difficulty in manufacturing the fuel, and the advent of reconnaisance satellite rendered the spy plane concept obsolete

[Brotoro]

49 minutes ago, ARS said:

When SR-71 travels at high speed, the body heats up (So much that according to the pilot's experience, they heat their rations by sticking it on the window) that the airframe is increased in length by one meter.

One meter seems excessive.

[ARS]

5 minutes ago, Brotoro said:

One meter seems excessive.

Well, that's according what the technician says, dunno if they exaggerated it or not, but the craft certainly increases in length when travelling at high speed

[wumpus]

1 hour ago, ARS said:

JP-7 is pumped and circulated across the airframe to act as a coolant. JP-7 is a coolant

Presumably this is done right before ignition much a hydrogen-cooled rocket.  Otherwise there doesn't appear to be anywhere that could work to radiate the heat.

[MaverickSawyer]

Okay, first things first, there's a lot of confusion here about basic turbine engine operation.

Turbine engine compressors are somewhat mislabeled. They're actually accelerating the air as it passes through each stage of rotors, and then using the stators to straighten out the airflow. This gets up to a pretty good head of speed before hitting the diffusor, just before the combustion chamber, where the air is decelerated and, according to Bernoulli's Principle, it gains pressure. Centrifugal compressors (aka impellers), though lacking the stators, still use the same basic principle, albeit much more dramatically. (Typical compression ratio across a single axial flow stage is ~1.2:1. Centrifugal is ~20:1.)

As for the "turboramjet", yes, the SR-71 had what amounted to one, albeit not as just the J58 engine alone. The complete intake/engine/exhaust system did function as a hybrid of a turbine engine and a ramjet. The intakes did ~80% of the compression work when at cruise, in a manner similar to that of a ramjet. However, this compressed air was then fed into the J58 turbojet engine, which then functioned largely as normal, albeit at very high temperatures. Some, but not most, of the air did indeed bypass the J58, but this was used mostly for cooling the outside of the engine and providing cooling air to the afterburner section, not to provide additional oxygen to support combustion.

Re: Blackbird and it's fuel... JP-7 was quite expensive, as it was only produced in small quantities for the Blackbirds. Most aircraft couldn't even run on the stuff, as it was exceptionally stable, even for kerosene derivatives. Much of this was to prevent the fuel from boiling at cruise, and so that it could be used as a hydraulic fluid for the engine. It was NOT circulated throughout the entire airframe as a coolant, though it was used to cool important parts of the engine and as the first stage of the bleed air cooling system. Also, the Blackbirds did stretch some in flight, though not a meter. That'd be about 3% of the length of the aircraft, which is far too much to align with the thermal expansion coefficient of titanium alloys similar to those that could be expected to be used under such conditions. A few centimeters would be more realistic. That said, yes, it could get quite hot in the cockpits if it weren't for the air conditioning they used. There is a story of an RSO whose AC failed for the cockpit, and the cockpit temperature was over 300*F, enough to curl the pages of his checklists and melt the grease pencil marks off them.

And turbofans... Yes, most of the thrust is provided by the fan air bypassing around the engine core. Much of the core flow energy is extracted by multi-stage turbines to drive both the compressor section(s) and the fan. Modern designs can achieve bypass ratios in excess of 20:1, i.e. for every pound of air that flows through the core, 20 flows through the fan bypass. Modern military aircraft engines for fighters are low-bypass, usually less than 4:1, or even less than even. (The F404 engine on the legacy F/A-18 Hornets had a bypass ratio of ~0.75:1, and were often described as "leaky turbojets".)

 

 

Lemme know if there's any other questions you have. I'd be happy to answer them.  

[Cheif Operations Director]

3 minutes ago, MaverickSawyer said:

Okay, first things first, there's a lot of confusion here about basic turbine engine operation.

Turbine engine compressors are somewhat mislabeled. They're actually accelerating the air as it passes through each stage of rotors, and then using the stators to straighten out the airflow. This gets up to a pretty good head of speed before hitting the diffusor, just before the combustion chamber, where the air is decelerated and, according to Bernoulli's Principle, it gains pressure. Centrifugal compressors (aka impellers), though lacking the stators, still use the same basic principle, albeit much more dramatically. (Typical compression ratio across a single axial flow stage is ~1.2:1. Centrifugal is ~20:1.)

As for the "turboramjet", yes, the SR-71 had what amounted to one, albeit not as just the J58 engine alone. The complete intake/engine/exhaust system did function as a hybrid of a turbine engine and a ramjet. The intakes did ~80% of the compression work when at cruise, in a manner similar to that of a ramjet. However, this compressed air was then fed into the J58 turbojet engine, which then functioned largely as normal, albeit at very high temperatures. Some, but not most, of the air did indeed bypass the J58, but this was used mostly for cooling the outside of the engine and providing cooling air to the afterburner section, not to provide additional oxygen to support combustion.

Re: Blackbird and it's fuel... JP-7 was quite expensive, as it was only produced in small quantities for the Blackbirds. Most aircraft couldn't even run on the stuff, as it was exceptionally stable, even for kerosene derivatives. Much of this was to prevent the fuel from boiling at cruise, and so that it could be used as a hydraulic fluid for the engine. It was NOT circulated throughout the entire airframe as a coolant, though it was used to cool important parts of the engine and as the first stage of the bleed air cooling system. Also, the Blackbirds did stretch some in flight, though not a meter. That'd be about 3% of the length of the aircraft, which is far too much to align with the thermal expansion coefficient of titanium alloys similar to those that could be expected to be used under such conditions. A few centimeters would be more realistic. That said, yes, it could get quite hot in the cockpits if it weren't for the air conditioning they used. There is a story of an RSO whose AC failed for the cockpit, and the cockpit temperature was over 300*F, enough to curl the pages of his checklists and melt the grease pencil marks off them.

And turbofans... Yes, most of the thrust is provided by the fan air bypassing around the engine core. Much of the core flow energy is extracted by multi-stage turbines to drive both the compressor section(s) and the fan. Modern designs can achieve bypass ratios in excess of 20:1, i.e. for every pound of air that flows through the core, 20 flows through the fan bypass. Modern military aircraft engines for fighters are low-bypass, usually less than 4:1, or even less than even. (The F404 engine on the legacy F/A-18 Hornets had a bypass ratio of ~0.75:1, and were often described as "leaky turbojets".)

 

 

Lemme know if there's any other questions you have. I'd be happy to answer them.  

What I do not understand is why not just use an impeller why the complex stator turbine system instead

Essentially a ramjet with a impeller

[ARS]

1 hour ago, Cheif Operations Director said:

What I do not understand is why not just use an impeller why the complex stator turbine system instead

Essentially a ramjet with a impeller

Individually, impeller cannot withstand the airspeed where the ramjet operates and ramjet cannot operate efficiently in impeller airspeed. Since impeller is designed mostly for subsonic flight, it'll be ripped apart in ramjet airspeed while ramjet is designed only for supersonic flight. A hybrid design such as SR-71 engine used the shock cone intake adjustment (pushing it forward or backward) to shift between subsonic (where low-speed air entering the turbine) and supersonic mode (where high-speed air directly enters combustion chamber)