Forum: Technology

Last edited by bigbird2 on Oct 05, 2010 - 07:43 PM; edited 2 times in total

Kryptid wrote:

So how are engines usually improved in thrust over their lifetime? By increasing their ability to withstand higher operating temperatures? Is that how they've improve the thrust output of the F100 series over time?

Jet engine is really a "chemical reactor", you mix air and fuel to get hot gas. You want to get the most "bang" out of your fuel.

The basic goal is simple, take as much air in for given intake, order the flow, mix it with fuel, then extract as much of those heat out for useful work.

a) you can try to put in "more" oxygen (eg. create better fan and intake system to suck in more air, compressor that spin faster or blade with better aerodynamic. Of course that means new material to withstand stress. This means higher pressure into the combustion chamber, and increase oxygen content. Another way cooling hot incoming air for eg. is classic way to increase oxygen density by spraying it with water midst. hence the term dry/wet thrust.)

making the engine bigger and design bigger plane is the easiest way really. because a plane power to weight ratio increases faster in that approach. (F-15 uses 2 F-16 engine)

b) better, more complete burn. (you want air that mix evenly with fuel, you don't want blob of fuel inside a messy turbulent. You have to slow down and smooth out incoming air. You always want to match oxygen coming in with amount of fuel injected. Very hard to do in combat maneuvering where jet nozzle can change direction abruptly. They use electronic engine control these days, a little like electronic fuel injection in car. Adding fuel additive is also possible.

of course turbine blades are not optimized for infinite range of air flow speed, so you have that problem. You want to make sure air coming in and out are at optimum speed, without telling the pilot to only keep flying level at mach 0.9 500 feet above sea level only. there is that.

c) you want it all in most compact/light weight/safe to operate volume. (what use is 100% perfect burn if the engine is the size of a ship and weight 50 tons? An airplane can't have 20 stages fans nor it can have 20 feet compressor fan.)

So, the common approach off the top of my head: intake design, better compressor fans, higher core pressure , better combustion control, bigger pressure differential between core and turbine, lighter material.. and try not to go bankrupt doing it. New material is expensive.

F-16.net Sponsor

milosh wrote:

@bigbird2

Did you even bother to read this thread :>
TEG explained it very well. Even if you upgrade mil power to AB level it still wouldnt go fast as plane with older engine on AB. He gave very good example of same tanker with different engines.

with enough force one can make anything fly faster. Put a rocket engine with half a million pound of thrust for 2 seconds I bet that F-15 will go mach 4 no problem. (not that I want to sit inside it, nor will I guarantee it will come back in one piece.)

But you know, ... there is no law of physics that says you can't do that.

Quote:

Another way cooling hot incoming air for eg. is classic way to increase oxygen density by spraying it with water midst. hence the term dry/wet thrust.)

As far as using water injection goes, here's how I've come to understand it: the exhaust gases contain large amounts of energy in the form of heat. When those gases are blown out the nozzle, that energy is gone and no longer of use in providing thrust. However, if one were to inject water into that hot gas, the water would absorb some of that heat energy as it changes phases from liquid to gas. Since gaseous water takes up much more space than liquid water, the extra pressure created by the phase change generates extra thrust. So basically, water injection allows for an increase in thrust while actually reducing exhaust temperature because some of that heat energy is converted into extra thrust.

I think that's how it works, right? Do any modern engine use water injection? I've read that the F-105 and B-52 have used it, but I've never heard tell of it being used on, say, the F-22. If not, is there any reason for it's lack of usage?

Because fuel is lighter than water and has more expansion potential it makes less sense to use water.

bigbird2 wrote:

with enough force one can make anything fly faster. Put a rocket engine with half a million pound of thrust for 2 seconds I bet that F-15 will go mach 4 no problem.
But you know, ... there is no law of physics that says you can't do that.

Sure perhaps a rocket, but we were talking gas turbine engines with higher 'non-afterburning' thrust. A rocket is a closed end afterburner without the pesky rotating stuff that tends to melt or fly apart. The exhaust velocity of a rocket makes afterburning turbines look like a joke.

What we're trying to convey here is this; if you managed to 'Frankenstein' a 98K thrust PW4098 into the rear of an F-15, it would NOT achieve the MACH 2.5 that 2 of the 24K (48K total) thrust F100-PW-100s can achieve. The net exhaust velocity of the PW4098 is lower than the twin F100s even without afterburner. Sure it may be accelerating MORE air, but at the LOWER overall speed. I'm sure it would climb like an SOB during take-off, and get a great range boost, but top speed would be much worse even though it would have over twice the thrust when starting off.

Keep 'em flyin' Thumb

TEG

Kryptid wrote:

Quote:

Another way cooling hot incoming air for eg. is classic way to increase oxygen density by spraying it with water midst. hence the term dry/wet thrust.)

As far as using water injection goes, here's how I've come to understand it: the exhaust gases contain large amounts of energy in the form of heat. When those gases are blown out the nozzle, that energy is gone and no longer of use in providing thrust. However, if one were to inject water into that hot gas, the water would absorb some of that heat energy as it changes phases from liquid to gas. Since gaseous water takes up much more space than liquid water, the extra pressure created by the phase change generates extra thrust. So basically, water injection allows for an increase in thrust while actually reducing exhaust temperature because some of that heat energy is converted into extra thrust.

I think that's how it works, right? Do any modern engine use water injection? I've read that the F-105 and B-52 have used it, but I've never heard tell of it being used on, say, the F-22. If not, is there any reason for it's lack of usage?

"wet" thrust, where water/alcohol is literally injected into the engine to boost performance is a "historical term". no modern engine does this anymore. These days it primarily means 'afterburner', injecting fuel in afterburner part of engine. But the term of art of injecting liquid stuck.

http://en.wikipedia.org/wiki/Water_inje ... engines%29

Use in aircraft

Water injection has been used in both reciprocating and turbine aircraft engines. When used in a turbine engine, the effects are similar, except that preventing detonation is not the primary goal. Water is normally injected either at the compressor inlet or in the diffuser just before the combustion chambers. Adding water increases the mass being accelerated out of the engine, increasing thrust, but it also serves to cool the turbines. Since temperature is normally the limiting factor in turbine engine performance at low altitudes, the cooling effect allows the engines to be run at a higher RPM with more fuel injected and more thrust created without overheating.[2] The drawback of the system is that injecting water quenches the flame in the combustion chambers somewhat, as there is no way to cool the engine parts without cooling the flame accidentally. This leads to unburned fuel out the exhaust and a characteristic trail of black smoke.

That_Engine_Guy wrote:

bigbird2 wrote:

with enough force one can make anything fly faster. Put a rocket engine with half a million pound of thrust for 2 seconds I bet that F-15 will go mach 4 no problem.
But you know, ... there is no law of physics that says you can't do that.

Sure perhaps a rocket, but we were talking gas turbine engines with higher 'non-afterburning' thrust. A rocket is a closed end afterburner without the pesky rotating stuff that tends to melt or fly apart. The exhaust velocity of a rocket makes afterburning turbines look like a joke.

What we're trying to convey here is this; if you managed to 'Frankenstein' a 98K thrust PW4098 into the rear of an F-15, it would NOT achieve the MACH 2.5 that 2 of the 24K (48K total) thrust F100-PW-100s can achieve. The net exhaust velocity of the PW4098 is lower than the twin F100s even without afterburner. Sure it may be accelerating MORE air, but at the LOWER overall speed. I'm sure it would climb like an SOB during take-off, and get a great range boost, but top speed would be much worse even though it would have over twice the thrust when starting off.

Well, this is afternoon musing, so don't take me seriously. But the idea of turning F-15 into some sort of hot rod is too tempting. I think a modified P&W J58, the engine that powered SR-71 isn't too far fetch.

J58 dimension:

Length: 215.940 in.
Diameter: 55.43 in.
Dry Thrust: 23,900 LB

SR-71 empty weight: 67,500 lb

compared to F100:

Length: 191 in
Diameter: 46.5 in
Dry Thrust: 17,800 LB

F-15 Empty weight: 28,000 lb

http://www.habus.org/revealed/J58.htm

I am having one big evil grin and wonder how fast a modified F-15 will go with J-58 engine (probably the flight will last ridiculously short and useless, running out of fuel before making first turn. nevermind airframe stress.)

TEG brings up an interesting problem, combining awesome initial climb with great high speed performance. It seems that fighters keep evolving more towards the lower overall exhaust speeds as engines evolve. Is it because the fighter spends the bulk of its career at these lower speeds or is it because the turning battle tends to take place at this lower velocity spectrum?

Seems like the modern use of supercruise will increase the need for high exhaust velocity, though.

There's not a distinct trend, it all depends on the technology available and the requirements. You want to go fast? Low bypass and high velocity. Want to go far? High bypass with low velocity.

F-4 J79- No bypass (Turbofan not a thing yet)
F-15 F100- Low bypass (Better fuel economy, exhaust velocity not limiting factor)
F-22 F119- Extreeeemly low bypass (Good for supercruise, like a Super-J79)
F-35 F-135- Bigger bypass again (Fuel economy, probably helps with reliability)

@bigbird2: 9 inches in an engine bay is a lot of inches...

To add some more detail Cheers

F-15 F100-PW-100 Low bypass (Better fuel economy, exhaust velocity not limiting factor)
F-16 F100-PW-200 Same bypass but optimized for better reliability and single engine operations, tad more thrust
F-15/16 F100-PW-220 Same bypass as -100/-200, but FADEC (DEEC) controlled, little less thrust but WAY more reliable lots less stalls or AB blow-outs.
F-15/15 F100-PW-229 Lower bypass (about half that of previous F100s) Even more reliable and more power at MIL/MAX

TEG

Arguably, the trend for next-gen air superiority fighters would appear to trend towards seeking engines capable of producing super cruise performance and high-acceleration, while also maximizing reliability. Perhaps in this regard (other than installing big, large thrust to weight ratio medium bypass engines, or designing a highly aerodynamic airframe), a Variable Cycle Engine (or similar type) could be just one trend further developed over the mid-term to achieve such performance goals?

Perhaps the trend is towards higher 'dry' thrust exhaust speeds, but I bet these pale in comparison to wet exhaust speeds.

Another trend seems to be a max speed limit of around the Mach 2.0 mark. I think this is probably due to engine inlet design issues pushing way past M2, only a variable inlet seemed to be able to handle the larger speed ranges up to M2.5 and beyond.

Can someone explain to me what is meant by "pressure recovery"? I've seen this term used a lot in reference to inlet design, but I'm not exactly sure what it means. I gather it has something to do with the pressure at the engine face versus - pressure, but I'm not sure.

Forum: Technology

Jet Engine Question