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Faro
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Higher Octane Fuel/Water Injection For Windshear

Sat Dec 29, 2007 10:35 am

Would it be technically/economically feasible to integrate into modern high-bypass turbofans:

i) water injection; and/or
ii) a higher octane fuel [than kerosene] reserve

to provide for an emergency power boost contingency in case of extreme maneouvres/windshear ,for let's say not more than 2-3 minutes?

Faro
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Buzz
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RE: Higher Octane Fuel/Water Injection For Windshe

Sat Dec 29, 2007 11:11 am

Hi Faro, Buzz here. I'm familiar with the inner workings of some of the high power piston engines as well as modern turbofans. I think you're mixing up the 2... some of the reasons that one works well don't really apply to the other kind of engine. Now if it makes economic sense, or technical sense... I'll let somebody else figure that out. (grin)

Water Injection was used on some of the high power piston engines, and older turbojet engines. In the piston engines the water / alcohol mix was used to reduce the temperature of the compressed air charge going into the cylinders - there was a slim margin before detonation would occur. Some smart people figured that cooling the incoming fuel-air mixture gave a good margin of (internal engine) safety before the fuel-air mixture would detonate while being compressed by the moving piston - before the sparkplug ignites it.

Maybe the idea of "higher octane kerosene" had confused me. Octane is used to measure piston engine resistance to pre-ignition / detonation. It's really not applicable to turbine engines... although there is a compression ratio of 27:1 in a 757 / Pratt 2000 engine which is similar to what a diesel does. Higher energy fuel?

Water Injection was used on some of the older turbojet engines to reduce the EGT margin... the difference between the EGT and the temperature where the turbine starts to suffer damage. When a turbine engine gets tired, you can only put so much fuel into it before the EGT gets to the normal max limit. If you were able to cool the EGT, then you could pour in more fuel... and hopefully get more thrust before you're maxed out in the EGT department.

Sometimes we'd get DC-10's that were EGT limited taking off on a nice warm afternoon in HNL (Honolulu Hawaii). The Flight Engineer had to reduce power a little to keep one of the engine's EGT from going over red-line. But you park it on a nice cool night at PDX (Portland Oregon) and we have no problem getting the normal Thrust / N1 before we get to max EGT. In short, the turbines were tired and leaking, not able to turn all the hot exhaust gasses into rotary motion to turn the big fan in the front. Maybe water injection would have helped... but they still would have had a tired engine and written it up.

When turbofan engines became common, water injected turbojet engines were retired. I recall hearing of JT3C-6 engines called "water wagons", some DC-8's had them. I don't know which model 707 used water injection. And I recall seeing diagrams of B-52's with water injection. When the TF33 / JT3-7 fan engine was installed they didn't need the extra thrust: it went from 12,000 - 14,000 lbs to 17,000 lbs.

So to add extra thrust to the airplane... most of the time it's not needed, most of the takeoffs are de-rated takeoffs: you "tell" the fuel control that's it's hot outside (how about 40 degrees C?) and the fuel flow is reduced, EGT is reduced, thrust is reduced. And there's still no problem getting off the runway.
I've been involved with a few bird strikes / mangled engines that happened just after takeoff, and the pilots had not prayed for more thrust... besides, it might make them fly in a big circle (grin).

g'day
 
Tristarsteve
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RE: Higher Octane Fuel/Water Injection For Windshear

Sat Dec 29, 2007 11:40 am



Quoting Buzz (Reply 1):
I recall hearing of JT3C-6 engines called "water wagons", some DC-8's had them. I don't know which model 707 used water injection. And I recall seeing diagrams of B-52's with water injection.

Air India had a B747 with water injection. I remember the poor guys filling the tank at BAH. GF operated BAC111 with water injection in the Spey engines, but the Jumbo took so much water that they were pumping it in for ages, and they had a hand pump!
I never saw any other B747 with water injection, so perhaps it was special to Air India?
The Trident 2E also had water injection in the Spey engines. BEA had big problems with the pumps that pumped the water into the engines. They were not used very often, and were very unreliable. In the end BEA removed the pumps from most of the aircraft, and dedicated a few Tridents to Nicosia, Tel Aviv, and Athens that needed them.
 
Pihero
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RE: Higher Octane Fuel/Water Injection For Windshear

Sat Dec 29, 2007 12:18 pm



Quoting TristarSteve (Reply 2):
I never saw any other B747 with water injection, so perhaps it was special to Air India?

Oh no ! All AF747-100s had it. Made for some interesting calculation time before flight should one needed the extra performance, especially on the cargo planes !

As for the OP, Faro seems to be quite worried about windshear !
Like all aviation problems, it's better tackled preventively, be it with ground detectors or airborne equipment, or with some good common sense and airmanship.
Last line of defense is, of course, the Airbus envelope protection  Silly
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Faro
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RE: Higher Octane Fuel/Water Injection For Windshear

Sat Dec 29, 2007 1:53 pm



Quoting Pihero (Reply 3):
As for the OP, Faro seems to be quite worried about windshear !

Yes indeed Pihero, I feel that it's one of those safety issues that always be searching for your elusive "last line of defense". I don't believe it will ever be adequately tackled simply because it i) cannot be 100% reliably detected and ii) can easily be overpowering. As for envelope protection, wouldn't it be handy with, say, a 20% emergency power reserve?

The AF 747-100 water injection is very interesting; how much boost did it give the engines, for how long and using how much water?

Faro
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KFLLCFII
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RE: Higher Octane Fuel/Water Injection For Windshear

Sat Dec 29, 2007 4:34 pm



Quoting Faro (Thread starter):
economically feasible to integrate into modern high-bypass turbofans:

i) water injection

Hauling around all that extra water (=weight penalty, =$$ penalty) for the off-chance that a flight might encounter windshear after a collective system-wide failure of ATC, LLWAS, and plain-old bad pilot judgement?

I hear a resounding "no".  Wink
"About the only way to look at it, just a pity you are not POTUS KFLLCFII, seems as if we would all be better off."
 
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Faro
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RE: Higher Octane Fuel/Water Injection For Windshear

Sat Dec 29, 2007 4:56 pm



Quoting KFLLCFII (Reply 5):
Hauling around all that extra water (=weight penalty, =$$ penalty) for the off-chance that a flight might encounter windshear after a collective system-wide failure of ATC, LLWAS, and plain-old bad pilot judgement?

I hear a resounding "no".

Understood, now how much do we mean by "extra" weight? Conceivably, all the water remaining in the galleys/toilets can count towards that extra though we would also have to quantify the weight of the conduits that would channel it to the engines. This I imagine would not be prohibitive.

As you rightly stated, there will remain the "off-chance" of windshear; why not cater to it if we possibly can? Technology/ATC/good airmanship will indeed do their bit but they have sadly proven not to be 100% reliable.

Faro
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tdscanuck
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RE: Higher Octane Fuel/Water Injection For Windshear

Sun Dec 30, 2007 7:02 am



Quoting Faro (Thread starter):
Would it be technically/economically feasible to integrate into modern high-bypass turbofans:

i) water injection; and/or
ii) a higher octane fuel [than kerosene] reserve

Both are technically feasible, although higher octane fuel has zero effect on a turbine engine so there's really no point. Economically I would strongly suspect both are non-starters. Water injection implies significant weight (water, plumbing, and control system) for very little gain. Alternate fuel has an even larger weight penalty since, currently, there is no place to put a segregated fuel.

Quoting Faro (Reply 4):
Yes indeed Pihero, I feel that it's one of those safety issues that always be searching for your elusive "last line of defense". I don't believe it will ever be adequately tackled simply because it i) cannot be 100% reliably detected and ii) can easily be overpowering. As for envelope protection, wouldn't it be handy with, say, a 20% emergency power reserve?

It's absolutely true that windshear isn't 100% detectable and can be overpowering. The question is, how many instances of windshear that are currently overpowering would be not overpowering if you had some kind of boost? Obviously, very strong windshear is going to overpower any reasonably possible system. As a result, we're talking about significant cost for modification to protect an airplane against what is already a 1e-9 per flight hour event and, even then, we're not providing must more than a 20-50% increase in protection. There's just no way I can see to have that make sense.

Quoting Faro (Reply 6):
Conceivably, all the water remaining in the galleys/toilets can count towards that extra though we would also have to quantify the weight of the conduits that would channel it to the engines. This I imagine would not be prohibitive.

There are some pretty hefty certification issues that would come up by coupling the engine to the galley/lav system that I don't think have ever been addressed. In concept, it could be done, but it would significantly hike the price and difficulty of implementation.

Quoting Faro (Reply 6):
As you rightly stated, there will remain the "off-chance" of windshear; why not cater to it if we possibly can?

Because everything in aircraft is a tradeoff. There is no such thing as a free lunch so you always need to compare the cost (weight, expense, difficulty, reliability, etc.) against the risk reduction. The risk from windshear is already very very small. That means the only way you can justify come preventative measure is if it relatively low cost. The options you've suggested don't appear to jump that hurdle. Just because we can do something doesn't mean that it makes sense to do it.

Tom.
 
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RE: Higher Octane Fuel/Water Injection For Windshear

Sun Dec 30, 2007 8:37 am



Quoting Tdscanuck (Reply 7):
Quoting Faro (Reply 6):
As you rightly stated, there will remain the "off-chance" of windshear; why not cater to it if we possibly can?

Because everything in aircraft is a tradeoff. There is no such thing as a free lunch so you always need to compare the cost (weight, expense, difficulty, reliability, etc.) against the risk reduction. The risk from windshear is already very very small. That means the only way you can justify come preventative measure is if it relatively low cost. The options you've suggested don't appear to jump that hurdle. Just because we can do something doesn't mean that it makes sense to do it.

Thanx Tom for your detailed and judicious feedback.

Faro
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RE: Higher Octane Fuel/Water Injection For Windshear

Sun Dec 30, 2007 8:55 am

Regarding water used with some jet engines on transport aircraft....

It was de-mineralized water, no alcohol added, and alcohol was not needed, because...

All the water was used up during takeoff, 2.5 minutes approximately.

However the ADI for piston engines was another story.
There, alcohol was necessary simply because the excess ADI fluid was carried to altitude, where it would freeze...not good.
Same for some turbopropeller engines, although a slightly different mix was used, compared to the piston variety.

And, to answer the original question...No.

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