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Water used to increase/augment thrust in fighters

kcran567

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Is the practice of using water to increase thrust still used? I know fighters in the 1950's could be modified to increase thrust this way in lieu of afterburners. Also, wasnt the f-4 modified to use water. Is this still practical? Would a modern fighter be able to carry water to inject/augment thrust from its engine? could this in theory increase range and engine power, or is is simply for a short burst of extra thrust? If say a modern high performance supercruising fighter like the f-22 carried a significant amount of water onboard for cooling the engine along with increasing thrust is this system practical anymore? Can it be used to increase range?
 

Abraham Gubler

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Water injection in turbojets is used to lower temperature so the engine can run at a higher rate without melting the turbine. The Pegasus engine on the Harrier still uses water injection.
 

chuck4

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kcran567 said:
Is the practice of using water to increase thrust still used? I know fighters in the 1950's could be modified to increase thrust this way in lieu of afterburners. Also, wasnt the f-4 modified to use water. Is this still practical? Would a modern fighter be able to carry water to inject/augment thrust from its engine? could this in theory increase range and engine power, or is is simply for a short burst of extra thrust? If say a modern high performance supercruising fighter like the f-22 carried a significant amount of water onboard for cooling the engine along with increasing thrust is this system practical anymore? Can it be used to increase range?

I believe the use of water injection in jet engines changed marginal combustion chemistry and results in a visible smoke trail.
 

RanulfC

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kcran567 said:
Is the practice of using water to increase thrust still used? I know fighters in the 1950's could be modified to increase thrust this way in lieu of afterburners. Also, wasnt the f-4 modified to use water. Is this still practical? Would a modern fighter be able to carry water to inject/augment thrust from its engine? could this in theory increase range and engine power, or is is simply for a short burst of extra thrust? If say a modern high performance supercruising fighter like the f-22 carried a significant amount of water onboard for cooling the engine along with increasing thrust is this system practical anymore? Can it be used to increase range?

1) Water injection was used to increase the thrust (usually for take-off) in early turbojet-engine aircraft both civil and military. Normally it was injected into the inlet of somewhere forward of the combustion chamber and had a two-fold purpose. The water would evaporate and decrease the temperature of the incoming airstream which would allow the compressor, combustor, and turbine sections to be run hotter and at higher speed. The cooler air was also denser and therefore provided more "mass-flow" within the engine which equaled higher thrust.

Water-injection also helped allow the early after-burner engines to operate for longer periods, (again such as take-off) without melting or damaging the exhaust pipe and nozzle.

2) Chuck-4 is correct that in early turbojet engines this addition of water moved the combustin process to "off-nominal" conditions and resulted in leaving a smoke trail when injection was used. However this was fixed with later modification that varied the fuel injection to more readily match the actual combustion chamber conditions. You are also correct that the early F-4 had water injection but it was removed when later Turbofan engines replaced the original Turbojet propulsion. (A "note" here: The F-4X recon project planned on using water and methanol injection prior to the compressor face to allow both higher thrust modes but also to increase the operational Mach number of the engines from a normal "top" speed of around Mach-2.1 to almost Mach-4)

3) Note that I pointed out this was used with early TURBOJET engines which are different from todays TURBOFAN engines. Turbojets differ in that they pass the majority of their compressed air through the combustion chamber, turbine and out the exhaust. Turbofan engines on the other hand "bypass" a varying amount of air AROUND the combustion chamber, and turbine and often (especially in the case of High, and Ultra-High bypass Turbofans) even have seperate exhausts.

In the case of fighter aircraft in the last 40 years the use of "low-bypass" turbofan engines in them have allowed higher thrust because of increased air-throughput and mixing of the cooler, high density, high pressure "by-pass" air with the hot exhaust air from the "core" engine.

The F-22 like all "modern" fighters does not use water-injection because even the "low-bypass" turbofans used in these aircraft are capable of much higher Thrust-To-Weight ratios without the need for water injection.

As noted above the early water injection could be used by itself or in conjuction with the aircraft afterburner, but in either case resulted in less-nominal fuel combustion. Part of the reason early afterburners were such fuel hogs is that by the time the incoming air had made its way through the entire Turbojet engine most of the oxygen was already used up. In modern "bypass" engines instead the afterburner now has access to large amounts of relativly cool, high-oxygen air and so less fuel is needed in the afterburner to ensure combustion and thrust augmentation.

Modern "Supercruise" fighters have the ability to "cruise" as supersonic speed without using injectents due to their bypass ratio and higher engine thrust-to-weight in "dry" (non-afterburner) mode.

4) Water injectin does NOT allow increases of "range" which is a function of engine fuel consumption. (In fact, overall range is probably slightly reduced due to the increased fuel needed to fully combust the densified and cooler incoming air) It DOES increase engine "power" at least thrust-wise, but it doesn't make sense for modern fighters due to the way Turbofan bypass works.
(It is FAR more "effective" with Turbojets than Turbofans)

5) Water injection requires tanks to store the water and an added system to inject it into the inlet in a proper, metered amount (too much is bad for combustion and water droplets impacting the high speed compressor blades can seriously damge them, to little and it isn't as effective) to have effect and these add weight and complexity to the aircraft. So in general use water injection is probably not very practical. However as noted with the F-4X above there are "specialty" uses where water (or fluid) injection has been suggested.

Water-injection alone was tested and found to double the basic thrust of the F100 (F-16/early F-15) Engine in both dry and afterburner modes. Adding LOX injection along with water injection was even more effective and allowed the normally Mach limited engines to a achive speeds a little over double their "limit" in testing. Both were to be used in the DARPA RASCAL project aircraft allowing possible speeds up to Mach-5 and alittude operation of the engines up to 100,000ft. (The "keyword" for this type of injection system is MIPCC, or "Mass-Injection-Pre-Compressor-Cooling if you do a web-search btw)

Water and other fluid injection has been tested in multiple small turbojet engines to test the possible effects for high-speed, high-altitude UAVs and Missiles with good results.Again though the systems add to the weight and complexity of the basic vehicle which usually involves trade-offs between increased performance and lower payload.

Here's some links with information:
http://books.google.com/books?id=VpJEm7cFVE4C&pg=PA182&lpg=PA182&dq=Water+Injection+To+Increase+Thrust+of+Turbojet+Engines&source=bl&ots=z3cSC6yxAM&sig=YuUFbG8PYT8aBYsnqm0tDCxoY1g&hl=en&sa=X&ei=0EU3UbDNPKHJygHD-IDgBw&ved=0CD8Q6AEwAw#v=onepage&q=Water%20Injection%20To%20Increase%20Thrust%20of%20Turbojet%20Engines&f=false

http://www.ewp.rpi.edu/hartford/~ernesto/SPR/Walkowski-FinalReport.pdf

http://www.grc.nasa.gov/WWW/RT/2005/PB/PBA-snyder.html

http://www.pulse-jets.com/phpbb3/viewtopic.php?f=2&t=4628

http://www.kls2.com/cgi-bin/arcfetch?db=sci.aeronautics.airliners&id=%3Cairliners.1996.265@ohare.chicago.com%3E

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930092063_1993092063.pdf

http://aerade.cranfield.ac.uk/ara/1950/naca-report-981.pdf

http://www.dtic.mil/dtic/tr/fulltext/u2/a263727.pdf

http://naca.central.cranfield.ac.uk/reports/1948/naca-rm-e7l16.pdf

http://naca.central.cranfield.ac.uk/reports/1957/naca-tn-3922.pdf

Hope that helps :)

Randy
 

AeroFranz

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RanulfC, excellent synopsys, and thanks for the refrences! ;)
 

RanulfC

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I Live to Serve ;D

(Well, that's what my wife tells me... Something about it being in the "Marriage Contract"... I really need to find my copy and read it again, I'm pretty sure I don't recall the portion where it says I'm bound in eternal servitude to a superior creature... Heck I don't even remember any Cats being involved when we first got hitched!)

Randy :eek:
 

Arjen

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“Women and cats will do as they please, and men and dogs should relax and get used to the idea.” Robert A. Heinlein
 

RanulfC

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Arjen said:
“Women and cats will do as they please, and men and dogs should relax and get used to the idea.” Robert A. Heinlein
The man knew of which he spoke/wrote :)

Back on-topic sort-of, I found this report during the search that is interesting:
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930084872_1993084872.pdf
http://aerade.cranfield.ac.uk/ara/1957/naca-tn-3922.pdf

"Analytical Investigation of the Effect of Water Injection on Supersonic Turbojet-Engine-Inlet Matching and Thrust Augmentation"

The basic idea is that along with the thrust augmentation effect, water-injection showed promise of allowing fixed supersonic inlets to operate more efficently in off-nominal conditions and at higher altitudes. In supersonic flight one issue with a fixed, or non-moving/non-adjustable inlet is that above a certain (design) speed the inlet begins to "spill" excess airflow that it can not "ingest" properly which increases drag (and hence fuel consumption) greatly.

The "simple" answer to this issue is to provide the inlet with the mechanical ability to "adjust" itself to match the airstream by having movable cone/spike/ramps or cowling/nacelle lips. These are usually (and especially early on, the report was published in 1957) complex and heavier than a simple "fixed" inlet however and since normally full-afterburner and/or thrust augmentation was needed to reach and maintain these higher speeds these systems were seen as probably being too much "effort" for what was going to be a "dash" or short-operation-time system. So the idea of using evaporative cooling to decrease the "spillage" (by increasing density) of the incoming air stream seemed like a good "short-duration" idea.

If I read this report correctly this study showed that the idea offered several advantages as well as some additional drawbacks of the concept. For one thing the additional thrust augmentation (noted to be between 20% to 130% that of a non-augmented fixed inlet design) allowed a greatly reduced fuel consumption (at the cost of having the water injection system and water which was "assumed" to replace fuel capacity in the first place) due to lower afterburner settings along with an overall "net" thrust increase due to less "spilled" airflow. At "equivalent" thrust levels fuel consumption for the water-injection engine dropped by between 16% and 60% of the compared types. Engine airspeed limits were found to be increased up to 25% with similar loss of ISP due to water (liquid) expenditure being added to the formula. Lastly added thrust capability of the water-injection engine allowed a considerable increase in flight altitude limits compared to non-augmented engines.

Increased engine performance and materials advances along with longer operational requirements for high speeds meant that this concept was not used in service. Mechanically adjustable inlets were adopted instead to allow a larger "nominal" operations envelope along with higher "bypass" ratio engines for increased thrust capability.

Randy
 

Arjen

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RanulfC said:
The man knew of which he spoke/wrote :)
So do you concerning the use of water injection. Most enlightening.
 

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