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