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Harrier water injection

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Harrier water injection

Old 30th Nov 2016, 22:22
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Harrier water injection

Halfway through the superb book on the Pegasus.
Can any bona jet drivers (or others who may know) clarify something about water injection.
I read on wiki that it cooled the outlets, allowing running at higher power and outlet temperatures for 90 secs max.
I assume it was sprayed on the inside of the outlet in some sort of mist and that the rear hot exhausts simply dumped super-heated steam overboard as part of the jet efflux?
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Old 30th Nov 2016, 22:41
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Tartare,

The distilled water was injected into the combustion chambers to provide a temporary reduction in turbine temperatures and higher thrust. No steam out of the engine, as I remember it from the deck.

The water injection manifold was located very close to the fuel manifold, and wrapped around the engine. The water tank was located just aft of the engine between the aft nozzles. Water was injected at a high rate using a special pump located up in the main wheel bay.

'Wet' operation of the engine was strictly limited, and whether for launch or recovery, was closely monitored.

Hope this helps

Engines
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Old 1st Dec 2016, 00:00
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Thanks engines - my question answered and from no better a source!
I'd thought the water injection might also temporarily increase the mass and thrust of the efflux, but then deduced if that was the case, then all supersonic aircraft would probably have water afterburners...
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Old 1st Dec 2016, 00:42
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I think as well as slightly reducing the turbine inlet temperature, water injection does also increase the mass-flow rate and hence thrust! The drawback is that you need lots of water, so water injection tends to be limited to the brief periods when maximum thrust is needed, ie take-off or, for the Harrier, hovering at high auw.
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Old 1st Dec 2016, 00:49
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Tartare,

To add to Engines reply, water injection sprayed de-mineralised water, at pressure, onto the HP turbine blades and reduced the Jet Pipe Temp (JPT) by 23-25C. This temperature suppression allows increased fuel flow (and therefore RPM under certain conditions). The net effect of higher RPM and increased mass flow is more thrust - Harrier only had 90 sec of aggregate water flow.

The 'wet' and dry engine lifts were governed by a number of factors such as water switch position and gear/PLAU positions, not by the contents of the water tank. You could be in a position where you might be operating the engine to the higher 'wet' limits but if you didn't have water (I.e. you had run out) you would be fatiguing the engine per second like billy-oh. Hence, there was some (but not necessarily crucial) priority to get the jet landed before the water was gone.

Importantly, you didn't always need water to hover (low ambient temp, high atmospheric pressure), but if you did - and ran out mid-hover - there were fortunately only a few conditions where you would actually descend whilst at max throttle position.
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Old 1st Dec 2016, 12:14
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In everyday use, was there a different water loading procedure than this?
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Old 1st Dec 2016, 12:35
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Originally Posted by MSOCS View Post
Tartare,
To add to Engines reply, water injection sprayed de-mineralised water, at pressure, onto the HP turbine blades and reduced the Jet Pipe Temp (JPT) by 23-25C. This temperature suppression allows increased fuel flow (and therefore RPM under certain conditions).
This is an almost universally misunderstood field. The following is a standard answer that was prepared by one of the gas turbine design specialists at RR:

Are you sitting comfortably? Good, then we shall begin ....

First of all, gas turbine water injection is a thrust augmentation device. The concept of injecting water into a gas turbine has got sod all to do with cooling the engine as one of the advantages of a gas turbine is that they are self cooling – ponder the extensive use of such beasts in stationary applications, such as on oil rigs, pipelines and for powering ships.

Now we’ve got that one nailed – just how can you increase an engine’s thrust by injecting water? At first glance, it seems an absurd thing to do. Well, it’s simple really, and there are three different ways of doing it:

1) Add the water at the front of the compressor
2) Add the water directly to the combustion chamber
3) Add the water immediately before the turbine section or just before the propelling nozzle.

Taking each one in turn:

1) Injecting water at the compressor face has the effect of lowering the temperature of the inlet air, (assuming the water is at a lower temp than the ambient temp, of course, but seeing as you will generally be using water injection on hot days, that's taken as read). Remember the old maxim of 'It's fookin' difficult to compress cold air and vladimir impossible to compress hot air' and you soon realise that lowering the inlet air temp allows you to get either:
a) the same level of pressure rise as before but from less power offtake or
b) more pressure increase for the same shaft power requirement.
Both of these effects give you greater thrust (via less power offtake or through higher pressure ratio respectively) but option 1b) is usually the one used. In a nutshell, you are fooling the engine into thinking the ambient temperature has suddenly gone down and gas turbines work best at low temperatures. Because you have lowered the inlet air temp then obviously you are lowering the compressor outlet temp as well. This allows you to add more fuel and gives you a greater delta t across the combustor. You are also putting more mass flow through your engine (because you've added the water and water is more dense than air), giving you greater thrust because thrust is directly related to mass flow. Additionally, you can utilise a water/ethanol mix if you so desire, with the ethanol being burnt in the combustor giving you even more bang.

From point 1b) you can see the problems that occur with gas turbines at high ambient air temperatures: Higher air temp = lower compressor efficiency = lower pressure ratio = less efficient combustion = lower resultant thrust because the turbine is using up all the available power to run the compressor = you ain't going nowhere.

2) Adding water directly to the combustion chamber is one for the theoretical physicists. What you are trying to do is induce blockage and temporarily reduce the volume of the chamber, thus increasing the pressure inside the combustion chamber as the efficiency of the combustion process is increased at higher pressure. It also has some other peculiar effects such as increasing the air flow speed which is not detrimental. This type isn't used much as it's difficult to model and understand and can lead to combustion instability, which is a bit of a bad thing (tm)

3) Adding water at the turbine face or just before the nozzle simply works by adding mass flow to the engine's exhaust thus giving you more stuff out the back = greater thrust.

Option 3 is the simplest and most straightforward whereas option 1b will most probably give you the greatest thrust increase. Sometimes you will get a water injection installation that gives you both compressor and nozzle injection to get even more increase at the expense of plumbing complexity.

If anybody ever says that water injection is for 'cooling the engine', just ask them exactly what a couple of gallons of water is supposed to do to a raging inferno at 1,200 degrees centigrade travelling at 200 meters per second. You should get a few blank faces in return...
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Old 1st Dec 2016, 13:30
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Vzlet - In everyday use, was there a different water loading procedure than this?
Taken during a holiday on the cruise ship 'Vincible' some years ago



Back at sunny Dunsfold we had a small diesel powered bowser for the Flight Line and a similar but elec powered one for Hangar use.

Edit just realised it is the same S Harrier as in your link Vzlet (just a few years later )
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Old 1st Dec 2016, 13:59
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Good to see the use of adequate sun protection! Use of a dedicated water source does seem much more efficient.

Interesting coincidence indeed with XZ439. Here on the US east coast, we're fortunate to benefit from its regular performances at air shows. The owner, Art Nalls, also now has T.8 ZD993
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Old 1st Dec 2016, 15:34
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I don't think MSOCS was trying to say that water injection cooled the engine. He's saying that, among other effects, it resulted in a 25C drop in JPT. He's absolutely correct in that statement.

Last edited by AutoBit; 1st Dec 2016 at 15:52.
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Old 1st Dec 2016, 16:21
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Originally Posted by Vzlet View Post
Interesting coincidence indeed with XZ439. Here on the US east coast, we're fortunate to benefit from its regular performances at air shows.
439 was one of the FA2 development batch (the other being 195 IIRC) which were part way through being converted for the JTIDS upgrade at the time the FA2 was taken out of service. IIRC this included some structural mods in the rear equipment bad and of course the replacement of the old BCIU (80186-based mission computer) with the vastly more powerful "Core Computer" (I think it was a box full of PowerPC604s, but that was a long time ago!).

I don't think the Core Computer was ever given a flight clearence (hardware or its OFP) so I've always assumed Art's flying is on purely mechanical instruments with no HUD. That in turn probably means no IMC clearence because I remember that all the SHARs had an IMC clearence that required HUD and mechanical instruments to be both serviceable and in agreement with eachother.

PDR

Last edited by PDR1; 1st Dec 2016 at 16:57. Reason: Keyboard can't spell
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Old 1st Dec 2016, 16:52
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For those with further interest on the Pegasus engine. .

The book "Pegasus, The Heart of the Harrier" is still available on Amazon for the bargain price of 99 pence Kindle edition of course.
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Old 1st Dec 2016, 17:14
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PDR1 - 439 was one of the FA2 development batch (the other being 195 IIRC)
Spot on PDR - ZA195 was the other development SHar.
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Old 1st Dec 2016, 17:30
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PDR, as my colleague Autobit says, I never said it cooled 'the engine'. It cools the turbine blades, resulting in a temp drop (oh, yes I'm afraid, it DOES!)

I have personally invoked this phenomena over 1500 times in my Pegasus-propelled flying career, and watched the JPT drop almost every time (except where the water pump failed or the tank was empty) I also gave account of the increased mass flow you mentioned in your cut-and-paste reply.

So, as I stated previously, there are a combination of factors that increase the thrust in the Pegasus: the cooling effect is real (on ~700C Turbine blades, not in the chamber at >1000C!); the drop in temp permits more fuel/RPM (about 3% in fact); the higher mass flow gives higher thrust for the same RPM; and the digital engine control system re-datums to provide higher 'wet' limits for constrained times (15 seconds or much lower at times)
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Old 1st Dec 2016, 17:53
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Reading the Kindle "Peggie" book as well. I would suggest that it is very desirable reading for ex-Harrier Force mates .
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Old 1st Dec 2016, 18:47
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PDR1 on this one I'm afraid you're wrong. The primary reason the Harrier and the Pegasus use water injection is to cool the turbine section of the engine so that in cooler weather you have a lower JPT for the same thrust or in warmer weather you do not lose thrust due to the engine TGT limiting.

I speak from experience with over 1000 hrs on the ac and an A2 QFI rating. I've got countless <90 second "Press Up" sorties in the log book in the back of the T Bird and the reason for this is because they could not achieve jet borne flight without the cooling effect of the water. Given that the 500 lbs of water was used in 90 seconds that gave you about 40 seconds in the hover which made instructing "interesting"!

The other thing to consider is that the thrust that comes out of the Hot and Cold Nozzles is totally different! The front cold nozzles are at about 200 degrees C and the air that is coming out has not gone through the combustion process, it has only been accelerated by the front fan. The rear hot nozzles are at about 700-750 degrees C and the air/water has. If the Water Injection were to increase the mass flow and thrust then this would only have an affect on the rear ones which would cause a nose down pitch effect when the water flowed and I can assure you this did not happen!

Finally if you ran out of water you did not lose thrust, the JPT simply rose by 25 degrees. Provided this was still below the short lift wet temp limit of 755 degrees nothing happened, you just put more Life Counts on the engine. If it did go above 755 the DECS would trim the fuel flow to maintain 755 degrees and you would lose thrust. You would not crash though as you could "Trip the Limiters" which muted the JPT signal into the DECS and the engine limited purely on RPM.

As a pilot of the Carbon Fibre Death Provider you had to have a VERY intimate knowledge of the engine logic or you were in trouble. We used to have a "Weeds Brief" on a regular basis to refresh us to keep us out of those weeds!!!!!!!!!!!!!!

Oh what happy days! Oh what an AMAZING aircraft built by pipe smoking and tea drinking legends!
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Old 1st Dec 2016, 18:56
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Please excuse defective memory banks.

Maybe, Short Lift Dry was 103%, 715Deg C for 10 seconds

Possibly, Short Lift Wet was 107% 745Deg C, also for 10 seconds.

The reality was 10 gallons used on a wet take off, leaving 40 gals for recovery to VL. Sometimes the water injection would slow the engine accel, possibly giving a water stagnation at about 95%. The fix was to select water on as the motor hit full power.

You only need wet Thrust (107%) on short TO or VTO and the TGT sort of caught up later as you turned it off. Hovering for a VL was temp constrained to avoid cooking the engine. 670C dry or 695c wet. ...but not quite that simple because you were burning 400lb/min petrol and 400lbs of water in the same minute - so all a lot lighter, all a bit easier.

The equation was only spoiled by gentle yellow lights as you approached limits and more unpleasant red flashes as you porked it. Resultant hypertension would lead to a bit overc0ntrolling which increased TGT as engine bleed to the puffers put in a bit more fuel.

Only feel sorry for the poor QFI in the bargelike T4 that was nearly always water dependent as the FNG hovered over the field site. 'You blew down all the soddin tents,Sir! You had to weave to do that.'
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Old 1st Dec 2016, 20:30
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Quite common a few years ago , water methanol on the Andovers RR Darts , on the early 747 with P&W JT9's and a few others i believe
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Old 1st Dec 2016, 20:39
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Quite common a few years ago , water methanol on the Andovers RR Darts , on the early 747 with P&W JT9's and a few others i believe
The 11-1s I used to fly had it as well.
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Old 1st Dec 2016, 22:37
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A broader question.
The common layman's view was that the Harrier was very difficult to fly, prone to yaw in the hover, etc.
Hence only the best of the best were selected to fly it.
Certainly the P.1127, Kestrel and first Harriers broke new ground exploring the VTOL envelope, thanks to Messers Farley et al.
Yet the Pegasus book seems to suggest that once flight protocols were developed it was relatively benign to handle, even in VTOL flight.
Is that an accurate view - or was it still a handful that could bite you badly?
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