How Can a Jet Engine Fly in the Rain?

As to the Qantas 380 event specifically, here’s the wiki on that model of engine:
What’s relevant here is the top two pictures.

Starting with the lower picture we see the man standing next to this beast, giving us a sense of the scale. Some of the stuff we see on or attached to the engine is test stand stuff, but most of it is part of the engine as it would be installed on an airplane. The intake end is at right and the front of the engine proper is about at the right edge of the black plastic sheeting. As installed on the airplane there’d be another 3-4 feet of cowling off to the right and then the lip of the intake.

The engine fuel shut-off valve is probably somewhere in that rat’s nest of gizmos and plumbing in front of the technician’s tummy & crotch. Which rat’s nest extends up the opposite of the engine about as far as it does on this side. So maybe from 4 o’clock to 8 o’clock.

The upper picture shows an engine ready to be installed into the nacelle and hung from the pylon. We can see where the engine proper joins the white intake cowling & lip. Up at the very top by the red box we see where fuel and hydraulic lines and wiring connectors wrapped in sheet plastic and blue tape will eventually connect to the pylon and the rest of the airplane. I bet the largest and rightmost tube is the fuel line. Just below where the lines run horizontally forward is a tan-ish box with a yellow cylinder attached to the upper aft side. The yellow cylinder is the oil filter and the tannish box is the oil tank.

From the other picture we get a sense of scale; the fuel line is about the cross-section of the man’s wrist and the oil tank is a couple feet above the man’s head and maybe 3/4ths the height & width of his torso. Although he’s a bunch thicker than it is. :wink:

From @Francis_Vaughan’s picture of the fire/rescue folks trying to stop the engine (post #8) we see that as installed the bottom of the engine cowling is some 6-8 feet off the ground. That’s the outboard engine. The inboards hang a little closer to the ground, say 4-6 feet.

It’s also worth noting that as large as the Trent 900 is, the very largest turbofans are about 25% bigger in size and power output.

I think I asked this same question quite some time ago on the boards, I saw some fighter aircraft in a book being described as all weather fighters and I wondered if rain or cloud would down a jet in earlier days.

A simple explanation might be that the water droplets that affect the steam turbine don’t exist in the steam at the turbine inlet. Whether the steam enters the turbine saturated or superheated, it should be “dry” steam either way. As the steam flows through the turbine stages, the pressure reduces and droplets begin to form in the latter stages resulting in the damage described.

(Less familiar with this part, but I’ll give it a try) The water droplets (rain) entering and moving through the turbo-jet engine are entrained in air (of whatever relative humidity). The pressure of the air/water mixture is raised in the compressor and then the temperature increases dramatically in the combustor, lowering the relative humidity. I’m thinking this would dry the air and reduce or eliminate the droplets altogether by the point in the process where the pressure is reduced as the flow moves through the turbine to the exhaust.

So the steam engine starts dry, but the process causes the steam to become wet with droplets and, while the turbo-jet starts “wet,” the process dries the air. (I’m not certain how the compressor blades at the inlet aren’t damaged by the entering droplets).

As to fighters, “all weather” refers to the ability to employ weapons while in the clouds. A fighter that needs to spot its adversary visually to close and engage is useless if the adversary is able to do its mission (including most of the transit from home to the target area and back) from inside clouds.

Likewise ground attack airplanes that need to see the target on the ground to employ weapons are useless if it’s too foggy or rainy or hilly where the target is.

Conversely an “all weather fighter” would be able to locate and engage aerial targets using radar only to both find the enemy, decide they really are enemy, and press the attack successfully. Likewise on the bombing side, if you can locate the target via radar, laser, GPS, whatever and deliver ordnance to that spot never seeing it visually, you’re all weather.

Back in WWII, even nighttime was a pretty good defense against the day-only fighters commonly used. About halfway through the war night fighters using large airplanes (e.g. Me-110, Bristol Beaufighter, P-61) carrying large primitive radars began to take a way the sanctuary of night & clouds.

And on the offense side, the Germans invented a number of blind bombing aids so their high altitude bombers didn’t need to see the target to attack it. The US never really invented much of that during the war. Nevertheless, it was still a matter on both sides of dropping hundreds of tons of bombs generally somewhere in the same county as the target and hoping 1 or 2 did some damage to something important.

Even today, “all weather” needs a bit of qualification. An airplane like a modern F-15 or subsequent is certainly able to attack both aerial and ground targets while it or they are within clouds or darkness. But that doesn’t mean it can happily penetrate a mature thunderstorm or volcanic cloud. Avoiding the worst weather on Earth is still necessary. But those airplanes also lack a radar able to detect weather, so entering any cloud carries some risk of fatally encountering a thunderstorm. Knowledge of the general area weather here and now can help assess just how risking flirting with clouds is: safe, foolhardy, or somewhere in between.

So a more detailed set of terms would have been “Day visual fighter/attack”, “day or night visual fighter/attack”, and “non-visual fighter/attack”.

Finally as a general matter their tactical effectiveness declines a bunch if forced to operate non-visually. Before the advent of radar guided SAMs, clouds were a pretty good sanctuary from anti-aircraft ground fire. Nowadays, operating in clouds within the range of radar guided SAMs is effectively suicidal. Once a SAM is in the air coming your way the best hope of survival is to outmaneuver it in the last few seconds before it passes by. If you can’t see it coming with your own eyes, that option is foreclosed.

They’re pretty well hardened against impact, but can be won down some by enough rain, or worse yet hail. A severe hail encounter can wreck an engine pretty quickly even nowadays.

The exhaust comes out of a steam turbine cold. All of the energy has been used up making the turbine spin, against the resistance of the generator.

A airplane where all the energy was used up just making the turbine spin, would fall out of the sky.

Completely wrong.

[Wouldn’t get into the sky in the first place!]

But I was thinking as a wrote that, “making the turbine spin” is an important process. A typical jet engine is going to have a rotating mass > ton ??? And takes a while to get it to spin up to speed?

A jet engine in a plane, even a high bypass engine doesn’t extract all the energy from the gas, A straightforward jet engine extracts only what is needed to power the compressor, whereas a bypass engine powers the fan with most of the energy. But there is still thrust from the remaining gas flow.
A steam turbine tries to extract every last drop of energy from the gas flow. So much so that the outlet exhausts into a near vacuum provided by the condenser. The last stage of such a turbine is large as it is operating with much lower pressure and high volume. The turbine disk has quite a large diameter and the blades are thus travelling quickly. I suspect that this high speed and the relatively low temperature of the steam are a big part of the problem. Designers will want to get as much energy out as they can, and this is in conflict with avoiding droplets condensing.

It’s certainly an essential part of the process in a pure turbojet engine. But like the camshaft and valves in an automotive ICE or diesel engine, it’s a necessary evil that consumes power that in a counterfactual magical world you could avoid diverting and instead use to make your engine yield more useful results.

As @Francis_Vaughan almost says, in a steam turbine the production of the working gas is external to the “engine”. And as he did say, the whole goal of the turbine there is to extract the absolute practical maximum of the energy in that gas. Rather than the pure turbojet turbine’s goal of extracting the bare minimum needed to make the rest of the engine’s compress-burn-expand cycle work in a self-sustaining fashion.

The more modern turbofan jet engines are in a sense a hybrid. The so-called “core” of compressor, combustor and turbine work as I described above: trying to extract the minimum energy needed to make the cycle self-sustaining. Then a separate fan is bolted on the front turned by a separate turbine bolted on the back.

That fan + secondary turbine rig is indeed designed to extract the absolute maximum power possible consistent with two limitations:

  1. Like a steam turbine, the environment you’re exhausting into limits you to below the ideal theoretical results you’d get exhausting into a vacuum at 0K. But unlike a steam turbine aboard a ship or fixed plant that exhausts into a condenser maintaining constant parameters in a closed system, a jet engine exhausts into the ambient atmosphere where we encounter very different combos of temperature and pressure. From sea level in the Persian Gulf at ~1050 millibars at 50C with 95+% humidity to high altitude at 200mb at -60C with 2% humidity. Given that (so far at least) all this is done with fixed geometry airfoils, a system that works acceptably everywhere is probably not optimal everywhere.

  2. Unlike a steam turbine, you can’t have the power turbine be so much of an obstacle to flow that it clogs up the output of the upstream core more than that core can stand. Much of the black magic of modern jet engine design is right there; making these two separate systems with often opposing demands co-exist and couple efficiently both at steady state operations and during transients of starting through to idle and idle through to full power.

Which bit about transients leads to your second point.

The rotating mass of the core and of the fan system is big and heavy, despite being built as lightly as possible given the durability goals. Engines on 737- or A320-sized airplanes weight ~2500Kg / 5,000# each. The Trent 900 on the A380 we discussed earlier is about 6,500Kg / 14,000# each. I don’t have a formal rule of thumb for the rotating mass, but big picture I’d bet it’s about 1/3rd, so a little under a tonne/ton per small engine and 2+ tonnes/tons for a big one.

For very round numbers jet engines at idle are spinning the core around 50% of max RPM and the fan at around 20% of max RPM. Takeoff is close to 100% on both and cruise is in the mid-high 90s on both. In absolute terms core RPM redlines are in 20,000-30,000 RPM bracket and fan RPM redlines are in the 12,000-18,000 RPM bracket.

So yeah, lots of heavy stuff spinning at insane speeds under fine tolerances in beastly heat & highly reactive chemical conditions.

Modern engines can get from idle to full power or vice versa in 5-7 seconds. The acceleration is very non-linear. Getting from e.g. 50% RPM to 80% RPM takes most of the time and getting from 80 to 100% RPM is the last little snip of time. The thrust output vs. RPM is also very non-linear with not much change happening in the lower RPM ranges and substantially all the incremental power occurring in the last 10-15%.

Part of the reason airliners have such large flaps is to allow the wing to fly more slowly so takeoff and landing speeds are low enough that runways don’t always need to be 3 miles long to accelerate or stop.

But another large part for landing is to generate a lot of drag so we can leave the power set pretty high during approach and landing so that we have very fast thrust response if we need to go around. Pushing up the throttles and having to wait 5 or 7 seconds for meaningful power would often be a few seconds too many. This was a very severe problem with the early (<1960) military jets of all nations.

Making the engine acceleration rate fast enough, but keeping the core/fan coupling efficiency high for steady state cruise is another constraint on design. During steady state ops the core needs some excess capacity to add first fuel to the existing airflow, that grows the fire, that grows the turbine RPM which then grows the compressor RPM to add more air to further grow the fire, all while being obstructed by the fan turbine extracting some of that extra output to accelerate the fan.

But every bit of that excess capacity represents lost potential efficiency during steady state ops. Much black magic is hidden in there. It’s very much akin to impedance mismatches and their baleful effects on large transients in non-digital electronics design.

Is anyone looking at constant-speed turbofan engines? That would require a variable-pitch fan, which isn’t used now but seems to be in development (i.e., RR UltraFan). They’re doing it for efficiency but it would have the side effect of faster response time if run at constant RPM (or maybe not, if thermals are the real issue).

As you suggest, the RR UltraFan is the only such project I’ve read of. I have a vague notion I once read something about a USAF-sponsored engine research project looking at variable pitch fans, but that was in the context of low bypass fighter engines.

A technological prereq for a constant speed fan is a geared fan. It was a pretty big bite to get to geared fans at all. Which P&W is now doing mostly successfully with their GTF or PW1000G series. That GTF engine has had its fair share of teething pains, if not more, but AFAIK none of them stem from the gearbox.

Pretty incredible to route 30,000HP through a 3:1 reduction in a package a couple feet in diameter and a foot or so long. The UtraFan gearbox is being sized to process about 80,000hp in a slightly larger package.

I’m not enough of a propulsion guru to know how a constant speed fan would affect engine output acceleration. For sure you lose the problem of adding fan RPM against rotational inertia. So at least in theory you can ramp power extraction by the fan as fast as you can change blade pitch. Assuming the power required is available in the inter-turbine exhaust stream to extract. Which comes back to avoiding choking off the acceleration of the core.

As a general matter of piloting experience, ISTM the core gets up to speed first and the fan (and thrust output) follows by a couple seconds. Which suggests at least that part of the acceleration time could be shaved by a constant speed fan. I have to assume for stability if nothing else that the fan blade pitch mechanism is very tight and very quick to react. Such that the time needed to alter blade pitch would not be an obstacle to acceleration time.

As a separate matter …
The idea of adding a variable area fan nozzle has been talked about forever and tried at least a little. It seems to cross the boundary between what’s the engine maker’s product and what’s the nacelle maker’s product. So may be as much a business problem as an engineering one.

I could see trading off a variable pitch fan and a variable area nozzle, or at least the range of variability of the nozzle, as part of balancing all the interacting flows of energy. Much like compressors have variable pressure relief ports in some stages to improve stall resistance under acceleration or deceleration.

@LSLGuy: excellent.

Definitely. When I first read about GTFs, I thought “Wait, they aren’t already doing that? Seems easy and obvious.” But then considering the power levels and compactness requirements, I realized it was actually pretty damn impressive.

Not sure who said it first or what the canonical version is, but I’ve always thought this carried some deep truth: “The flaws in the product match the flaws in the organization that built it.” That is, if you ever spot some area where there seems to be an obvious optimization: a mass trade, or a cost trade, or a risk trade, or combining redundant systems, or an improvement that requires coordinated effort, and so on–then the odds are high that the uncrossable barrier matches something in the organization as well. Either one of the bits got outsourced, or the organization is so big and the internal silos so powerful that they might as well be different companies.

I think the popular RR model uses a 3-spool design, where the turbine shaft goes to the fan and drives the middle compressor at a higher speed, which drives the final compressor at an even higher speed. That is a bit of extra hardware, but it seems they are getting a lot of output and pretty good fuel efficiency with that design. They probably have maintenance crews diligently keeping the engines operating reliably.

In theory the 3-spool design allows tighter optimization of all 3 for their respective role in what becomes a 6-stage bucket brigade with a fire in the middle. At the same time each interaction between the spools needs to have stall resistance built in. Avoiding those “impedance mismatches” I mentioned earlier is not easy or cost free. Something is also lost at each stage transition.

Whether net, net the extra complexity buys enough real world improvement to pay for itself seems unclear to me. Of the major manufacturers, only RR has developed and persisted for 30+ years with their 3-spool design. Nobody else sees the same need on even their experimental future tech engines.

Although as @Dr.Strangelove just said, is that difference a sign of organizational flaws? Either Not Invented Here syndrome for everyone other than RR or conversely for RR “this is the only secret sauce we have so of course we ride it as hard as we can as long as we can; even well past its sell-by date?” Hellifino.

A variation on Conway’s Law, as it turns out:

Any organization that designs a system (defined broadly) will produce a design whose structure is a copy of the organization’s communication structure.

With a specific example relating to computers:

If you have four groups working on a compiler, you’ll get a 4-pass compiler.

I think we can predict that Rolls Royce has three groups working on spools.

That’s my thinking. The two spool design deals with airflow management with variable ( compressor ) stator vanes and low pressure compressor bleed valves, both mechanically operated, using servo fuel pressure as muscle. The former being quite complex with an amazing amount of moving parts. The RR RB211 uses the 3 spool design but still employs LPC bleed valves, although simple poppets actuated by compressor bleed air. It would be interesting to compare the two philosophies via some metrics ( engineering, material, weight, need of supporting subsystems, manufacturing processes etc ) of judging which is the better of the two, especially since they both work very well in service.


There was an English Composition teacher in my high school who assigned her class to build a story or essay around the line, “Wet birds never fly at night.” Several of the posts here would have truly dazzled her.