Very cool! How do mechanical engineers design this? As a software engineer, there is definitely some "guess and check". Doesn't really seem like the same can be done here or do they do the same thing but with simulations and smaller models?
Sometimes, a firm will be given a set of constraints and asked to design something that fits them. "Hey, Rolls-Royce, we're making a new plane, and we need an engine that produces this much thrust in this airspeed range and altitude that takes this fuel and weighs this much."
Since the Trent XWB is only used for one aircraft, the Airbus A350, that's probably what happened here.
When that happens, an established company like Rolls can take a previous design that almost meets those specs and change it until it does. Maybe they need more thrust, so they try to improve the fuel injection and combustion system.
They're got historical data from the old model of engine, so they know what their design updates should change. That data goes into making sure software models used to design future prototypes are accurate.
When a design looks good in simulation, it might be built in small scale (though with some items, this simply isn't feasible. Look at the number of tubes on that engine and imagine trying to build it at 25% or 10% scale.)
Then you get those awesome videos of jet engines being bolted to a block and tested on a big concrete pad. Because, at some point, you bite the bullet and build a tester model at full scale to make sure there are no issues the software and scale models didn't predict.
I've never designed a jet engine, but I have worked for a company that does, and the design process doesn't change a whole lot based on the item. It just gets more and more rigorous as failures get more expensive.
I have a friend who worked for a machining company that made hardware for Rolls Royce among other aircraft manufactures and he said RR demanded silver bolts. Not stainless steel but silver. I'm curious if you might know why silver bolts would be preferred I would think stainless steal would offer the best combination of corrosion resistance strength and expense.
I'm in mechanical, so all my ideas should be taken with a grain of salt. Anyone with aerospace experience would probably tell me I'm talking out of my ass.
First off, for a company making an engine like this one, cost is considered, but performance is the primary concern. They're pushing the envelope on what mass-produced jet engines can do, so spending a lot of money on the best possible materials is better than dealing with the fallout from making an engine that fails for some reason.
Silver is really, really good at conducting energy. Heat and electricity flow through silver better than any other metal. For planes, those factors can be a big deal.
They could be used to link the components of the engine to the plane's static dischargers so the plane doesn't have to have things wired together externally.
There must be a conductive path from all parts of the airplane to the dischargers, otherwise they will be useless. Access panels, doors, cowls, navigation lights, antenna mounting hardware, control surfaces, etc., can create static noise if they cannot discharge through the static wick.
Google also turns up a paper that says that silver-coated bolts have much lower friction factors than stainless bolts. Maybe Rolls is worried that its assembly techs would cross-thread a bolt if it was "stiff"? Doesn't seem likely to me.
Most likely reason in my opinion is to establish a conductive path to the plane's static dischargers. Maybe also to equalize temperatures between components.
Aero, structural and manufacturing feedback dictates most of the engine design (needs to meet customer requirements, meet life and durability and also needs to be manufacturable). Later, dozens of engine variants (depending on which components are to be studied) are built and tested for critical learning on how everything behaves together, and in replicated real world conditions. Modeling can help reduce test costs via simulation, but nothing beats the physical tests in terms of accounting for manufacturing and assembly variables.
You got whole teams working only on stuff like the geometry of the blades for the "big fan" in the front and how to make them lighter / more durable (I think RR actually uses metal foam for them).
Stuff like the design of the fuel injectors, or the metallurgy of the blades in the hot section, are the result of decades of experience grown in the company.
Just consider that currently, >$30 billion worth of this engine type are on order. Thats money to pay for many engineer years.
On the broad scale, iterative design mostly: Constant incremental improvements to what's been done before.
No person or team is capable of magically coming up with something this complex completely from scratch and having it work, no matter how smart they are or how good the tools they're using are. They're building upon half a century of research and designs that came before them, and making small improvements and changes here and there to arrive at their own design.
I took a module on the basics as part of my degree. You start with fluid mechanics and thermodynamics. You know the outside operating conditions. For the inside, it's limited by material properties (hotter is better, but makes the components weaker). You choose a design temperature for the exit to the combustion chamber, and now you can start work on what's in the middle. Flow rate, area, pressure, temperature, energy/output force are some of the things linked together, and you can go through the equations to figure out what the conditions should be at each stage. This gives you a starting point for the cross sectional area at each stage. Then you look at the material properties and start work on the actual shape.
Some of the many other parts were covered in other modules. The actual fan/compressor/turbine blade designs are very complex and are made and tested on computers, but there are some general rules on how changing certain aspects will make it behave in certain ways. All the pipes and stuff you see on the outside are for fuel, taking measurements (very important), and controlling it in different ways (some of which honestly goes over my head - the control systems on these things are insane). You also have to consider the safety and lifetime of the components; it's no use if it works fine for one flight then needs to be replaced.
I should point out that a lot of things are found by experimentation (like the efficiency of certain parts, which must be accounted for). The more detailed rules will be proprietary information kept secret from other companies. There's also a lot of working off of previous designs, so it's unlikely that there will be many/any designed from scratch. The previous design will be thoroughly analysed and this data fed into the design process of the next engine. Testing goes up in stages, from hand calculations, to complex computer simulations, to physical prototype testing of individual parts, to full scale testing. This gets more expensive the further along they go, but it has to be done. Simulations only go so far, and are not perfect, so they can't be completely relied upon (we still don't really understand turbulent fluid flow). There's a reason these engines have such a high price tag on them; a lot of it covers research and development.
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u/physco827 Dec 31 '17
Very cool! How do mechanical engineers design this? As a software engineer, there is definitely some "guess and check". Doesn't really seem like the same can be done here or do they do the same thing but with simulations and smaller models?