I am wondering what the real state of the P&W F-135 engine is these days? While the relevancy of simulations are debated, some of the relevance depends on what the engine is really capable of delivering. What makes me wonder is this information from the commercial engine business side as stated to Aviation Week by the new President of P&W regarding their new GTF engine for the A-321neo:
Rotor bow, or thermal bowing, is normally due to asymmetrical cooling after shut-down on the previous flight. Differences in temperature across the shaft section supporting the rotor lead to different thermal deformation of the shaft material, causing the rotor axis to bend. This results in an offset between the center of gravity of the bowed rotor and the bearing axis, causing a slight imbalance and potentially reducing the tight clearance between the rotor blade tips and the compressor wall. Maintaining this clearance as closely as possible is critical to engine efficiency. Slower starting allows more even heating, eliminating differential thermal deformation.
“On an A320 it takes about 150 to 160 seconds to start both engines, and those are (IAE) V2500 kind of numbers,” says Leduc. “The initial PW1100G engines we put into service on Lufthansa take about 350 seconds. All engines have a bit of rotor bow. The V2500 had it, the PW2000, PW4000 and the CFM(56) had it, there’s not an engine built that doesn’t have a certain amount of rotor bow. We have taken an incredibly conservative approach here. We basically have dictated start times to ensure ourselves we will never rub a rotor out,” says Leduc.
Pratt is adopting a two-pronged attack on the issue. All production standard engines now feature a damper on the third and fourth shaft bearings to help stiffen the shaft. “The engines we are building in the factory today are to this bill of material and the first 20 engines we built were not. With these 20 we have a plan with the customers to go back and modify them as needed,” says Leduc.
The second prong is to collect data from engines in service and under accelerated testing, and to gradually reduce the start time based on real experience. “We are basically going to be able to match rotor clearance, engine by engine. And we are going to do it with an algorithm that will have greater and greater fidelity over time. By the time we get to June, it will be down to 200 seconds for start time and by the time we get to December we will be down to 150 seconds for start time. It’s all about how much data we accumulate and the level of fidelity in the algorithm,” he adds.
I might add, the rotor shaft bending can also be due to the weight of components attached to either end, the dumbbell effect. Regardless of where the rubbing takes place, be it blade tips or internal seals, it isn't an easy problem to deal with. It results from miscalculation of structural support design. It is true all engines do have some rotor bow, but the ones that have the least have good structural support throughout the length of the engine. Opened clearances to solve the rubbing problem reduces engine efficiency, and that effects performance items such as range of operation and true developed engine thrust, items that remain to be confirmed on the F-35. BTW, the CFM56 LEAP engine starts in 50 seconds per engine.