Aren't their engines terrible?

The Russian engines they were initially prototyped with, or the Chinese engines they're planning for the production version?

I'm not sure it's hugely relevent though. The F35 procurement was structurally unsound. The result is an aircraft that's a jack of all trades and a master of none..

...or nearly none. I understand it currently holds a marginal and very expensively won advantage at long range air-to-air. Or at least it will once all the kinks are ironed out.

Top speed is not very useful in modern air combat.

F-35 does something like Mach 1.6 where the F-15 Eagle hit ~Mach 2.5 and F-14 Tomcat hit ~Mach 2.4.

That's not because we can't make a much faster jet, just other compromises are vastly more useful.

Top speed matters a lot in modern air combat, maybe more than ever. With modern fire and forget, datalinked missiles, the shooter aircraft can often immediately turn around after firing and run away from the missile coming at him. An F-15 can turn tail and run at Mach 2.5. If the AAM has a speed of Mach 4, the missile closes at Mach 1.5. Against the F-35, it’s closing at Mach 2.4. This means that the enemy’s AAMs have an effective range about 70% higher against the F-35 than the F-15. Now, this is of course dependent on being able to maintain long range tracking of the F-35, but my understanding is that against a high-end opponent, stealth is not a panacea.

F-35 as a plane is mediocre compared to say Su-57. I'm willing to grant that if Su-57 is to challenge F-35, F-35 loses. The thing is F-35 carried through wont be challenged by Su-57 as F-35 is a basically a flying platform. Where Su-57 is optimized for single unit operation ( and Soviets and Russians absolutely excel in that ), F-35 is optimized to be able to operate in a diverse group. What it sees, C&C sees. Until Russia or China get there, F-35 will be massively superior to anything else. For F-35 all fighting is supposed to be BVR.

> F-35 as a plane is mediocre compared to say Su-57

Su-57 is still in infancy stage and judging from Russian problems, there's high chance it won't be around soon.

You can't turn very quickly at high Mach. So, depending on range and heading it can make things worse. Remember, aircraft don't turn by hitting reverse if you are going east a Mach 2 (686 m/s) you can't just subtract 98 m/s in a 10g you need to go through a curve in the air taking a lot longer than 7 seconds to stop going east.

Start combat at lower speeds and it's going to take even longer to hit high Mach.

Which is not to say it's useless, just not as useful relative to other tradeoffs.

Yes the Chinese are having trouble producing high performance engines, and rely upon Russian ones currently. But speed (and acceleration) is extremely important in ACM. A more powerful, compact engine can supercruise, both extending range and reaction time while imparting a lot of kinetic energy to missiles, extending their range significantly.

And for the F-35, it's easy to think of it as a dogfighter, but it's really intended more for the ground attack role. It can defend itself, but it's not going to be performing CAP when the F-22 or F-15s are around.

For reliability, sure, but that doesn't matter when:

1. lives are relatively cheap

2. there is an ejection seat

3. most flights will complete without a failure

You can fight a war even if every flight has a 1% chance of a random engine explosion. There were worse odds over Europe in World War II.

GE decided to make passenger jet engines in China. There should be no doubt that the reliability (likely a function of the factory) will find its way back into fighter jet engines.

Regarding performance, that ought to match the Russian engines that were cloned.

The Chinese haven't been able to clone the Russian engines yet. Metallurgy at those levels is very tough. And Russian engines don't match what's currently used in US fighter aircraft.

Passenger jet engines don't really map to fighter engines. It's like comparing an F1 race car with a Volvo semi.