> Every attempt to replace it has been (or will be) retired before its EOL.

Though that's probably because it's design is basically "big, boxy, utilitarian sky truck," which is so flexible you can always find something to use it for. Everything else seems to make tradeoffs for some flashy capability, which don't age as well.

How long have the B-52 jet engines had monocrystalline fan blades? That must have an enormous impact on efficiency and performance. They became available in the late 60s.

https://www.americanscientist.org/article/each-blade-a-singl...

They're still using the original TF33 engine they were delivered with in 1961. Presumably the fan blades have been replaced many times over during maintenance, but I don't know if there's any advantage to switching to better blades if you don't change the operating parameters to take advantage of them.

There's a plan to replace all the engines with a modern turbofan (RR F130).

It's interesting that even though the bypass ratio changed from 1.4 to 4.2 (so it's quite a different engine, lot quieter for sure) the fuel consumption only went down 15% (22g/kN.s to 18.6g/kN.s)

Hmm, I'm not familiar enough with jet engines to say whether that is expected or not. 15% doesn't sound like a huge improvement, I agree.

But I'll note the F130 is a medium bypass business jet engine producing 'only' around 75kN. 8 of those is about 600kN, which could be achieved with two of those newest generation civilian high bypass turbofans used e.g. in the B787 or A350. I suspect the motivation was to choose a new engine that is as similar to the existing one in order to minimize work needed to install them on the plane. There being AFAIU less than 100 B52's in service, I guess minimizing engineering cost has better payoff than minimizing fuel consumption with a high bypass engine.

These engines are also way larger diameter => redesign nacelles => higher drag and moment => engine mounts + wings ...

Also this family of engines (F130/BR700) has insane reliability - something like one unplanned engine removal per 100,000 hours. Some have reached 1 million hours

So what happens if four newest-generation high-bypass turbofans are installed on a B-52?

Is the airframe at risk in doubling the thrust at full throttle?

Doubling the thrust is likely overall a bad idea, for the reasons you mention.

Also looking at other posts in this discussion, it seems the 4-engine option (4x 150kN class size engines to provide about same overall thrust) was discarded because the asymmetrical thrust in case an engine gives out at a critical moment (takeoff, say) would mean that the plane would need a bigger tail in order to have enough rudder authority. Which in turn would mean that the airframe would need strengthening to take the added weight and stress from that bigger rudder. Etc. etc. leading to the cost of the engine replacement program spiraling out of control. So in that sense as close to a 1:1 as possible is the right decision.

The plan in the 90s was to use 4 high bypass engines and replace the tail. At the time improving the -52 was not favored by the USAF because it would threaten the B-1. I still think it is the correct approach

Of great importance was not rewiring or modifying all the engine controls: the new engines had to slide right in and attach to the controls the old engines used. This alone demanded 8 small engines. Read about how they solved the flutter problem on the 747 and you’ll appreciate how important it is not to change the aerodynamics of the wing in any way. Finally, competition for the contract was fierce as there were many different engines that approximately fit the profile: it was very close to competing business jet designs with successful production histories. Rolls Royce, I think, was a surprise winner.

Turbine blades, not fan blades. The single crystal metallurgy is to avoid high-temperature creep (i.e. slow deformation over time under an applied load), which is a life-limiting factor in turbine blades. Especially the first row or two which see the highest temperatures.

Platonic question: is it still a B-52 then? cf. the Ship of Theseus

At least in civilian aviation, it is a very relevant and practical question, and a major contributor to the Boeing 737-MAX fiasco.

There is a thing called "type rating", that is qualification pilots need to have in order to fly a specific type of plane. For example, pilots need a type rating for the Boeing 737 in order to fly a Boeing 737.

Manufacturers try to do everything they can not to require another type rating when they make changes to an existing aircraft, because type ratings require expensive and time-consuming training that pilots and their companies would rather avoid, and it influences purchase decisions. One of the root causes of the B737-MAX crashes is that Boeing tweaked some settings in order to make the B737-MAX behave like the original B737 when it was, in fact, a different plane, and they did it wrong.

> One of the root causes of the B737-MAX crashes is that Boeing tweaked some settings in order to make the B737-MAX behave like the original B737 when it was, in fact, a different plane, and they did it wrong.

This is wildly wrong. The only thing you got right is that the MAX was designed to avoid needing a new type certification.

Boeing radically shifted the Cg to accommodate changing trends in the commercial airline industry which were resulting in Airbus's planes being more competitive. They shifted it well outside what would be considered prudent.

This required flight control software bandaids to keep the plane stable (or perhaps better put, from entering a flight envelope where loss of control would happen), but they then also cheaped out of providing redundancy for some critical sensors... AND cockpit annunciators to alert on failures of the sensors.

Boeing then passed off the Cg shift and additional systems as very minor changes in the filing for their airworthiness certifications, the FAA rubber-stamped everything because they didn't have the staff to review all these certificate filings so they largely trust manufacturers...

...and then making things even worse, Boeing did very little to disclose the additional stability system and sensors to the airline's chief pilots who develop the in-house training. To say they were furious to learn they hadn't been told of the new systems would be an understatement.

The MAX crashes would have been substantially less likely if the pilots had benefitted from their airline's chief pilots knowing of the safety systems and having training on how to handle failurs, as well as warning annunciators for sensor failures....but Boeing wanted to save a thousand bucks or so per plane - or more likely, wanted to avoid drawing attention to the situation and having people ask "why is a critical flight control system dependent upon a single sensor?", which is likely why they also did little to inform airlines of the new systems, train the chief pilots on it, etc.

I generally agree with you, except for the fact that it was more of an aerodynamic issue than a center of gravity issue: the large body of the engine produced lift at higher angles of attack, making the nose go up and worsening the problem. The fact the engine was more powerful also contributed to that problem: the higher thrust produced more upwards torque.

And I am not saying that the software bandaid and the goal of avoiding a new type certification is a bad thing. In fact, it is done all the time with no ill effect.

What I am saying is that they did it wrong, and that's the real problem. Your explanation on what they did wrong is, I think, entirely correct.

My hypothesis is that if they didn't care about keeping the same type rating, they probably could have skipped the bandaid and trained pilots to just keep out of that dangerous configuration, just like pilots are trained to recognize, avoid and recover from stalls. It didn't look like an unmanageable problem for pilots, more like something they needed to be aware of and train for.

I mean in this case yes. Since it is a brand and not a single B-52 physical plane which is being improved all these years.

To eliminate confusion, the current operational B-52Hs were originally manufactured in 1960-1961.

They have undergone significant upgrades over the years, on all systems, including the airframe itself, since then.

The earlier models were manufactured in the 1950s.

Most of the H models were manufactured in 1960 and 1961, but they have undergone significant airframe improvements, several engine improvements, and of course, avionics improvements.

It's still a B-52, but it's a "B-52H Block $whatever_design_block_they_are_on_these_days" model.

That's a little disingenuous, regarding the F-15 -- there is the A/B series, with A being single-seat and B being the two-seat trainer, and the C/D series, with the same distinction between the two models. Both the F-15A and the F-15C are air superiority fighters, with an exclusively air-to-air role. The F-15E is a multirole fighter-bomber, not an air superiority fighter. The F-15X is an F-15E repurposed as a beyond visual range (BVR) missile truck, with data links to AEWC planes that can provide targeting solutions for those missiles. All of the letters between E and X have not been used.

The more useful identifier is the block number which identifies production changes.

I think it is but they do rev the sub model number it seems.

Wonder what a from-scratch B-52 equivalent would look like nowadays. I don't think they could resist the urge to give it a gimmick. Who wants to design a bomb truck?!?

You mean like a giant, tailless, stealth, flying wing? Or would that would be too gimmicky?

The B2 is great.

But, while stealth is really useful, it does require some trade-offs. Since B-52 is only useful in cases where stealth isn't required, and it seems to still be in use, I guess if the B-52 were hypothetically retired there'd be room for a non-stealth replacement. Unless the techniques they developed to come up with the F-22 and F-35 opened up a possibility of a zero-compromise stealth design (I'm skeptical, but I don't know anything about plane development).

The gimmick is that it's really big, I suppose.

> It's honestly a fantastic case for incremental improvements over time.

The trick is to design something solid enough to be a good base for incremental improvements over time.

I don't think you would say that if you had ever been inside one. They are ancient -- and very greasy.