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Comment by jabl | Hacker News Reader

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.

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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.

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