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Much to discuss here.....
The airframes are still the airframes. Really, the only thing that has been changed in the H models over the years is the avionics. In fact, this re-engining will be one of the first major upgrades that is not on the electronics side of the house.
From simply the H model, this is true.
But from B thru G, this is not. Wing rake, incidence, and a little things like root fairings have all been changed along the way. Not to mention the huge tail rebuild early on. That's probably not what you were saying, though.
The thing is when the B52 was designed there was no such thing as stress modeling and stress optimization. So, some aircraft were massively over-designed and had fatigue lives well in excess of initially calculated (the B-52 and KC-135), and some had fatigue lives that were not extendable (C-141s an A-6).
The thing is, this is not entirely correct.
While FEM and the resultant FEA did not exist until the 60s, that has little to nothing to do with the design - which, BTW, has been well analyzed many times since.
The airframe was designed without quite so much oversight by government or shavetail kids as we have today. Please humor me.
Management looked for the most experienced Engineers, not the ones that would work Saturdays for free.
(Yes, I've been down this road many times before - having nearly this same discussion, though you don't sound quite as Junior as most.)
The newest and coolest tools have been found errant more than once on several big projects. 737 Tail comes to mind. There was a time we understood the difference. Thanks to corporate management structure - all planes are built by corporations - we no longer understand these differences.
As opposed to being calculated in NASTRAN et. al. in a virtual environment, the thousands of B-52 parts were analyzed by formula then pulled and stretched in a real environment with large met-lab machines. This work is being done to this day doing exactly those kinds of yank tests to B-52 structure continually since day one. It's possible we know more about B-52 fatigue than any other flying vehicle after testing it for over 50 straight years.
Computers are only JUST NOW bringing that kind of certainty to the table after many years of added uncertainly margins in the low-level code. Lawyers, you know. They're lurking around every corner.
The code we finally have in the last 5 years with the element resolutions available to be processed are pretty darned impressive. They're carrying enough data finally to put together really nice conclusions - after the users get enough training. So the corner has been turned, just not back when we thought we had.
....so more invasive and expensive repairs are required such as new wings and other rebuilds of aircraft structure. These are expensive, almost as expensive as building a new airframe.
As it isn't public knowledge, you may not be aware that the Boeing design crew in OKC adjacent to Tinker have been busily redesigning the entire wing box. This won't be just a re-engine with new controls like the journalists report.
What you may also have overlooked is that NO ONE ever cares if something is "too expensive" when it comes to a front line bomber. "Too expensive" is a common, pedestrian, acceptable excuse for "insufficient political clout" to rise above other companies more enthusiastic about winning available monies. No one likes the real explanation, so expense is tossed around.
This BUFF re-engine contract has been in the works since 1981 that I know of and likely before that.

There are other aircraft that are capable of performing the A-10's mission, in fact, there is nothing a A-10 can do that an F/A-18E, AV-8B, or an F-35 can't do as well...
The A-10 had comprehensive "austere environment" requirements that, to my knowledge, the F18,35, or ANY F-series, new or old, is completely incapable of.
"Austere" as in takeoff and land from a gravel road with potholes. Seen an F35 do that? And that's not including uphill, downhill, and tree obstruction operational requirements. I don't recall seeing an F18 take off from an uphill slanted dirt road.
The publicity people didn't just brag about the tough conditions, those were Pentagon requirements they turned into talking points. But hey, they met them, anyway.
They had req's like do a complete turnaround with only 3(?) ground crew. Seen an AV8 do that? They can't fuel these things without 5 people or more, let alone reload ordnance, service fluids, and feed the pilot. Full turnaround.
Now sure, in fairness, they usually don't, but they can if need be.

3) Survivability. The A-10 is more likely to become damaged due to enemy ground fire that other faster aircraft. Yes the design is very robust and can absorb hits without falling out of the sky, but honestly, what good is it if you have to spend three weeks repairing and airplane after every sortie?
This is a long-standing debate, you're right.
IMO, it's the difference between "survivability" and "operational spec" and it's twofold.
An A-10 pilot will fly into crap no fighter would ever consider. Not just speed, but because those jocks simply don't do such things and you can't make them. The "titanium bathtub" he knows will protect him so he takes on MORE fire than any sane human would willingly encounter and he loves it.
And he survives quite well. Survivability is way up there. In fact, shot-for-shot, projectile-for-projectile, it's far higher suvivability than any fighter which has a very low ballistic tolerance. The fighter is simply a harder target for kinetic weapons to hit.
BUT, bringing the aircraft back to full operational spec is a long chore, you're right. It could go out on a bunch more sorties as-is (and they sometimes do) and each one becomes survivable with no less trouble. But then when someone has to sign off on the next mission, they balk at all the flak damage and won't sign off until it's fixed. Yup. 3 weeks downtime. The plane could fly and do just fine, but nope, it's not up to spec. It's the 'unintended consequences' of the high survivability aspect of the design.

Anyway, unlike the days of when the BUFF was designed and deployed, there are countless new links in the chain with many more agendas than ever before. If the B-52 did not already exist, there's no way we could build it today. Oh, we have possibly the Engineering talent and fabrication processes, but it would NEVER make it through the countless meetings and committees these days. I know these things because I've sat in these meetings. And what would remain would look and act more like an obese 747 than a B-52.
We saw this happen in spades on the V-22 tiltrotor that weighs roughly twice what it could ever dream of weighing - until the Pentagon 'oversight' people got into it.
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