As the subject line says, why is the B-52 fitted with eight modest engines instead of, say four bigger ones?
Well, the design is pretty old.
Perhaps reliability was an issue, & therefore redundancy was viewed as desirable?
IIRC, the engines were about as powerful as they could make them back then, and 8 were needed for the desired performance. I think some were re-engined later, with 4 more modern turbofans.
I think because “bigger ones” wasn’t an option when the B-52 was conceived and designed toward the end of the 40s and early 50s. There were actually two proposals to upgrade the B-52 to four more modern engines instead of the eight older ones (which themselves became more powerful with subsequent iterations). Even as recently as the Boeing 747, engines were somewhat underpowered in the earliest models.
amazing that plane and the C-130 are still in active use due to big upgrades. In some cases they strip down to bare metal frame and rebuild or replace everything. If you look at fighters or small bombers some recent models have already been retired such as the stealth F-117
Interesting there is now a plan actually moving forward to reengine them. After various failed plans over teh years.
The current plan is with 8 new engines off bizjets rather than 4 engines off an airliner. That minimizes everything else that needs changing.
This 2-page pdf is a decent potted history of the various re-engining efforts including the current one.
Not quite the same thing. C-130s are still being built new now. The last B-52 was built in 1962.
The current C-130, the J model shares a general shape and size with the original A model. But little else. A common heritage, but the J is more like the grandchild of the early C-130s.
Conversely, the “newest” upgrades to the B-52 are applied to 50 year old aircraft. Instead of sending the C-130 grandchild into battle, we’re giving granddad a pacemaker, 2 new hips, and a modern M4 Carbine to replace his M1 Garand. Good luck Gramps; don’t forget your naptime.
FWIW, the bomber was designed in 1948, and didn’t fly until 1952.
And more interestingly, the Pratt & Whitney TF33 turbofan engines are a retrofit from the original P&W J57 turbojets. So clearly jet engine technology wasn’t quite there when the plane was designed.
And for what it does, it’s perfectly good if a bit on the fuel inefficient side. Grandpa isn’t competing in the aircraft decathlon or anything.
The Air Force is still (or once again) looking for an up-engining solution.
Anyone choosing to read this thread might be interested in this 49 min docu about the Antonov AN-225, the largest operating aircraft in the world and from 2020. The entire video is fascinating but if you wanted to watch a particularly interesting clip, jump to the control systems with wire bundles as big as my leg. Avionics and navigation stuff has always been cutting edge but this was the height of analog.
The late forties early fifties era turbojets did not have the power and reliability to be able to fulfill the performance specifications demanded of the of the proposed bomber.
Their was a genuine proposal during its design phase to have four large turbo props instead. As was done with its contemporary the Tu-95.
The F-117 still lives. A few are kept operational and “primarily” used as stealth agresser opponents for training. Articles from 2019 and 2020.
A really interesting thread.
The approach of stripping back to the airframe and building up from there - does that represent a significant saving over making the entire aircraft from scratch? Or is it more a managerial solution that Fix-our-Existing-Frontline-Planes money is a lot easier to find than Lets-Design-A-New-Plane-From-Scratch or Let’s-Scrap-Old-Planes-And-Buy-New-Ones budgets?
I’m curious why the B-52 seems to be immune to metal fatigue and can last a century while other much newer aircraft have to be retired in 1/3 the time. Stronger ability to withstand pressurization?
Not exactly a retrofit; more of early model vs late model thing.
All the B-52s from the first prototype to the last B-52G produced flew with various flavors of J57s. The last model, B-52H, were all built with TF33s. Only H models survive today.
Interestingly enough, the early engine design for the TU-95 was a dual turboprop rig. Where each of the 4 engines was to be a pair of turboprop compressor/combustor/turbine modules tied to a common gearbox driving the two contrarotating props. So in a sense, that would have been 8 engines turning 8 props installed on 4 concentric shafts housed in 4 nacelles. Much like the B-52 ended up.
Reliability problems in the combining gearbox drove the Soviets to give up that plan early on and design a single much larger gas turbine from scratch. So the TU-95 is truly a 4-engine airplane with each engine swinging a dual contrarotating prop.
That same combining gearbox concept is used a lot now in helicopter gas turbines.
IMO mostly this. Reduced uncertainty, better politics, apparently more bang for the immediate buck no matter how much trouble you’re building up for downstream.
Is it really less expensive for the Air Force to keep B-52s running than to develop a new heavy bomber?
I do not mean a new stealth bomber or supersonic bomber (read expensive). Just a workhorse heavy bomber (read relatively cheap) which the B-52 is and whose value has been clearly shown. It is often stated the planes are now a fair bit older than those who fly them.
I cannot imagine constantly stripping down these planes and replacing stressed airframe parts that are no longer made would be cheaper than a whole new heavy bomber. No need to have the manufacturers reinvent the wheel. Have the requests for proposal demand everything is off the shelf parts except for a new airframe. I mean, they clearly have all the stuff now which lets the B-52 fly. Just re-purpose that stuff.
IIRC the B-52 is even a little small by today’s standards (relatively speaking…it is certainly a big plane with a large payload). Surely we can do better.
AIUI, the Air Force has taken an approach that it’s simpler and easier to just build one bomber type to face all problems, rather than make a cheap flying bomb truck like the B-52. They are going with the B-21 Raider, which is basically another B-2, but a bit more advanced. It’s overkill for carpet-bombing the Taliban in Afghanistan, but it might save more money than pursuing multiple bomber variants.
Many fewer cycles (pressurization and take-off/landing). LSL Guy and Richard Pearce have touched on this before. A B-52 may fly a 12 - 15 hours mission but it’s just one cycle. Then, maybe again the next week. A commercial jet like a 747 is similar with maybe 1/2 flights a day. A Southwest Airlines 737 may have 6-8 cycles or more a day and repeat for the entire week [pre-pandemic]. As for pilot proficiency, a large military aircraft like a B-52 or C-5A will make the required take-off and landing requirement but not actually touch down - tires are expensive and labor intensive to replace. They won’t cycle to pressurization altitude.
And gradual replacement of critical parts like your grandfather’s axe that’s gone through 3 handles and 4 heads but is still “original”.
Yes, it is. There is a lot of overhead in design and requirements engineering; compatability and interface definitions; materials design, specifications, and testing; environmental and safety regulations; and contractual obligations to start with, if you’re a government contractor*–and that’s all before the prototyping and testing costs–to start from scratch. It’s more cost effective to maintain, upgrade, and recycle parts than start an entire new redesign.
Or you could be Elon Musk and dispense with all that crap and just start assembling things.
Tripler
*Note: This is not an all inclusive list.