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Old 24th Oct 2022, 04:02
  #41 (permalink)  
 
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Another reason for keeping 8 engines was the design of the systems to get multiple redundancy from several sources for hydraulics, electrics, air etc.

Losing 1/2 of the electrics for an application is better than losing it all.
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Old 24th Oct 2022, 20:56
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Originally Posted by ORAC View Post
That would mean a total pylon/ wing redesign along with hydraulics, electrics etc along with flight testing for engine out performance etc.
They opted for swap 8 for 8 to minimise the changes required.

https://theaviationist.com/2022/09/2...nacelles-test/

https://www.airandspaceforces.com/ar...-for-the-b-52/


While USAF once considered four large-fan commercial engines instead, it stuck with eight to avoid substantial redesign of the wing, cockpit, and other components, and to minimize risk and delay.
At the end of the day it is always cost vs. benefit. The advantage of 4 bigger modern engine compared to 8 modern Business Jet engines won't be terribly big. And for this little extra advantage you have to redesign A LOT of systems and validate structure and aeroelasticity of the wing and in the worst case make structural changes to the wing structure. And have less redundancy plus potential directional control problems in case of engine failure of the outer engines. And all this for <5% less fuel burn (0.657 vs 0.627 lb/lb/h cruise sfc RR725 vs CFM56-7).
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Old 13th Jan 2023, 06:44
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Originally Posted by henra View Post
At the end of the day it is always cost vs. benefit.
Looking just at the thrust numbers of the new B-52's and the B-2's engines (and assuming the latter will be similar in B-21) I wonder why they decided not to use the Raider's engines as replacements.

Last edited by Bahrd; 13th Jan 2023 at 09:06.
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Old 13th Jan 2023, 07:30
  #44 (permalink)  
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Still speculation as to whether the B-21 has 2 or 4 engines, let alone which type they are.
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Old 13th Jan 2023, 09:06
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"I wonder why they decided not to use the Raider's engines as replacements."

IF they are different its probably because the B21 engines have to fit in a stealth airframe and environment

The B-52 is so unstealthy you just fit the ones that are the best deal on cost, maintainability, economy etc as it makes no effective difference to the radar signature
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Old 13th Jan 2023, 11:38
  #46 (permalink)  
 
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Originally Posted by ORAC View Post
Still speculation as to whether the B-21 has 2 or 4 engines, let alone which type they are.
I think four engines in B-21 are more likely (B-2 has four, and one of the reason B-52 has (and will have) eight engines is the redundancy (both are strategic bombers) and "rudder authority" issues (in case of an engine failure)).
Originally Posted by Asturias56 View Post
IF they are different its probably because the B21 engines have to fit in a stealth airframe and environment
From the available sources (like e.g. Northrop Grumman B-21 Unveiling May Answer Lingering Mysteries | Aviation Week Network and Air Force Unveils New B-21 Stealth Bomber After Seven Years in the Making - Defense One) one can learn that B-21 has been designed with modularity in mind and with the help of as many off-the-shelf components as possible.

I am sure US AF, together with Northrop Grumman and Boeing, have examined the single engine type option in both bombers and just wondered what the reason would be not to choose it.
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Old 13th Jan 2023, 11:53
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There are references to F135 derived engines in B-21 that would suggest only 2 are required.

I believe ground clearance is a major issue with a 4-engined B-52.
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Old 13th Jan 2023, 12:42
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If the B21, which has no fin and no rudder, is a twin, the single engine handling case must have been a significant design challenge!
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Old 13th Jan 2023, 14:10
  #49 (permalink)  
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Not sure why, two centrally mounted engines with a centreline exhaust shouldn’t prove a major issue with a single engine failure.

Might need a titanium divider between them though - as in the F-111.
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Old 13th Jan 2023, 17:30
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""one of the reason B-52 has (and will have) eight engines is the redundancy"

no - when it was designed/built (70 years ago) they needed 8 engines of the time to make it work
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Old 13th Jan 2023, 18:54
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Originally Posted by unmanned_droid View Post
I believe ground clearance is a major issue with a 4-engined B-52.
When I was involved in a B-52 re-engine exercise in the early 2000's, the leading contenders were the PW2000 and RB211-535 (i.e. 757 engines) - one per pod. While ground clearance was a concern, we had ways of dealing with it (one idea was getting rid of the outrigger gear on the wingtip and incorporating something into the outboard engine nacelles).
According to 'former' poster Ken V, the big issue with going with four engines was messing up the ability to carry wing mounted weapons.
Engine maintenance was not considered to be much of a concern. The time-on-wing between overhauls is so high on modern engines (15-20,000 hours between overhauls being pretty common) that the airframe would wear out before the engines...
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Old 13th Jan 2023, 23:05
  #52 (permalink)  
 
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Originally Posted by ORAC View Post
That would mean a total pylon/ wing redesign along with hydraulics, electrics etc along with flight testing for engine out performance etc.

They opted for swap 8 for 8 to minimise the changes required.

https://theaviationist.com/2022/09/2...nacelles-test/

https://www.airandspaceforces.com/ar...-for-the-b-52/


While USAF once considered four large-fan commercial engines instead, it stuck with eight to avoid substantial redesign of the wing, cockpit, and other components, and to minimize risk and delay.
believe many eons ago they were offered jt8-219s for reengine. Too expensive, gas is cheap.
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Old 14th Jan 2023, 19:41
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Yeah, I'd say having to incorporate landing gear into nacelles represents a ground clearance issue and extensive engineering effort in the solution. It sounds like a bad solution to me.

You are correct, weapons carriage on wing was another issue that was mentioned.
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Old 14th Jan 2023, 20:09
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Originally Posted by unmanned_droid View Post
Yeah, I'd say having to incorporate landing gear into nacelles represents a ground clearance issue and extensive engineering effort in the solution. It sounds like a bad solution to me.
The outrigger gear on the BUFF wingtips are not intended to carry significant load - they are basically there just to support the wing statically - the main body gear are intended to take all the landing loads (they don't even touch the ground unless the wing is carrying lots of fuel). So in short, the structural requirements were not that big of deal - the mass and thrust of the engines was a far more significant design consideration (for example, the gyroscopic loads of a rapidly spinning turbofan engine at rotation are massive).
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