1) Should the B52 have 4 or 8 Engines?

2) What should the replacements be?

I believe the GE Passport BizJet Engine would be a wonderful replacement for the 8 Guzzling Tubes currently on the BUFF: "It is designed to produce 10,000 to 20,000 pounds-force (44 to 89 kN) of thrust for large business jets and regional jets, in place of the General Electric CF34. It is selected to power the Bombardier Global 7000/8000."

http://www.airpowerstrategy.com/2016/11 ... w-engines/

A post from this thread,

viewtopic.php?f=10&t=1027655&start=50

It is a good analysis.

Slug71 wrote:

http://www.airpowerstrategy.com/2016/11/22/old-dog-new-engines/

A post from this thread,

viewtopic.php?f=10&t=1027655&start=50

Dutchy wrote:

It is a good analysis.

It appears to be good but it notes the re-engine idea does makes sense. It basically states that the re-engining only makes sense if it removes all TF33 engines from service, which certainly makes sense. But it seems to imply you can't do that because it is in service on other aircraft, however as near as I can tell the number of "other aircraft" still in use with it is very small now. And that could certainly be addressed by extending any refit developed for the BUFF to those few aircraft as well or simply allowing them to age out as they are set to do without upgrades. (It also speaks to the upper wing surface service life which is also valid but a separate issue really.)

Tugg

A couple of things here:

As quoted in the linked article, and as I've seen elsewhere, the B-52 has issues with directional stability in the Yaw at low speed. Going from 8 engines to 4 engines represents a massive change in the single engine out power characteristics of the airframe and will MASSIVELY increase the scope of this project. It is almost an absolute neccessity to the costliness of this project for it to remain an 8 engine bomber.

Accepting that the 52 must remain an 8 engine bomber, we have to keep the engines relatively similar to the existing ones in their major dimensions.

The TF33 is:
Length: 142 in (3607 mm)
Diameter: 54 in (1371.6 mm)
Dry weight: 4650 lb (2109 kg)

the original J57s were a bit longer, and about 4200 lbs.

So, any replacement engine should be less than or equal to those engines in mass to be an "easy" installation onto the existing pylons. Why? Any additional weight must be accounted for in the wing structure, requiring that the wings be torn up and extra reinforcements to be added. This will increase weight further, reducing the gains by the increased engine efficiency. This will drastically increase cost as it will require a lot of design and engineering work and lots of labor for each bomber. It will also require an extensive testing program, which will add many extra hours to the airframes that they just don't have to spare if they are to go to 2040.

Any new engines must have an outside fan diameter that is less than the current engines. This is to keep ground clearance to the same or greater amount to the existing engines, reducing the testing and engineering load and retaining program cost benefits. If the diameter is reasonably close, it can also make some of the aerodynamic work on the new nacells easier. If they can get close enough, they may be able to just reconstruct the internal structure of the nacells without altering the exterior of them, also saving a lot of money.

The engine should be currently in productionfor new aircraft. We don't need another long testing and teething program, we need something that's already flight proven. We need something that doesn't have a problem with staying in storage for long periods of time and then making a long distance flight. We need something that has long overhaul cycles to keep maintenance costs low.

While I've seen suggestions for the GE passport, that engine is still not in regular commercial use. Its definitely an innovative and efficient engine, and it's also roughly of the dimensions that are needed, but its got very little wing time, and that's only in a testing environment as far as I can tell.

For western engines, this leaves just one engine family, the RR BR700. Specifically, the BR700-725. The critical stats of the BR725 are:
Length: 130 inches
Weight 3600 lbs
Fan Diameter: 50 inches.
This engine has a lot of pylon time on the Gulfstream 650. The life of a business jet is often similar to a bomber, with lower total hours on average, long waits between flights, and long flights after those waits.

The engine is significantly lighter than the TF33, by about 1000 lbs each. This will reduce the dry weight of the plane by about 4 tons, returning take off performance that was lost when the TF33s were mounted. The reduced weight will also improve overall efficiency as additional weight requires extra fuel to lift to altitude. This will give fuel savings over and above just the improvement to SFC that the new engines will bring. Also, a reduction in empty weight will improve cruise fuel consumption as well, but by a rather small amount.

The efficiency improvements will easily extend the ferry range to 10,000+ nautical miles, basically eliminating any needs for IFR for any ferry missions between ay two US airforce bases. This ferry range is essential globe spanning as, for example, the farthest airport from Barksdale AFB is just over 10,000 miles away in Australia. Mission range will be a bit over 5,000 nautical miles, which allows an unrefueled strike to over 95% of the populated land mass of the planet from the closest AFB to them. As has been discussed many times, tanker fuel is massively more expensive than ground loaded fuel, and to esentially never have to use it would both be a wartime boon and a financial savings. For loiter missions, the loiter time will be up to or above 18 hours (based on range and cruise speed). But its not just the range that will improve, it will also be the reduced fuel expense. While fuel is currently cheap, it may not always be. As we saw less than a decade ago, it can get quite expensive, and the savings can be substantial, even for low frequency use.

The other advantage that the BR725 will bring is maintenance support. Gulfstream 650s are still rolling off the assembly line with new BR725s. Those planes will have an expected service life of 30 years, meaning engine support for that long. Also, the DOD is not unfamiliar with the BR700 series. The C-37B (gulfstream 550) is currently used by the USAF, USN, and US Army and currently has the highly related BR710. There is precedent for the DOD to use this engine family.

Choosing any other engine for this program would be a costly mistake.

LightningZ71 wrote:

A couple of things here:

As quoted in the linked article, and as I've seen elsewhere, the B-52 has issues with directional stability in the Yaw at low speed. Going from 8 engines to 4 engines represents a massive change in the single engine out power characteristics of the airframe and will MASSIVELY increase the scope of this project. It is almost an absolute neccessity to the costliness of this project for it to remain an 8 engine bomber.

The analysis when the USAF evaluated re-engining the B-52 with 4 large turbofan engines suggested a way around it; derate all of the engines by 75% (which also improves engine life), and use the FADEC and the avionics to detect when there is an engine out scenario, to automatically advance the throttles on the failure side to 100% thrust, and derate the engines on the other side to compensate.

The original Boeing proposal to the USAF was for a four engined variant using the RB211-535E4B engine and updated avionics to counter any engine-out scenario.

The proposal was rejected after the USAF used the wrong data to analyse cost savings against program costs. The price of fuel at the pump was used and not fuel offloaded from a tanker in flight, which is much higher and would be saved by the new engine.

Channex757 wrote:

The original Boeing proposal to the USAF was for a four engined variant using the RB211-535E4B engine and updated avionics to counter any engine-out scenario.

The proposal was rejected after the USAF used the wrong data to analyse cost savings against program costs. The price of fuel at the pump was used and not fuel offloaded from a tanker in flight, which is much higher and would be saved by the new engine.

I would simply dust off that proposal, and update the engine to the PW F117 engine, as used on the C-17. That way, the engine can be shared with the C-17, meaning that the USAF doesn't have to operate yet another engine depot for another engine type.

Slug71 wrote:

http://www.airpowerstrategy.com/2016/11/22/old-dog-new-engines/

A post from this thread,

viewtopic.php?f=10&t=1027655&start=50

This article convinced me that a 4 engine solution - which I thought was the way to go - is the more expensive one due to airframe limitations and thus cost or risk.

The problem with the 4 engine re-engine is it is an engineering nightmare! First of all, the H model has barely enough rudder control to handle 4 engines out on one side. Now we are going to exacerbate the problem with higher thrust engines. The rudder, in that case, will have to be modified.

The other problem is the pylons will have to be redesigned. The current pylons will not allow for the installation of large high bypass fans on them, they aren't strong enough for that.

Finally, the USAF/DOD will have to address the issue of the upper wing skin. Right now, that is the limiting factor on the remaining life of the Buff. If the aircraft is re-engined, the that would be the time to do it. Otherwise, I hate to say it, the boneyard is the only real option.

LightningZ71 wrote:

A couple of things here:

As quoted in the linked article, and as I've seen elsewhere, the B-52 has issues with directional stability in the Yaw at low speed. Going from 8 engines to 4 engines represents a massive change in the single engine out power characteristics of the airframe and will MASSIVELY increase the scope of this project. It is almost an absolute neccessity to the costliness of this project for it to remain an 8 engine bomber.

Accepting that the 52 must remain an 8 engine bomber, we have to keep the engines relatively similar to the existing ones in their major dimensions.

The TF33 is:
Length: 142 in (3607 mm)
Diameter: 54 in (1371.6 mm)
Dry weight: 4650 lb (2109 kg)

the original J57s were a bit longer, and about 4200 lbs.

So, any replacement engine should be less than or equal to those engines in mass to be an "easy" installation onto the existing pylons. Why? Any additional weight must be accounted for in the wing structure, requiring that the wings be torn up and extra reinforcements to be added. This will increase weight further, reducing the gains by the increased engine efficiency. This will drastically increase cost as it will require a lot of design and engineering work and lots of labor for each bomber. It will also require an extensive testing program, which will add many extra hours to the airframes that they just don't have to spare if they are to go to 2040.

Any new engines must have an outside fan diameter that is less than the current engines. This is to keep ground clearance to the same or greater amount to the existing engines, reducing the testing and engineering load and retaining program cost benefits. If the diameter is reasonably close, it can also make some of the aerodynamic work on the new nacells easier. If they can get close enough, they may be able to just reconstruct the internal structure of the nacells without altering the exterior of them, also saving a lot of money.

The engine should be currently in productionfor new aircraft. We don't need another long testing and teething program, we need something that's already flight proven. We need something that doesn't have a problem with staying in storage for long periods of time and then making a long distance flight. We need something that has long overhaul cycles to keep maintenance costs low.

While I've seen suggestions for the GE passport, that engine is still not in regular commercial use. Its definitely an innovative and efficient engine, and it's also roughly of the dimensions that are needed, but its got very little wing time, and that's only in a testing environment as far as I can tell.

For western engines, this leaves just one engine family, the RR BR700. Specifically, the BR700-725. The critical stats of the BR725 are:
Length: 130 inches
Weight 3600 lbs
Fan Diameter: 50 inches.
This engine has a lot of pylon time on the Gulfstream 650. The life of a business jet is often similar to a bomber, with lower total hours on average, long waits between flights, and long flights after those waits.

The engine is significantly lighter than the TF33, by about 1000 lbs each. This will reduce the dry weight of the plane by about 4 tons, returning take off performance that was lost when the TF33s were mounted. The reduced weight will also improve overall efficiency as additional weight requires extra fuel to lift to altitude. This will give fuel savings over and above just the improvement to SFC that the new engines will bring. Also, a reduction in empty weight will improve cruise fuel consumption as well, but by a rather small amount.

The efficiency improvements will easily extend the ferry range to 10,000+ nautical miles, basically eliminating any needs for IFR for any ferry missions between ay two US airforce bases. This ferry range is essential globe spanning as, for example, the farthest airport from Barksdale AFB is just over 10,000 miles away in Australia. Mission range will be a bit over 5,000 nautical miles, which allows an unrefueled strike to over 95% of the populated land mass of the planet from the closest AFB to them. As has been discussed many times, tanker fuel is massively more expensive than ground loaded fuel, and to esentially never have to use it would both be a wartime boon and a financial savings. For loiter missions, the loiter time will be up to or above 18 hours (based on range and cruise speed). But its not just the range that will improve, it will also be the reduced fuel expense. While fuel is currently cheap, it may not always be. As we saw less than a decade ago, it can get quite expensive, and the savings can be substantial, even for low frequency use.

The other advantage that the BR725 will bring is maintenance support. Gulfstream 650s are still rolling off the assembly line with new BR725s. Those planes will have an expected service life of 30 years, meaning engine support for that long. Also, the DOD is not unfamiliar with the BR700 series. The C-37B (gulfstream 550) is currently used by the USAF, USN, and US Army and currently has the highly related BR710. There is precedent for the DOD to use this engine family.

Choosing any other engine for this program would be a costly mistake.

If a re-engining project had been undertaken in the 70's, 80's, or 90's, these issues would have been addressed. Now that the topic is being revisited in the 2010's, it's necessary to reduce the developmental risk and potential for cost escalation. The shorter tails of the G and H models probably makes developing a quad engined variant of the B-52 more difficult.

Considering the fact that the engines are arranged with two engines hanging off each pylon, I can't understand why the B-52 was not designed to handle the case of an uncontained engine failure of one of a pair of engines causing the failure of the other engine in the pair.

Dutchy wrote:

Slug71 wrote:

http://www.airpowerstrategy.com/2016/11/22/old-dog-new-engines/

A post from this thread,

viewtopic.php?f=10&t=1027655&start=50

This article convinced me that a 4 engine solution - which I thought was the way to go - is the more expensive one due to airframe limitations and thus cost or risk.

Same.

The initial design of the B-52 had a taller vertical stabilizer which could provide more yaw control. Coupled with lower thrust engines, the effect from an uncontained engine failure taking out a second engine was much lower with respect to yaw control.

Yes, a fadec system can auto compensate for an engine failure, however, that whole setup brings us to another issue. The TF33 has 17,000 lbs of thrust. A replacement 4 engine setup will need to produce 34,000 lbs of thrust. If that's after a 25% derate then they needed to be rated for about 45,000 lbs before the derate. What is currently in production in the west in that thrust range? Not much. The largest single aisle commercial engines are over 10,000 lbs short on thrust. The smallest widebody engines are too large in thrust, weight, and diameter. You'll either sacrifice reserve engine out thrust, or have to develop a new engine. Both are unacceptable.

The BR715 is as close to a drop in as it gets.

LightningZ71 wrote:

The initial design of the B-52 had a taller vertical stabilizer which could provide more yaw control. Coupled with lower thrust engines, the effect from an uncontained engine failure taking out a second engine was much lower with respect to yaw control.

Yes, a fadec system can auto compensate for an engine failure, however, that whole setup brings us to another issue. The TF33 has 17,000 lbs of thrust. A replacement 4 engine setup will need to produce 34,000 lbs of thrust. If that's after a 25% derate then they needed to be rated for about 45,000 lbs before the derate. What is currently in production in the west in that thrust range? Not much. The largest single aisle commercial engines are over 10,000 lbs short on thrust. The smallest widebody engines are too large in thrust, weight, and diameter. You'll either sacrifice reserve engine out thrust, or have to develop a new engine. Both are unacceptable.

The BR715 is as close to a drop in as it gets.

The Pratt F117 as found on the C-17 does 40,400 lb; later versions of the PW2000 series engines, the PW2043, does 43,000 lb of thrust. That's pretty close, and it is an in service engine with the USAF.

The Rolls Royce Trent 500 is another more powerful option; the engine was originally certified to 60,000lb of thrust, but early versions that powered the A340-500 and A340-600's were derated to 53,000lb and 56,000lb respectively.

Remember, the original proposal that used a 4 engine configuration that was rejected by the USAF used the Rolls Royce RB211-535E4B, which was rated at 43,100 lb of thrust. Obviously, if Boeing thought they could make it work with that engine, they could probably make it work with the Pratt F117 (I would probably even share the same pylon as the C-17's for more commonality there).

Regardless of the engine choice, there will be changes to the avionics to control the engines. They could program in the throttle management system for engine out scenario's at the same time as re-configuring the avionics.

The F117 is also at the end of its lifecycle. What is it currently being deployed on? Also look at its price. The C-17 program was playing north of 8 million a pop for them. BR700s go for less than 1/3rd of that. That's a potential cost difference of over 600 million dollars just for engines.

Thanks for that LightningZ71! It does seem like a good fit. Is there a power difference?

LightningZ71 wrote:

The F117 is also at the end of its lifecycle. What is it currently being deployed on? Also look at its price. The C-17 program was playing north of 8 million a pop for them. BR700s go for less than 1/3rd of that. That's a potential cost difference of over 600 million dollars just for engines.

The F117 also is in the USAF's inventory on the C-17, and the PW2000 series engines also exist on the C-32's. As such, Pratt will continue to support the engine for the life of the C-17 program, which will be for decades to come. Plus, the USAF already has a depot and maintenance personnel trained on the F117 engine; they don't have that capability with the BR715, and thus would be required to stand up a depot to maintain the BR715's.

The cost are not out of line compared to other aircraft engines; the CFM56-7 is about $11 million per aircraft. I actually doubt the numbers you are quoting for the BR715; I'm reading the unitary cost of the engine to be closer to $7 milllion each. And you are buying 8 of them, instead of 4 F117's.

You guys don't seem to understand some basic aero stuff. While the F117/PW2000 is a great engine, putting it on a B-52 would require a massive amount of engineering work for the redesign of the rudder. The H with the TF33 engines is barely capable of handling an situation where all engines are out on one side. That is because the G/H are not "tall tails". It's been a long time since I've flown a buff, but the D had a taller tail and more rudder area, less thrust put out by the engines and it could handle a 4 engine out situation. The G had no problem since it was a J-57, the rudder was also hydraulically powered. The H was right on the edge and putting 2 F117 engines on one side, the H would not be able to handle the additional thrust. The pylons would have to have a major redesign also. The electrics and hydraulics would also have to have a major rework. Then unless the upper wing skin is addressed, you have a very short lifespan for such a large capital investment.

https://www.youtube.com/watch?v=vHdIRwKtnig

If Boeing have a compelling case for the re-engine then why not invest for the returns it would bring? Less money spent on overhauls, tankers and a reduction in AOGs.

Maybe a solution with similar-thrust CF34s can be developed. Boeing did favour the reliability and on-wing time of the RB211 though.

I do have to Yield to Point blank on price numbers on the BR. The numbers that I was using appear to have been a decade or more old and we're referencing an earlier, lower thrust version of the BR700. Though, the numbers on the F117 I was using are also just as old.

My point for this is that the military is at this time looking at divesting all of its F117 using birds, except for the C-17, in the next decade or less. This will reduce fleet commonality. Also, while not in house for Depot maintenance, there is an existing maintenance program for the br725s and institutional experience with them.

No choice is ideal, but the BR725 is, I think, the most logical choice from a program cost standpoint. Remember, if you go with the heavier and larger f117, you have to replace almost all of the ground handling equipment for engine service on the 52s. That is not trivial.

Here's a thought. What about the GE Passport?

They are being built at a thrust rating of 18,000 pounds dry (naturally), and GE have stated there is growth potential if needed. Using the Passport would on-shore the project with local construction of the engine.

mmo wrote:

You guys don't seem to understand some basic aero stuff. While the F117/PW2000 is a great engine, putting it on a B-52 would require a massive amount of engineering work for the redesign of the rudder. The H with the TF33 engines is barely capable of handling an situation where all engines are out on one side. That is because the G/H are not "tall tails". It's been a long time since I've flown a buff, but the D had a taller tail and more rudder area, less thrust put out by the engines and it could handle a 4 engine out situation. The G had no problem since it was a J-57, the rudder was also hydraulically powered. The H was right on the edge and putting 2 F117 engines on one side, the H would not be able to handle the additional thrust. The pylons would have to have a major redesign also. The electrics and hydraulics would also have to have a major rework. Then unless the upper wing skin is addressed, you have a very short lifespan for such a large capital investment.

Boeing didn't think it was an insurmountable issue when they originally made the proposal to the USAF to reengine the B-52 with 4 Rolls Royce RB211 engines in 1996. Pratt also did a study to reengine the B-52's back in the 1980's with the PW2000 engine as well.

From what I've read of the topic, all of options used the FADEC and the avionics to compensate in an engine out scenario by altering thrust levels on all of the engines automatically (the earlier Pratt proposal was different as they mounted 2 PW2000's on a single pylon). Since the flight deck avionics will need reconfiguration anyways to accommodate a new engine, this should not be something that isn't expected anyways.

Theoretically, what the USAF could do is take advantage of the fact that the Boeing 757 is retiring for many users and harvest the engines and pylons off of the retired aircraft. Perhaps the Delta and United fleets would be a good candidate as they are Pratt powered aircraft, and thus harvesting the engines and pylons off of retiring 757's, refurbishing them would provide considerable cost savings at acquisition. Then, the B-52's would have an engine option that is common with another large aircraft fleet, reducing the costs of engine overhauls by not duplicating depots.

I did not say it was insurmountable, it becomes very expensive. I don't know how much time you have flying a B-52, but I have just over 2500 in a D, G and H. So, I might not be an expert, but I am well versed in the flight characteristics.

First of all, I am not talking about one engine out, but all engines on one side. The H can barely handle the Vmca problem with the TF33s. So mounting F117s, or any other engine like that becomes problematic. The biggest issue is the limited life remaining on the upper wing skin. The fatigue on that area is the limiting factor and unless that is addressed, you are throwing money away. In addition, the electrics, hydraulic and pneumatic systems will have to be redone no mater what engine is selected, if at all.

As much as I enjoyed flying that aircraft, it's time to send it to the boneyard.

mmo wrote:

I did not say it was insurmountable, it becomes very expensive. I don't know how much time you have flying a B-52, but I have just over 2500 in a D, G and H. So, I might not be an expert, but I am well versed in the flight characteristics.

First of all, I am not talking about one engine out, but all engines on one side. The H can barely handle the Vmca problem with the TF33s. So mounting F117s, or any other engine like that becomes problematic. The biggest issue is the limited life remaining on the upper wing skin. The fatigue on that area is the limiting factor and unless that is addressed, you are throwing money away. In addition, the electrics, hydraulic and pneumatic systems will have to be redone no mater what engine is selected, if at all.

As much as I enjoyed flying that aircraft, it's time to send it to the boneyard.

The problems with engine out scenario will also be magnified by any proposed 8 engine reengining. All of the talked about engines (Rolls Royce BR715, GE's CF-34's, etc) have more thrust than the old TF33's, so whatever the engine configuration is, the problems are just going to get magnified. Some sort of engine out management system that involves altering the engine thrust levels automatically is going to be needed, regardless.

The current TF-33 engines are rated at 17,000 lbs of thrust. The RR BR725 is rated at 16,900 lbs and, according to RR, is software upgradeable to higher thrust ratings. The multiple engine out scenario on one side is a tough one for the H to manage. The problems with changing to a four engine configuration are not total thrust related though. Larger Turbofan engines can be derated in software to evenly match the total thrust of two TF-33s at 34,000 lbs. Thrust is not the issue. The problem here is two fold: How likely is a total engine loss on one side and what is the post loss drag like?

With 4 engines, a single uncontained turbine failure event in the #2 or #3 engine can easily spew engine parts into the intake flow of the outboard engine. While the outboard failing can also potentially do the same, the likelihood of that happening is much lower. In an 8 engine arrangement with smaller engines, a failure of the outboard pod outboard engines is largely shielded from the rest of the plane by the outboard pod inboard engines. Failure of the outboard pod, inboard engine is less likely to throw parts forward, and the inboard pod inboard engines is protected by the inboard pod outboard engine. The possibly worst case failure would be the inboard pod outboard engines, but the outboard/outboard engine is still relatively protected in that failure. Failure of the inboard/inboard engine is much more likely to throw parts into the fuselage and has its own risks, however, parts from a smaller engine are more likely to do less damage on a mass basis alone.

The other issue is drag. With a four engine setup, failure of any of the engines will result in considerably more drag than using eight smaller engines. In a dual engine out scenario, you've got considerably more drag on one side than in the current setup, which will require opposite side engines to provide even more thrust to keep the aircraft in the air, exacerbating the thrust imbalance. No matter how you do it, going to a four engine setup will require a significant modification of the tail of the aircraft that will not only be expensive, but will also require considerable additional mass in the form of a larger vertical stabilizer and increased drag from the extra surface area, which will just serve to reduce the benefits of the whole program.

Using a smaller engine like the BR725 or the Passport, should it be far enough along in development, will all but eliminate any need to modify control surfaces as the engines are functionally equivalent replacements for the existing engines while bringing all the benefits that are desired.

Why is the US so keen on keeping the B52? I mean if they wanted they could certainly have come up with an RFP for a new Low Cost Bomber for the Missions the B52 is doing...

I am somewhat astonished that this ancient design still is so much in use despite the well known drawbacks.

TheSonntag wrote:

Why is the US so keen on keeping the B52? I mean if they wanted they could certainly have come up with an RFP for a new Low Cost Bomber for the Missions the B52 is doing...

I am somewhat astonished that this ancient design still is so much in use despite the well known drawbacks.

Couldn't you say that about 90% of all the aircraft in every military on earth? Tu-95's? Mig-Whatevers?, pretty much most of China's aircraft? It's not as cheap to develop new aircraft today as it was back then.

I always thought the new/improved E-8 JSTARS engine ... JT8D-219...would be a 'best option'... but design/integration issues on the aging B-52 struts would still add some complexity. http://www.af.mil/News/Article-Display/ ... on-engine/

Understand that AFMC is done testing P&W improvements to the B-52 TF33 variant for reduced noise, fuel consumption, smoke, etc... and improved maintainability... and that these mods are to be incorporated current TF-33 PDM, starting within a year or two. https://www.flightglobal.com/news/artic ... en-424327/

The BR-715 for the HGW 717 variant at 21,000 lbs would be ideal, and matches the upgraded variant of the TF-33. In fact, I believe this is the model they were looking at for more recent re-engineing proposals. I never heard anything about the BR-725 being proposed.

SCAT15F wrote:

The BR-715 for the HGW 717 variant at 21,000 lbs would be ideal, and matches the upgraded variant of the TF-33. In fact, I believe this is the model they were looking at for more recent re-engineing proposals. I never heard anything about the BR-725 being proposed.

I wouldn't hold your breath.

I think the US government still feels salty (I know i would) about US engines being shut out of the A400M constest, so to make it right I would think they would really only consider an American made engine. Hell we let RR get into the V-22, but got slapped in the face with the A400. Fair is fair.

Andre3K wrote:

SCAT15F wrote:

The BR-715 for the HGW 717 variant at 21,000 lbs would be ideal, and matches the upgraded variant of the TF-33. In fact, I believe this is the model they were looking at for more recent re-engineing proposals. I never heard anything about the BR-725 being proposed.

I wouldn't hold your breath.

I think the US government still feels salty (I know i would) about US engines being shut out of the A400M constest, so to make it right I would think they would really only consider an American made engine. Hell we let RR get into the V-22, but got slapped in the face with the A400. Fair is fair.

Not to nitpick but wasn't the PW turboprop that was not selected to power the A400M, actually a PWC, C standing for 'Canada'? If so I have hard time understanding what you are implying...

ITMercure wrote:

Andre3K wrote:

SCAT15F wrote:

The BR-715 for the HGW 717 variant at 21,000 lbs would be ideal, and matches the upgraded variant of the TF-33. In fact, I believe this is the model they were looking at for more recent re-engineing proposals. I never heard anything about the BR-725 being proposed.

I wouldn't hold your breath.

I think the US government still feels salty (I know i would) about US engines being shut out of the A400M constest, so to make it right I would think they would really only consider an American made engine. Hell we let RR get into the V-22, but got slapped in the face with the A400. Fair is fair.

Not to nitpick but wasn't the PW turboprop that was not selected to power the A400M, actually a PWC, C standing for 'Canada'? If so I have hard time understanding what you are implying...

Touche`. Though don't some significant parts of the engine come from the US?

Andre3K wrote:

SCAT15F wrote:

The BR-715 for the HGW 717 variant at 21,000 lbs would be ideal, and matches the upgraded variant of the TF-33. In fact, I believe this is the model they were looking at for more recent re-engineing proposals. I never heard anything about the BR-725 being proposed.

I wouldn't hold your breath.

I think the US government still feels salty (I know i would) about US engines being shut out of the A400M constest, so to make it right I would think they would really only consider an American made engine. Hell we let RR get into the V-22, but got slapped in the face with the A400. Fair is fair.

Well, the RRs on the V-22 were an Allison project, so pretty much British in name only after they bought the company.

And I suspect that Boeing would be happy to partner with RR if some kind of US cover were needed. The task would require pylon integration along with the nacelle and engine.

I am having a hard time finding a cost on the BR725, I have seen $2M (from an A.net post several years ago). Does anyone know now what it is?

Tugg

I haven't seen which BR 700 that they are proposing, but I personally wouldn't use anything other than the BR 725. The fan diameter is the closest to the TF33, as is the length. It has the best fuel burn numbers of the family, is the youngest, and is rated at 16,900 and of thrust in the gulfstream that is its main current application. That is within 100 lbs of the TF33 in the 52H. Its all about simplifying the development work and it just won't get any simpler than that.

As for price, I saw somewhere that the cost of one for the Gulfstream is around the 4 million mark, but that may have been a lifecycle number and not just a walk away unit cost.

LightningZ71 wrote:

I haven't seen which BR 700 that they are proposing, but I personally wouldn't use anything other than the BR 725. The fan diameter is the closest to the TF33, as is the length. It has the best fuel burn numbers of the family, is the youngest, and is rated at 16,900 and of thrust in the gulfstream that is its main current application. That is within 100 lbs of the TF33 in the 52H. Its all about simplifying the development work and it just won't get any simpler than that.

As for price, I saw somewhere that the cost of one for the Gulfstream is around the 4 million mark, but that may have been a lifecycle number and not just a walk away unit cost.

Would you be cool with a deal where all of the engines are manufactured here under license?

Andre3K wrote:

LightningZ71 wrote:

I haven't seen which BR 700 that they are proposing, but I personally wouldn't use anything other than the BR 725. The fan diameter is the closest to the TF33, as is the length. It has the best fuel burn numbers of the family, is the youngest, and is rated at 16,900 and of thrust in the gulfstream that is its main current application. That is within 100 lbs of the TF33 in the 52H. Its all about simplifying the development work and it just won't get any simpler than that.

As for price, I saw somewhere that the cost of one for the Gulfstream is around the 4 million mark, but that may have been a lifecycle number and not just a walk away unit cost.

Would you be cool with a deal where all of the engines are manufactured here under license?

Why would the USAF want to just keep the same thrust levels with replacement engines for the B-52 fleet? Boosting the thrust would improve runway and climb performance.

They already don't use the full available thrust of the existing TF33s due to issues with the vertical stabalizer. The G and H model B-52s have a shortened vertical stabalizer due to the emphasis their design had on low level penetration bombing. That reduced the available yaw authority of the the vert. stab. to a point where a multiple engine out scenario on the same side right at V1 is essentially unrecoverable. By limiting the maximum thrust generated by the engines during take-off, the possible yaw effect of the thrust imbalance is kept within the envelope that the plane can be potentially recovered from. This isn't something that I'm just yanking out of thin air, this has been written about by actual B-52 flight crews.

As it stands right now, the B-52H is volume limited, not weight limited. It can carry almost anything that you can shove in its belly and hang on its pylons and doesn't need any more thrust. Due to the location of the added IR and Optical targeting pod that was added to its chin, it is also limited in maximum cruising speed. This is because it creates regions of transonic airflow around the nose, causing vibration and drag. So, it can't use extra power on take-off, and it can't use it to go any faster during cruise. About the only place where additional engine power might come in handy would be climb performance when at maximum war load, but even then, it is still cruise speed limited. To climb much faster than it currently does, it would need to climb at an unneccessarily steep angle of attack, which just isn't needed. Any additional power is essentially wasted. It's better to keep the listed power on the BR 725 or make a minor software adjustment to precisely match the TF33 power output and keep the engine well within its operational envelope, maximizing its on-wing life.

As for having it license built in the US or built by a NATO ally, I don't personally have a skin in the game. The USAF already uses Gulfstream aircraft for transporting VIPs around that are equiped with BR 700 series engines that were made by RR directly, so this is not without precedent.

Pull the TF33s, make the needed interior modifications to the engine nacells, wire up the new FADEC system and replace the upper wing skin while you're at it. Once you're done, you'll have a bomber that, once fully fueled and loaded with bombs, can essentially cover 95% of the planet from the closest US Air Base without needing to refuel. It will have modern, reliable engines that are both quieter, cleaner, and more fuel efficient than the existing ones.

What's not to love?

LightningZ71 wrote:

They already don't use the full available thrust of the existing TF33s due to issues with the vertical stabalizer. The G and H model B-52s have a shortened vertical stabalizer due to the emphasis their design had on low level penetration bombing. That reduced the available yaw authority of the the vert. stab. to a point where a multiple engine out scenario on the same side right at V1 is essentially unrecoverable. By limiting the maximum thrust generated by the engines during take-off, the possible yaw effect of the thrust imbalance is kept within the envelope that the plane can be potentially recovered from. This isn't something that I'm just yanking out of thin air, this has been written about by actual B-52 flight crews.

As it stands right now, the B-52H is volume limited, not weight limited. It can carry almost anything that you can shove in its belly and hang on its pylons and doesn't need any more thrust. Due to the location of the added IR and Optical targeting pod that was added to its chin, it is also limited in maximum cruising speed. This is because it creates regions of transonic airflow around the nose, causing vibration and drag. So, it can't use extra power on take-off, and it can't use it to go any faster during cruise. About the only place where additional engine power might come in handy would be climb performance when at maximum war load, but even then, it is still cruise speed limited. To climb much faster than it currently does, it would need to climb at an unneccessarily steep angle of attack, which just isn't needed. Any additional power is essentially wasted. It's better to keep the listed power on the BR 725 or make a minor software adjustment to precisely match the TF33 power output and keep the engine well within its operational envelope, maximizing its on-wing life.

As for having it license built in the US or built by a NATO ally, I don't personally have a skin in the game. The USAF already uses Gulfstream aircraft for transporting VIPs around that are equiped with BR 700 series engines that were made by RR directly, so this is not without precedent.

Pull the TF33s, make the needed interior modifications to the engine nacells, wire up the new FADEC system and replace the upper wing skin while you're at it. Once you're done, you'll have a bomber that, once fully fueled and loaded with bombs, can essentially cover 95% of the planet from the closest US Air Base without needing to refuel. It will have modern, reliable engines that are both quieter, cleaner, and more fuel efficient than the existing ones.

What's not to love?

1) Where are you coming up with your information about engine thrust being limited? Both the G and H can use full Mil power for the H and full Wet thrust for the G. You are correct about the tail, but misinformed about the performance. The G/H has a powered tail, while the A-F have a tall tail which is not powered. The powered tail compensated for the shorter tail, thus VMCA is not a factor for engine out scenarios. You do run into a VMCA issue on all engines out on one side, but that is easily resolved by computing the VMCA speed.

2) I am intrigued by your comments about being "volume limited". Could you elaborate on that? Last time I looked, that was not true. Are you saying the H go with full fuel, a full bomb bay and both wing stations full?????

Hey, I was as shocked to find out that as you seem to be here, but, alas, that's where we are. To start with, go to the top of the thread and look at Slug71's link in the second post. It details both of those issues...

http://www.airpowerstrategy.com/2016/11 ... w-engines/

Even if we're arguing the rare edge cases here, one thing that the B-52H is not is lacking thrust. The engines are more than sufficient for any load that the structure is rated for and increasing the available engine thrust to any significant amount will require major internal modifications to the engine pod pylons to handle the increased force, the wing structure to handle the increased force, and will require major reworking of the tail to accommodate the increased asymmetric thrust forces that will result in engine out scenarios. Just doing it in the FADEC and flight management systems likely won't be sufficient due to the likelihood that, if you've increased thrust and MTOW to take advantage of it, you'll also need more thrust available at critical points in the flight just to stay airborne in a controllable state. If not, why did you invest in the extra capability in the first place?

As for being volume limited, I didn't specify, but meant that it can take off with a full load of munitions and sufficient fuel to get airborne and rendezvous with a tanker. The B-52 has a a MTOW of 488,000 lbs and an empty weight of 185,000 lbs. With a listed fuel capacity of just under 48,000 gallons of fuel, which would weigh just under 312,000 lbs (JP4=~6.5 lbs per gallon), it can tank more fuel than it can take off with (312K + 185K=497K, which is greater than 488K lbs) without any weapons load at all. Volume limited means war load in this context, not fuel. Having more efficient engines means that it can get farther with a full war load if needed, or requires less fuel from the tanker to complete the mission. It means that you don't have to fly the vulnerable tanker closer to the enemy that has comprehensive long range A2/AD systems. It means that you don't have to fly the tanker farther away from the air base, wasting its own fuel in the process. I'm sorry that I wasn't specific or clear enough in my original statement.

1) You wrote several things which you are now contradicting yourself on. Oh, where to start.

2) I think you might want to go back and re-read where ever you are getting your information. Using 4 large thrust engines will require a rudder re-engineering as that is where all engines out on one side presents the problem. Using 8 lower thrust engines would allow the existing rudder to be used. The pylons will have to be redesigned no matter what is hung on them. The pneumatic and electrics will have to be redone.

3) In your posting you state "They already don't use the full available thrust of the existing TF33s due to issues with the vertical stabalizer". I am saying you are wrong. The TF-33 and H model can use full rated thrust on all engines if the situation dictates. There are NO restrictions at all. I am not sure where you got that info, but it is dead wrong. Both the G and H can use full rated thrust. The G was actually faster than an H with wet numbers.

4) You wrote, in your first post, "As it stands right now, the B-52H is volume limited, not weight limited." and in the second post you wrote, "As for being volume limited, I didn't specify, but meant that it can take off with a full load of munitions and sufficient fuel to get airborne and rendezvous with a tanker.". That is Weight limited! So, which do you mean?????

When I was on active duty, the 488,000 was actually tested to a 550,000 inflight weight. The alert sorties were all just at 488,000 and there was some concern about the tanker availability, the thought was given to have an increased onload for the first A/R. The aircraft flew just fine up to 550,000 and was actually more stable. The D was a 450,000 MTOW and it was flown to 525,000.

5) Finally, you keep writing about things being done in the FADEC and FMS. The Buff doesn't have those luxuries. The FADEC and FMS is the copilot in the right seat.

I will gladly defer to your operational experience with respect to what you pointed out.

In your experience, did you find that the B-52H was hindered by a lack of thrust in any situation?

As for the FADEC that I'm referring to, I am well aware that the 52H, as it exists today, is still largely a manual plane with respect to engine management. Any re-engining program that uses modern engines will be required to make the significant investment in time, labor and engineering to install a FADEC system and integrate it with all the relevant systems. Modern engines are designed around FADEC systems and derive at least some of their efficiency gains from the finer grained engine management techniques that it brings. Trying to convert them to manual systems just isn't worth the time and effort, both in engineering and qualification.

One of the main reasons I am pushing the BR725 is based on its dimensions and mass. While any change will have engineering considerations, reducing the engine mass while also keeping the major dimensions within a few percent of the existing engines means that you have less risk of needing major changes in wing and pylon structure. Yes, the nacells will need to be redone, but the exterior dimensions can be maintained, meaning that there is no need to do a major airflow analysis on them. The biggest pain will be routing the wiring for the FADEC system. The existing fuel system will be more than sufficient in capacity with respect to tubing routing. Nothing else should have to change (though I'm sure it will all be looked at).

Just for your info, I am extremely familiar with the FADEC/EEC, I only have about 18,000 on the 744, 757, 777, 787 and A-320. So, I have had a look around the systems a couple of times!

With respect to the H and all the other models, personally, I felt it was underpowered at MGW behind a tanker. The reason was refueling was done at 255KIAS, which at heavy weights was on the back side of the power curve. Bumping it up to 280KIAS, made all the difference in the world.

The airspeed restrictions you referred to a few posts ago was not the result of the targeting pod which is not in the nose but the FLIR and EVS turrets in the chin.

So, then the final question would be, is it worth a significant increase in the cost of the project to use increased thrust versions of the br725 (due to the structural engineering work required to safely handle the extra thrust)? Or, do you think that the nearly 8000 lbs of weight saved by switching to engines that are nearly half a ton lighter each will make enough of a difference that the extra thrust won't be needed?

LightningZ71 wrote:

So, then the final question would be, is it worth a significant increase in the cost of the project to use increased thrust versions of the br725 (due to the structural engineering work required to safely handle the extra thrust)? Or, do you think that the nearly 8000 lbs of weight saved by switching to engines that are nearly half a ton lighter each will make enough of a difference that the extra thrust won't be needed?

I've got a feeling that there might actually be a need to ballast the engine nacelles, otherwise, it could cause problems with flutter with the wing control surfaces and problems structurally as you change the load balance.

LightningZ71 wrote:

So, then the final question would be, is it worth a significant increase in the cost of the project to use increased thrust versions of the br725 (due to the structural engineering work required to safely handle the extra thrust)? Or, do you think that the nearly 8000 lbs of weight saved by switching to engines that are nearly half a ton lighter each will make enough of a difference that the extra thrust won't be needed?

Again, the current TF-33s provide enough thrust so having additional thrust isn't a requirement. It would add more problems when you consider the rudder issues and the engineering required to make the required changes to handle the additional thrust. The money should go to reskinning the upper wing. That is the limiting factor on the life of the aircraft. Reskinning would essentially give you an aircraft with lots of life left.

I happen to whole heartedly agree with the reskinning. You're going to have to have a lot of the wing opened up to run the wiring for the engines in the first place, its the perfect time to do the upper wing as well.

Wtih respect to the mass change on the engines having a negative effect on the wing flutter characteristics of the airframe, instead of just adding straight ballast to the pods, could the weight be put to more constructive use? Perhaps adding a layer of thermal insulation to the inside lining of the nacells to reduce the thermal signature from the sides and below? Or, could a layer of splinter armor be added between the engine itself and the outer nacell skin to reduce the effects from the explosion of a nearby annular blast fragmentation warhead or to reduce the damage caused by an uncontained turbine failure? You're looking at between 800 and 1000 lbs per engine to play with, surely something at least marginally useful could be done with it?

LightningZ71 wrote:

The initial design of the B-52 had a taller vertical stabilizer which could provide more yaw control. Coupled with lower thrust engines, the effect from an uncontained engine failure taking out a second engine was much lower with respect to yaw control.

Yes, a fadec system can auto compensate for an engine failure, however, that whole setup brings us to another issue. The TF33 has 17,000 lbs of thrust. A replacement 4 engine setup will need to produce 34,000 lbs of thrust. If that's after a 25% derate then they needed to be rated for about 45,000 lbs before the derate. What is currently in production in the west in that thrust range? Not much. The largest single aisle commercial engines are over 10,000 lbs short on thrust. The smallest widebody engines are too large in thrust, weight, and diameter. You'll either sacrifice reserve engine out thrust, or have to develop a new engine. Both are unacceptable.

The BR715 is as close to a drop in as it gets.

Actually, the TF33 Fn is 21,400 lbf.. The BR715 would be a Fn drop in, but would require a new Nacelle, which is why RR is proposing a BR725, even though I is a 4K defecate in Fn, per engine.

New Nacelle requirements has made many of the re-engining offering in the past, a bad business case. And, even RR is not going to invest in a derivative of either engine for a potential of a 400 engine sale, tailored to just the B-52, unless they see this as their opening into the USG's big engine military market.

The replacement engine on the B-52 should be whatever engine is in the B-21. To deal with the issues around the engine swap the plane should be redesigned to look like a B-21 as well. It should probably get the same internal systems too.

Honestly though, if having bombers is important just buy enough B-21 to replace the lot of them and move on. The B-52 is a great plane but it's time to move on. The more B-21 we buy the cheaper their operating and acquisitIon cost are likely to be. We can't keep buying bombers in tiny batches forever. Commit to buying a dozen or so a year for the next 10-15 years, give the program some certainty.

If a B-52M project was undertaken, then maybe some of the best examples (stored or not) could go through a remanufacture with a composite upper wing skin treatment as well as engines. There are certainly better Al-Li alloys out there now if composites are a stretch too far.

I would compare it to the C-5M rework that seems to have been a success so far. There isn't a need to reinvent the wheel when the B-52 is a pretty good plank to hang your munitions off, so investment in that would cut a lot of development costs out. Weight on the wing seems to be an issue that doesn't need to over-complicate matters, so a lighter engine type would make sense.

The GE Passport would tick the boxes for me, but if that's a bit too advanced what about the CF34? That's a motor the Air Force knows inside out. Literally. Pairs of the version from the bigger regional jets would offer the kind of thrust needed but keep weight down.