B-52s to be re-engined by Rolls Royce!

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Those are all part of the DC-9 family. I don't know if there's any way to say they don't all build upon later generations.
Yes there is. The 717 is almost entirely structurally the same as the DC-9-30 except for some added frames, which is different then the MD-80. The MD-90 is based on the MD-80 but the 717 is not.

Avionics is a different story. The 717 avionics is based on the MD-11.
 
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Possibly of interest, I have operational experience with the Pratt 815, Rolls Royce BR710 and BR725 (F130) engines. It's my opinion that the BR725 (F130) is the top performer. The three are very similar and in the same thrust class. However, in real world use, the high altitude performance of the 725 is markedly better than the Pratt 815. This is likely due to the bypass ratio of the Pratt 815 being 5.5-5.6, and the 725 (F130) being 4.1.

Despite Pratt's 5 turbines driving the fan, (vs RR's 3) the Pratt is unable to create sufficient high altitude thrust to match the Rolls. We see this as a poor high altitude climb rate, terrible climb rate at altitude with anti-ice on, and an inability to achieve sufficient airspeed at altitude to maintain a safe AOA (angle of attack) (too slow and nose up at FL510)

By way of comparison, the RR BR725 can push a very similar, but ever so slightly larger and heavier airframe to M 0.92 at 49,000 feet and the Pratt 815 manages only M 0.88-89. Problem is, 0.88 is too slow and results in an AOA of nearly 0.8, well into the yellow range.

Furthermore, the Pratt is more thirsty in cruise flight. We've been noticing 100 pounds per hour more fuel flow at similar speeds and thrust settings.
 

JHZR2

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Would it really need to be "militarized" for anything other than the controls and the design of the nacelles? A B-52 is really more like a cargo plane designed to drop its cargo in anger. I thought the primary goal here was to find something that didn't really need much modification, and where commonality with a commercial engine would reduce acquisition and maintenance costs.

There are "military" aircraft where the engines aren't any different than the civilian equivalent other than the paint job. Like the KC-10 and various VIP transport aircraft.

Didn't RR just buy Allison from GM to start their American operations?
Materials in the hot section are different, particularly for marinized military engines. Shock, vibration, and other situations need to be designed and qualified to.
 

4WD

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Would it really need to be "militarized" for anything other than the controls and the design of the nacelles? A B-52 is really more like a cargo plane designed to drop its cargo in anger. I thought the primary goal here was to find something that didn't really need much modification, and where commonality with a commercial engine would reduce acquisition and maintenance costs.

There are "military" aircraft where the engines aren't any different than the civilian equivalent other than the paint job. Like the KC-10 and various VIP transport aircraft.

Didn't RR just buy Allison from GM to start their American operations?
Thought the Allison engine deal was in the mid 90’s

The transmission company is from Indy land right ?
 
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Thought the Allison engine deal was in the mid 90’s

The transmission company is from Indy land right ?

It was. However, weren't Allison Engine and Allison Transmission considered separate entities by the time RR bought the aircraft engine division?

I remember Allison transmission because they were making hybrid drivetrains for buses. The first one I rode on was the Yosemite Valley shuttle back in 2005, which were Gillig Low Floor models with the Allison EP40/H 40 EP transmission with a Cummins engine. They were really quiet too coming from a stop, but then the diesel engine kicked in as it got up to speed.

 
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Sure. Why didn't someone think of that earlier? And find some way to hang ordinance from the unused engine mounts.

Interesting concept, even though I'm sure you're joking.
Simple solutions aren't easy to come by. Jokes aside it is sarcasm. A lot goes into fitting engines and incorporating a plethora of variables.

Just good to see how one engine (GE90 115) generates, give or take, 15,000 lbs less thrust than all those current engines on the b52 combined.
 
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Simple solutions aren't easy to come by. Jokes aside it is sarcasm. A lot goes into fitting engines and incorporating a plethora of variables.

Just good to see how one engine (GE90 115) generates, give or take, 15,000 lbs less thrust than all those current engines on the b52 combined.

The current engines are tiny though. I always wondered why they went with 8. Still - they've gone from turbojets to turbofans so it's not as if they haven't been through all sorts of modifications over the years.
 
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It's interesting that the winning think outside the box solution ended up being to keep 8 engines. And it was practical since engines that "drop in" replace the TF-33 size and thrust are mass manufactured for large corporate jets and small regional jets.

I wonder how well a GE90 would run at 51,000 feet altitude. The 777 is only designed for 43,000.
 
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The current engines are tiny though. I always wondered why they went with 8. Still - they've gone from turbojets to turbofans so it's not as if they haven't been through all sorts of modifications over the years.
Its interesting. The B-52 was designed in the 50's and I suppose the answer back then was 8 engines. Maybe with the structure of the existing wings its just better to stay with the distribution of power across the wing as it was designed. I think its cool to have those small jet engines. Can I get one for my dragster?
 
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Its interesting. The B-52 was designed in the 50's and I suppose the answer back then was 8 engines. Maybe with the structure of the existing wings its just better to stay with the distribution of power across the wing as it was designed. I think its cool to have those small jet engines. Can I get one for my dragster?

If you've ever looked at a 707, they went with four tiny engines. The USAF still operates a few 707 variants with more modern turbofans.
 
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Those engines were state of the art in the 1950s.

It took quite a while for the industry to work up to the very large engines that make a two-engine airliner possible, since that means in case of failure the remaining single engine alone has to have enough thrust to keep the plane flying.
 
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Those engines were state of the art in the 1950s.

It took quite a while for the industry to work up to the very large engines that make a two-engine airliner possible, since that means in case of failure the remaining single engine alone has to have enough thrust to keep the plane flying.

The 737 came out in the early 60s. I don't think thrust was so much the issue but rather worries about reliability if an engine was out and no place to land.
 
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It was. However, weren't Allison Engine and Allison Transmission considered separate entities by the time RR bought the aircraft engine division?

I remember Allison transmission because they were making hybrid drivetrains for buses. The first one I rode on was the Yosemite Valley shuttle back in 2005, which were Gillig Low Floor models with the Allison EP40/H 40 EP transmission with a Cummins engine. They were really quiet too coming from a stop, but then the diesel engine kicked in as it got up to speed.

Yep, they were separate by then. When my Dad was at GM, he did long stints at both. 45 years, total, beginning in 1943 or 44 when doing a co-op program at General Motors Institute, which is now Kettering U.
 
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I was thinking P&W(Raytheon) or GE was going to get the bid. But, the military’s been burned by P&W on a fighter deal, and the PW1000 has been giving A320neo customers a sore eye(and some have been switching to the CFM Leap).

Looks like the military is familiar with this Rolls-Royce engine, it’s also used on the G6 and the Boeing 717/MD-90.
 
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It was. However, weren't Allison Engine and Allison Transmission considered separate entities by the time RR bought the aircraft engine division?

I remember Allison transmission because they were making hybrid drivetrains for buses. The first one I rode on was the Yosemite Valley shuttle back in 2005, which were Gillig Low Floor models with the Allison EP40/H 40 EP transmission with a Cummins engine. They were really quiet too coming from a stop, but then the diesel engine kicked in as it got up to speed.

Allison Engine and Transmission were once the same until GM divested from turbines. GM kept Allison Transmission and Detroit Diesel until the latter was sold to Penske(and now Daimler) and the former was sold to a equity firm. GM wanted to use turbines to power cars, like Chrysler did.

In the Bay Area, Allison hybrid bus drivetrains didn’t really take off. BAE Systems enjoys the Bay Area client base(SFMTA/AC Transit/VTA).
 
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