JT4D/TF75

isayyo2

isayyo2

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Just wanted to start a small thread on a project that came too late, and since the Flight archives are dead. JT4D was to be a low bypass-turbofan variation of the JT4A/J75, in similar vain to the J57/JT3C and TF33/JT3D family. Copied from the article:
Flight, 30 Oct 1959:

Largest of the new Pratt & Whitney fans is the TF75 (commercial designation JT4D), which is a straight conversion of the J75/JT4A, following the formula proven with the J57/JT3. The company have no contract for this work and are unlikely to take it to the hardware stage without military backing or assurance of substantial civil sales. The simplest conversion is the addition of the front fan of the JT3D-1 on to a basic J75 or JT4A-9. This is rendered possible since the hub diameter of the two compressors is essentially the same. Compared with the JT4A-9 this somewhat compromised turbofan will have 8 per cent better s.f.c. It is designated JT4D-3 and would be rated at 22,500 lb. The optimum JT4D-1 would have a new ad hoc fan of appreciably greater diameter. The sea-level rating would be 25,000 lb, and cruise s.f.c. would be approximately 15 per cent better than that of the straight turbojet.

The military TF75-P-1, corresponding to the JT4D-1, is the engine specified by Douglas for the larger of their two CX military freight aircraft, in which four of these big engines would be installed in twin pods. In their submission to the U.S.A.F. for this aircraft last August, Douglas said: "The turbofan power system was chosen for the future cargo airplane because it provided the most favourable balance between speed, productivity and direct operating cost for the design cargo/range mission. A truboprop propulsion system would have provided more range or more cargo and better take-off performance. The turbofan provided greater speed, and takeoff performance comparable to current jet aircraft.
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Credit to: https://www.airliners.net/forum/viewtopic.php?t=750861#p10836281
 
What interests me is the fighter applications for the engine. 25,000 pounds dry thrust is damn near as much as the J75 put out in burner (26,500). Slap a burner on it and we're probably looking at something like 32-33,000 pounds of thrust. Imagine a Tomcat equipped with those right off the bat instead of the TF-30s that it got
 
SSgtC said:
What interests me is the fighter applications for the engine. 25,000 pounds dry thrust is damn near as much as the J75 put out in burner (26,500). Slap a burner on it and we're probably looking at something like 32-33,000 pounds of thrust. Imagine a Tomcat equipped with those right off the bat instead of the TF-30s that it got
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Yes that'd be quite the sight!
Though your Tomcat may look a bit different since the TF75 would have been significantly larger in diameter and heavier than the TF30.

I would be interested in existing types like the F-105, F-106, or U-2 receiving the engine. 25,000 lbs of thrust would have been a boon to the P6M program too.
 
SSgtC said:
What interests me is the fighter applications for the engine. 25,000 pounds dry thrust is damn near as much as the J75 put out in burner (26,500). Slap a burner on it and we're probably looking at something like 32-33,000 pounds of thrust. Imagine a Tomcat equipped with those right off the bat instead of the TF-30s that it got
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The 26,500 was only during water injection on takeoff for the F-105. Other than that it was the same 24,500 as on the F-106. Regarding the TF30 there were actually variants considered all the way up to 30k.
 
isayyo2 said:
Yes that'd be quite the sight!
Though your Tomcat may look a bit different since the TF75 would have been significantly larger in diameter and heavier than the TF30.

I would be interested in existing types like the F-105, F-106, or U-2 receiving the engine. 25,000 lbs of thrust would have been a boon to the P6M program too.
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The XF8U-3 Crusader 3 would have been interesting too. (And they'd planned on upgraded J75s for it too.)
 
Does anyone know what the fan diameter was going to be on the proposed engine for the CX? I'm trying to see something on what it does to the engine diameter.
 
Scott Kenny said:
We call those KC-135Rs.
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Returning to this thread...

A TF75 might not be as efficient as the CFM56, but would be available by 1960 which leads to some interesting "what-if" thoughts. I only recently learned that the 707-320 actually started with JT4As, so a drop-in kit would allow for an easy conversion.

Would a TF75 consume noticeably more fuel than a TF33?

Could the TF75 make an appearance on new variants of the B-52 and C-141?
 
isayyo2 said:
Would a TF75 consume noticeably more fuel than a TF33?
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Yes and no. Assuming P&W got the full 15% reduction in SFC, the TF75 would burn .629 lbf•h while the TF33 burned .74 lbf•h. So in terms of SFC, the TF75 would be significantly more fuel efficient.

But here's where it gets a little fucky. The TF75 would burn some 500 pounds/hr more fuel than the TF33 would during cruise due to the higher thrust and greater fuel burn overall from it. It also imposes a weight penalty. The baseline J75 weighed in around 5,100 pounds. Figure that's going to grow to at least 5,200 pounds with the larger fan. The TF33 would be some 600 pounds lighter. That means a TF75 equipped 707 is going to burn more fuel than a TF33 equipped one is just from having to lug around an extra 2,400 pounds of engine (600lbs/ea times 4 engines). So 2,000 pounds more fuel burn per hour at cruise just from the engines, plus probably another 1-200 pounds per hour from the heavier engines.

So in total, yes. A TF75 equipped aircraft will burn a lot more fuel per hour, despite having better SFC. On the flip side, that same aircraft will have significantly better performance than it's TF33 equipped counterpart (better short field performance, better "high and hot" performance, better climb performance (and this might mediate some of the increased fuel burn at cruise, by letting you get to cruise faster and burn less fuel climbing), a likely higher cruise speed at the same EPR settings, etc). Basically, you're trading increased fuel burn for 100,000 pounds of thrust verses 68,000 pounds of thrust. That's a huge increase.
 
MadRat said:
So three engines essentially could have done work of four?
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Yes and no. You'd need to design a new trijet for them vs just a simple engine swap. What you're more likely to see is the extra thrust used for either a faster cruise speed of the existing aircraft, or you'll see it used for a High Gross Weight version or possibly even a stretched version to provide additional passenger and freight capacity. Another option would be a larger twin jet to complement the 737 and possibly you'll see that in place of the trijet 727 (though I'm doubtful, airlines didn't trust twins to fly long distances).
 
airlines didn't trust twins to fly long distances
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Yup, ETOPS rules for twin jets didn't got relaxed until the mid-90's and the 777. Smart move by Boeing back then, they hit right between A340 (too many engines) and A330 (two engines, but smaller than the 777).
 
SSgtC said:
Yes and no. You'd need to design a new trijet for them vs just a simple engine swap. What you're more likely to see is the extra thrust used for either a faster cruise speed of the existing aircraft, or you'll see it used for a High Gross Weight version or possibly even a stretched version to provide additional passenger and freight capacity. Another option would be a larger twin jet to complement the 737 and possibly you'll see that in place of the trijet 727 (though I'm doubtful, airlines didn't trust twins to fly long distances).
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I'd expect it to be the high gross weight, or maybe a hot&high specialist.
 
Archibald said:
Yup, ETOPS rules for twin jets didn't got relaxed until the mid-90's and the 777. Smart move by Boeing back then, they hit right between A340 (too many engines) and A330 (two engines, but smaller than the 777).
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1985, actually. TWA was granted 90 minute ETOPS approval for their 767-200s. This allowed twin engine aircraft to cross the Atlantic.
Scott Kenny said:
I'd expect it to be the high gross weight, or maybe a hot&high specialist.
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My guess too. I'd expect to see an HGW or stretched 707/DC-8/etc over anything else.
 
SSgtC said:
Yes and no. Assuming P&W got the full 15% reduction in SFC, the TF75 would burn .629 lbf•h while the TF33 burned .74 lbf•h. So in terms of SFC, the TF75 would be significantly more fuel efficient.

But here's where it gets a little fucky. The TF75 would burn some 500 pounds/hr more fuel than the TF33 would during cruise due to the higher thrust and greater fuel burn overall from it. It also imposes a weight penalty. The baseline J75 weighed in around 5,100 pounds. Figure that's going to grow to at least 5,200 pounds with the larger fan. The TF33 would be some 600 pounds lighter. That means a TF75 equipped 707 is going to burn more fuel than a TF33 equipped one is just from having to lug around an extra 2,400 pounds of engine (600lbs/ea times 4 engines). So 2,000 pounds more fuel burn per hour at cruise just from the engines, plus probably another 1-200 pounds per hour from the heavier engines.

So in total, yes. A TF75 equipped aircraft will burn a lot more fuel per hour, despite having better SFC. On the flip side, that same aircraft will have significantly better performance than it's TF33 equipped counterpart (better short field performance, better "high and hot" performance, better climb performance (and this might mediate some of the increased fuel burn at cruise, by letting you get to cruise faster and burn less fuel climbing), a likely higher cruise speed at the same EPR settings, etc). Basically, you're trading increased fuel burn for 100,000 pounds of thrust verses 68,000 pounds of thrust. That's a huge increase.
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While the increased weight of the engines would require additional thrust, it wouldn’t be much. The TF75 would be running at a lower percentage of its capability in cruise than the TF33, so the improved SFC would likely result in better cruise fuel consumption, all else being equal.

Of course, with the additional available thrust, things are likely not to be equal. Increased thrust can increase climb rate, reducing the time to climb offsetting higher fuel burn at high power. You might push the throttles and cruise faster, burning more fuel. Or higher, offsetting the higher power setting. And most likely, the aircraft is stretched to increase payload at a higher gross weight.
 

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