Fighter: Flop or Not

[Shortround6]

Why not just use a pair of J47's? They are similar in weight to the J35, they produce more thrust, and have a better SFC.

The prototype Penetration fighters (XF-88 and XF-90) used Westinghouse J 34 engines which were much lighter than the Allison/GE J-35. They were also much smaller in diameter.
The early J34s went around 1200-1250lbs without afterburners and were a nominal 24 in in diameter.
The early J35s were around 2400lbs without afterburners and were a nominal 37in in diameter.
The early J47s also were around 2400lbs without afterburners and were a nominal 37in in diameter. However they are later in timing.
Both the J34 and J35 seemed to take a while to develop into more powerful versions (and to increase LBO).

I don't know what engines they were originally supposed to use and/or if the J34s were used in the prototypes because they were available and would fit. Both planes seem to have been intended to use afterburners but afterburners were still pretty much in the experimental stage in the late 40s. There are couple of CS sheets for the F-90 showing a calculated (never built) version using the Westinghouse J46 engine.
This engine was one of the two engines that sank Westinghouse as a jet engine manufacturer. While the paper work shows an intended use as of Jan 1950 the J46 didn't actually fly until July of 1953 in a Vought F7U Cutlass and that may have been non-afterburner version. First 16 F7U-3 using non-afterburning J46s. I would note that even in 1954/55 the J46 was providing less power than the Jan 1950 data sheet shows.
The J47s have very little difference in SFC over the Allison J35. as far as overhaul life goes. from wiki so usual disclaimer:
"Overhaul life for the J47 ranged from 15 hours (in 1948) to a theoretical 1,200 hours (625 achievable in practice) in 1956. For example, the J47-GE-23 was rated to run 225 hours time between overhauls. As installed on the F-86F, it experienced one in-flight shutdown every 33,000 hours in 1955 and 1956." If you are planing a long range fighter in 1948/49 is the engine you want to use?

AS far as approving or canceling programs goes a number of these early engines (Westinghouse in particular) took a number of years to go from great promise to dismal failure and and many programs were caught switching engines. Some programs were simply caught by rapid progress. The P&W J57 engine used in the B-52 was on the drawing boards in the late 40s, first run in 1950 and first flown in 1952.
It's power to weight ratio and SPC was in a class of it's own among american engines. GE scrambling to come up with the J79.
Sometimes the manufacturers and the Air Force/Navy simply threw out a program and started over.

 

[Graeme]

Many of these sheets can be found here : Standard Aircraft Characteristics Arcive

Many thanks for that link SR!

This may be of use to you and others interested in the early jets (dunno how to make a PDF- there is also a European chart) - from somewhere around '54.

Interesting thread and I'm looking closer at these early jets - but the more I look and learn the F-84 in my opinion was not a flop in the early role as a fighter.

Cheers.

,

 

[Shortround6]

That site has been linked to by others. Tomo, for one, but deserves to be brought up again as it has so much good information.

Thanks for the chart. A lot of the period periodicals/books were operating under security restrictions. While the P & W J57 started work in 1950 and ran on the bench in 1952 (?) and flew in 1953 any real details (including dimensions and weight) were restricted from publication for several more years, real details like pressure ratio and air flow not showing up until 1955-56 (?) at least it was not in 1954-55 Janes.

I would also note that most (all?) of the early after burners were not throttle-able . they were all or nothing = on/off.
There is some talk of "electronic" controls of the afterburner but given the dates this means either vacuum tubes or resistors/capacitors and relays/switches or both? Using the afte burner required a different exhaust nozzle configuration and miss matching the nozzle opening to the engine operating condition could damage the engine, not just give less than optimum thrust.

 

[Zipper730]

The prototype Penetration fighters (XF-88 and XF-90) used Westinghouse J 34 engines which were much lighter than the Allison/GE J-35. They were also much smaller in diameter.

There is definitely an advantage in having an engine with a high T/W ratio, a low frontal area and diameter, as well as a low specific fuel-consumption.

The J34 had a good power-to-weight ratio for the time, it had good thrust-to-frontal area and diameter as well, and it's SFC was also quite good. The problem is that many of the penetration fighters were larger than their standard designs and required more power to do the job.

The J47s have very little difference in SFC over the Allison J35.

That's interesting, I thought they did.

The P&W J57 engine used in the B-52 was on the drawing boards in the late 40s, first run in 1950 and first flown in 1952.

Started out as the XT-45 in 1946. I'm not sure when they converted it into a turbojet, but it was effective regardess!

 

[Shortround6]

There is definitely an advantage in having an engine with a high T/W ratio, a low frontal area and diameter, as well as a low specific fuel-consumption.

The J34 had a good power-to-weight ratio for the time, it had good thrust-to-frontal area and diameter as well, and it's SFC was also quite good. The problem is that many of the penetration fighters were larger than their standard designs and required more power to do the job.
That's interesting, I thought they did.

Getting good fuel consumption figures is hard as sometimes the fuel consumption (SFC) is not only given at different ratings, full power or normal or cruise, but in some cases one companies "Normal" was another companies "cruise". Add that in the early days (the 40s and early 50s) the SFC was also measured on a test stand with the engine stationary (static) they got no benefit from the pressure rise in the intake (RAM for piston engines). This means actual flight numbers could vary and vary with installation, speed and altitude much more than piston engines did. Most of these engines (including WW II enignes) had a rated SFC of between 1.0 and 1.3 without after burner and at cruise or normal. Again this is for service engines, a few prototypes fall outside the range. the late 40s engines were pretty much between 1.0 to 1.1 to 1 without afterburner.

Compression ratio or pressure ratio is one of the main factors in SFC and during the 40s the pressure ratios of the compressors was usually between 4 and 5 to 1. Older and smaller engines sometimes dipping into the 3 to 1 range and only a very few of the newest axial flow engines breaking 5 to 1 for a service engine. At this time the advantage of the axial flow compressor was largely an illusion. The J 57 started as a 6 to 1 ratio in the design process, went to 8 to 1 as a design 2 spool engine then 10 to 1 and finally underwent a total redesign to the "Wasp waist" configuration and the 12.5 to 1 ratio of the first service engines.

Good thrust to frontal area is good in theory but in practice it tends to fade as you need so much fuel that you can't make the fuselage that skinny anyway. The J34 was never intended as a single engine powerplant, it wasn't powerful enough. The idea for the J 34 was usually two engines in the wing roots to get enough power, avoid losses due to long intake and exhaust ducts and leave room in the fuselage for fuel.
The XP-88 and XP-90 both carried over 900 gallons of internal fuel so the question of 24in diam or 28in or 37 in dia engines becomes rather moot even if the engines were pushed to the rear or lower rear of the aircraft. The F2H-2 Banshee had tankage for 789 gallons of fuel in 3 fuselage tanks so while having slim engines compared to centrifugal engines was certainly a plus it was hardly critical.

 

[Zipper730]

Getting good fuel consumption figures is hard as sometimes the fuel consumption (SFC) is not only given at different ratings, full power or normal or cruise, but in some cases one companies "Normal" was another companies "cruise". Add that in the early days (the 40s and early 50s) the SFC was also measured on a test stand with the engine stationary (static) they got no benefit from the pressure rise in the intake (RAM for piston engines).

I didn't know that...

Compression ratio or pressure ratio is one of the main factors in SFC and during the 40s the pressure ratios of the compressors was usually between 4 and 5 to 1. Older and smaller engines sometimes dipping into the 3 to 1 range and only a very few of the newest axial flow engines breaking 5 to 1 for a service engine. At this time the advantage of the axial flow compressor was largely an illusion.

I didn't know that, but it makes sense...

The J 57 started as a 6 to 1 ratio in the design process, went to 8 to 1 as a design 2 spool engine then 10 to 1 and finally underwent a total redesign to the "Wasp waist" configuration and the 12.5 to 1 ratio of the first service engines.

Wait, I thought the J57 started out as the XT-45 which was a twin-spool turboprop, which was then reconfigured to pure-jet configuration instead.

Good thrust to frontal area is good in theory but in practice it tends to fade as you need so much fuel that you can't make the fuselage that skinny anyway.

I was talking about the engine itself, not the plane...

The J34 was never intended as a single engine powerplant, it wasn't powerful enough.

As I understand it, the roots of the J34 were in the J30: The J30 was designed as an auxiliary powerplant at first, because the US Navy did think turbojets were suitable for operating off a carrier-deck. They eventually had a change of heart and built, what would become, the FH-1 Phantom.

Since the engine was fairly small at first, they needed six to eight engines: They realized it would be a dumb idea to have a fighter powered by eight engines, so they began bulking up the engine and also let the engine manufacturer have a role in the design process. What previously required six-to-eight engines, now required four, and eventually two.

There had been a thought of putting one large engine in the aircraft (presumably something J34 sized), but McDonnell did not wish to go through with it because they'd already worked out the basic fuselage configuration and the fuel-tank layout.

The idea for the J 34 was usually two engines in the wing roots to get enough power, avoid losses due to long intake and exhaust ducts and leave room in the fuselage for fuel.

The F2H was basically a super Phantom. The Phantom was impressive, but it was only capable of achieving 500 mph in level flight under limited situations, it didn't have cannon armament, and the ejection seat seemed like a good idea. Range was also a problem with early jets, so they decided they'd like more of that too.

The XP-88 and XP-90 both carried over 900 gallons of internal fuel so the question of 24in diam or 28in or 37 in dia engines becomes rather moot even if the engines were pushed to the rear or lower rear of the aircraft.

I'm not sure I follow you there, I suppose a thinner engine would allow more room for fuel if that's what you are getting at.

The F2H-2 Banshee had tankage for 789 gallons

Wasn't it used as an escort briefly?

 

[wuzak]

I'm not sure I follow you there, I suppose a thinner engine would allow more room for fuel if that's what you are getting at.

I believe he was saying that a large fuselage engine would cut into the space allocated for fuel that the twin small engine configuration allowed.

 

[Shortround6]

Wait, I thought the J57 started out as the XT-45 which was a twin-spool turboprop, which was then reconfigured to pure-jet configuration instead.

Well, it did but the XT-45 was an evolutionary step. P & W was looking for a replacement for the large piston engine and had decided to develop 3 products, a small turboprop, a large turbo jet and and a large truboprop in 1945. The large turbojet was a single spool 6 to 1 pressure ratio engine (JT3-6) of 7500lbs but it was never run. It morphed into the two spool XT-45 (PT-4 also never run) and then by loosing the propeller section/parts it was turned into the two spool JT3-8 (X-176) which was first run in the late spring/early summer of 1949. This projected started work on the JT3-10 and about the same time as the JT3-8 ran work started on the JT3-A with the wasp waist. Please note that most of these projects overlapped to a considerable extent. Work on the XT-45 didn't stop when the JT3-8 started. It stopped when the JT3-10 started and the work on the JT3-8 didn't stop until over a year after the JT3A was started. The JT3A was the J57.

JT stands for jet engine, the number 3 stands for the design number (3rd jet engine) and the -6, -8, -10 stands for the pressure ratio. PT stands for turboprop. These are P &W designations while J57 and T45 are government designations.

I was talking about the engine itself, not the plane...

Low frontal area got a lot of "play or press" in the 1930s with the V-12 vrs radial engine arguments. A bit over blown as a sitting pilot (even with legs out stretched) has more frontal area than most V-12 engines although he does have to have his head (and shoulders) above the engine. But then some of these aircraft only carried 100-180 gallons of fuel. Once you switched to jets that need hundreds of gallons per engine just to stay in the air for a couple of hours the fuselage became so fat that small differences in diameter are irrelevant.
On long range jet fighters it gets worse. For an extreme example

1150 gallons of internal fuel. J33 engines about 50 in diameter. or 13.9 sq ft frontal area each. Changing to J35s of 37.5 in D and 7.7sq ft would obviously have helped some. going to Westinghouse J34s of 24in diameter and 3.1 sqft with that fat fuselage?
On single engine aircraft the fat centrifugal engines could be hidden inside the fuselage of most planes that held around 400 gallons or more of fuel,

I would note that even the Folland Gnat used an engine of 32.4 in diameter and 5.7sqft frontal area and carried 240 gallons of internal fuel.

two seat trainer may have carried less?

as a counter point you have the Vought F6U Pirate, J34 24in diameter engine but 370 gallons of internal fuel

tip tanks held 140 each so the fat fuselage may not be all due to the internal fuel.

As I understand it, the roots of the J34 were in the J30: The J30 was designed as an auxiliary powerplant at first, because the US Navy did think turbojets were suitable for operating off a carrier-deck. They eventually had a change of heart and built, what would become, the FH-1 Phantom.

Since the engine was fairly small at first, they needed six to eight engines: They realized it would be a dumb idea to have a fighter powered by eight engines, so they began bulking up the engine and also let the engine manufacturer have a role in the design process. What previously required six-to-eight engines, now required four, and eventually two.

There had been a thought of putting one large engine in the aircraft (presumably something J34 sized), but McDonnell did not wish to go through with it because they'd already worked out the basic fuselage configuration and the fuel-tank layout.

Mostly right but there may be some confusion as to which engine/s were intended for the 6-8 engine layouts. The J30 ( Westinghouse 19A=19in diameter) was giving 1135lbs on the first test run so 8 such engines seems a bit like overkill. Westinghouse did build and run a 9.5in diameter engine (the Yankee 9.5 or J32) which gave around 275lbs of thrust and may have had drawings for an 11.5in and/or another intermediate size. No actual work (metal cut/shaped) being done on these sizes. The 19A was changed the 19B when the compressor went from 6 stages to 10 stages.
There was some excess enthusiasm for a short period of time during WW II for jet engines as (over simplified) they thought that such a simple basic engine, only one moving part could be scaled up or down with relative ease to suit different applications. after a few years of laborious development that idea went out the window and only returned after they got a much firmer grasp on materials, manufacturing techniques, and airflow through the compressor combustion chambers and turbines. Even in the 70s and 80s engines were sometimes adjusted in power by adding/subtracting compressor stages rather than changing diameters. At any rate Westinghouse found they had quite enough on their plate just getting the A19 to run properly. Combustion problems lead to uneven temperatures around the combustion chamber lead to short life of the combustion chamber and turbine. This lead to restrictions in performance and while the FH-1 Phantom was supposed to have a 41,000ft ceiling it rarely operated over 30,000ft. Range was less than book figures and it is unknown if it ever reached it's book speed of 479mph at sea level.

 

[Old Wizard]

 

[swampyankee]

Zipper, I was a development test engineer for gas turbines, and later worked on performance simulation software for them. Bearings and seals are major development concerns with gas turbines; they may be easier than reciprocating engines, due to lack of things banging back and forth, but they also tend to operate at high DN numbers.

 

[Zipper730]

I believe he was saying that a large fuselage engine would cut into the space allocated for fuel that the twin small engine configuration allowed.

Understood

Well, it did but the XT-45 was an evolutionary step. P & W was looking for a replacement for the large piston engine and had decided to develop 3 products, a small turboprop, a large turbo jet and and a large truboprop in 1945. The large turbojet was a single spool 6 to 1 pressure ratio engine (JT3-6) of 7500lbs but it was never run. It morphed into the two spool XT-45 (PT-4 also never run) and then by loosing the propeller section/parts it was turned into the two spool JT3-8 (X-176) which was first run in the late spring/early summer of 1949. This projected started work on the JT3-10 and about the same time as the JT3-8 ran work started on the JT3-A with the wasp waist. Please note that most of these projects overlapped to a considerable extent. Work on the XT-45 didn't stop when the JT3-8 started. It stopped when the JT3-10 started and the work on the JT3-8 didn't stop until over a year after the JT3A was started. The JT3A was the J57.

Okay, I gotcha.

Low frontal area got a lot of "play or press" in the 1930s with the V-12 vrs radial engine arguments.

So the issue was more fineness ratio than actual frontal area itself?

Once you switched to jets that need hundreds of gallons per engine just to stay in the air for a couple of hours the fuselage became so fat that small differences in diameter are irrelevant.
On long range jet fighters it gets worse. For an extreme example

1150 gallons of internal fuel. J33 engines about 50 in diameter. or 13.9 sq ft frontal area each. Changing to J35s of 37.5 in D and 7.7sq ft would obviously have helped some.

Yeah, but I'm not sure how much. The XP-83 was an interesting design which had many problems, but it did prove the following

• You could make jet-fighter with a large fuel-fraction that could withstand g-loads similar to a contemporary fighter (P-80)
• Control loads though a little heavier than the P-80, were still considered acceptable

On single engine aircraft the fat centrifugal engines could be hidden inside the fuselage of most planes that held around 400 gallons or more of fuel

The F-80 for example; the F9F actually had an internal load that was like 680-685 with 120 gallons on the tips.

Mostly right but there may be some confusion as to which engine/s were intended for the 6-8 engine layouts. The J30 ( Westinghouse 19A=19in diameter) was giving 1135lbs on the first test run so 8 such engines seems a bit like overkill. Westinghouse did build and run a 9.5in diameter engine (the Yankee 9.5 or J32) which gave around 275lbs of thrust and may have had drawings for an 11.5in and/or another intermediate size.

I figured it just grew from the aux powerplant requirement to the ultimate design whether work was done on them or not.

There was some excess enthusiasm for a short period of time during WW II for jet engines as (over simplified) they thought that such a simple basic engine, only one moving part could be scaled up or down with relative ease to suit different applications.

That makes sense

Zipper, I was a development test engineer for gas turbines, and later worked on performance simulation software for them. Bearings and seals are major development concerns with gas turbines; they may be easier than reciprocating engines, due to lack of things banging back and forth, but they also tend to operate at high DN numbers.

DN?

 

[swampyankee]

DN is shaft diameter times rpm.

 

[Shortround6]

Sort of the speed of the surfaces in the bearing?

Although that would truly be diameter times Pi times rpm but since Pi is a constant (a 3 in dia shaft would have 50% more speed than a 2in shaft of the bearing surfaces for the same rpm) perhaps the simpler formula is just as effective?

Just guessing so if I am wrong I welcome the correction.

 

[swampyankee]

Sort of the speed of the surfaces in the bearing?

Although that would truly be diameter times Pi times rpm but since Pi is a constant (a 3 in dia shaft would have 50% more speed than a 2in shaft of the bearing surfaces for the same rpm) perhaps the simpler formula is just as effective?

Just guessing so if I am wrong I welcome the correction.

You're correct; it's a measure of speed of the bearing surfaces.

 

[Zipper730]

DN is shaft diameter times rpm.

Like gear-ratio...

 

[wuzak]

Like gear-ratio...

No!

I have to say that when I saw the term DN I wondered what pipe sizes had to do with anything.

 

[Zipper730]

Okay, regarding the F-84 the following that I get out of it from my position and others is the following

A. Positives
1. Range: Though designed largely to interceptor specification (excluding the earliest internal studies of a jet-powered P-47), Republic was often committed to long-ranged aircraft-designs, and the F-84 emerged with a range superior to that of the F-80 and F-86
2. Ruggedness: Borne both out of the earliest internal studies, and structurla problms early on, the operational F-84 variants proved to be quite rugged and damage resistant, making them useful for low altitude operations
3. Effective as a fighter-bomber: Though designed to be an interceptor, it's range at low altitude, rugged construction, and bomb-carrying capability made it quite suitable as a fighter bomber, and eventually a nuclear-strike plane once IFR was added.
4. Aerial-Refuelling: It was the first USAF plane to be so configured.

B. Negatives
1. Early Problems: It had structural issues and bad-stall characteristics. Not sure if the stall characteristics were ever ironed out, though a RAF test pilot said the plane was viceless in handling by the time he flew it, so it might be yes.
2. Climb-performance was inferior to the Gloster Meteor, DH Vampire, F-80 Shooting Star, Supermarine Attacker, and Hawker Seahawk.
3. Turning performance, while able to ironically beat out a MiG-15 at low altitude (whether it be due to aerodynamics, strength, or control load), it was generally inferior to the F-80 under most circumstances.
4. While it was used as an escort-fighter, it didn't seem to be very good at it

Personal Opinion
While I personally believe it would have been better off had it been designed smaller and lighter for the interception mission and constructed with more care early on, it would have likely been rugged enough to survive combat and agile enough to not get hit too often: I would not consider it a flop, though I would not say I think highly of the design as a fighter.

 

[tyrodtom]

You keep posting the F-84 was designed as a interceptor, and was a failure as a interceptor.

It was designed to a late 1944 General Operational Requirement for a day FIGHTER capable of 600 mph, and a 700 mile range.
Never designed as a interceptor, never attempted to be used as a interceptor, so never failed as a interceptor.

 

[Shortround6]

2. Climb-performance was inferior to the Gloster Meteor, DH Vampire, F-80 Shooting Star, Supermarine Attacker, and Hawker Seahawk.
3. Turning performance, while able to ironically beat out a MiG-15 at low altitude (whether it be due to aerodynamics, strength, or control load), it was generally inferior to the F-80 under most circumstances.
4. While it was used as an escort-fighter, it didn't seem to be very good at it

2. Once again you fail to say which versions of which aircraft you are comparing to. Climb-performance of the F-84 varied considerably between the B-C- early D models (around 4100fpm for initial climb) to the late D-E (6000-7050fpm) to the G (initial climb unknown but time to 35,000ft 9.4 minutes).
Of course the Meteor MK 8 out climbs the early F-84, a pair of 3600lb thrust engines in a similar weight plane, however the MK had a major shortcoming, lack of range. it held 420 imp gallons for it's two engines. there was a 175 imp gallon belly tank and one has to be rather careful as performance figures are often for internal fuel only. There were two 200imp gallon under wing tanks.
Yes the Vampire could outclimb the early F-84s. But it was a small, light aircraft which also had a fuel problem and was slow to boot. max speed 540mph?
Supermarine Attacker is a real stretch, 3rd prototype doesn't even fly until Jan 1950. Pretty poor interceptor in 1948-49-50 if it is not in squadron service. tail wheel landing gear and a wing with a lower mach number than a Spitfire. It's 226sq ft wing also means a wing loading not much different than an F-84.
Hawker SeaHawk, doesn't enter squadron service until 1953.
If I was picking an ideal interceptor for 1948-50 and could use a time machine to use 1953 fighters I too would pick something other than an F-84.

I would also note that the first duty of an escort fighter is to actually show-up where the bombers are. Then we can talk about how effective they are.

 

[Shortround6]

It is often hard to separate the the performance of an aircraft from it's engine. Unfortunately for the F-84 it's engine essentially stagnated in thrust development for over 4 years while reliability/durability issues were straightened out.
Also unfortunately the US only had 2 other engines of the even close to the power needed in flyable condition/status until 1948/49. The J33 was too fat to replace the J35 without major redesign and the J34 made less thrust than the J35. The P& W J42 only shows up at the end of 1948 and it also too fat. By 1951 the J42 is setting new standards in both durability (first turbine engine rated at 1000hrs between overhauls) and reliability (ingesting foreign objects and battle damage).

What Allison was promising behind closed doors as far as thrust development is unknown. F-84s with 3750lb thrust engines were last delivered in April 1949 with last of the "D"s. The 4900lb thrust engines being installed the next month with the first Es. By June of 1951 the Gs were being delivered with 5600lb thrust engines. about a 50% increase in power in two years. None of these used afterburners Many of the D's were re-engined with the -17B engine if 4900lbs thrust.
4000lbs had been set as a target goal in the summer of 1943. The engine exceeded 3000lbs the first time it ran in 1944.

I would note that even in 1947 P&W was going to refuse to license and produce the Nene (J42) unless Rolls-Royce designed and did initial development on a more powerful engine. P&W got what they wanted with the Tay (J48)