Pratt & Whitney J-91 vs General Electric J-93 GE-1, GE-3, and GE-5 Questions

[KJ_Lesnick]

Normally, I would just look through my books but I don't have very much information about the J-91 other than it had a 9-stage compressor, a twin-stage turbine, a 7:1 pressure-ratio, a trans-sonic compressor, a 55-inch diameter, and that the J-58 was an 80% scaled down version with a higher pressure-ratio and a variable IGV to lower it's (the J-58's) pressure-ratio at high supersonic speeds, and that it had an afterburner that could burn either JP-4 or HEF-3 -- Plus I've heard conflicting data regarding the thrust output of the J-93's from 27,000 lbf to 31,500 lbf


I'm wondering the following questions...

1.) How the thrust levels of the J-91 using conventional fuel (JP-4 in combustor and afterburner) compared to the J-93 GE-1 using conventional fuel (JP-4 in combustor and afterburner)?

2.) How the thrust levels of the J-91 using conventional fuel (JP-4 in combustor and afterburner) compared to the J-93 GE-3 using conventional fuel (JP-4 in combustor and afterburner)?

3.) How the thrust levels of the J-91 using conventional fuel (JP-4 in combustor and afterburner) compared to the J-93 GE-3 using JP-6 (in both combustor and afterburner)?

4.) How the thrust levels of the J-91 using high-energy fuel in afterburner compared to the J-93 GE-1 using high energy fuel in the afterburner?

5.) How the thrust levels of the J-91 using high-energy fuel in afterburner compared to the J-93 GE-5 (GE-3 used only JP-4 or JP-6) using high-energy fuel in the afterburner?


KJ Lesnick

 

[F119Doctor]

I see that this thread never had a response, maybe there are some experts out there that can fill some of the blanks for you:

1. From what I have read, the J91 was designed for an airflow of 400 pound per second. The J58 was scaled down to approximately 300 pps, so with all else equal the J91 ratings would be ~30% higher than the J58

2. These are sea level ratings. At M3, inlet heating has raised the inlet temperature to 600F+. Limit for the J58 was 800F (427C). As the inlet temperature goes up, the effective rotor speed slows down. At M3.2 cruise, even though the J58 mechanical rotor speed was being held at 100%, the corrected rotor speed was 65%, just above idle airflow. At low corrected airflow and pressure ratio, the compressor is pushed toward stall, with the back end of the compressor being too small for those conditions. The inlet guide vanes going cambered and the mid compressor bleed provided the stall margin necessary. The genius of Ralph Abernathy was to capture the bleed flow and reintroduce the air into the engine flow in the afterburner. The bleed bypass and the IGV cambering occurred at approximately M2.2 where it had little effect on engine airflow to prevent causing the SR-71 inlet to unstart.

3. I don’t think the high energy fuels made much difference to these engine thrust levels, since you are limited by the amount of oxygen to burn. But you could go further since you didn’t need as much of the fuel to produce the same heat release. But there were a lot of impacts to the use of the boron ZIP fuels, and they were never pursued beyond experimental testing.

 

[sferrin]

I see that this thread never had a response, maybe there are some experts out there that can fill some of the blanks for you:

1. From what I have read, the J91 was designed for an airflow of 400 pound per second. The J58 was scaled down to approximately 300 pps, so with all else equal the J91 ratings would be ~30% higher than the J58

2. These are sea level ratings. At M3, inlet heating has raised the inlet temperature to 600F+. Limit for the J58 was 800F (427C). As the inlet temperature goes up, the effective rotor speed slows down. At M3.2 cruise, even though the J58 mechanical rotor speed was being held at 100%, the corrected rotor speed was 65%, just above idle airflow. At low corrected airflow and pressure ratio, the compressor is pushed toward stall, with the back end of the compressor being too small for those conditions. The inlet guide vanes going cambered and the mid compressor bleed provided the stall margin necessary. The genius of Ralph Abernathy was to capture the bleed flow and reintroduce the air into the engine flow in the afterburner. The bleed bypass and the can cambering occurred at approximately M2.2 where it had little effect on engine airflow to prevent causing the SR-71 inlet to unstart.

3. I don’t think the high energy fuels made much difference to these engine thrust levels, since you are limited by the amount of oxygen to burn. But you could go further since you didn’t need as much of the fuel to produce the same heat release. But there were a lot of impacts to the use of the boron ZIP fuels, and they were never pursued beyond experimental testing.

This book has a several pages on the J91. Doesn't say anything about the -7 anywhere else. Maybe it was the variant they wanted to use for the nuclear propulsion effort? The J89 appears to be the Allison rough equivalent to the J91.

 

[F119Doctor]

The Jack Connery’ book is a good read!

 

[CaseyKnight]

I'm kind of surprised how little information there is on the J91 outside this site, to be honest.

From what I have read, the J91 was designed for an airflow of 400 pound per second. The J58 was scaled down to approximately 300 pps, so with all else equal the J91 ratings would be ~30% higher than the J58

Checks out, with 80% the mass-flow, thrust should be 25% higher for the J91 over the J58.

These are sea level ratings. At M3, inlet heating has raised the inlet temperature to 600F+. Limit for the J58 was 800F (427C). As the inlet temperature goes up, the effective rotor speed slows down. At M3.2 cruise, even though the J58 mechanical rotor speed was being held at 100%, the corrected rotor speed was 65%, just above idle airflow.

Though I knew that RPM did change with mach-number (on the J79 this was called the T-2 reset), I didn't know the RPM would slow down that much at higher speed.

 

[sferrin]

I'm kind of surprised how little information there is on the J91 (particularly in regards to dimensions), to be honest.

 

[CaseyKnight]

@sferrin,

Truthfully, the amount of data available outside of this site is quite small, though what I sad probably doesn't make me look terribly bright. From what I recall, I don't believe I've ever found anything for inlet diameter though I could probably make an educated guess (provided the 55" figure is accurate) based on the fact that mass-flow and frontal-area tend to scale together (all other things being equal).

I edited my post.

 

[F119Doctor]

Though I knew that RPM did change with mach-number (on the J79 this was called the T-2 reset), I didn't know the RPM would slow down that much at higher speed.

The mechanical RPM remains constant as the inlet temperature increases, but the effective speed slows down. We call this corrected RPM. The relationship is the square root of the absolute temperature ratio. If you double the absolute inlet temperature while maintaining the same mechanical RPM, the effective speed will be reduced by the square root of 2 (divide RPM by 1.412). The airflow and compressor performance will be the same as if it was turning that slower speed at the colder standard day temperature. By the way, this is the exact same relationship as Mach number vs air temperature. At the higher inlet temperature, the speed of sound goes up with the square root of the absolute temperature ratio, so you can say the Mach number of the compressor blade tips is going down while turning the same mechanical tip velocity.

The J58 does have a small mechanical speed reset, going from 102% at colder inlet temperature to 100% at hotter temperatures (I’m guessing to reduce centrifugal stress at the higher temps), but that is not what we are talking about with the low corrected rotor speeds at M3+.

There is a document:

“Course Ae107 – Case Studies in Engineering – The SR71 Blackbird,” Lockheed and California Institute of Technology

that you can Google - you will see this reduced airflow with increased inlet temperature referred to as “turn down”.

 

[Scott Kenny]

There is a document:

“Course Ae107 – Case Studies in Engineering – The SR71 Blackbird,” Lockheed and California Institute of Technology

that you can Google - you will see this reduced airflow with increased inlet temperature referred to as “turn down”.

No ebook available, not finding any pdfs via google.

 

[F119Doctor]

No ebook available, not finding any pdfs via google.

Try this link: https://authors.library.caltech.edu/records/6s4e6-b2j60

It is broken up into a series of PDF files that you have to download individually. Not sure if there is any export filter on that site.

 

[Scott Kenny]

Try this link: https://authors.library.caltech.edu/records/6s4e6-b2j60

It is broken up into a series of PDF files that you have to download individually. Not sure if there is any export filter on that site.

Thanks!

 

[charleybarley]

@sferrin,

Truthfully, the amount of data available outside of this site is quite small, though what I sad probably doesn't make me look terribly bright. From what I recall, I don't believe I've ever found anything for inlet diameter though I could probably make an educated guess (provided the 55" figure is accurate) based on the fact that mass-flow and frontal-area tend to scale together (all other things being equal).

I edited my post.

I'm guessing scaling mass flow and area means the mass flow per unit area has to be the same which I think it would have been since the J91 compressor "was about right for a Mach 3 engine" ref J Connors book, with a new high for flow/sq ft (lowest hub/tip ratio) for a P&W compressor with its front 4 transonic (P&W's first) stages. As already said the engine proposal (J58) for US Navy aircraft Mach 3 capability was a scaled J91 and the CIA/Lockheed-required-engine retained the compressor and turbine aerodynamics.

 

[charleybarley]

I didn't know the RPM would slow down that much at higher speed.

F119Doctor has explained the importance of air temperature to the way compressors handle their air. In picture form (compressor map) we have it explained by Bob Abernethy in his patent https://patents.google.com/patent/US3344606A/en, Fig.4/5 and all the explanatory words. Note that there is no compressor speed or inlet temperature specified because the location on the map depends on the ratio of the two. eg a low rpm (say idle) and a low temperature (say sea level) have a similar location (meaning compressor and air interact in similar fashion, eg stalling and choking) as a high rpm (say max) and high temp (say Mach 3 ram rise).

Hang in there. It's getting more tangible and interesting I hope. Remember operating at left of map (low corrected speeds) means stalling and choking (unless prevented with comp bleed or variable vanes (YJ93) or pre comp cooling MIG25)

A pre-Blackbird J58 proposal for a Navy aircraft had problems typical of most compressors starting and running at low power. The solution, again typical but specifically for the J58 was 4th stage compressor bleed open for starting and accelerating to higher thrust levels. See ring of "start" bleed ports ref "SR-71 Blackbird",James Goodall.



When Bob Abernethy came up with his '4th stage to afterburner' solution for stalling and choking at high mach (left side of map) he said "The same problem exists when starting the engine and P&W solution was to open "start" bleeds. ref roadrunnersinternationale.com More never told tales of P&W.
The start bleeds were retained , known as 'start bleed doors', although more elongated (x12), or external bleeds (ie to nacelle) and the bleed bypass tubes added, known as internal bleeds, each having their own schedule, see Flight Manual "Compressor Bleed And IGV Shift Schedule".
This photo shows start bleed doors in front of bleed tube elbows.

Jet engine performance - Wikipedia

en.wikipedia.org

 

[charleybarley]

Can we necessarily believe all that we read?

The J91 was proposed for a Mach 3 aircraft and "it was a Mach 3 engine" and "the compressor was about the right pressure ratio and configuration for a Mach 3 engine." Also "the overall performance was consistent with what was predicted".
Apparently its 400 lb/s size was chosen as the biggest engine that could run at Mach3 in the Willgoos altitude chamber, and it seems in its 500 hours testing it did run in the chamber, presumably up to Mach 3. Then it was cancelled.

It was subsequently scaled down for a proposal for a Navy Mach 3 aircraft as the J58-P2. So presumably this too was a Mach 3 engine. (It ran for 700 hours before being cancelled).

Then the story seems to fall apart. When the J58 was resurrected for Lockheed a performance program was written (so it had run for 700 hours without one?) which showed it wouldn't even get to Mach 2.5 so it can't have run at Mach 3 at Willgoos.

So it wasn't a Mach 3 engine after all? Did its Mach capability not come over with the J91 scaling?

Any comments? Perhaps I'm out to lunch.

 

[aim9xray]

There is some scant evidence that the J58-P-2 was planned for a A3J Vigilante derivative where the non-afterburning P-2 would have replaced the the afterburning J79s, with an accompanying reduction in fuel consumption and increase in range - yes, supercruise at Mach 2+. But not Mach 3.

 

[Archibald]

https://www.secretprojects.co.uk/threads/north-american-xb-70-valkyrie.16322/page-9#post-643494

 

[charleybarley]

The J91 was a Mach 3 success in testing. I wonder what crutches the compressor used to run efficiently at M3 (ie at only 66% of take-off airflow) bearing in mind what its 80% scale version would subsequently need to get beyond M2.5 (JT11D-20 for Blackbird).

 

[F119Doctor]

I’m just guessing:

It is possible that the J91 was using a limited amount of overboard compressor bleed to maintain compressor stall margin. There may be a difference between operating at M3 inlet conditions on a test cell and making usable thrust at that condition. In a real aircraft, the inlet will still be creating a large pressures recovery and they may not have determined what to do with the bleed air besides dumping it in the nacelle - a problem for later…

The other possibility is that the M3.2 design point for the J58 is significantly hotter than M3 for the J91, making the issue just that much more severe.

 

[Scott Kenny]

I’m just guessing:

It is possible that the J91 was using a limited amount of overboard compressor bleed to maintain compressor stall margin. There may be a difference between operating at M3 inlet conditions on a test cell and making usable thrust at that condition. In a real aircraft, the inlet will still be creating a large pressures recovery and they may not have determined what to do with the bleed air besides dumping it in the nacelle - a problem for later…

The other possibility is that the M3.2 design point for the J58 is significantly hotter than M3 for the J91, making the issue just that much more severe.

Given that the J58 was using "directionally solidified" aka single-crystal castings and was still thermally limited to... 472degC(? IIRC), I suspect that the J91 would also have issues. Especially if the J91 wasn't using single-crystal castings.

edited for emphasis

 

[F119Doctor]

Given that the J58 was using "directionally solidified" aka single-crystal castings and was still thermally limited to... 472degC(? IIRC), I suspect that the J91 would also have issues. Especially if the J91 wasn't using single-crystal castings.

edited for emphasis

427C is the inlet temperature limit, not the turbine temperature limit where the directionally solidified blades are located.

The J58 control system included an electronic EGT limiter / trimmer along with rotor speed governor using exhaust nozzle area. When the engine was at M3+ cruise conditions in AB, the rotor speed was held constant by the nozzle. If the rotor speed dropped below the target speed, the nozzle opened to reduce pressure aft of the turbine, increasing the pressure drop across the turbine, which increased the rotor speed. The EGT trimmer allowed the pilot to bump up fuel flow to reach the cruise target EGT. Increasing fuel flow increases RPM, the nozzle trims closed to hold RPM, AB pressure and thrust increases. When the SR-71 hit a cold patch of air at cruise altitude, the inlet temperature and EGT would drop in response, allowing a significant increase in engine exhaust pressure and thrust. On a really cold day at altitude, the aircraft could hold cruise speed at close to minimum AB. On a hot day, it might not hold M3.2 at Max AB.

I believe the 427C limit is to protect the titanium blades in the front half of the compressor, and not any turbine temperature limit