NGAD engines

[Gums]

Salute!

I moved this to the Technology sub-forum, but need to get the engine Doc, TEG and Spurts to join.

I am looking at the two competitors, the 102 and 103, and the "variable" this and that, and modes that resemble what Pratt did 50 years ago with the J-58. That sucker shifted/diverted a lot of air and used intake geometry and exhaust bypass and other stuff to get the Blackbird up to and cruise at Mach 3.

Tap, tap, motor Doc and TEG, and what Spurts can contribute and so forth.

Gums sends...

[sprstdlyscottsmn]

Salute!

Tap received.

Not only did the airflow management on the J58 allow M3 cruise, but the sucker was more efficient at M3.2.

Makes sense to revisit that tech for efficient supersonic performance.

[f119doctor]

Yes, you can say that the J58 was a variable cycle engine, transitioning from a straight turbojet below M2.2 to a low bypass single shaft turbofan above M2.2. And the entire propulsion system (inlet, engine, nacelles, and nozzle) effectively functioned as a ramjet at M3.2 cruise with 90+% of the system pressure ratio occurring in the inlet.

But reason for the bleed bypass was not primarily for fuel efficiency, but to make the engine function with an 800F inlet. Even with the rotor turning at 100% mechanical RPM, the compressor is functioning as if it was only at 65% RPM (what is known as corrected speed), which just a little above Idle, with low airflow and pressure ratio. At these low compressor pressure ratios, the rear end of the compressor is too small to accept the brought in at the front, resulting in choked flow at the rear, which overloads the front end causing compressor stall. The bleed system vents the excess front end air to prevent the compressor stall. The genius of the bypass tubes was to capture the bleed air and re-introduce it to the exhaust flow at the front of the AB to contribute to thrust and cooling of the AB duct.

The other J58 / A-12 SR-71 invention was the blow in door ejector nozzle. At subsonic speeds, the blow in doors brought in additional air that filled the large divergent section of the nozzle. At supersonic speeds, the increased nozzle pressure ratio pushes the blow in doors shut, the nacelle cooling air (from the inlet aft bypass doors) is entrained around the convergent nozzle, and the large divergent flaps would float outward to efficiently accelerate the exhaust flow with the ever increasing Nozzle Pressure Ratio as Mach number increased. The TF30 on the F111 also used this nozzle design (turns out it is good for slow speed and very high speed, but draggy at transonic)

When I get home next week, I can go into some other variable cycle history and challenges of variable cycle engines.

[hornetfinn]

1960s was a really crazy decade for technology development. SR-71 is one of those things that just blows your mind and there is definitely a lot to learn from that project today as lot of that high-speed stuff was bit forgotten after it.

[Gums]

Salute!

Thanks for your help, Doc. Maybe I could get TEG to join up.

You got it right, Finn, was a good time to be in the bidness.

We found ways to get better range with new wings and better motors, use the new computing technology, guide bombs with lasers, have look-down-shoot down radar and to reduce RCS!!!!

Gums sends...

[f119doctor]

Single shaft engines like the J58 are simple in one respect. The power extracted by the turbine is controlled by the pressure drop across the turbine. The upstream pressure is set by the pressure / temperature of the combustor airflow thru the fixed area 1st stage nozzle vanes, and the downstream pressure is controlled by the exhaust nozzle area. If you open the nozzle, the exhaust pressure goes down, the pressure drop across the turbine increases, resulting in the rotor speed increasing. If you are governing the rotor speed, fuel flow has to be decreased to reduce the combustor temperature and pressure. I'll get back to this later.

A 2 shaft turbofan is relatively easy to make variable bypass, but to do it efficiently is another story. Both the F100 and F119 are variable bypass engines, over a small range. Below Mil power, they open the nozzle a bit to increase airflow and lower exhaust pressure/velocity to increase propulsive efficiency at subsonic speeds. But only a little bit.

On a two shaft mixed flow turbofan, the core turbine power is controlled by the fixed 1st stage area upstream, and the fixed LPT inlet vane area downstream. The LPT power has the fixed LPT inlet vane area upstream, and the variable exhaust nozzle area downstream. If you open the nozzle, the Fan / LPT rotor speeds up and the exhaust pressure goes down. To maintain LPT speed, you have to reduce core fuel flow, which slows down the core rotor speed, airflow, pressure, and temperature. The reduced exhaust pressure reduces exhaust velocity and thrust, which is good for subsonic propulsive efficiency, but the reduced core pressure (OPR) and temperature are bad for thermal efficiency. And on a conventional mixed flow turbo fan, opening the nozzle to increase bypass ratio mis-matches the core and duct bypass flow, with increasing duct flow becoming restricted in narrow duct passage and causing flow disturbances where they combine in the AB duct.

The GE YF120 had a more sophisticated approach, with a 2 stage low rotor fan with a fan duct exit shutoff, a core mounted 3rd stage fan with a variable inlet vane , and a variable ejector system (VABI) for smoothing the aft end bypass flow combining with the core exit. In the low bypass mode, it made very good supercruise thrust, but in the high bypass mode it still ran into the problem of the core running in a very low OPR mode that limited its subsonic thermal efficiency.

I am not familiar with all of the tricks in the latest XA100/101/102/103 adaptive engines. The third bypass stream is new method of increasing the bypass ratio and airflow for subsonic propulsive efficiency. But the real challenge will be how to make a large core with the temperature and speed margin necessary to produce high velocity thrust at the elevated inlet temperature condition of supercruise, while making it think it is a small, high OPR core at subsonic cruise. The tyranny of the fixed turbine inlet vane area causing low OPR in high bypass operations may be an area that is being addressed by these designs.