Can high-bypass turbofans cruise at high altitude?

Question

They have talked for years about re-engining the B-52 with something more modern than its TF33-P-3/JT3D turbofans, which are fairly low-bypass (1.42:1 for the JT3D).

Modern engines are much higher bypass - like the GE Passport at 5.6:1, similar to the CF34 - in the same weight class and thrust as the TF33. A larger engine like the LEAP has an even higher bypass ratio, e.g. 11:1.

Now, the B-52 has a published operating ceiling of 50,000 feet. The 737, A320, Bombardier RJs etc. have a ceiling of 41,000.

However the KC-135s, re-engined with semi-modern CFM56 high-bypass turbofans, have a ceiling of 50,000 feet.

So I am confused. Does the bypass ratio have an impact on the practical ceiling of the aircraft? Could the B-52 fly to current spec with engines like the GE Passport?

A side question... the TF33 claims a specific fuel consumption of 0.56 lb(lbf-h). I can't seem to find that figure on the most recent high-bypass turbofans. Is it inapplicable to high-bypass turbofans? If not, is there a viable way to figure what a modern engine would do to the B-52's range?

Answer 1

Yes in principle, but some modifications are advisable.

What limits the maximum operating altitude of a jet engine (besides the thrust needed to climb up there) is the length of the combustion chamber and the absolute pressure of the air entering it. Since the atmospheric pressure drops with altitude and the compression ratio of the compressor stays constant, the absolute pressure in the combustion chamber drops with altitude.

For combustion to take place, first the fuel droplets injected into the air stream need to evaporate. This is made easier by higher temperature and pressure, and the lower both are, the more residence time of the fuel-air mixture (caution, PPT link) in the combustor is required for good combustion. Longer combustors have higher pressure losses and weigh more, so engine designers try to limit their length.

Just throttling down can already cause the ignition to stop at high altitude. If a flame-out occurs, the engine cools down quickly and a restart at a lower altitude might not be possible. Turbofans suitable for high altitude operations use special measures to stabilize the flame. When they are in place, there is no reason the B-52 can't continue to fly at 50,000 ft.

Answer 2

If we define thrust T of a turbofan as

$$ T = \dot{m} \cdot (V_e - V_0)$$

with:

• $\dot{m}$ = total mass flow through both the gas generator and the fan
• $V_e$ = average exhaust speed
• $V_0$ = airspeed

we can see that thrust is proportional to mass flow. At higher altitude the air density is lower, so mass flow through the engine is lower, but as long as combustion can be maintained, the engine at altitude can still deliver a fraction of its sea level thrust. This fraction needs to be sufficient to propel the aircraft at cruise speed of course, plus excess climbing power for reaching the cruise altitude, so TO thrust needs to be sufficient for this. Just a matter of dimensioning the engine.

The operational ceiling of an aircraft is a function of the wing geometry: wing loading, aspect ratio, wing profile, taper ratio, twist. For a given aircraft that is re-engined, operational ceiling would not change.

The bypass engine is powered by a gas generator, identical to a single stream turbojet - the higher bypass engine operational limits are the same as those for a single stream turbojet.

Answer 3

Although of course the engines will be important in allowing an aircraft to reach its service ceiling, the choice of service ceiling has more to do with the aircraft than the engines in this case.

Airliners are designed to to be economical to operate. Flying higher puts more stress on the fuselage to maintain the same cabin pressure. To guard against the higher risks in the case of decompression, regulations regarding emergency oxygen supplies may have higher requirements for aircraft flying this high. Peter discusses the issues in engine design for these altitudes. All of these costs are just not worth any benefits of flying higher. See also: Why are many jet aircraft designed to cruise around FL350-370?, including another good answer from Peter.

Military aircraft are designed to different requirements. They will not accumulate flight cycles nearly as fast as an airliner, reducing stress put on the fuselage. In the case of the B-52, only the crewed areas in the forward fuselage are pressurized, reducing the amount of reinforcement needed. Thus the costs are lower, and military aircraft have more emphasis on performance, providing the incentive for higher ceilings. Business jets have the same low-cycle high-performance requirements.

You mention the CFM56 and GE Passport. As you note, the 737 and A320 use the CFM56 engine and have lower ceilings, while the KC-135R has a ceiling of 50,000 feet. The GE Passport powers the Bombardier Global 7000/8000 which have service ceilings of 51,000 feet. So there is no reason to doubt that high bypass turbofans can function at these altitudes.

Thrust Specific Fuel Consumption (TSFC) is very dependent on conditions. It's available if spend some time looking but you have to be careful that you are comparing the same conditions. This site lists military aircraft engines. The JT3D/TF33 of the original KC-135 and the B-52 is around 0.535 lb(lbf-h). The F108-CF-100 on the KC-135R is listed at 0.363 lb(lbf-h).

This table lists specs for commercial jet engines.