Great article about the B-52 reengining saga. Changing an engine type on an existing platform is always problematical for various reasons. One good
example is the DC-8. They took the Dash 60, with JT3D engines, and turned them into the Super 70, with CFM56 engines. They found that they actually
lost some performance on takeoff due to the new engines, even while saving in fuel burn, and reducing the noise footprint.
The KC-135 changed to the TF33 engines in the 1980s, and saw a 14% savings in fuel, along with a 7,000 pound increase in thrust, and 20% more offload.
Then in the 1990s, they changed to the CFM56 engine. The aircraft saw a 100% increase in thrust over the original engine, 25% increase in
efficiency, 60% increase in range, 50% more offload, and 25% decrease in operating costs.
But then you run into the serious problems associated with changing the engine types. The KC-135 had to have an Engine Failure Assist System (EFAS)
added to the aircraft, because if they lost an engine on takeoff, the pilot wouldn't be able to react fast enough to save the aircraft. Which ties
in directly with the major problem the B-52 would encounter. When the B-52 was designed, it was designed for high altitude flight and bombing, so it
was designed with a high tail, and small rudder. The tail had to be tall to offer directional stability at high altitude. The rudder and elevator,
contrary to most aircraft that have a 25% chord to give sufficient effectiveness, the B-52 has a 10% chord on both. This is because it was found that
the rudder and elevators would exceed the critical Mach number before the wing.
But when the F model was redesigned to the G model, the aircraft was to be used to fly at low level, which meant the tail had to be cut in half, and
widened to reduce flutter conditions at low level. This means that the stability at high altitude is reduced, especially during in flight refueling.
The rudder, as it is now, can only compensate for an asymmetrical condition caused by an engine out on takeoff, under a very small speed window, and
only with certain engines. If they don't immediately put in full rudder, then minimum control speeds can increase as much as 25 knots. The TF33
engines currently in use, had to be derated, because if they were operated at full power on takeoff, then the rudder wouldn't be able to compensate
for an asymmetrical condition if an engine failed.
All of this plays a big role in the way forward for replacing the TF33 engines on the B-52. If they were to go to a four engine setup, the rudder
would need a complete redesign. It would not be capable of controlling the aircraft in an asymmetric condition. If they do a one to one replacement,
they'd have to either derate the engines again, or design a new rudder.
It seems that the USAF cannot get through a year without being inundated with calls to replace the engines on the B-52. Recent calls from Air
Force Global Strike Command, Boeing, General Electric and Pratt & Whitney have renewed the vigor of the program despite the general lack of
funding and enthusiasm for large dollar aircraft procurement. Fuel savings and increases in capability are touted as the driving factors in these
programs. However, for the B-52 these arguments show little merit when they are studied with any depth.
Where it concerns the B-52, we are fortunate that there is a long and well documented history of the design process. Not only do we know the specifics
of the design, we know why it was designed a specific way. Its strengths and weaknesses have been well vetted through a half century of combat. That
history can guide us towards a full understanding of what a re-engine program may entail.
Upgrades to existing systems are done for a variety of reasons. Typical upgrades are used to correct a deficiency or to create a required capability.
There have been cases where upgrades are done to ensure a particular system remains viable in a new environment. Sometimes upgrades solve logistical
problems as certain parts are no longer produced and an upgraded system is necessary to continue using the whole aircraft. For example, in the early
2000s the B‑52 upgraded its 1970s‑era beryllium ball inertial navigation system with a modern ring-laser gyro system in order to correct a failed
supply line, not to increase its navigation performance.