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Hanging New Engines On 60-Year-Old B-52 Bombers Will Raise Some Serious Risks

The U.S. Air Force wants to continue operating the venerable B-52 bomber through 2050. As I have written recently, that would make it the longest continuously operating jet in history.

In order to accomplish this, Air Force leaders plan to make major upgrades to the 76 B-52H bombers in their fleet, including the installation of new engines.

Today’s commercial turbofans are so much more advanced than the TF33 engines currently powering the bombers—there are eight on each plane—that the Air Force believes it can get a 30% gain in fuel efficiency and (given less weight) a 40% gain in range.

A request for proposals issued to industry in May also specified big gains in engine reliability, so that the new engines would never need to be removed from wings during the bombers’ remaining time in service.

The plan to buy new engines is called the Commercial Engine Replacement Program, or CERP, and if all goes as planned, the savings from operating with new engines would cover the cost of installation.

So what’s not to like?

The only real issue, as the Air Force’s solicitation acknowledged, is that there is “significant risk” in hanging new engines on such an aged plane. Those risks haven’t gotten much of a public airing, so what follows is a laundry list of issues that might arise.

The case for going forward with CERP is compelling. But if you think it’s going to be a piece of cake, read on.

Engineering data. Most of the history of manned aviation has unfolded since the B-52 was conceived in the late 1940s. The “H” variant of the bomber, which is the only remaining version in the Air Force fleet today, ceased production in 1962. There are no 3D models of the airframe that can facilitate the application of digital engineering methods to the reengining project. It will be harder to predict where structural stresses might appear in the process of adapting the airframe to new engines than it would be on more recent planes.

New nacelles. B-52 engines are podded in dual nacelles, with each structure containing two engines. No matter which engine is selected for the CERP effort, new nacelles will be required. Because the nacelles and engines are located well forward of the wing, the introduction of new structures could impact air flow on control surfaces such as the flaps, leading to stresses. Coping with those stresses could require modifications to the wing. The bigger the new engine and nacelle, the more likely such stresses become.

Power output. General Electric
, Pratt & Whitney, and Rolls-Royce all say they will offer engines for the CERP effort. However, all of the engines being proposed are significantly different from the TF33 currently powering the bomber. The TF33 is a first-generation turbofan derived from a pre-existing Pratt turbojet. If the new engines generate significantly more power than the existing engines, that could create structural stresses for pylons attaching the engine nacelles to the wings.

Engine controls. The B-52 bomber ceased production long before digital engine controls were introduced. The industry standard today for optimizing engine performance and safety is called a “full authority digital engine control,” which allows a computer to manage engine parameters based on sensor readings taken many times per second. It is much more advanced than the hydromechanical engine controls on B-52s today. The planes will need to be wired with sensors, displays and other hardware to implement the digital controls required for new engines.

Cyber safety. Hydromechanical controls are impervious to cyber intrusions, but electronic engine controls are driven by software that is potentially vulnerable to being compromised. The Air Force and contractors will need to implement cybersecurity measures to assure that the introduction of digital engine controls does not create operational weaknesses on the planes. Full-authority electronic controls do not typically permit a manual backup to manage engines if their software is impaired.

Certification challenges. B-52s will need to be recertified for flight when they receive new engines. The service would prefer that recertification be limited to the engines themselves. However, the more the engine deviates from the thrust and dimensions of the existing TF33, necessitating structural and other modifications, the more likely a “re-cert” of the whole airframe will be required. Recertification of whole planes can be a protracted, laborious process that turns up problems not previously anticipated.

Supply chain. The engines likely to be offered by industry for the CERP project vary considerably in terms of their age and flight hours. Newer engines usually have a robust supply chain, whereas mature engines approaching the end of their life cycle are more likely to have parts and supplier issues. Thus the selection of an engine needs to take into account the likely viability of engine supply chains as the B-52 continues operating to mid-century, and perhaps beyond. The Air Force has had chronic problems with parts obsolescence and suppliers exiting the market on older planes in its fleet.

Sustainment process. Closely related to the supply chain concern is the challenge of sustainment—keeping the engines ready for combat on short notice—over a multi-decade period. The existing TF33s are fully integrated into the Air Force’s sustainment system and well understood by maintainers. That is not going to be the case with the commercial engines that replace them, so there will be a learning curve likely to impact aircraft availability. Aircraft operated in small numbers such as the B-52 generally have higher per-aircraft sustainment costs than those operated by the hundreds, and picking the wrong new engine could exacerbate this problem.

The Air Force has already anticipated many of these issues by funding offerors to assess the risks associated with reengining the B-52 and devise solutions. Ideally, what it needs is an engine that mimics the thrust and dimensions of the legacy engine while offering greater fuel efficiency and reliability. What it doesn’t need is a reengining program that requires complex modifications, structural fixes, and cyber innovations potentially leading to a prolonged certification process.

B-52 integrator Boeing
and engine manufacturer Pratt & Whitney contribute to my think tank.

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