Some aircraft just go on forever. The DC-3/C-47s in SAAF service are 80 years old and – as ‘Dakeltons’ – are still expected to meet South Africa’s huge maritime patrol requirement. This is especially because chronic underfunding has reduced South Africa’s ability to patrol from St Helena to the Antarctic to wishful thinking.
IN THE USA THE equivalent of our ‘Dakelton’, despite the massive defence budget, is the B-52 which after 60 years, is finally getting new engines – to keep it current to 2050, by when it will be 100 years old.
The big surprise, at least to me, is that they are keeping the eight small-engine configuration, rather than going for the presumably much cheaper four medium-sized engine option.
Since the 1960s the U.S. Air Force and Boeing have been investigating ways to replace the engines. “The oldest suggestion of a potential re-engine for the B-52 that I’ve seen was from 1969,” says James Kroening, Boeing’s B-52 programme manager. That wasn’t long after U.S. Strategic Air Command began flying the B-52B in 1955, carrying nuclear bombs to deter the Soviet Union.
Now, after starring roles in the Cold War, the Vietnam War, the Persian Gulf War and the wars in Afghanistan and Iraq, equipping the B-52 fleet with new engines is an idea whose time has finally come.
At least four formal proposals to re-engine the bomber and more than twice as many studies on the subject date back to the 1970s. Up to now the closest it came to happening was in 1996 when Rolls-Royce and Boeing jointly proposed fitting each B-52 with four leased Rolls-Royce RB211-535 engines. This would have involved replacing the eight Pratt TF33s with a total thrust 136,000 lb with four RB211 engines with a total thrust 148,000 lb. The extra thrust and reduction in fuel consumption would cost approximately US$2.56 billion for the whole fleet.
‘the eight TF33s could be replaced with just one GE9X engine’
As a side note, it is amazing to note how far jet engines have come – the total of 136,000 lbs of thrust from the eight TF33s could be replaced with just one Boeing 777X GE9X engine which produces 134,300 lbs of thrust. It also makes me wonder why they don’t just convert a fleet of Boeing 747s or 777s into bombers. But I suppose the airlines might not be too happy with their airliners being the same as bombers.
Nonetheless, a workable solution to keep the B-52 going was needed, and that meant new engines. But each proposal failed, primarily because the USAF always believed the B-52 would be replaced by newer bombers, specifically the B-1 and B-2. The USAF views B-52s as more versatile than B-1Bs or B-2s, because the B-52s can carry a wide range of conventional munitions as well as the nuclear-capable Long Range Standoff cruise missile that could debut in 2030. Plus, B-52s are cheaper to fly and require fewer maintenance man hours than the 1970s-designed B-1B and 1980s-designed B-2.
Understanding the limitations of the B-1 and B-2, the Defense Science Board (DSB) had urged the Air Force to re-engine the aircraft without delay, saying doing so would not only create significant cost savings, but reduce greenhouse gas emissions and increase aircraft range and endurance. An interesting aspect is the cost of aerial refuelling; the DSB estimated that refuelling in the air cost $17.50 per gallon, whereas the Air Force had only priced on the ground fuel at $1.20 per gallon.
The Air Force also suspects that the stealthy advantages of the B-2 will have waned by the 2030s, noting in its “Bomber Vector,” as it calls the bomber plan released along with the 2019 budget request, that the B-2 “will see its technological advantages diminish in the not-too-distant future.”
by 2030 the engines will be “unsustainable.”
The impasse remained however until in April last year when the USAF admitted that by 2030 the engines will be “unsustainable.” That was a problem, because the USAF announced that by the mid-2030s, it plans to fly just two kinds of bombers for conventional and nuclear-deterrence missions: The Big Ugly Fat F – cker, or BUFFs, and a planned fleet of 100 B-21 Raiders, the Northrop Grumman stealth bombers.
And so here we are, sixty years later, with the ‘Buff’ still with its smokey old eight Pratt & Whitney TF33 engines. Now with the latest announcement, the Buff will live on – for another 30 expected years, until 2050 no less, thanks to the long, long-awaited announcement that they will be getting new engines. And not good old American Pratt & Whitneys or General Electric engines – but Rolls Royce.
The key to modern aircraft are the engines and massive electrical power generation – to run the demands of modern avionics and aircraft systems. “If you step back and look at this re-engine program, electrical power is probably the single biggest area of improved performance that is a requirement,” says Kroening, Boeing’s B-52 manager.
The USAF is not saying how much extra electrical capacity the Air Force wants, but greater electric power is needed to run a host of future improvements to the B-52, from defensive directed energy weapons systems and hypersonic weapons to new sensors and avionics.
Much work has already been done to bring the analogue B-52 into the digital age. This includes new combat network communications technology, a weapons bay improvement programme that will enable the B-52 to carry smart weapons internally, Link-16 tactical data link capability and upgraded GPS interface units.
With the need to operate to 2050, more electrical power and greater fuel efficiency, early in 2020 the USAF released a request for proposals for 608 commercial engines plus spares and support equipment, with the plan to award the contract in May 2021. And as noted – the decision had been made to retain the eight-engine configuration. This Commercial Engine Re-engining Program (CERP) saw General Electric propose its CF34-10s and Passport turbofans, Pratt & Whitney its PW800, and Rolls Royce its F130. They are all engines currently used in bizjets: the Rolls Royce on the Gufstream G650.
With hindsight, it now seems sensible to retain the eight-engine design. Large diameter high bypass turbofans would not have enough ground clearance on the outer pylons, especially considering the lack of stability of the aircraft’s narrow tandem wheel undercarriage design. Another less obvious challenge is the effect on the wing harmonics of having just four or even two engines. Furthermore, it would require large structural modifications, including to the empennage, to provide adequate control.
In late September 2021 it was announced that Rolls-Royce had won the competition with their F130 engine. Using a British/European engine was perhaps not such a big surprise as the F130 was already in use by the E-11 and C-37 so it’s a known quantity and will reduce the support costs. Also, it is already actually built in the USA.
There are many other noteworthy benefits to the re-engining. The Air Force’s quick-start requirement for a nuclear mission is one.
The B-52 lacks onboard starting capability. The bomber’s original TF33 engines are started in two ways: for normal operations the B-52 starts pneumatically. A ground crew wheels a cart-mounted auxiliary power unit, or APU, and attaches a hose to a fitting on the inboard pod on the left wing that houses the No. 3 and No. 4 TF33s. From the APU compressed air is supplied to rotate engine compressor spools. With sufficient pressurised air in the combustion chamber, fuel is introduced to the engine, ignited and the turbofan starts. Once these engines are stabilised, bleed air is sent from them to start the others. In addition, the ground APUs supply electric and hydraulic power to the aircraft without the need to start the engines.
Alternatively, explosive cartridges can provide quick engine starts for B-52s assigned to the nuclear alert mission. Gunpowder cartridges are inserted into breaches on the engines. Firing each cartridge ignites fuel supplied to each engine and starts all eight simultaneously.
While a large increase in power was readily available, aerodynamic and structural considerations made the USAF decide on keeping the total power largely unchanged. The Air Force decided on the same 17,000-pound thrust class as the TF33 to produce “no change to current minimum control airspeed” of the B-52 while “maintaining the aircraft’s current combat ceiling and takeoff performance.”
The new engines had also to be compatible with the existing B-52 electrical, hydraulic, pneumatic and fuel systems and be capable of being changed quickly if necessary. External weapons carriage should be “unaffected” by the new engines. “Struts and nacelles will be replaced, but it is our intent that the size, weight, thrust capacity, etc., of the engines are such that handling characteristics that are impacted won’t be a significant actor,” explains Kroening.
Each of the three bidding engine makers were confident their commercial powerplants would meet the Air Force’s requirements. The challenge is to adapt them for military use. For example, military engines and control systems must be nuclear-hardened. The radiation associated with a nuclear blast can physically damage semiconductors in electronics causing a variety of aircraft systems, including engines, to malfunction. Nuclear hardening makes electronic components resistant to radiation by using different manufacturing techniques and radiation-tolerant materials in the production of semiconductors.
They must also perform in circumstances commercial engines are not designed for, including aerial refuelling.
There are also issues to be solved with packaging commercial engines in a form that doesn’t deviate too much from the current pod configuration that houses two TF33 engines. The challenges are formidable.
Critics of these aging designs have often had to swallow their words. I am grateful to Martin Strümpfer of the SAAF WhatsApp group for reminding us that as far back as 1964 Gen. Curtis LeMay said “I am afraid the B-52 is going to fall apart on us before we can get a replacement for it.”
As a final thought, the old joke about ‘Methuselah aircraft’ remains particularly true for the B-52: “The B52 may well be doing the flypast when the B2 withdrawal from service ceremony happens.”