Rolls-Royce To Begin Unblended SAF Tests

 - November 24, 2020, 12:00 PM
Engineers prepare a Rolls-Royce Trent test engine for operational evaluation on 100 percent SAF today at the company's facility in Derby, UK. The test program will also involve one of the manufacturer's Pearl business jet engines, and test flights. (Photo: Rolls-Royce)

With the intense focus of the aviation industry on the lowering of its carbon footprint, Rolls-Royce is looking ahead to the day when sustainable aviation fuel (SAF) becomes more widely available and will begin exploring the use of 100 percent SAF on one of its engines this week.

The powerplant maker will conduct ground testing with one of its Trent engines on a stand at its Derby, UK test facility, using SAF produced by World Energy in California and delivered by SkyNRG.

The program is aimed at determining how the company’s current engine designs operate on “neat” or unblended SAF and what future modifications could benefit them. According to Simon Burr, the company’s director of product development and technology, Rolls-Royce will induct an engine from its Pearl familywhich powers the in-flight-test Gulfstream G700 and Bombardier Global 5500 and 6500—into the 100 percent SAF test program in the new year at its Germany location. “I would just say I am less worried about relight and the operability of the engine,” Burr told AIN. “I don’t think those things are a major consideration, it’s mainly the issue of [fuel system] sealing in particular that is the prime anxiety.”

Currently, to be considered a drop-in replacement for standard jet-A and to meet the specification for ASTM D7566, SAF must be blended to ensure the presence of proper amounts of aromatic compounds, which are generally lacking in neat SAF.

“These fuels have low aromatic content, and that will give a positive in that you are likely to get less emissions of particulates,” Burr explained. “The downside is that in sealing systems, certain types of seals, particularly nitriles, they swell in jet fuel and the problem is if you go and switch from jet fuel to SAF with a low aromatic content, the seals can then shrink and the consequence to that is you get leakage.”

Burr noted that in the company’s modern engines, nitrile seals have been supplanted with those made from synthetic materials such as fluorocarbon and fluorosilicon, which are unaffected by aromatic content. “I’m confident the fuel we’re testing would work quite happily on all our current engines,” he said, adding that he would expect to soon see an airframer such as Gulfstream announce a test flight on 100 percent SAF.

“But we have to support our engines going back decades and so we have to be diligent and say, 'What would we require if we want to run 100 percent SAF on an engine that was built 30 years ago?'" As the test program continues, Rolls-Royce will continually reassess the status of the market to decide if it needs to begin backtesting its legacy engines.

In addition to lower lifecycle carbon emissions, SAF has other benefits over conventional jet fuel, including slightly higher energy density and better thermostability.  “Jet-A is actually quite a wide spec, and sometimes the fuels are not terribly thermally stable, so you end up with gumming for the fuel system and there’s more maintenance required,” Burr said. With 100 percent SAF offering improvement in that area by as much as 50 degrees, that could open up new possibilities for engine-makers in terms of engine architecture, “but it would be a brave person that just designed for an SAF-only world at this moment in time,” he added.

It isn’t only the engines that need to be considered when contemplating the use of unblended SAF, but the aircraft’s entire fuel system as well. Those systems might also contain nitrile-based seals, which could cause leakage concerns. While airframers may incorporate specific changes into their new designs to accommodate the eventual use of 100 percent SAF, until then that presents other concerns.

“The issue we’re sensitive about is you don’t want to have a situation where you have different infrastructure for two different types of fuel, the kind of diesel-petrol thing where people are worried about putting the wrong stuff in their machine, with all the safety hazards that go with that in flight,” said Burr. “In a perfect world, the oil companies will be able to create a formulation of [neat] SAF that is a drop-in replacement, so it will have enough aromatic content so it's just a subset of jet-A, you can plug it in, mix it up, it doesn’t matter.”

The Trent engine selected for the test is equipped with the manufacturer’s new ALECsys (advanced low emissions combustion system), part of its UltraFan next-generation high-bypass ratio engine demonstrator program that is anticipated to offer a 25 percent fuel savings over the standard Trent. Burr insists that the test results, which Rolls-Royce will share, will be applicable to regular engines. “Just to be clear, the test has nothing to do with the combustion system, it just happens we’ve got an engine test vehicle with very sophisticated emissions measurement on it, so we’re using that.”

With the total SAF available at present only equating to 0.1 percent of global jet fuel production, but with aspirations to boost that level to 2 percent in the coming decade, Burr acknowledged that these tests are looking out over the horizon, in terms of furthering the industry’s goals as well as Rolls-Royce’s own efforts, even as the world remains fundamentally changed. “Covid squeezes every budget,” he said. ”The fact that we’re doing this tells you that we regard it as important.”