How quickly can aviation make the transition to green power?

Avi­a­tion has yet to end its 70-year depen­dence on the high­ly pol­lut­ing cock­tail of hydro­car­bons known as kerosene, but sev­er­al ini­tia­tives are pro­pelling the indus­try towards a clean­er future. These are focused on sus­tain­able avi­a­tion fuel (SAF), liq­uid hydro­gen and elec­tric pow­er, both pure and hybrid. 

SAF can be obtained from a range of renew­able sources – for instance, forestry waste, used cook­ing oil, food pack­ag­ing and even dis­pos­able cof­fee cups that would oth­er­wise be des­tined for land­fill. Since much of the renew­able bio­mass that’s used in its pro­duc­tion absorbs car­bon from the atmos­phere, SAF has “the poten­tial to reduce green­house gas emis­sions by up to 80% com­pared with con­ven­tion­al jet fuel”, accord­ing to Shell.

Many air­lines have pledged to adopt SAF. For instance, BA’s own­er, the Inter­na­tion­al Air­lines Group, has com­mit­ted to pow­er­ing 10% of its flights with the fuel by 2030. In May it took deliv­ery of a batch of SAF pro­duced in the Lin­colnshire refin­ery of US oil com­pa­ny Phillips 66. This will be added to the pipeline infra­struc­ture that feeds the UK’s air­ports.

Sim­i­lar­ly, Unit­ed Air­lines has agreed to buy up to 52.5 mil­lion gal­lons of SAF from Finnish refin­er Neste over three years to fuel its ser­vices from Amsterdam’s Schiphol air­port.

Unit­ed Air­lines’ chief sus­tain­abil­i­ty offi­cer, Lau­ren Riley, reports that “demand from cus­tomers to lim­it their fly­ing emis­sions is grow­ing expo­nen­tial­ly. This agree­ment means that cus­tomers tak­ing flights from Ams­ter­dam and, poten­tial­ly, oth­er air­ports will be part­ners in our sus­tain­abil­i­ty efforts.” 

The search for a sustainable alternative

Despite these lead­ing exam­ples, more con­cert­ed action is still required to ensure indus­try-wide uptake, accord­ing to Simon Burr, direc­tor of engi­neer­ing and tech­nol­o­gy in Rolls-Royce’s civ­il aero­space arm. Bar­ri­ers to the large-scale use of SAF remain, he says. Chief among them are its cur­rent lack of “avail­abil­i­ty, scal­a­bil­i­ty and afford­abil­i­ty. We there­fore need action on a glob­al scale in forums such as the gen­er­al assem­bly of the Inter­na­tion­al Civ­il Avi­a­tion Orga­ni­za­tion. It is vital that gov­ern­ments agree on a long-term decar­bon­i­sa­tion tar­get.”

Pro­mot­ing the use of SAF is a key part of Rolls-Royce’s sus­tain­abil­i­ty strat­e­gy. The com­pa­ny has suc­cess­ful­ly test­ed sev­er­al of its engines to ver­i­fy their com­pat­i­bil­i­ty with the fuel. Its next-gen­er­a­tion engine demon­stra­tor, Ultra­Fan, is set to under­go sim­i­lar test­ing. 

“Thanks to rig­or­ous test­ing, we know our engines can oper­ate on SAF,” says Burr, who adds that, if pro­duc­tion can be scaled up suf­fi­cient­ly, “uptake will improve afford­abil­i­ty and cre­ate an envi­ron­ment where more air­lines can adopt SAF as part of their own flight to net zero.”

Liq­uid hydro­gen is anoth­er clean­er alter­na­tive to kerosene that’s come on to the industry’s radar. The British gov­ern­ment is pump­ing £27.2m into a scheme led by GKN Aero­space to devel­op a hydro­gen propul­sion sys­tem for small­er air­craft that could be scaled up. The H2Gear project is aim­ing to enable flights by 2026, using a sys­tem that con­verts hydro­gen into elec­tric­i­ty using a fuel-cell sys­tem. The process emits no CO₂.

Part­ners in the ini­tia­tive include Aeris­tech, Intel­li­gent Ener­gy and the uni­ver­si­ties of Birm­ing­ham, Man­ches­ter and New­cas­tle, while easy­Jet will pro­vide insights into oper­a­tional require­ments and options for flight demon­stra­tions.

“Tech­nol­o­gy is a key dri­ver to achieve our decar­bon­i­sa­tion tar­gets, with hydro­gen propul­sion a front-run­ner for short-haul air­lines like us,” says easyJet’s direc­tor of flight oper­a­tions, David Mor­gan. His air­line is opti­mistic that “it could begin fly­ing cus­tomers on planes pow­ered by hydro­gen com­bus­tion, hydro­gen-elec­tric pow­er or a hybrid of both by the mid to late-2030s”.

Making old machines green

While the arrival of new hydro­gen-pow­ered air­craft will cre­ate a new cat­e­go­ry of clean­er air trav­el, anoth­er approach is to retro­fit exist­ing planes with the tech­nol­o­gy.

ZeroAvia’s HyFly­er II project is devel­op­ing hydro­gen engines for demon­stra­tion on a 19-seater air­craft that it hopes will fly next year. The Anglo-Amer­i­can com­pa­ny will work with Canada’s MHIRJ Avi­a­tion Group to design and equip region­al jets (air­craft designed to car­ry few­er than 100 peo­ple on short-haul flights) with a 600kW pow­er­train. 

At the annu­al sum­mit of the World Eco­nom­ic Forum at Davos in May, the founder and CEO of ZeroAvia, Val Mif­takhov, declared that the hun­dreds of region­al flights tak­ing place dai­ly across North Amer­i­ca “can and should be zero-emis­sion well before the end of this decade. This agree­ment is a giant step for­ward in deliv­er­ing hydro­gen-elec­tric engines to the region­al jet seg­ment.”

The ven­ture has secured air-safe­ty exper­i­men­tal cer­tifi­cates for its pro­to­type air­craft from UK and US air­wor­thi­ness author­i­ties, passed sig­nif­i­cant flight tests, and estab­lished part­ner­ships with orig­i­nal equip­ment man­u­fac­tur­ers and glob­al air­lines. It’s on track to start com­mer­cial oper­a­tions in 2024. 

Uptake will improve afford­abil­i­ty and cre­ate an envi­ron­ment where more air­lines can adopt SAF as part of their own flight to net zero

ZeroAvia is also devel­op­ing a 5MW mod­u­lar pow­er­train for use in 40- to 80-seat tur­bo­prop air­craft – a project with back­ers includ­ing Alas­ka Air and Unit­ed Air­lines. 

As urban road net­works strug­gle to accom­mo­date com­muter traf­fic, the only way is up. That’s the ratio­nale behind the evo­lu­tion of a new cat­e­go­ry in avi­a­tion: advanced air mobil­i­ty – a mash-up of urban air mobil­i­ty (fly­ing taxis) and com­muter air­craft.

“These air­craft will intro­duce all-elec­tric fly­ing, which is qui­eter and more effi­cient, enabling us to reduce trav­el times dra­mat­i­cal­ly,” says Matheu Parr, cus­tomer busi­ness direc­tor at Rolls-Royce. 

The com­pa­ny is plan­ning to intro­duce all-elec­tric and hybrid-elec­tric engines for ser­vice by the mid-2020s. To this end, it’s using next-gen­er­a­tion demon­stra­tor air­craft, includ­ing the Spir­it of Inno­va­tion. Pow­ered by a 400kW unit, it can claim to be the world’s fastest all-elec­tric vehi­cle, hav­ing topped 387mph dur­ing a test flight. 

Rolls-Royce has also been work­ing with man­u­fac­tur­ers Tec­nam and Rotax to com­plete flight-test­ing of a hybrid-elec­tric plane pow­ered by par­al­lel-hybrid propul­sion – tech that could be applied to larg­er air­craft. 

Data gleaned from tests is already being applied in the urban air mobil­i­ty mar­ket. A Rolls-Royce unit has been cho­sen by Ver­ti­cal Aero­space for its all-elec­tric VX4 air­craft, which is designed to trans­port a pilot and four pas­sen­gers, emis­sion-free, over 100 miles at 200mph. The Bris­tol-based firm is aim­ing to secure cer­ti­fi­ca­tion for VX4 in 2025 and has gar­nered con­di­tion­al orders and pre-order options for up to 1,350 air­craft from play­ers includ­ing Amer­i­can Air­lines and Vir­gin Atlantic.

Electric aircraft versus sustainable aviation fuel

Whichev­er alter­na­tive ener­gy source becomes the preva­lent choice, one cer­tain­ty is that tomorrow’s skies will fea­ture a broad­er spec­trum of vehi­cle types. This presents an oppor­tu­ni­ty for dis­rup­tors to shape the future of ener­gy-sav­ing propul­sive tech­nol­o­gy.

Bed­ford-based SME Blue Bear Sys­tems Research is lead­ing a sev­en-mem­ber con­sor­tium that’s devel­op­ing “a high­ly pow­er-dense, qui­et and effi­cient propul­sion mod­ule with zero tailpipe emis­sions”. The design can be adjust­ed for gen­er­al avi­a­tion air­craft, large car­go drones, air taxis and region­al planes. 

Co-fund­ed by the government’s Aero­space Tech­nol­o­gy Insti­tute pro­gramme, the Inte­grat­ed Flight Con­trol, Ener­gy Stor­age and Propul­sion Tech­nolo­gies for Elec­tric Air­craft (Incep­tion) project is focused on opti­mis­ing the con­ver­sion of elec­tri­cal ener­gy into thrust. 

The project start­ed in Jan­u­ary 2021 and pro­duc­tion of the com­plet­ed design is under way, with wind-tun­nel test­ing due to start immi­nent­ly. The plan is to install the mod­ule on an air­craft plat­form, aim­ing for air­wor­thi­ness cer­ti­fi­ca­tion in 2026.

The indus­try needs to have more faith in dis­rup­tive new entrants if it’s to have a green­er future, argues Blue Bear’s CEO, Dr Yoge Patel. 

“The UK is one of the most inven­tive nations, attract­ing a lot of invest­ment – and Blue Bear has had great gov­ern­ment sup­port,” she says. “But how do you get that brand cred­i­bil­i­ty when you’re a start­up?” 

The next big chal­lenge, once you’ve estab­lished a foothold in avi­a­tion, is stay­ing in the game, Patel says. And the third is to grow your busi­ness, either organ­i­cal­ly or through invest­ment, with­out los­ing the inno­v­a­tive cul­ture that “keeps you sharp, stops you becom­ing com­pla­cent and pre­vents you from being process-dri­ven”. 

Her firm, for instance, “set out to become an agile sys­tems inte­gra­tor. It was also impor­tant to cre­ate an agile cul­ture and not to be depen­dent on any of the large com­pa­nies”.

Ver­ti­cal inte­gra­tion has been cru­cial to the Incep­tion project. As well as pro­duc­ing the engine, Blue Bear pro­vides all the test­ing and eval­u­a­tion infra­struc­ture too. 

“We aren’t depen­dent on a third par­ty for that. We also do our own cer­ti­fi­ca­tion and assur­ance,” Patel says. “We have ver­ti­cals that we’ve joined togeth­er to allow us to go from an idea to the deliv­ery of flight-test­ed goods.”

That lev­el of con­trol is unusu­al for an SME. And, while this has giv­en the firm free­dom, there is a cru­cial fac­tor over which it has no sway: the reg­u­la­tion of the next gen­er­a­tion of ener­gy-effi­cient engines that will serve many forms of avi­a­tion, poten­tial­ly shar­ing the same air­space.

“The rules and pro­ce­dures will change,” Patel says. “The way the indus­try sup­plies air­wor­thi­ness evi­dence also needs to change if we’re going to do this more quick­ly and thor­ough­ly. Along with fund­ing, that is an absolute bar­ri­er.”

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