SABRE is a hy­brid rocket and jet, billed as ‘the big­gest break­through in aerospace propul­sion tech­nol­ogy since the in­ven­tion of the jet en­gine’

SP's Airbuz - - Table Of Contents - BY JOSEPH NORONHA

MOD­ERN JET EN­GINES ARE ar­guably among the most com­plex ma­chines ever made. Years of toil and bil­lions of dol­lars go into de­sign­ing and de­vel­op­ing these de­vices that are worth their weight in sil­ver. And since com­mer­cial en­gines must keep ex­pen­sive airliners with hun­dreds of pas­sen­gers flaw­lessly fly­ing, they have no room for er­ror. In­deed, jets are built not just at Six Sigma qual­ity stan­dards (3.4 de­fec­tive fea­tures per mil­lion in­stances), but when pos­si­ble, even bet­ter. En­gine tech­nol­o­gists strive to ma­chine thou­sands of com­po­nents to pre­cise spec­i­fi­ca­tions us­ing ever lighter and tougher ma­te­ri­als, and pack­ing them into the small­est pos­si­ble space. Their aim is to de­liver ul­ti­mate lev­els of power and ef­fi­ciency.

How­ever, power comes at a price. Avi­a­tion fuel is now much cheaper than in the mid-2014, when it con­sti­tuted al­most half an air­line’s op­er­at­ing cost. But the air­lines know that an­other sharp rise is cer­tain, sooner or later. That is why there is no slack­en­ing of de­mand for new and even more fuel-ef­fi­cient en­gines like Pratt & Whit­ney’s PurePower PW1000G, a geared tur­bo­fan en­gine that en­tered com­mer­cial ser­vice in Jan­uary this year and CFM In­ter­na­tional’s Lead­ing Edge Avi­a­tion Propul­sion (LEAP) tur­bo­fan that should fol­low in July.

Power also needs to be ‘pu­ri­fied’ be­cause of con­cerns over cli­mate change. Ac­cord­ing to the in­flu­en­tial Air Trans­port Ac­tion Group (ATAG), avi­a­tion pro­duced 770 mil­lion tonnes of CO in 2 2015 out of over 36 bil­lion tonnes gen­er­ated by hu­man ac­tiv­ity. Al­though com­mer­cial avi­a­tion is re­spon­si­ble for just two per cent of world­wide CO emis­sions, these may have a sub­stan­tial im­pact 2 on the at­mos­phere be­cause they are re­leased at high al­ti­tude. There­fore, pres­sure is mount­ing on the in­dus­try to achieve per­cep­ti­ble progress in im­prov­ing fuel ef­fi­ciency and re­duc­ing emis­sions and noise. And it has set an am­bi­tious tar­get: to slash net car­bon emis­sions to half of their 2005 level by 2050. This re­quires huge in­vest­ments and a mas­sive leap in tech­nol­ogy.

FAN­NING FRENZY. For the first few decades of avi­a­tion, it was the pis­ton en­gine cou­pled with the whirring pro­pel­ler that pro­vided power for flight. The Jet Age dawned in the early 1950s with fuel-guz­zling tur­bo­jets. Then tur­bo­jets mor­phed into tur­bo­fans in the 1960s and it took only a short pe­riod for these fu­el­ef­fi­cient en­gines to power prac­ti­cally ev­ery com­mer­cial jet.

In a tur­bo­fan, most of the cold air is sent around the en­gine while a small pro­por­tion flows through the core. This is be­cause it is more ef­fi­cient to ac­cel­er­ate a large amount of air a lit­tle than a small amount a lot. The pro­por­tion of air flow­ing around the core against that pass­ing through it is called the ‘ by­pass ra­tio’. Over the years, air in­takes have be­come ever larger to ac­com­mo­date larger and bet­ter fans. And mod­ern tur­bo­fans have by­pass ra­tios of up to 9:1 — one mea­sure of their ul­tra-ef­fi­ciency. But the longer fan blades mean their tips are now mov­ing at close to the speed of sound. And that is not heart­en­ing be­cause the re­sult­ing shock waves could trig­ger po­ten­tially dan­ger­ous vi­bra­tions.

Pratt & Whit­ney be­lieves that con­ven­tional en­gines have prac­ti­cally reached their limit of im­prove­ment and new think­ing is nec­es­sary. It spent about 30 years and over $10 bil­lion to de­velop its PurePower brand. PurePower en­gines use large fans (81 inches in di­am­e­ter on the Air­bus A320­neo), but a spe­cial gear­box makes the fan turn slower than the tur­bine. This is pos­si­bly the big­gest ad­vance in propul­sion phi­los­o­phy since the ar­rival of the tur­bo­fan. The gear­box keeps the ef­fi­ciency of each com­po­nent at its peak and in­creases the by­pass ra­tio to 12:1. Over­all, the A320­neo’s PurePower en­gine prom­ises 15 per cent lower fuel con­sump­tion than the stan­dard A320 as well as a dra­matic re­duc­tion in en­gine noise — up to 75 per cent lower on the ground. Nat­u­rally there is a price to pay — in­creased weight and aero­dy­namic drag. And the gear­box adds com­plex­ity to the en­gine, height­en­ing the risk of some­thing go­ing wrong some day.

Apart from be­ing of­fered as one of two options on the A320­neo, vari­ants of Pratt’s PurePower en­gine will also be in­stalled in Bom- bardier’s CSeries re­gional jets, the Mit­subishi Re­gional Jet (MRJ) and Embraer’s E-Jets E2 re­gional jets. It will also be of­fered as an op­tion on Rus­sia’s Irkut MC-21, a 150- to 212-seat nar­row-body air­liner.

CFM In­ter­na­tional, owned by Safran and Gen­eral Elec­tric, prefers the tried and tested route. It feels it can make a bet­ter en­gine than PurePower us­ing con­ven­tional tur­bo­fan ar­chi­tec­ture, with­out the added weight and drag of a gear­box, not to men­tion its com­plex­ity. The CFM LEAP is a high-by­pass tur­bo­fan, of­fered as an op­tion for the A320­neo and as the ex­clu­sive en­gine for the Boe­ing B737 MAX as well as the Com­mer­cial Air­craft Cor­po­ra­tion of China’s new COMAC C-919 air­liner. LEAP has more than 4,000 dif­fer­ent parts and some are sub­jected to tem­per­a­tures of ap­prox­i­mately 2,700 de­grees Cel­sius while turn­ing at 2,400 RPM. CFM copes with these harsh con­di­tions by us­ing ad­vanced light­weight com­pos­ite ma­te­ri­als such as car­bon fi­bre fan blades, achiev­ing what it calls ‘the ul­ti­mate re­fine­ment of the tra­di­tional tur­bo­fan en­gine.’ The tur­bine shroud, an­other highly stressed por­tion, is made from ce­ramic ma­trix com­pos­ites (CMC). CMCs have been around for decades but only re­cently has the tech­nol­ogy ma­tured enough to make it prac­ti­cal to use in jet en­gines.

All in all, the PurePower and LEAP are tech­no­log­i­cal mar­vels that seem set to power the bulk of the global com­mer­cial fleet for a decade or more. As to which will fare bet­ter in the mar­ket, air­line head hon­chos are no­to­ri­ously con­ser­va­tive and usu­ally pre­fer ice­cream in “any vari­ant of vanilla”. That is per­haps why the LEAP has more than 10,500 firm or­ders and com­mit­ments against over 7,000 or­ders and options for PurePower. How­ever, these are early days and the ri­vals are yet to prove them­selves in com­mer­cial con­di­tions. GE9X: BIG­GEST AND BEST? Since nar­row-body planes dom­i­nate avi­a­tion, tech­nol­o­gists mostly con­cen­trate on de­vel­op­ing new and bet­ter en­gines to power these. For the big jets, the un­der­de­vel­op­ment Rolls-Royce Trent 7000, des­ig­nated the ex­clu­sive

en­gine for the Air­bus A330­neo, has a 112-inch fan di­ame­tre and 10:1 by­pass ra­tio. Gen­eral Elec­tric is also go­ing all out to in­cor­po­rate ad­vanced tech­nolo­gies in the un­der­de­vel­op­ment GE9X power plant. It claims the GE9X will be its most fuel-ef­fi­cient en­gine ever, with a by­pass ra­tio of 10:1. It will have a sig­nif­i­cant fuel burn sav­ings: 10 per cent bet­ter spe­cific fuel con­sump­tion (SFC) than the GE90-115B en­gine that pow­ers the Boe­ing B777-300ER and five per cent bet­ter than any com­pa­ra­ble en­gine in ser­vice in 2020. It gen­er­ates thrust of 1,05,000 pounds (467 kN). Its fan is 133.5 inches in di­ame­tre — a world record. This helps draw in more air us­ing less en­ergy and op­er­at­ing qui­eter. The en­gine uses CMC ma­te­ri­als in its core for the fan case and fan blades. CMCs have twice the strength and bet­ter ther­mal ca­pa­bil­i­ties than their metal coun­ter­parts yet weigh just a third. The GE9X, that will power the Boe­ing B777-8 and B777-9 airliners, has al­ready at­tracted over 700 or­ders and is sched­uled to en­ter ser­vice around 2020. DOU­BLE BUBBLE. Jet en­gine tech­nol­o­gists like to do their thing — mak­ing com­plex ma­chines de­liver the most power with the greatest ef­fi­ciency — work­ing in­de­pen­dently. At the same time, air­frame designers sep­a­rately strive to build aero­dy­nam­i­cally ef­fi­cient struc­tures and worry about the power plant only at the ap­pro­pri­ate time. In fact, they usu­ally of­fer two or three dif­fer­ent en­gines as options to cus­tomers. For in­stance, the stan­dard A320 may be pow­ered ei­ther by CFM In­ter­na­tional’s CFM56 or by the V2500 from In­ter­na­tional Aero En­gines. Inevitably, when air­frame and en­gine are mar­ried up, there is a drop in pre­dicted per­for­mance which is in­con­se­quen­tial. How­ever, now that fu­el­ef­fi­ciency is a burn­ing is­sue, avi­a­tion tech­nol­o­gists have re­alised that to­day’s tube-and-wing design is near­ing its limit of ex­ploita­tion and in­te­grated en­gine-air­frame design is in­dis­pens­able.

In­te­gra­tion is what the Mas­sachusetts In­sti­tute of Tech­nol­ogy’s D8 fu­ture air­craft design con­cept is all about. It is sim­i­lar in size to the Air­bus A320/Boe­ing B-737 and claims to of­fer po­ten­tially huge ben­e­fits like 71 per cent re­duc­tion in fuel burn, 60 ef­fec­tive per­ceived noise level in deci­bels (EPNdB) less noise and 87 per cent re­duc­tion in land­ing and take-off (LTO) cy­cle emis­sions of ni­tro­gen ox­ides (NOx) com­pared with a B-737-800 air­craft. Ev­ery part of the air­frame and en­gine will be metic­u­lously re­con­fig­ured to max­imise ef­fi­ciency and min­imise op­er­at­ing costs. The fan will be large and fea­ture an im­pres­sive by­pass ra­tio of 20:1. The wide ‘ dou­ble-bubble’ fuse­lage will gen­er­ate in­creased lift, so smaller wings can sup­port the air­craft’s weight and a lighter land­ing gear and smaller tail will suf­fice. In turn, smaller en­gines and less fuel ca­pac­ity will be needed. The twin en­gines will be in­te­grated with the fuse­lage for a clean high-as­pect-ra­tio wing with low drag. Mounted at the rear end, they will re-en­er­gise the slow-mov­ing bound­ary layer flow over the fuse­lage and pro­mote bound­ary layer in­ges­tion (BLI) which in­creases ef­fi­ciency. This too means smaller en­gines, which re­duces weight and fuel car­riage even more. In­deed, it is a clas­sic vir­tu­ous cir­cle and may en­dow the D8 with hard-to-beat ef­fi­ciency. How­ever, the D8 will not hap­pen in a hurry be­cause nu­mer­ous tech­no­log­i­cal chal­lenges re­main to be over­come. It is ex­pected only around 2035, a time­frame that NASA calls ‘ N+3 gen­er­a­tion’. Be­yond that, what? SABRE RATTLING. Mod­ern jets sat­isfy the hu­man need for speed over short dis­tances, but for longer jour­neys, time in the air can be rather irk­some. For in­stance, de­spite Emi­rates’ ef­forts to make its pas­sen­gers com­fort­able, how many would fancy spend­ing 17 hours 15 min­utes to cover 14,203 km on its Auck­land to Dubai Boe­ing B-777-200LR flight? In a cou­ple of decades, the same trip may be ac­com­plished in a frac­tion of the time, thanks to SABRE.

UK’s Re­ac­tion En­gines is de­vel­op­ing a re­us­able space plane, Sky­lon, pow­ered by a Syn­er­getic Air-Breath­ing Rocket En­gine (SABRE). SABRE is a hy­brid rocket and jet, billed as “the big­gest break­through in aerospace propul­sion tech­nol­ogy since the in­ven­tion of the jet en­gine.” Al­though SABRE will pro­vide an eco­nom­i­cal way to place satel­lites in or­bit, it can also func­tion as a com­mer­cial air­liner. The first full ground-based en­gine test is sched­uled for 2020 and the first un­manned test flight should hap­pen about ten years from now. The UK Gov­ern­ment has in­vested £60 mil­lion to­wards this next-gen­er­a­tion en­gine that is likely to make low-cost high-speed flight fea­si­ble.

Un­like rocket en­gines that must carry their own oxy­gen thus sig­nif­i­cantly in­creas­ing weight and drag, SABRE will ‘ breathe’ air from the at­mos­phere. It will fea­ture two op­er­at­ing modes: ini­tially the air-breath­ing mode cruis­ing at Mach 5 through the at­mos­phere; then if re­quired, us­ing stored oxy­gen and hit­ting Mach 25 as a con­ven­tional rocket in space. A SABRE-pow­ered air­liner will take off from a run­way, reach any point on the earth in less than four hours and land on a run­way. Now that is near-per­fect power!


A cut­away view of PurePower PW1000G en­gine

CFM LEAP uses ad­vanced light­weight com­pos­ite ma­te­ri­als such as car­bon fi­bre fan blades, achiev­ing what it calls ‘the ul­ti­mate re­fine­ment of the tra­di­tional tur­bo­fan en­gine’

Cut­away of the SABRE en­gine and (inset) Sky­lon in flight show­ing the SABRE en­gine

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