RB199 devel­opment: the engine that started it all

“With­out the RB199, there would be no MTU Aero En­gines.” This one sen­tence un­der­lines the im­por­tance the de­vel­op­ment of the Tor­na­do en­gine had for MTU. But this alone does not do jus­tice to this pi­o­neer­ing achieve­ment. Be­cause no oth­er sto­ry il­lus­trates quite as ef­fec­tive­ly what MTU had been about since day one: us­ing high-cal­iber tech­ni­cal in­ge­nu­ity to push the bound­aries of what’s pos­si­ble—in short, to in­no­vate. Back at the start of RB199 de­vel­op­ment in 1969, this was known as en­gi­neer­ing in­ge­nu­ity, says en­gine de­vel­op­er Arthur Schäf­fler. This savvy was ab­solute­ly nec­es­sary in or­der to mas­ter the chal­lenges MTU en­gi­neers faced back then with the new jet en­gine, notes 81-year-old Schäf­fler, who was part of RB199 de­vel­op­ment from day one and whose ca­reer would take him all the way to the role of tech­ni­cal di­rec­tor for Eu­ro­jet.

“The mis­sion re­quire­ments for the Tor­na­do could hard­ly have been any big­ger,” re­mem­bers Schäf­fler. “On the one hand, the mul­ti­role fight­er had to in­ter­cept en­e­my air­craft at Mach 2.2, on the oth­er mas­ter op­er­a­tions at low al­ti­tude.” And of course the jet should be able to take off and land on short run­ways. In oth­er words, the RB199 not on­ly had to gen­er­ate a great deal of thrust while be­ing as light­weight as pos­si­ble—and con­sume low amounts of kerosene and en­able peak ac­cel­er­a­tion in a short time­frame—it al­so had to have a thrust re­vers­er. The idea was to have a three-shaft en­gine with 12 com­pres­sor stages, with a com­pres­sor pres­sure ra­tio of 23:1 and a tur­bine in­let tem­per­a­ture of some 1,300 de­grees Cel­sius. “Con­sid­er­ing what was state of the art back then, this was a mas­sive leap in both of these re­spects,” Schäf­fler says.

Grown-up chal­lenge for the young MTU

Schäf­fler re­mem­bers how the Tur­bo-Union con­sor­tium, a joint ven­ture be­tween Rolls-Royce (40 per­cent), Fi­at (20 per­cent) and MTU (40 per­cent) that was found­ed specif­i­cal­ly to build the RB199, ea­ger­ly set about work. The di­vi­sion of la­bor meant that the MTU team, made up of en­gi­neers from Daim­ler-Benz and MAN Tur­bo­mo­toren GmbH, “land­ed a ma­jor deal con­sid­er­ing what they knew at the time,” says Schäf­fler. “We were re­spon­si­ble not on­ly for the medi­um- and high-pres­sure com­pres­sors, the medi­um-pres­sure tur­bine and thrust re­vers­er, but al­so for the in­ter­nal air and oil sys­tem for cool­ing and lu­bri­cat­ing the high­ly stressed cas­ing and chan­nel­ing cool­ing air to the tur­bine blades. Such tech­no­log­i­cal re­quire­ments were not the on­ly un­chart­ed ter­ri­to­ry for MTU. Up un­til that point, the Mu­nich-based com­pa­ny and its pre­de­ces­sors had main­ly man­u­fac­tured li­censed en­gine parts and had nev­er had the re­spon­si­bil­i­ty for de­vel­op­ing such ma­jor por­tions it­self. “Be­ing sole­ly ac­count­able for an as­sem­bly al­so meant we were un­der a fair amount of psy­cho­log­i­cal pres­sure,” Schäf­fler says.

The mod­est per­for­mance of the first RB199 test run met with long faces among Schäf­fler and his col­leagues. “The ef­fi­cien­cy was con­sid­er­ably be­low par,” he says. While the medi­um-pres­sure com­pres­sor quick­ly at­tained “very sat­is­fac­to­ry” per­for­mance, the high-pres­sure com­pres­sor ran in­to com­plex prob­lems. These in­clud­ed the rel­a­tive­ly thick ro­tor blades of the rear stages—which were made from heavy nick­el al­loys—ex­pand­ing sig­nif­i­cant­ly slow­er than the cas­ing un­der fast-chang­ing op­er­at­ing con­di­tions such as ac­cel­er­a­tion and de­cel­er­a­tion. This meant that over long phas­es, the ra­di­al clear­ance be­came too large. The re­sult was re­duced per­for­mance pa­ra­me­ters for the high-pres­sure com­pres­sor and an im­per­mis­si­ble low­er­ing for the aero­dy­nam­ic sta­bil­i­ty thresh­old. “It was a long time be­fore we found a de­sign that suc­cess­ful­ly slowed the ex­pan­sion of the cas­ing,” Schäf­fler says. At least all the en­gine man­u­fac­tur­ers were in the same boat at that stage, says the en­gi­neer. “The clear­ance prob­lem was not well known—we just had to ac­cept the long time it took to find a so­lu­tion.”

Fail­ing the bird strike test

Just as sur­pris­ing was the dis­cov­ery that the sur­face of the blades for all rear stages of the high-pres­sure sec­tion was much too rough. “This meant that the sys­tem wasn’t achiev­ing its full aero­dy­nam­ic po­ten­tial,” Schäf­fler says. Here too, it would be years be­fore they suc­ceed­ed in man­u­fac­tur­ing the thumb­nail-sized blades in a suit­able qual­i­ty. An­oth­er thing Schäf­fler re­mem­bers very well is how the blades had to be re­designed af­ter the first bird strike test: “The bird passed through the low- and medi­um-pres­sure com­pres­sors with­out do­ing much dam­age be­fore com­plete­ly de­stroy­ing the high-pres­sure com­pres­sor.” Com­pared to the oth­er is­sues with the RB199, Schäf­fler says this was easy to fix: “When we made the blades for the high-pres­sure com­pres­sor’s first ro­tor stage, we sim­ply ex­tend­ed the chord length by some 30 per­cent. The strength­ened blades sub­se­quent­ly with­stood the strike, as did the down­stream stages,” Schäf­fler says. Step by step, MTU de­vel­oped valu­able knowl­edge in all ar­eas.

“Even though the jour­ney was some­times ar­du­ous and caused us to curse reg­u­lar­ly, the grasp of the over­all sys­tem that we gained over this time proved ex­treme­ly use­ful to MTU,” Schäf­fler says. Schäf­fler’s time al­so saw an in­no­v­a­tive leap in en­gine con­trol tech­nol­o­gy. In the fall of 1987, the ana­log reg­u­la­tor first used for the RB199 was re­placed with a much more re­li­able, flex­i­ble and amenable dig­i­tal con­trol unit, the DE­CU 2000. The mod­el was su­per­seded in 1995 by the DE­CU 2020 fea­tur­ing im­proved proces­sors. “This con­trol unit was in pro­duc­tion un­til 2003 and suc­cess­ful­ly em­ployed by the Ger­man and Ital­ian air forces,” Schäf­fler says. Around 700 were made, “but I couldn’t tell you the ex­act num­ber,” he laughs. But this is not the case when it comes to the num­ber of RB199 en­gines built. “There were 2,504,” Schäf­fler says. “And many are still in ser­vice.” Schäf­fler says he is of course proud of the RB199, which de­spite all its chal­lenges did ul­ti­mate­ly be­came one of MTU’s ma­jor suc­cess­es.

The 81-year-old, who un­til a few years ago still gave in­tro­duc­to­ry cours­es to MTU em­ploy­ees on how jet en­gines work, says he was lucky to be em­ployed at MTU. “It was the ide­al life for an en­gi­neer.”