Guy Norris/LOS ANGELES
General Electric has started work on the first J85 upgrade kits for the US Air Force's Northrop Grumman T-38 trainer fleet following the award of a $601 million contract for the modification of 1,202 engines.
The T-38 propulsion modernisation programme, which includes new ejector nozzle kits, is designed to extend the trainer's life to 2040.
Alan Ewing, manager of market analysis for GE Aircraft Engines says: "We're ready to go with the first two kits, and they should be ready by the end of the year." Key programme targets include a 3:1 improvement in low cycle fatigue to give a 5,400h life, a 5:1 cut in maintenance man-hours and a cut in the number of parts to maintain, from more than 850 to around 100.
The engine element of the T-38 upgrade was prompted by the loss of an aircraft due to an uncontained eighth stage compressor disk failure in 1995. Growing corrosion issues with the compressor rotors, which have become the main maintenance cost driver, were related to the failure.
To combat these, and to reduce the maintenance burden, the USAF, GE and ABB Alstom of the UK, teamed to develop a spooled rotor made primarily from corrosion-resistant Inconel 718.
The design replaces the eight individual disks with one disk and two spools, and allows individual blades to be replaced without the need for rotor disassembly or rebalancing. Flight testing of the compressor was completed last year on a Learjet 24 fitted with a modified CJ610-6, the civil J85.
The revised ejector nozzle incorporates free-floating doors and replaces the original configuration which was sized for the maximum afterburner condition. This exit area caused over-expansion of the exhaust plume at military power, causing excess drag and reducing net thrust. The aerodynamic actuation system controls the plume expansion, resulting in a higher net thrust of 1-2% at take-off conditions and up to 10% at military power. The performance improvement can be traded off for reduced fuel consumption or increased thrust, adds GE.
Other upgrade elements include a new stator casing, a revised cast mainframe, longer life combustion liner, an improved afterburner, new turbine casing and an updated ignition electrical system.
GE believes the J85 contract work could open the door to additional upgrade business, with more than 3,500 military and 3,000 civil J85/CJ610 and related CF700 family engines in operation. The engine powers all versions of the Northrop Grumman F-5 and Cessna A-37A/B as well as early Learjet 24/25s, Israel Aircraft Industries Westwind, Dassault Falcon 20 and Rockwell Sabre 75 business jets.
Source: Flight International
