PAUL LEWIS / WASHINGTON DC
The US Navy's unmanned combat air vehicle (UCAV-N) programme is set to progress to the critical flight-demonstration phase with the release of a request for proposals (RFP) by the Defence Advanced Research Projects Agency (DARPA). Meanwhile, the competitors, Boeing and Northrop Grumman, are striving to pick an off-the-shelf engine for the designs while wrestling with the conflicting needs for fuel efficiency, a small diameter fan and margin for growth.
The RFP divides the UCAV-N Phase 2 programme into two subphases extending to mid-2005. Under Phase 2A, Boeing and Northrop Grumman will receive funding for further studies through to the third quarter, when a Phase 2B contract award is planned for one of the companies to flight test a demonstrator vehicle. DARPA does not rule out the possibility of both contenders being asked to fly.
Neither Boeing nor Northrop Grumman has revealed their engine choices. Both are looking at 8,000lb-thrust (36kN) class engines, slightly larger than the revised requirement for Boeing's X-45B US Air Force UCAV-AF candidate. Contenders are understood to include the General Electric F404, Honeywell AS907, Pratt & Whitney Canada PW300 series and a growth version of the Rolls-Royce AE3007.
The UCAV-N fan size is thought to be similar to the X-45B's 0.66m (27in) diameter, whereas current versions of the candidate engines have fans larger than 0.76m. Engine manufacturers contend that, if the UCAV-N is to achieve the necessary fuel efficiency for the 12h endurance missions required by the USN, a bypass ratio of 4:1 or larger will be required. "The small fan diameter points towards almost a turbojet, but you're then trading poor fuel performance for thrust," says one manufacturer.
Thrust demands are expected to increase as the vehicle grows in size and weight and electrical power requirements multiply. The X-45B is already larger and will need almost 50% more thrust than the smaller Honeywell F124-powered X-45A. Most manufacturers agree that, short of developing an all-new engine to a different cycle with higher pressure stages, adapting powerplants will demand performance compromises.
Source: Flight International
