http://www.janes.com/aerospace/military/news/idr/idr010504_1_n.shtml(Jane's Military Aerospace News of Boeing X-45 from May 2001)
The tailless X-45A design shares some features with the X-36 experimental UAV, flown in 1996. The wing shape is similar, as are the trailing-edge control surfaces and the yaw-axis-vectoring exhaust nozzle. The low-observable nozzle has no external moving parts and is still nominally classified, although a 1994 McDonnell Douglas patent shows a similar nozzle with internal moving ramps. The X-45 is considerably larger than the X-36 and is autonomous, rather than being remotely piloted, but there are enough similarities to reduce some of the basic design risks.
Next year, following the initial tests, the second aircraft will join the first for Block 2 tests, intended to evaluate the operational concept for the UCAV. Both aircraft will be fitted with electronic surveillance measures (ESM) equipment supplied by Northrop Grumman's Defense Systems Division in Rolling Meadows, Illinois, and satellite datalinks, and will be used for a series of increasingly complex tests. These will culminate in a series of 'graduation exercises' in which the two X-45As will work together, under the control of a single operator, to perform pre-emptive and reactive SEAD missions.
Command and communications, and the ability to make the UCAV as intelligent and autonomous as possible, are the core of these demonstrations. "When the enemy cuts the communications link between the UCAV and the controller - and they will - we want to be able to prosecute the mission," comments DARPA program manager Colonel Mike Leahy. Hunting and targeting relocatable SAM systems, the UCAVs are working in a dangerous environment, where a combination of stealth and tactics are essential for survival. This means, for example, that the UCAV will have to be able to change its ingress and egress route if its ESM detects a new threat, ensuring that it keeps its least detectable aspects towards the radar.
ESM is an important technology for UCAVs in the SEAD mission. In 1990, the state-of-the-art in combat ESM was the Litton Amecom ALD-11, a system that weighed more than 700kg and cost tens of millions of dollars, but could locate and identify a radar emitter in real time. Smaller, compact radar warning receiver (RWR) systems could provide only a rough bearing measurement. In the past few years, however, the EW industry has made great strides in creating small, low-cost receiver systems which provide full-scale ESM capabilities for the weight and cost of an RWR.
The UCAV system is intended to use co-operative tactics to locate and destroy targets. Although their ESM sensors will have some ability to provide precision location data, a pair of UCAVs will be able to pin down a target's position more quickly and more accurately if they each detect it from different angles. Operational UCAVs will use a 'spotlight' synthetic aperture radar (SAR) to help positively identify targets and further refine their location data: tactically, it may make sense for one UCAV to pop-up and image the target while its robotic wingman delivers the weapon. The key, says Col Leahy, is to make sure that the UCAV team can do this within a timeline defined by the threat's ability to move. Block 2 tests will cover preemptive and reactive SEAD; the latter tests will include manned aircraft to show that the UCAV can effectively escort the manned strikers.
Within a few months, the DARPA/Boeing team expects to start detailed design of the third UCAV prototype, the X-45B, using funds added to the program by Congress. The X-45B will resemble the two X-45As externally, but will incorporate a number of important differences. It will be built more like a production aircraft, with a low-cost, almost-all-composite airframe. It will also incorporate a complete suite of LO materials, and will be used for tests to show that the UCAV's LO systems can be maintained economically.
The X-45B could join the program as early as mid-2003, for Phase III of the program, including three test blocks. "The big change," says Col Leahy, "is the switch in emphasis from technical feasibility to military utility".