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UCAVs spread their wings
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| 4 May 2001 |
Bill Sweetman
Since
1997, when the Uninhabited Combat Air Vehicle (UCAV) was a gleam in the eye of a few advanced researchers,
the concept has made remarkably rapid and solid progress, writes Bill Sweetman.
The first designed-from-the-ground-up UCAV is being prepared for its first flight, and the program is on track to support the start of engineering and manufacturing development (EMD) in 2008. A second UCAV program, to develop a larger and more versatile UCAV for the US Navy, is gathering speed.
Both projects reflect a consensus on what UCAVs should be designed to do. Suppression of enemy air defenses (SEAD) is the first mission to be tackled, followed by precision strike. Both UCAVs strike a similar balance between cost and capability. While they cost a fraction of the price of a manned fighter, they are certainly not toys: the DARPA/Boeing X-45 is the size of an advanced trainer like the BAE Systems Hawk, and the navy's UCAV will be larger still.
The larger UCAVs are big enough to carry off-the-shelf weapons and have as great a range as any tactical fighter. They carry onboard sensors, including active radar, and are designed to operate as autonomously as the rules of engagement will permit. With the advance of computer power in the last few years, designers are now reasonably confident that UCAVs will be able to respond in a quasi-intelligent manner to new threats and targets. Both UCAVs take advantage of small size, non-afterburning engines and the absence of a cockpit to achieve inherently stealthy designs, probably without using the most advanced and sensitive materials.
The proof of the pudding
Next year, the first UCAV is due to start realistic mission testing, and this will indicate whether the vehicles are indeed survivable, lethal and smart enough to carry out useful military missions without generating as many headaches for their users as for the enemy.
Early
in April, the first DARPA/Boeing X-45A UCAV prototype was undergoing engine
runs at NASA's Dryden Flight Research Center in California, and the second
X-45A was being readied for painting in St Louis. Developments in the program
reflect steady progress with this ambitious and - so far - promising program.
The first aircraft is expected to start Block 1 flight tests later this year, intended to validate the aerodynamic performance of the tailless air vehicle, expand the flight envelope and demonstrate that the basic flight-test datalink, using an ARC-210 radio, functions properly.
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. 
The
two X-45As are externally representative of a production-model UCAV, but
have some internal differences. The prototypes use more metal (the airframes
are 45% composite, whereas the production aircraft will be 90% composite)
and one of the two internal weapon bays is occupied by an avionics pallet.
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". The Phase III tests will be more complex and challenging, and will demonstrate a greater level of autonomy, reducing dependence on the vulnerable datalink. These tests, according to current plans, will reduce the program risk to the point where an engineering and manufacturing development (EMD) program could start, possibly at the beginning of FY05.
Navy milestone
Results from the DARPA/Boeing effort have been encouraging enough to support the launch of a program to develop a UCAV for the US Navy, also with an initial operating capability date around the end of the decade. The Navy UCAV (UCAV-N) project is also a DARPA-led effort, and passed an important milestone at the beginning of April with the start of Phase 1B, which will last until the end of the year and encompasses the preliminary design of the entire system, including the air vehicle, support equipment and infrastructure. Boeing and Northrop Grumman are the competing prime contractors in the program.
Phase 2 starts early next year and is due to run for three-and-a-half years, until late 2004 or early 2005. The goal is to demonstrate the critical technologies for UCAV-N, including prototype flight tests. DARPA hopes to keep both competitors in the program through this phase, if funding permits. After 2005, the navy will take over the program and will spend time bringing key technologies to maturity before starting EMD in 2008.
UCAV-N is more ambitious than the land-based UCAV in several respects. It is carrier-based, which imposes a different set of operational challenges and constraints. The navy is looking for an aircraft with a greater range and weapon load than the USAF is requesting, and also wants the UCAV to perform a quite demanding surveillance mission. Also, the navy's concept of operations for the UCAV is substantially different from the USAF's. The result is that the navy UCAV will be rather larger than the USAF aircraft.
Some basic decisions about UCAV-N will be confirmed this year, according to DARPA program manager Bill Scheuren. Both contractors plan to offer catapult-launched, arrested-landing designs. "We could do a short-take-off, vertical landing version," says Scheuren, "but it would be expensive". For the time being, too, both Boeing and Northrop are looking at a single vehicle configuration for all three missions - precision strike, SEAD and surveillance. This is a challenge in terms of vehicle cost and size, but offers the operator more flexibility than a mixed fleet of strike/SEAD and surveillance aircraft.
The competitors are working to define a size for the vehicle. The navy wants to carry 1,800kg of weapons on an 1,110km-radius strike mission - which is actually as much as many current manned fighters can do - and also wants to perform a 12h surveillance mission at the same radius, carrying sensors including a high-resolution radar. There is no navy requirement to make the vehicle air-transportable, but the service wants to be able to ferry the aircraft from shore bases to the carrier. The size limit is set by spot factor rather than dimensions. The UCAV-N is to have a flyaway cost one-third of that of the navy JSF (that is, around US$12-15 million) and half the operations and support costs of the F/A-18C.
Carrier operations are a make-or-break issue for UCAV-N. Traditionally, naval commanders regard any jet-powered, explosives-carrying unmanned vehicle approaching their vessel as a missile, and respond accordingly. UCAV-N will be required to bring back unused ordnance to the ship, and will have a jet-like (120kt-plus) approach speed. A ramp strike or loss of control on approach could be catastrophic. For this reason, Northrop's privately funded Pegasus test vehicle has now become formally part of the DARPA program.
If there is one system that makes UCAV-N practical, it is the Shipboard Relative Global Positioning System (SRGPS), which provides landing guidance with enough accuracy to permit automated carrier landings. SRGPS is under development as a precision landing guidance system for all US Navy aircraft. It is an extension of differential Global Positioning System (DGPS), which compares the aircraft's GPS data with a second GPS at a fixed, pre-surveyed reference point to determine and eliminate sources of error. With SRGPS, the aim is not to determine an absolute position for the aircraft but to provide an accurate position relative to the ship deck. The system takes the ship's motion (forward, heave and sway) into account. Tests at sea, using an F/A-18 and the carrier Roosevelt, were due to start in April. SRGPS is designed to provide guidance to within 40cm accuracy, and values as low as 20cm have been achieved in tests.
Deck handing will present interesting challenges. The navy is insisting that the UCAV-N should have no special support requirements. The service wants to fly manned and unmanned aircraft in the same landing pattern, and to be able to disengage the UCAV from the arrester wire and move it off the landing area as quickly and easily as any manned aircraft. (One possibility is a remotely controlled steering system.)
Another
very important difference between the navy and USAF aircraft is in the way
that they will be used. The USAF plans to keep most of its UCAVs in containers
until they are needed for combat, and to conduct the minimum amount of actual
flying in support of training, tests and exercises. UCAV-N, however, will
be flown routinely in peacetime as part of normal carrier operations, because
of its surveillance role. Also, because UCAV-N launch and recovery are an
integral part of the complex ballet that is carrier operations, the navy
takes the view that the UCAV must always be operational. As a result, UCAV-N
will be designed for a much longer and harder operational life than the
USAF vehicle. Boeing has not disclosed details of its UCAV-N design. Northrop Grumman, however, regards UCAV-N as an important opportunity and has unveiled a reduced-size test vehicle, Pegasus, that is intended to prove the aerodynamics of the basic design and demonstrate its compatibility with the carrier.
Ideal shape
The
kite-like Northrop Grumman UCAV-N design (for further details see article
Northrop Grumman
unveils Pegasus naval UCAV demonstrator) is very close to an ideal stealth
shape. The entire vehicle lies within a perimeter formed by four straight
sides, with sharp sweepback on the leading edges and significant forward
sweep on the trailing edges. The aircraft is therefore a 'four-lobe' design;
its peak radar cross-section (RCS) occurs when the illuminating radar is
on a bearing that is at right angles to one of the edges, but this phenomenon
will be transient because of the movement of the aircraft. There are no
other edges in the shape - there are no flat body sides or vertical tails
- and the body is flared outwards towards the edges so that the slope angles
immediately behind the edges are very shallow. This minimizes reflections
from the edges. The aircraft has what appears to be one of the stealthiest inlets ever designed: a shallow, wide slit, V-shaped in plan view and aligned with the trailing edges. Ahead of the inlet is a raised hump which helps to direct air into the inlet while masking much of its area from view - in fact, the inlet is almost completely masked from any angle below the vehicle. The Pegasus test vehicle has a fixed, circular nozzle, but the definitive design has a V-shaped exhaust, which does not appear to feature thrust vectoring.
The
Northrop Grumman vehicle has a simple flight control system, comprising
a pair of one-piece elevons and two sets of upper- and lower-surface 'inlay'
flaps. The elevons are used for pitch and roll control and the inlay flaps
work like the split rudders on the B-2, providing yaw control and added
drag to decelerate and increase descent angles. The Pegasus demonstrator's
principal task is to show that this configuration can be made to land safely
on a carrier, which requires very reliable and accurate control of the landing
flightpath. The Pegasus is being built by Burt Rutan's Scaled Composites company in Mojave, California. It is a scaled-down version of the UCAV-N, powered by a Pratt & Whitney Canada JT15D engine, and will have a maximum gross weight of 3,290kg. It will have a specially designed, carrier-compatible landing gear (the full-size replica unveiled in March had a T-38 landing gear borrowed from a museum) and an arrester hook. The prototype is being fitted with a scalable, open-architecture vehicle management system developed by BAE Systems Binghamton, New York, unit.
Pegasus is due to fly later this year and is expected to perform 12 or more missions at the navy's weapon development centre at China Lake, California. The missions will include carrier-type approaches, touchdowns and arrested landings.
Other UCAV programs are continuing to emerge. Boeing and the US Army are looking at a UCAV based on Boeing's Canard Rotor-Wing (CRW) concept. The DARPA/Boeing CRW flight-test vehicle, powered by a Williams F112 cruise-missile engine, has completed its fabrication stage at the Phantom Works in St Louis and has been transported to Mesa, Arizona, for assembly and flight test in the second half of the year.
The CRW concept (for further details see article Flights of fancy take shape) - which uses a two-blade, jet-driven rotor which stops and converts to a wing for cruising flight - is simple enough to make sense for a 2-2.5 tonne tactical UAV or UCAV. Boeing, which is also active in the Army's Future Combat Systems project aimed at developing advanced land vehicles, has conducted simulations of a CRW UCAV in the land-warfare environment. "It brings a different form of agility to the land force," according to Phantom Works president-designate George Muellner. "It can operate in the vertical mode and transit at 400kt." The CRW can land and take off vertically, hover to take advantage of terrain cover, and yet move around the battlefield faster than a helicopter.
So far, UCAVs have evolved with remarkable speed. The challenge is now to take the technology through some demanding and critical tests, and convince some sceptical users that the new systems will work.
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| X-45A prototype at St Louis; flight tests are scheduled for later this year. (Source: Boeing) |
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| Northrop Grumman's kite-like design is very close to an ideal stealth shape. (Source: Northrop Grumman) |
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| The two X-45A prototypes are 45% composite - production vehicles will be 90% composite. (Source: Boeing) |
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| Northrop Grumman's Pegasus test vehicle has one of the stealthiest inlets ever designed. (Source: Northrop Grumman) |
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| A mission control station for Boeing's UCAV. (Source: Boeing) |
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| Boeing is using the T-33 as a piloted surrogate for the UCAV to test sensors and communications systems. (Source: Boeing) |
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| An air vehicle storage container shown by Boeing. (Source: Boeing) |