Unmanned combat aircraft development in the US of A has taken an unusual turn. After the success of the pioneering X-45A UCAV demonstration, the follow-on J-UCAS programme was cancelled when the US Air Force withdrew. This left the US Navy leading a smaller programme to demonstrate a carrier-based low-observable UCAS.
On the face of it, the US gave up its lead in unmanned combat aircraft, giving Britain's Taranis, Europe's Neuron and even Russia's Skat a chance to catch up. But while the Navy's X-47B UCAS-D programme may be smaller in dollar terms than J-UCAS, it tackles two extremely difficult challenges: co-existing with manned aircraft and operating from a carrier.
The US Navy does not have the luxury of keeping its manned and unmanned combat aircraft separate, in the air or on the carrier flightdeck. Every Naval UCAS that goes on a ship will displace a manned aircraft, so they have to be interchangable. This creates challenges a land-based UCAVs don't need to face. Northrop Grumman has developed a presentation - a "day in the life" of N-UCAS - that goes some way to describing the challanges, and solutions.
The day begins with N-UCAS in the hangar deck...
Wings folded, the kite-shaped N-UCAS has a space-efficient spotting factor of 0.87 versus the F/A-18C. The wingtips of the X-47B UCAS-D have been clipped to provide 20in of folded clearance beneath the drop tanks suspended from the hangar ceiling. This allows the landing gear struts to extend 20in so that the Pratt & Whitney F100-220 engine can be removed and replaced in the shadow of the aircraft. "You don't jack an aircraft at sea," says Northrop.
Before the N-UCAS is moved on to the elevator, the wing fold is tested. The aircraft is controlled via a laptop computer. A challenge for the N-UCAS is to maintain low-observability with wing folding. The design must eliminate any external bump and maintain electrical continuity across the fold. On the flightdeck, the folded wing must withstand wind or jet blast up to 60kt without breaking. "It's a very rigid wing," the company says.
With no overhang, the tailless vehicle is easy to manoeuvre out of the hangar and on to the elevator. There is no power on the aircraft, and no hydraulics, so an air-pressure system is used for braking, the number applications available displayed to crewman handling the vehicle. Three N-UCAS can fit on the elevator, tail-outboard, where only two F/A-18s can be carried.
On the flightdeck, preparations for the day's mission are accomplished in the shadow of the aircraft. The N-UCAS is fuelled through a port in the starboard main landing-gear well, avoiding extra openings in the LO airframe. Weapons are loaded into the two bays with the same trapeze system used on the F-35C Joint Strike Fighter. Alternatively, these bays can carry sensors or even fuel, allowing the N-UCAS to act as a aerial refueling tanker.
The key to N-UCAS is "not to change any procedures on the flightdeck", says Northrop. "This aircraft taxies and moves like every other aircraft on the deck." Aircraft lighting indicates its status: green, it is under control of a deck officer; blue, the N-UCAS is ready to launch and under the control of the mission operator below decks in the combat information centre.
In Northrop's operating concept, each deck officer has a control display unit on his wrist, linked to a wireless computer on his belt. As he takes control of the N-UCAS on the carrier deck, he has a "dead man's switch". To move the aircraft, he must make the switch. If he lets go, the brakes go on and hook comes down to let everyone know the vehicle is not under control.
Manoeuvring the aircraft to the catapult, the deck officer lowers the launch bar on the nose gear and engages the shuttle. Power is added, the holdback bar raised and the catapult tensioned. Lights change from green to blue to signal the vehice is ready for launch, and control has transferred to the mission operator. Launch is no different than for a manned aircraft. "The Navy has launched aircraft hands off from carrier decks for 20 years," says Northrop.
In the cramped confines of the carrier's CIC, the mission operator can handle several N-UCAS using two displays providing situational awareness inside and outside the autonomous vehicles. Designed for persistent surveillance and strike missions, the N-UCAS has an unrefuelled endurance of 12-14h, increasing to 50-100h with aerial refuelling.
Returning from its mission, the N-UCAS enters the carrier control area at 200nm from the ship. Recovery data from the vehicle, including its fuel status, is datalinked to the carrier's air traffic control centre for display alongside information from manned aircraft. From 200nm to 50nm, the system generates recovery order, creating individual vectors to flow the aircraft in.
At 50nm, manned and unmanned aircraft alike begin receiving vectors - heading, airspeed and altitude - to recover to the carrier. "N-UCAS is in amongst the gaggle," says Northrop. Coupled to the carrier's precision ship-relative GPS landing system, the autonomous aircraft begins its approach. At 3.25nm, if the landing signal offcer and air boss agree, the vehicle tips over on to a 3.5° glideslope for its final descent.
At 0.75nm, if the approach looks good, the LSO makes a switch and sends a signal to the aircraft to keep on coming. If the deck is fouled, the air officer hits a button and waves off the N-UCAS, which adds power and executes a pre-programmed flightpath to come around again. If the deck is clear, the aircraft keeps coming, aiming to catch the third wire.
As it senses weight on wheels, the aircraft powers up and reconfigures the flight controls ready for a "bolter", or go-around, if it misses the wires. If it feels the deceleration of a successful arrested landing, it retards the engine to idle, drops the wire and raises the hook. Control transfers to the deck officers, who taxi and park the vehicle. Mission accomplished.