Game-Changer: USA Developing UAV Aerial Refueling

2007 AAR experiment
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Quick question: what’s the biggest limiting factor in today’s aircraft? Answer: the pilot. Fortunately for pilots, they’re also an aircraft’s greatest advantage, which will keep them in the mix, and in the cockpit, for some time to come. Those limitations are bringing unmanned aircraft into the combat picture, however, especially when it comes to the greatest limitation a pilot places on an aircraft: aerial endurance. Remaining awake, active, and effective in a manned fighter aircraft for 72 hours straight is simply not within the realm of possibility. On the other hand, a UAV with that endurance level, flown by pilots on the ground or at sea who can hand the aircraft off to a colleague while they depart for a coffee, bathroom break, or sleep, could easily remain aloft that long. All it needs is an appropriate level of mechanical reliability – and, of course, the ability to take on fuel from an aerial tanker aircraft.

That simple concept has profound implications for the ways in which airpower might be used.

UAV AAR: Implications

X-47B Concept
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Imagine, for instance, a carrier with UCAS-D X-47s on board. The ship receives a crisis call for events 3,000 miles away – and immediately launches its armed complement of X-47 stealth UCAVs toward the area, while the rest of the carrier steams over to catch up. Pilots fly the aircraft via secure satellite links from duty stations within the ship, while USAF aerial tankers keep the X-47s fueled to maintain their surveillance and targeting “orbits” over the crisis area, or even slightly outside of that area. The aircraft are on station much faster, and remain so for a far longer period of time than would normally be possible. The result is effective power projection that attracts as little or as much attention as it chooses, while remaining available for very fast use in the target zone.

UAVs are already being flown over Afghanistan from duty stations near Las Vegas, so part of this scenario has already come true. Unmanned stealth fighters are in development, as the X-47 is improved and tested under the Navy’s UCAS-D program. The missing link for the rest is aerial refueling. Looks like it won’t stay missing for much longer.

AAR Program Goals: Phase II

KC-135 & RNoAF
F-16, Afghanistan
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Work began with a 2005 contract to add this capability to its canceled X-45C UCAV, and was followed by successful 2006 and 2007 exercises that used a machine-piloted Learjet as a surrogate. These Phase I exercises demonstrated that a single UAV could safely maneuver among 7 refueling positions behind a tanker aircraft, and conduct a breakaway maneuver.

Now, the Phase II contract looks to extend Boeing’s efforts – and take another step toward the kind of UCAV future outlined above. Automated Aerial Refueling also has implications for manned aircraft, which could also benefit from systems that can automate risky maneuvers like air-air refueling.

Phase II Spiral 1 involves designing and testing a multichannel precision-GPS relative navigation system, an automated flight control system, and the required command and control additions to guide the unmanned receiver to the manned tanker. Tests were originally set to use only the air force’s boom system, but also ended up supporting probe-and-drogue positions.

Under a planned Spiral 2, the team will evaluate non-GPS sensors to augment the Spiral 1 system, and conduct successively more advanced tests.

The Navy’s UCAS-D program, which uses Northrop Grumman’s developmental X-47B unmanned attack aircraft, is also involved, and plans are moving toward autonomous refueling capability from both USAF (boom) and Navy (drogue) aircraft. The goal was to successfully refuel an F-16, acting as an unmanned surrogate, by 2011. Flight tests did begin in December 2011, using a Calspan Learjet equipped with X-47B hardware and software, but the actual refueling looks set to be delayed into 2013. The UCAS-D program’s current goal is to run 2 more surrogate test sets, then run a full test using the X-47B in 2014.

A separate DARPA/NASA program called KQ-X will involve the next step beyond, and test air-air refueling where both aircraft involved are UAVs. Northrop Grumman’s RQ-4 Global Hawks will be used as the refueling and receiving platforms.

Contracts and Key Events

X-47B UCAS-D
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Aug 28 – Sept 6/13: Testing. Calspan has modified a Learjet with a non-functioning aerial refueling probe, and Northrop Grumman added X-47B UCAV added hardware and software for navigation, command and control, and vision processing. Its challenge? To fly behind an Omega K-707 tanker, and demonstrate its ability to hold correct positions and operate with the installed systems. Testing went well.

The next step will using the kind of digital messaging and navigation processes that were demonstrated by the UCAV’s recent carrier landings, with Rockwell Collins TTNT datalink, and Precision Relative GPS (PGPS) algorithms. The final goal? A complete autonomous rendezvous, approach, plug, and safe separation. No fuel will be transferred to the Learjet, which isn’t equipped to receive it anyway, but the ability to fly that kind of evolution is enough challenge all by itself. Sources: US NAVAIR, “Navy autonomous aerial refueling tests underway”.

Dec 21/12: X-47B test prep. Northrop Grumman Systems Corp. in San Diego, CA receives a $9.7 million cost-plus-incentive-fee contract modification for Autonomous Aerial Refueling (AAR) demonstration activities in support of the N-UCAS program. Services will include completion of Delta Critical Design Review (DCDR), surrogate testing with manned aircraft, preparation for the X-47B demonstration, travel, and support technical data for the AAR demonstration activities.

Work will be performed in Manhattan Beach, CA (70%) and Patuxent River, MD (30%), and is expected to be complete in December 2013. All contract funds are committed immediately (N00019-07-C-0055).

Aug 9-17/12: KQ-X. DARPA and Northrop Grumman have flown Global Hawk UAVs in refueling formation while equipped with their respective drogue and probe, but haven’t been able to do more than that.

AviationWeek later explains that KQ-X is a reversal from usual probe-and-drogue configurations. The tanker is fitted with a refueling probe on the nose, and trails behind the receiver who uses hose-drum unit under the fuselage. The receiver trails the hose, the tanker comes up behind and plugs into the drogue, then it pushes fuel uphill to the receiving aircraft. They reportedly picked this approach because it minimizes the number of UAVs they’d have to convert for tanker roles. Flight International | Aviation Week.

Jan 21/12: Testing. NAVAIR/AFRL’s AAR program completes a series of ground and flight tests that began in November 2011, using a Calspan Learjet surrogate with X-47B hardware and software, and a Omega Air Refueling K-707 aerial tanker. The tests included simulated flight demonstrations of both boom/receptacle (USAF) and probe-and-drogue (Navy & European) aerial refueling techniques, but no fuel was actually transferred, and Calspan’s Learjet wasn’t equipped for that anyway. The tests were all about correct positioning and coordination, beginning at a position 1 nautical mile from the K-707, and allowing autonomous guidance to move the Learjet into the 3 air-air refueling positions: observation, contact, and re-form.

Navy UCAS program manager Capt. Jaime Engdahl says that the next big step will involve using the actual X-47B. The team plans to conduct 2 more surrogate test periods before a planned refueling demonstration with the X-47B in 2014. NAVAIR | Northrop Grumman.

Nov 7/11: Boom & Drogue. Inside the Navy reports [subscription] that the US Navy will be expanding the X-47B’s planned aerial refueling capability, to autonomously refuel while in flight with both USAF Air Force and USN aerial tankers.

The USAF uses KC-135s and KC-10s, but many of the KC-135s need to place an attachment on the refueling boom, in order to refuel probe-carrying aircraft. The US Navy has KC-130 Hercules aerial tankers, and its F/A-18E/F Super Hornets can become “buddy refuelers” with special wing tanks.

July 18/11: AAR P2. Northrop Grumman Systems in San Diego, CA receives a $25 million cost-plus-incentive-fee contract modification for UCAS-D autonomous aerial refueling technology maturation and demonstration activities. They’ll provide “air systems, air vehicle segment, and mission management segment requirements definition; integration planning and verification planning; and definition of certification requirements and approach.”

Work will be performed in San Diego, CA, and is expected to be complete in December 2012. US Naval Air Systems Command in Patuxent River, MD manages the contract (N00019-07-C-0055).

Next step beyond
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June 30/10: KQ-X. Northrop Grumman announces a $33.3 million DARPA contract to take the next step forward: UAV-to-UAV aerial refueling. The “KQ-X” program will use a pair of the company’s RQ-4 Global Hawk high altitude UAVs, modified to add a standard Navy hose-and-drogue system. The refueling will be completely automated and autonomous, and will take place at high altitude. Not only would this program mark the first UAV-to-UAV refueling, it would be the first time that Global Hawks or any other HALE UAV have even flown in formation.

Work will be performed at Northrop Grumman’s Unmanned Systems Development Center in Rancho Bernardo, CA near San Diego (82%), Salt Lake City, UT (9%); and Davenport, IA (9%). Sargent Fletcher, Inc. (aerial refueling systems) and Sierra Nevada Corporation are major KQ-X subcontractors, and pilots from NASA, NOAA, and Northrop Grumman will command the Global Hawks from the NASA Dryden Flight Research Center at Edwards Air Force Base, CA. Bids were solicited on the World Wide Web, with 341 bids received by DARPA’s CMO group in Arlington, VA (HR0011-10-C-0076). Northrop Grumman Aerospace Systems VP Advanced Concepts, Carl Johnson:

“Demonstrating the refueling of one UAV by another is a historic milestone… It adds aerial refueling to the list of capabilities that can be accomplished autonomously by Global Hawks; it opens the door to greatly expanded operational utility for UAVs; and, as a side benefit, it promises to increase the safety and reliability of aerial refueling between manned aircraft by reducing pilot workload.”

KQ-X with Global Hawks

April 12/10: Testing. Northrop Grumman touts the performance of its equipment in recent tests:

“Employing a Learjet as a surrogate for an unmanned aircraft and a modified refueling tanker, Northrop Grumman’s relative navigation software and LN-251 embedded global positioning system (GPS)/fiber-optic inertial navigation system (INS) provided the precise positioning information required in support of the simulated aerial refueling mission. A series of 10 flight tests, which concluded on March 18, demonstrated that Northrop Grumman’s relative navigation software could be successfully hosted in the LN-251. The test also exercised the LN-251’s newly-added Ethernet interface as well as its embedded 24-channel GPS receiver with enhanced tracking capabilities.”

Jan 26/10: AAR P2. Northrop Grumman Integrated Systems Sector in San Diego, CA received an $11 million not-to-exceed modification to a previously awarded cost-plus-incentive-fee contract for autonomous aerial refueling technology maturation and demonstration activities in support of the Navy UCAS-D.

Work will be performed in El Segundo, CA (60%) and Rancho Bernardo, CA (40%), and is expected to be complete in November 2010 (N00019-07-C-0055).

Boeing AAR concept
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Jan 12/09: Testing. Jane’s confirms that the X-47 UCAS-D program will begin aerial refueling tests performed in 2010, using surrogate aircraft.

Additional Readings

• DID – Naval Air, Unmanned: US Navy Flying Toward N-UCAS. The X-47B is step one.

• DID – Ride on the Ray: Boeing’s X-45 UCAVs. Boeing has its own long-term stake in these contracts.

• DID – USAF Wants a New Bomber… Or Does It? Unmanned options are under review.