Technical advances have the on the verge of refueling operations with no human present.

idair refueling is unmanned air vehicles has grown in about to change—and recent years,” said Jim McCormick, the it has nothing to do Defense Advanced Research Projects with the new KC-46 Agency program manager for KQ-X, a tanker program. This program testing Global Hawk remotely Mchange is something elemental. piloted as unmanned tankers Since became routine and receivers. in the late 1940s, USAF pilots have Of course, there are distinct tech- learned the fine skills of flying their niques for aerial refueling. Three major aircraft to contact with a tanker—one of approaches have all chalked up suc- the most sensitive in-flight maneuvers. cesses. The first approach driven by Normally, it takes eyes-on from the NASA and DARPA matured the concept pilot in the cockpit, the of optical tracking for automating the on board the tanker, or both. probe and drogue “Navy-style” refuel- But over the last decade, advances ing. The Air Force Research Laboratory in precision navigation and automated has spurred extensive industry work in technology have opened up a new realm: refueling remotely piloted aircraft from automated refueling, where sensor feed- USAF tanker booms. On top of this, back routines control the contact between DARPA now has a new program under receiver aircraft and tanker aircraft way to demonstrate that one unmanned without control inputs from pilots. Global Hawk can act as a tanker to refuel Flight tests beginning in the mid- another Global Hawk at high altitude. 2000s have pioneered methods for Automated air refueling required automation routines. And more is com- technology to advance beyond basic ing. Summer 2012 may see tests of one RPA control. In the late 1990s, several unmanned aircraft refueling another. developments pointed toward the pos- “In-flight refueling has proven in- sibility of autonomous aerial refueling. valuable to manned , First was the widespread use of Predators and there’s no reason to expect that the in the Balkans and other locations. Next, same wouldn’t be true for unmanned the Global Positioning System systems, especially as the demand for constellation reached full operational ca- Artist’s concept by Erik Simonsen

Pilot Dick Ewers and engineer Leslie Molzahn keep their hands off the controls as NASA F/A-18 #845 pulls up to the refueling drogue during an autonomous refueling demon- stration flight in 2007. NASA photo by Lori Losey

36 AIR FORCE Magazine / March 2012 A KC-135R refuels a future remotely piloted aircraft in this artist’s concept.

Refueling the

RPAs By Rebecca Grant pability in 1995. GPS provided a means and react with finely shaded control to Most unmanned aircraft operations for more reliable flight and autonomous changes such as wake flow turbulence. are remote, where pilots and sensor positioning. However, aerial refueling for RPAs operators fly aircraft by transmitting Soon the idea of refueling unmanned on intelligence-surveillance-reconnais- commands over radio or satellite com- vehicles took root. “Making UAVs air sance missions was not a pressing pri- munications links. Aircraft—manned refuelable would double or triple the ority because those unmanned aircraft and unmanned—also have automated loiter time, allowing a single UAV to already boasted long endurance. controls and subroutines that assist hu- perform the missions of two or three The real impetus toward automated man control or, as with autopilot, take unrefuelable UAVs,” concluded Maj. air refueling came from research in over in prescribed situations. Jeffrey L. Stephenson in a 1998 master’s the early 2000s on a Joint Unmanned True autonomy is a different beast. degree thesis for the School of Advanced Combat Air System program, dubbed It stems from command routines based Airpower Studies aptly titled “The J-UCAS. This program ultimately did on sensor inputs exclusive of human Aerial Refueling Receiver That Does not proceed, and part of it was spun intervention. Automation is “hands- Not Complain.” off to create the Navy UCAS dem- off” work done machine-to-machine. Stephenson sketched out the benefits onstrator now flying as the Northrop The Automation Federation defines and challenges of automated refueling X-47B. it as “the creation and application of for remotely piloted aircraft such as However, the seed was planted. How technology to monitor and control the Predators. One big unsolved problem would a stealthy but heavy, and possibly production and delivery of products and was how to handle the fine control armed, long-range RPA get maximum services.” That’s easier said than done, required for joining hose and recep- endurance? Midair refueling was the especially with objects such as aircraft, tacle. Remote piloting and automatic answer. But it could not rely only on which move in a dynamic environment waypoint flying were adequate for ground controller inputs because of of wind and weather. getting unmanned aircraft from point the time lag over the satellite link. Achieving autonomy crosses many A to point B. To refuel, though, they’d Unmanned aircraft refueling had to be functional domains and “involves a very need to move in close to the tanker automated. broad range of technologies, including AIR FORCE Magazine / March 2012 37 robotics and expert systems, telemetry ceiver’s probe with the refueling basket of the bouncing basket in order to catch and communications, electro-optics, proceeded via optical tracking, which it. But the optical tracker did just that, cybersecurity, process measurement and used a system of cameras and emit- gradually falling into rhythm with the control, sensors, wireless applications, ters to make the minute corrections basket so that movements were syn- systems integration, test measurement, necessary to achieve lock. Basically, chronized. and many, many more,” according to the it took the place of what pilots have By 2007, the Autonomous Airborne Automation Federation. been doing for decades. Refueling Demonstration was logging Advances in many of these disciplines “Skilled pilots can actually save full success. DARPA announced that made automated air refueling possible. some tricky, last-second movement the system had demonstrated the abil- DARPA and NASA began the Autono- the basket has a habit of making,” ity to “join the tanker from up to [2.3] mous Airborne Refueling Demonstration commented NASA Dick miles behind, 1,000 feet below, and 30 by gathering data on how the tanker probe Ewers. But, he added, they often “set degrees off heading.” Specifically, that and receiver acted in the stream of air. themselves up for a basket strike, rip- meant an unmanned aircraft could fly Contact was key. ping off the basket from the hose or first to a designated waypoint using GPS “Autonomous in-flight refueling sometimes breaking the probe or parts and then switch to a fully autonomous using a probe-and-drogue system of the airplane.” refueling mode. is basically a docking situation that Intriguingly, the automated systems The Air Force also wanted to develop probably requires centimeter-level handled the process differently. Pilots something different, namely, an auto- accuracy in the relative position of the learned not to try to follow every move mated system suitable for its boom- equipped tankers. The main advantage of a boom is greatly increased fuel flow rates of up to 1,200 gallons per minute. It’s important when fighters are waiting turns to refuel or large aircraft such as , AWACS, JSTARS, or even

USAF photo by Bobbi Zapka other tankers need fuel. While some Air Force tankers carry both probe and drogue and boom systems, refueling from the boom has long been the norm for USAF pilots. Automating the boom operation was a different challenge, especially since the boom was regarded as not nearly so forgiving as the basket. Two potential approaches were tried in models and found wanting. The first was to use GPS to edge an unmanned receiver into position. This seemed to

refueling probe (from the receiving aircraft) with respect to the drogue USAF photo (from the tanker) during the end game,” explained a team of engi- neers from Texas A&M and Tech in a 2007 paper. In making the contact, pilots had “to ensure that the tip of the probe contacts only the inner sleeve of the receptacle and not the more lightly constructed and easily damaged shroud,” the team added. Now, it would be up to an automation routine to do the same. The first break- through came in 2006 when a NASA F/A-18 engaged with a contract Omega Air Refueling Services tanker while rely- ing on an autonomous system. However, these flights still required pilot consent at points in the maneuver. The process relied on a combination of technologies. Inertial navigation assisted by GPS guided the receiver A Calspan Learjet, configured to fly like an RPA, maneuvers into refueling position aircraft toward the refueling airplane. under a KC-135R in these photos of automated aerial refueling demonstration test Once in close, the mating of the re- flights. By 2007, the system was considered fully successful. 38 AIR FORCE Magazine / March 2012 A Global Hawk refuels another RQ-4 in this NASA artist’s concept. DARPA believes Global Hawk is the obvious choice of test aircraft for the Autonomous High-Altitude Long-Endurance Refueling program. NASA envisions that the tanker will fly NASA illustration behind the receiving aircraft.

work for formation flying. However, “These tests show that we are making Demonstration of the fine skills for it did not fully cope with “distortions great advancements in system integrity, station-keeping opens the possibility due to wake effects from the tanker,” continuity, and availability through im- for all aircraft, manned and unmanned, found the Texas A&M team. proved relative navigation algorithms, to refuel under autonomous control. Another discarded approach was control laws, and hardware,” Equipment installed in the test air- pattern recognition. It didn’t work in program manager David Riley com- craft enabled eight straight days of low lighting conditions and threatened mented in December 2007. unmanned air refueling tests. Test to take up too much on-board comput- In 2009, Boeing again won the Air officials said the system blurred the dis- ing power. Force Research Laboratory’s contract, tinction between traditionally piloted Method three was picked. This was this time worth $49 million for a full test and autonomous aircraft, comparing a technique broadly based on optical program. Reports at the time hinted that it to a safety feature. “The pilot can recognition, with the help of algorithms part of the reason for AFRL’s interest let go, and it relieves fatigue. Planes to improve prediction. The guinea pig was to explore automated refueling of can be manned or unmanned—it’s op- was a specially modified Learjet flown an optionally manned new long-range tional,” said Lt. Col. Lee Grunberger, out of Niagara Falls, N.Y., by Calspan strike . The technology required who was one of the test coordinators. Corp. as part of an overall contract led a boom system depending directly on Success with manned aircraft tank- by Boeing’s Phantom Works. advances in optical tracking. The key ers refueling autonomously operated “The goal is to be able to fly some- was to steer the boom using an image receivers was not the end. The next thing without a pilot in it within 40 feet placed on the receiver RPA. hurdle was the unmanned tanker. of a manned vehicle,” an AFRL official Then in 2010, Northrop Grum- Global Hawks were the natural can- said of the program in an interview. man demonstrated its capability for didate for the KQ-X program. The Flight tests in 2007 showed major positioning aircraft. “The success of high-altitude surveillance airplanes progress. Although the Learjet had an this flight test is especially notable had been flying in combat all during automated air refueling system, pilots because it demonstrates the ability of the 2000s with proven reliability. Their handled takeoff then turned the jet an embedded GPS/INS to host relative internal fuel capacity, of about 17,300 over to the system to demonstrate a navigation processing,” said Alex Fax, pounds, made the Global Hawk a suit- refueling rendezvous. The automated director of positioning, navigation, and able “tanker” that could carry aloft system guided the Learjet into position timing solutions at Northrop Grum- enough fuel for both its own missions behind a KC-135 tanker. There it ran man’s Navigation Systems Division. and potential offload. through seven air refueling positions A series of tests carried out by the According to DARPA, the program is including contact, precontact, left and 190th Air Refueling in late 2010 addressing the challenges of unmanned right inboard observation, left and and early 2011 marked a new era. A systems, sensing, and aerodynamics right outboard observation, and the Learjet test aircraft once again played to a much greater degree than AARD. all-important break away. The Learjet the role of unmanned aircraft. Pilots “Tackling these complexities in a fully held contact position for 20 minutes flew the airplane to altitude then turned unmanned refueling scenario, with and was guided by the autonomous it over to the automated system, which the real-world Global Hawk system, system for a total of one hour and 40 moved the airplane into position for the should increase our confidence that minutes of flight time. tanker boom operator. unmanned systems can be autono- AIR FORCE Magazine / March 2012 39 whether to add autonomous refueling capability to current platforms. For example, converting legacy RPAs to take on fuel in flight depends on the aerodynamics and durability of each

NASA photo Landis Tony by system. At a minimum, each must be able to handle a single point refueling receptacle. Then there are the flying characteris- tics to consider. Early model Predators were designed for a limited envelope, not including extreme turbulence, weather, and high-altitude operations. On the other hand, the Global Hawk’s inherent flying characteristics are far better suited to midair refueling. The F/A-18 research aircraft follows a pickup carrying an airborne tanker drogue Tactics are another consideration. image down a runway in early AARD testing. An RPA’s approach to the boom is broadly based on optical recognition, aided by algorithms to improve prediction Planning tanker orbits, especially for accuracy. high-intensity air campaigns, is an art in itself. Day-to-day tactics and training mously refueled in a safe, flexible, and “When you add autonomous flight of will have to sort out the most efficient affordable manner,” said McCormick, both aircraft into the mix, ... you gain systems for rendezvous, for example. the program manager. a capability that has mission applica- Stephenson advocated “en route ren- As early as 1998, Stephenson had cited tions far beyond just aerial refueling,” dezvous [allowing] both the UAV and the range, fuel payload, and high-altitude said Geoffrey Sommer, KQ-X program tanker to enter the air refueling track on operating characteristics and deemed it manager for . a straight-line course.” In this case, “the ideal for top priority in the unmanned The concept for double unmanned tanker will not have to orbit and waste aerial refueling mission. The problem at refueling was a bit different from the rou- valuable time waiting to hook up with the time was how to compensate for as tine scenario of receiver trailing tanker. the UAV,” he pointed out. More experi- much as a 3.5-second delay in a satellite In this case, plans called for the tanker ments—and a dose of experience—will control link during rendezvous. Global Hawk to fly behind the receiver be needed to clarify these points, but the DARPA took up the challenge with Global Hawk. “We want the aircraft progress is promising. its new KQ-X program in 2010—and with the smarts and the maneuvering Refueling will be essential for un- followed a pragmatic approach. “We’re capabilities in the rear,” Northrop Grum- manned deep reach aircraft on strike using proven Global Hawk aircraft and man official Mark Gamache explained or ISR missions. For example, the ground stations, algorithms developed to news site Xconomy at the Navy’s UCAS-D stealth demonstrator under AARD, and off-the-shelf refueling outset of the program. logged successful flights by two test air hardware,” McCormick explained. The According to McCormick, 2012 is vehicles in 2011. Although it is just a DARPA program takes full advantage of a make-or-break year. “We plan to demonstrator, UCAS-D’s estimated the work carried out over the previous complete, this summer, a convincing range of 1,726 to 2,417 miles would be decade and will use it to reduce risk. demonstration that includes repeated greatly expanded by aerial refueling. “We’re mostly avoiding new technology, transfer of fuel,” he said. “In the process, The ability to top off with fuel from so we can focus on the challenges of we will learn better how this type of a “recovery” tanker could become integration and unmanned operation,” aircraft operates in close formation and important in operations around the he noted. gain valuable experience with complex carrier—or even over land bases. The The Autonomous High-Altitude unmanned operations.” same would hold true for unmanned Long-Endurance Refueling program strike aircraft. Refueling is essential set out to demonstrate “repeatable prob- Unmanned Fleet? for moving from the relatively light ability of success with limited flight In preparing a 2011 study of au- ISR payloads to toting munitions out performance aircraft under high-altitude tonomy, the Defense Science Board to deep strike ranges. conditions, redundant safe separation, observed, “Dramatic progress in support- The technology of visual recognition and unmanned flight operations,” stated ing technologies suggests that unprec- for close-in guidance may also pay off DARPA officials. edented, perhaps unimagined, degrees in other applications going well beyond “We think this is important because of autonomy can be introduced into air refueling, such as complex RPA a next generation HALE platform de- current and future military systems.” formations. signed to refuel may be much more The Pentagon urged the DSB to identify Given these advances, there seems affordable, capable, and effective,” Mc- opportunities for “more aggressive ap- little doubt that the Defense Science Cormick said. plication of autonomy.” Board’s predictions about new oppor- Two older Global Hawks operated by Still, several questions remain before tunities in autonomy are being proved NASA were designated for the program. USAF finds itself conducting hands-off right when it comes to automated aerial Step 1 was risk reduction. Northrop refueling on a regular basis. First is refueling. n Grumman’s Proteus test aircraft flew within 40 feet of the NASA Global Rebecca Grant is president of IRIS Independent Research. Her most recent article Hawk while at 45,000 feet. for Air Force Magazine was “Black Bomber Blues” in the January issue. 40 AIR FORCE Magazine / March 2012