Technical advances have the Air Force 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 aircraft as unmanned tankers Since aerial refueling 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 boom operator 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 military aviation, 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 satellite systems, especially as the demand for constellation reached full operational ca- Artist’s concept by Erik Simonsen Pilot Dick Ewers and flight test 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 Grumman 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 test pilot 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 bombers, 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 aerospace engi- neers from Texas A&M and Virginia 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.