Aerial tankers can fuel up hungry jets, turboprops and even helicopters equipped with a refueling probe by trailing a hose and drogue behind them. In smooth air, such a mating dance can be a challenge, in rough air and bad visibility it can become downright scary. A new ‘smart drogue’ system in development may finally help make ‘plugging in’ a less terrifying affair.
The ‘hose and drogue’ method of aerial refueling is far more common around the globe than the USAF’s ‘big tanker’ boom and receptacle method. The advantage to the boom and receptacle method is that it is faster and more reliable, especially under harsh conditions, and the rigid boom can pump fuel into the receiver aircraft at much higher rates than a flexible hose can. Still, this system requires large and complex dedicated tanker aircraft, and interoperability during international operations can be an issue. Meanwhile, tactical hose and drogue refueling capability can be acquired cheaply via purchasing ‘buddy refueling pods’ for existing fighter aircraft and it can even be procured on a strategic level at a low cost from ‘pay by the hour and pound’ commercial aerial tanker providers.
A aerial refueling boom operator inspects the “Iron Maiden” basket that flies with a hose adapter behind the KC-135R’s rigid boom. This basket is notorious for causing damage to aircraft and being tough to plug into, hence its other nickname: “The Wrecking Ball”
During hose and drogue tanking operations, a deployed drogue, or ‘basket,’ can hop around in turbulence and it can even be effected by the slipstream and ‘bow wave’ of the connecting aircraft. Additionally, minor asymmetry or damage to the basket can make it wobble about and oscillate rapidly, making plugging into it nearly impossible.
Even in good conditions, extreme pilot concentration is needed to couple with the basket, which can be a taxing affair after a long combat mission or while flying through stormy weather after repeatedly trying to land on an aircraft carrier at night.
Just like how munitions have become independently guided over the last half century, now the aerial refueling drogue may be getting an intelligent upgrade of its own in the not so distant future. The idea is to create a smart, stabilized drogue that can attach to modern hose and drogue style refueling systems. This smart drogue has sensors onboard that constantly assess the basket’s movement, and small winged surfaces behind the basket rapidly adapt to any errant motion. The result of which is something of an active buffer, that can greatly reduce the movement of the drogue even during intense turbulence and shearing airflow.
In a way, similar ‘active’ dampening systems are designed into ships to minimize the effects of wave motion on the vessel. Some aircraft, such as the B-1B Bone, have small canards vanes that do the same thing during low-level, high-speed interdiction missions. Even our cars have stability assist traction systems that buffer sliding and loss of traction via the use of the car’s ABS system. Adapting a similar active stability system to aerial refueling seems like a no-brainer and, if anything, it is probably quite overdue considering the money and stakes involved with the act of aerial refueling to begin with.
The idea behind what the US Navy deems the Actively Stabilized Drogue Refueling System, which was officially opened as a program in 2009, is to not only make refueling easier for manned receiving aircraft, but to also greatly enhance the reliability and ease of operation when it comes to refueling future unmanned aircraft. The video posted above was taken in the UAV Lab at Western Michigan University in 2011. The sub-scale test was deemed a success, which will hopefully lead to a full-scale test in the future.
As a whole, hose and drogue refueling technology has began to become more user friendly over the last couple of decades. Another area where this is happening is in variable speed baskets that can refuel a wider range of receivers.
In the past, different baskets had to be fitted based on what speed the receivers operate at. This means that a KC-130 tanker could not refuel fighters and a loaded down MH-53s in the same mission. This is now changing with companies like Cobham making variable speed baskets. Currently, this is what the company has available when it comes to refueling baskets:
- Low speed VDD: 100-180 kts (Compatible with helicopter and tilt rotor probes)
- High Speed VDD: 180-325 kts (Compatible with NATO STANAG 3447 compliant probes)
- Variable Speed VDD: 105-215kts (Developed for the 48-000 pod on USAF C-130 Aircraft)
Cobham is now working on an all-speed basket that can refuel the whole range of combat aircraft without being changed. Such a system, fitted with the Actively Stabilized Refueling System, would make life much easier for both tanker crews and receivers and will allow for less tankers being needed to support more disparate aircraft on a single mission. It will also allow for faster, more reliable tanking, which in itself allows for more fuel being available for receivers on a single tanker mission.
Emerging unmanned combat air vehicles, like the Navy’s X-47B and the UCLASS production aircraft that will follow it, have a great advantage in range over similar sized manned systems. Just because they can fly farther does not mean more range would not be beneficial, in fact it would only multiply their usefulness. Thus, aerial refueling of these unmanned systems is a very relevant capability to have. By providing a stabilized drogue system, that is more resistant to turbulence and airflow variations and thus basket movement, unmanned subsystems used for autonomous refueling will be more reliable and may even exceed the capabilities of manned aircraft as their abilities evolve.
Over the last decade, NASA, in conjunction with the DARPA, have tested autonomous aerial refueling with both their F/A-18B Hornets and with RQ-4A Global Hawks. The Hornets, loaded with the same sub-systems that can be installed on unmanned aircraft, plugged into the basket multiple times, and incredibly smoothly at that, without the pilot’s hands touching the controls. The Global Hawks flew in perfectly tight formation, just short of making contact between the basket and the probe, to demonstrate that even high-altitude aerial refueling is possible with an aircraft as fragile and that operates in such a tough aerodynamic environment as the Global Hawk.
Turning some Global Hawks into tankers would massively extend the range of other missionized Global Hawk cousins. Such a technology could lead to high-altitude, long-endurance unmanned systems ‘orbits’ going from upwards of a day and half in length, as they are today, all the way to multiple days or possibly even weeks.
Currently, one fully missionized Global Hawk has to take the place of another once its fuel runs low in order to maintain a consistent surveillance ‘orbit.’ This limits the effectiveness of a relatively small fleet that is in high demand. Sending a basic, unmissionized Global Hawk tanker to refuel the missionized one on-site, or even better, having the missionized Global Hawk drop altitude and refuel from a standard USAF tanker, makes the most of the limited Global Hawk fleet size on hand.
This concept is not just relegated to High Altitude, Long Endurance unmanned systems like the Global Hawk, but any unmanned surveillance aircraft with a decent endurance would benefit from aerial refueling.
More persistence equals better intelligence and affordability, which then ripples down through the decision making chain resulting in better choices by commanders. In other words, higher quality intelligence, which can be affected by length of observation time as much as the fidelity of the sensors observing a target, leads to better information for which leaders in the field and in Washington have to make major tactical or even strategic decisions with. Being able to park a Global Hawk on the border of a country for days at a time, instead on a few dozen hours, changes the standoff surveillance game completely. This can be accomplished through aerial refueling or by building many more Global Hawks.
Calspan’s unmanned aircraft surrogates (Learjet 25s) are leading the realm when it comes to tactical unmanned aircraft aerial refueling capabilities. Using Precision Relative GPS and a similar data-link architecture as what allows the Navy’s experimental X-47B Unmanned Combat Air Vehicle (UCAV) to land on an aircraft carrier, Calspan’s Lear acts just as an X-47B would while approaching the tanker and suckling its basket for gas. During tests in 2013, the Calspan Learjet 25 fitted with a refueling probe and with X-47B’s avionics, autonomously connected with a KC-707 tanker as if it were refueling operationally.
The work that Calspan is performing was supposed to lead directly to live testing with the X-47B as part of the drone’s overall program goals, but the objective was cut in 2013 due to budgetary restrictions. Today, the X-47Bs continue to serve in flight testing, although it is not clear if the Autonomous Airborne Refueling Demonstration will ever take place under the program. Still, in many regards, this technology is fairly proven and somewhat mature, and it will only get easier to apply operationally if an Active Stabilized Drogue System were fitted to the tanker’s basket.
Update 4/22/15: X-47B has autonomously refueled from a KC-707
All these hose and drogue tanker developments are a good thing, as they undoubtedly will result in aerial refueling capability for operational unmanned systems and easier refueling for manned systems in the future. In fact, some say that the USAF has been providing aerial refueling for shadowy unmanned aircraft with its boom and receptacle method for sometime, with the capability presumably being proven in the black-budget world long ago.
The truth of the matter is that the tankers of the future, not just the receivers, will most likely be unmanned. On a tactical level an Unmanned Combat Air Vehicles (UCAV) will be very well suited for being a Carrier Air Wing’s tanker aircraft, as they possess a superior range to begin with than manned fighters, such as the Super Hornet or F-35C. With its internal weapons bays filled with gas tanks and two more tanks carried on its wings, stealthy carrier borne UCAVs that may have struck targets deep inside enemy territory one day will be able to act as an escort, launch or recovery tanker the next.
On a larger scale, one day we may see aircraft as big as the upcoming KC-46 Pegasus tanker become optionally manned, or totally unmanned, to refuel their unmanned UCAV and even bomber brethren autonomously. Presumably, such a unmanned tanker would be able to do so with remarkable efficiency, as it would be networked with the rest of its unmanned swarm, which will work to make all their collective movements and actions as efficient as possible in an effort to complete their common objectives with the highest chance of success.
Although a fully unmanned vision of air combat and aerial refueling may still be some time out into the future, in the mean time, an aerial refueling basket that moves less when a receiver is trying to plug into it would be a godsend for combat aviators around the globe and it would save money through efficiency. The idea of a stabilized drogue, maybe one that can accommodate a vast array of aircraft operating at a wide range of speeds, also begins to take away some of the luster of the more expensive but efficient boom and receptacle aerial refueling method. In the end, these new technologies—stabilized drogues, all-speed baskets and unmanned refueling—will help unlock a new user friendly era of air combat, even if that user ends up being a a flying computer networked to dozens, or even hundreds, of other flying computers.
Tyler Rogoway is a defense journalist and photographer who maintains the website Foxtrot Alpha for Jalopnik.com You can reach Tyler with story ideas or direct comments regarding this or any other defense topic via the email address Tyler@Jalopnik.com