Aerial Refueling of UAVs



OBJECTIVE: Radically increase mission length and on-station availability of Unmanned Air Vehicles (UAVs) by developing capability to conduct air-to-air refueling (AAR) of Groups 4 and 5 UAVs with calibrated airspeeds of 130 KCAS or less. 

DESCRIPTION: Unmanned air vehicles play an important military role for locating time critical targets, reporting enemy positions and movements to battlefield commanders, and destroying strategic targets or lethal ground systems. Additionally, these unmanned systems are being designed to remain in flight for time periods of multiple days or more. Unfortunately, returning to base of operations to obtain additional fuel creates a deployment and logistics challenge. Boom-and-receptacle and probe-and-drogue are the two hardware configurations and methods commonly used for AAR. In the former, a refueling boom on the rear of the tanker aircraft is steered into the refueling port on the receiver aircraft. In this method, the job of the receiver aircraft is to maintain proper position with respect to the tanker. With the probe-and-drogue method, the tanker aircraft trails a hose with an aerodynamically stabilized flexible basket or drogue. The receiver aircraft has a probe that must be placed or docked into the drogue. This is the preferred method for small, agile aircraft because the equipment is small and lightweight, and a human operator is not required on the tanker aircraft. One of the challenges faced in AAR is minimum airspeed. For this effort the focus will be Groups 4 and 5 UAVs with maximum airspeeds of 130 KCAS. In order to maximize potential on-station capability, it is desirable to minimize the distance the UAV must travel in order to reach the refueling orbit. Since many military UAVs operate inside hostile territory, bringing a large manned tanker into this arena is problematic. Because of this, an unmanned, automated tanker could a potential solution. There are five (5) key technical challenges associated with AAR of UAVs: 1. The refueling procedure will require the UAV to operate in close proximity of the tanker aircraft. Therefore, it is critical for at least one of the aircraft to know, with a high level of accuracy, where it is relative to the other. 2. UAV and tanker must avoid collisions with each other. 3. One or both the tanker and UAV must respond quickly if an unsafe refueling condition occurs. 4. From a cost, maintenance, and availability point of view, minimize modifications to both the tanker and the UAV, e.g., avoid changes to structural members, and try to keep size and weight of refueling hardware under 3%. 5. The refueling system must operate under broad weather conditions and day and night conditions. This list of technical challenges is not all inclusive. Rather, there are additional technical challenges such as, redundancy or contingency management, beyond line of sight communications between the tanker, UAV and their operators, and full airspace collision avoidance. These issues are beyond the scope of this program and will not be addressed; however, the architecture chosen should lend itself to accommodate these challenges in the future. For this effort, offerors should consider both the tanker and UAV for how a potential refueling system will work. One element of the problem is for the system to be applicable to unmanned refueling aircraft, rather than existing manned tankers (e.g. KC-135). Offerors should also consider automation of probe/drogue deployment and retraction, coordination between unmanned systems (cooperative control), and possible influence (flight dynamics or other) of a smaller tanker aircraft, if any. 

PHASE I: The contractor shall conduct an AAR concept study of Groups 4 and 5 UAVs with maximum airspeeds of 130 KCAS that demonstrates: (A) feasibility of proposed system architecture, (B) minimal impacts to proposed tanker/UAV combination, (C) feasible AAR concept of operations, (D) capability to control the systems, and (E) risk analysis of proposed concept. 

PHASE II: The contractor shall demonstrate the following: (A) Ability for both aircraft to rendezvous at the designated refueling point, (B) Ability to safely operate both aircraft in close proximity to each other, (C) Ability to transfer fuel from the tanker aircraft to the UAV, and (D) Ability to conduct mission planning. All of this will be done and require minimal modifications to either aircraft. Cooper Harper Ratings or a similar approach for qualifying goodness shall be used. 

PHASE III: The contractor shall pursue commercialization of the various technologies developed in Phase II for potential government applications. There are potential commercial applications in a wide range of diverse fields that include cargo, resupply, airdrop, or ISR type missions. 


1. Nalepka, J. P., and Hinchman, J. L., Automated Aerial Refueling: Extending the Effectiveness of Unmanned Air Vehicles, AIAA Paper 2005-6005, 15“18 Aug. 2005.

2. Tandale, M.D., Bowers, R., and Valasek, J., Trajectory Tracking Controller for Vision-Based Probe and Drogue Autonomous Aerial Refueling, Journal of Guidance, Control, and Dynamics, Vol 29, No 4, July-August 2006, pgs 846-857.

3. U.S. military UAS groups. In Wikipedia. Retrieved September 19, 2016, from

4. Cooper-Harper Rating Scale. In Wikipedia. Retrieved September 19, 2016, from


KEYWORDS: Air To Air Refueling, Probe And Drogue, Refueling Boom, Autonomous Operation, Flight Controls, Unmanned Air Vehicle, Unmanned Aircraft System, Vision Based Sensing, Remotely Piloted Vehicle, Aircraft Conversion, Drone 

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