Conformal Air Refueling Tank System

By Eric Hehs Posted 19 October 2010

The continuing evolution of the F-16 took another step forward in the fall of 2009 when a Fighting Falcon was aerial refueled from a probe-and-drogue refueling system for the first time. While all production F-16s are equipped with receptacles designed for refueling from tankers equipped with flying booms, CARTS, which is short for Conformal Aerial Refueling Tank System, allows F-16s to be aerial refueled from the more prevalent probe-and-drogue systems.

Probe-and-drogue refueling on the F-16 has been studied for many years. Some of the previous efforts to install the capability on the F-16 came as the result of Operation Desert Storm in 1991 when many US Air Force KC-135 boom-equipped tankers were retrofitted with probe-and-drogue refueling systems to support aircraft with refueling probes. The problem then became that a refitted tanker could only support aircraft equipped with refueling probes and not those with receptacles. The KC-10 tankers have always had the capability to support both probe-and-drogue and boom refueling systems during a single mission.

The F-16 program investigated fourteen different types of refueling probes during these early studies. The design choices were based on probe type (fixed, pivoting, telescoping), mounting location (over-wing, underwing, strake, fuselage, external tank), and probe shape (straight, small dogleg, large dogleg). Two designs were flight tested on F-16s: a straight probe mounted on top of the wing and a large dogleg probe mounted on an external, underwing, fuel tank. The flight tests, in the early 2000s, were limited to flying qualities and dry hookups with a variety of tanker aircraft. None of the hookups involved the actual transfer of fuel from the tanker to an F-16.

The most likely air forces to incorporate CARTS into their F-16 fleets are those that operate probe-and-drogue tankers and that have F-16s equipped to carry conformal fuel tanks. The capability, however, received additional impetus more recently with the advent of the competition for the Indian Air Force Medium Multi-Role Combat Aircraft, or MMRCA.

“All contenders for MMRCA had to demonstrate an inflight probe-and-drogue refueling capability,” explained Don Thompson, ADP’s project manager and engineer for CARTS, who is leading the current design effort. “So we took all the various concepts that had been studied previously and chose the most viable one. We then designed, built, tested, and demonstrated the system in less than eighteen months.”

How It Works

The most viable candidate was based on a modification to the conformal fuel tank system found on the most recent versions of the F-16. CARTS uses a straight probe that telescopically extends and retracts from the forward section of the right-side conformal fuel tank. The right side was chosen to avoid engineering issues associated with the internal gun, which is mounted on the left side of the aircraft. The mechanically driven probe quickly moves the probe’s nozzle to its fully extended position just in front of the pilot’s eye position. The probe tip location provides the appropriate clearances between the drogue basket and airframe.

Incoming fuel does not enter the fuel reservoirs within the conformal fuel tanks directly. Instead, fuel flows directly to the F-16 refueling manifold. This approach optimizes flow rates and pressures. In essence, the aircraft does not know whether it is being refueled by a probe-and-drogue system or by a flying boom. During refueling, the F-16 digital flight control laws adjust automatically to provide the pilot with optimum flight control response for air refueling.

The system used for the flight demonstration in 2009 required a new conformal tank structural design and minor modifications to the Block 60 F-16 used for the testing. “We replaced the forward section of the right segment of a conformal tank with a new tank that contained the telescoping probe,” said Thompson. “The outer mold line of the new tank matched the outer mold line of the existing tank to avoid affecting the aerodynamic performance of the aircraft.”

The aircraft required an additional fill port and a control valve for the fuel manifold as well as some electrical modifications and a control panel in the cockpit.

The Project

Lockheed Martin funded the project, which covered the design, procurement, construction, assembly, testing, and demonstration of two CARTS tanks and three aircraft modification kits. The modification of the F-16 plumbing system, exterior lighting, and cockpit control for the demonstration was performed in the United Arab Emirates.

“We needed a test aircraft version of the Block 60 aircraft,” Thompson said. “The test Block 60 aircraft in the US was tied up with avionics modification and integration to support MMRCA field trials, and we needed a dedicated aircraft for the CARTS portion of the field trials demonstration. We modified the aircraft in the UAE because it was easier and quicker than bringing another Block 60 F-16 into the US to modify and then send it right back to the UAE on the way to India for the field trials.”

During the flight testing and MMRCA demonstrations, the CARTS-equipped F-16 successfully received fuel from both a modified DC-10 and an Indian Air Force Ilyushin Il-78 tanker aircraft and made approximately forty aerial refueling contacts. The speed envelope, as tested, is in the range of 250 to 300 knots. The pilot flies the probe into the refueling basket of the drogue from just below at a closing speed of no more than ten knots.

Production Version

The demonstration version of CARTS is being further refined for production. “We are modifying the structure of the conformal fuel tank by moving the tank bulkheads and designing some new bulkheads,” Thompson explained. “The additional strength is needed for the probe/tank interface. We are also adding some lightning strike protective measures to the production design.” The production version will have an electrical rather than a hydraulic actuation to simplify the design and eliminate the extra demand on the hydraulic system. The production version will also be compatible with night vision goggles.

The production design is expected to be finalized in late 2011 and fielded by 2015.

The overall design and development effort is a collaboration between four companies from four nations: Lockheed Martin is responsible for overall system engineering, integration, aircraft modification, and project leadership; Hindustan Aeronautics Limited in India is responsible for CARTS tank design and probe integration; Cobham Mission Systems in the United Kingdom is responsible for the telescoping fuel probe design and integration; and Israel Aerospace Industries Ltd. in Israel is responsible for technical consulting associated with the tank design.

Eric Hehs is the editor of Code One.