As the U.S. Navy ponders future purchases of the F/A-18, it is showing growing interest in the comprehensive structures, systems and propulsion upgrade package proposed by industry earlier this year that would extend the combat life of the current fleet.

One element of Boeing's Advanced Super Hornet plan receiving close attention is large conformal fuel tanks (CFT). In flight tests, they have shown the potential for marginal drag reduction and other benefits over the unmodified aircraft, despite their size and capacity. With the ability to hold 3,500 lb. of fuel, the tanks could add 260-nm combat range to the Super Hornet and, in the case of the EA-18G Growler electronic-attack version, would extend time on station and bring-back weight.

“Boeing looked at ways of bringing the Super Hornet into the 2030 threat environment in an affordable way, and Northrop Grumman's main role in this was development of the CFT,” says Northrop Grumman F/A-18 programs director and chief engineer, Bob Walke. Revealing more details about the CFT development and plans for the production version, Northrop says the prototype units went from “napkin to first flight” in just 10 months. “The effort began in 2010 with low-level trade study work until 2012, when the decision was made to make a prototype happen quickly,” says Walke. Following a go-ahead in September 2012, the tank design was completed in January 2013, assembly began in May, delivery started in early July and flight tests on a leased F/A-18 were underway in August.

The CFTs sit atop the upper fuselage on either side of the central spine and run 24 ft. in length from the aft cockpit to the leading edge of each vertical tail. At 4.3 ft. wide at the broadest point, the CFTs hug the upper fuselage surface, rising to a maximum height of only 1.8 ft. just forward of the wing leading edge.

Shaped for low drag and stealth, the test CFTs were not filled with fuel but were otherwise aerodynamically representative of the production tanks.

“We knew it was essentially a zero-drag configuration and, although there was no content in the tanks, we could measure drag through fuel flow. We actually saw a little better performance, as it improves transonic transition,” Walke adds.

In flight tests this August back-to-back comparisons of a “clean” F-18 against the CFT-configured F-18 showed “same or better” fuel flow for the modified aircraft at the same gross weight and Mach 0.84 cruise conditions at 34,000 ft.

Although they look the same, the prototype CFTs weighed 1,500 lb.; production fuel tanks would weigh 870 lb. “We simplified the design substantially,” says Walke. “For instance the prototype had metal lower skins and floors, whereas the production version would be composite like the upper skins. It would also have fewer aluminum frames.”

If sanctioned, the production version of the tank also will be modified to include plumbing for pumping fuel, as well as enclosures and openings for cooling and conditioning ducts.

“We have three openings,” says Walke. “One for the fuel oil heat exchangers, another for the liquid-cooling system ground fan, which is used for the radar system ground check out, and the third, which is an environmental control system auxiliary scoop that opens up on the ground and below Mach 0.5.”

“The CFT is production-ready now should the Navy make that decision,” says Northrop Grumman F/A-18 program manager John Murnane who adds that “in flight tests, pilots reported it flew just like a Super Hornet. It accomplished everything we wanted it to do. The Navy is showing strong interest in this and we're ready to do it.”

Capt. Frank Morley, program director for the Navy, confirms the service's interest. Commenting at a recent ceremony marking the 35th anniversary of the F-18, he says the CFT option has “certainly got our interest,” among all the various upgrades available.

The company is optimistic the Navy will indicate its interest more substantially in the next program objective memorandum, a planning document for pending procurement. Production could begin “maybe by 2016,” says Murnane, with deliveries starting about three years after receiving the official go-ahead. The CFT is also attracting interest from Australia, the other operator of the Super Hornet and Growler, and it is designed to be retrofittable for new-build aircraft. “The intent is to be able to install it in a shift,” says Walke. The CFT bolts onto the structure at three attachment points per side, which are designed to keep loads isolated from the rest of the structure and vice versa.