Raytheon has disclosed the existence of a new project to develop a purpose-built, heavy fuel engine under 20hp (15kW) to power small unmanned aircraft systems.

The project is initially aimed at powering the Swift Engineering KillerBee-3, Raytheon's selected platform competing for the US tri-service, small-tactical UAS (STUAS)/Tier II contract.

But Raytheon also plans to offer the engine to the wider small UAS market, seeking to capitalise on fast-growing demand for a class of aircraft that can remain aloft for up to 20h carrying a payload up to 13.6kg (30lb).

Raytheon has declined to name its engine supplier, but Flight International has reported that Swift will install a small heavy fuel engine made by XRD. Raytheon's partner is cleared to offer the new heavy fuel engine for the small UAS market, says Ryan Hartman, Raytheon's capture lead for the STUAS/Tier II programme.

Five prototype engines have entered a bench-test programme, but Raytheon is planning to develop the engine at a moderate pace. "The key is not rushing this into production," Hartman says. "The key is maturing it methodically."

Heavy fuels are fast becoming a mandatory requirement for military operations. Such fuels are less prone to accidental combustion and are already ingrained in the military's logistics system.

Raytheon's approach to develop a purpose-built engine stands in sharp contrast to its competitors for STUAS/Tier II.

In particular, Boeing/Insitu has adopted a very different philosophy for Scan Eagle, already flying on a fee-for-service basis in Iraq. Rather than develop an all-new engine, Insitu has partnered with Maryland-based Sonex Research to adapt the Scan Eagle's gasoline engine to operate using kerosene-based heavy fuels, such as JP-5, JP-8 and diesel.

Sonex's process involves major component changes to adapt an engine built for hobbyists to standards required for military operations and for use with less-combustible heavy fuels.

Despite the challenges, Sonex boasts achieving a 25% reduction in specific fuel consumption and a proven track record in battlefield conditions.

"We had 350 [flight] hours in theatre in the first of operations with no problems," says Sonex founder and chief executive Andrew Pouring.

On the other hand, Raytheon's Hartman questions whether an adaptive engine can ever be as reliable as a system designed for burning heavy fuels: "We have made a conscious decision not to go down that path. I just don't see how you expect that to be a reliable engine source."

Source: Flight International