One of the biggest problems for those designing the next generation of unmanned combat air vehicles (UCAVs) is how to define, choose and incorporate a powerplant. Rolls-Royce believes it has the answer in a new type of engine system that has a much hotter core and provides not only power to the airframe, but also manages the entire power requirement of the UCAV. However, it doesn’t have the money for the program–at least, not yet.

“Future platforms have some real challenges: increased persistence, much greater electrical demand and thermal management,” said Graham Hopkins, director of engineering and technology for Rolls-Royce Defence Aerospace. Moreover, he noted, although the requirement for low-observability drives the size and shape of the vehicle, today’s military engines impose a practical limit on just how stealthy a UCAV can be.

The UCAV designers must ensure that the rotating parts of the engine are invisible to radar. This requires hiding the compressor face within an “obscuration inlet,” such as an S-curve. The exhaust must be similarly shielded and its infrared signature must also be reduced.

A single, Rolls-Royce Turbomeca Adour Mk951 powers both the Neuron and Taranis UCAV demonstrators that are now taking shape. This engine is an upgrade designed for the latest versions of the BAE Hawk jet trainer. However, inside a UCAV, the engine must cope with the airflow distortions mandated by stealth; presumably, the Mk951 has sufficient stall margin and surge limits to cope.

Rolls-Royce is a partner in the UK-only Taranis effort and therefore will gain practical experience of the interaction between powerplant and UCAV. As Hopkins already knows, in the arrow-shaped designs favored by stealth engineers, the inlet, engine and exhaust together define the minimum length of the vehicle, which in turn defines the wingspan and the cross section is essentially driven by the fan diameter, which is driven in turn by the bypass ratio required to provide cooling air to the exhaust.

If the engine core could run hotter, it could deliver the same thrust for a smaller size and could have a smaller fan–leading to a smaller air vehicle. Rolls-Royce has calculated that for every 100-degree Kelvin increase in the core, there could be a 10-percent reduction in the size of the platform. If the platform were 10 percent smaller, it would be 20 percent stealthier, according to Hopkins. Moreover, the “recuperator” that would be part of the engine design can improve specific fuel consumption by 15 percent and reduce exhaust temperature by 200 degrees K.

Integrated Power System
Engineers would design such an engine as an integrated power system (IPS), said Hopkins. Today’s engine accessory gearboxes can’t cope with the amount of power that a production UCAV would require, he said. For instance, phased-array radars need short bursts of high power. An IPS could be designed with intelligent controls that could, for instance, divert power from the UCAV’s actuators when the radar needs it to illuminate targets. The generators would be mounted directly on the engine shaft, perhaps on the high-pressure as well as the low-pressure spools.
An IPS might also play a role in “active” thermal management on board a UCAV.
“Using fuel to absorb heat may not be enough in the future,” Hopkins noted. An IPS could also be designed so that the bypass duct is part of the aircraft’s structure.
Because of the potential for improved specific fuel consumption and cooler exhausts, Rolls-Royce is doing some work on hotter cores, with funding by the UK government under the environmentally friendly engine (EFE) program. The UK Ministry of Defence (MoD) has put only a small amount into the EFE and needs to spend more on high-temperature technology, according to Hopkins. The MoD’s official Defence Industrial Strategy states that it will “continue to invest in propulsion technology where this shows potential application to our future needs.”

The problem for Rolls-Royce is that the MoD has not yet decided whether it really will need UCAVs in the future. That, after all, is the purpose of the Taranis demonstrator program.