Rolls-Royce believes that advanced engines will be the single most important design factor for future fighter aircraft.
Conrad Banks is R-R’s chief engineer for defence future programmes. In his 30-year career he has worked on projects such the Pegasus for the BAE Systems Harrier and the EJ200 for the Eurofighter Typhoon. Now his role is to look towards the future.
"My job is to get the best technology developed, so whichever platform goes forward – future fighter, UCAVs, combinations of, growth variants – we are in a position to exploit and develop the best system," he says.
"We're putting advanced technology on the shelf. When the politicians agree to collaborate on certain programmes we are ready to press the button. When sixth technology [fighter] programmes are launched, we'll be ready with solutions."
Banks believes that the physical characteristics of fighters, whether manned or unmanned, will largely be driven by the shape of the propulsion system – which comprises the engine as well as the intake and exhaust.
This is a departure from the past, where airframers would develop an aircraft, and call on engine-makers to provide an engine of a certain thrust. The change is largely driven by the premium placed on stealth, which demands that engines be buried deep in an aircraft. S-shaped inlets fore and aft ensure that radar waves cannot reach engine parts without bouncing around the intake or exhaust. Shaping, however, while crucial, is not enough: radar absorbent materials (RAM) are also necessary to absorb radar waves.
"By the time you've got these S-shaped intakes front and back, the length of the propulsion system is very long," says Banks. "The airframers want it to be as short as possible because then the aircraft does not have to be unnecessarily large, and they've got more space for weapons and fuel."
THE LEGACY OF TARANIS
Should the intake be scrunched up too much, the engine could be prone to surges. Banks says a great deal about the relationship between engine, airframe, and aerodynamics was learned from the secretive BAE Systems Taranis technology demonstrator, which is powered by an off-the-shelf Adour engine. Jointly funded by the UK's Ministry of Defence and industrial partners including BAE Systems and Rolls-Royce, the unmanned Taranis remains shrouded in secrecy since its first flight in 2013.
A key focus area for R-R is the development of distortion-tolerant military fans, which are better able to deal with separation of air flow issues. Banks says the length of the intake in front of the engine is determined by fan-size. If the length is less than four times the fan diameter, "very, very high distortion levels" occur.
"What you want to do is crumple [the intake] up as much as possible, and do the same with the exhaust," says Banks. "The more you crumple this up, the shorter the length of the propulsion system, and the more flexibility the airframer has to design the aircraft."
Another focus area is an embedded starter-generator that could save space and provide the large amount of electrical power required by future fighters. Banks says power demand on fighters will rise exponentially, with electricity needed for powerful radars, electrically-actuated control surfaces and directed energy weapons.
Existing aircraft engines generate power through a gearbox underneath the engine, which drives a generator. In addition to adding moving parts and complexity, the space required outside the engine for the gearbox and generator makes the airframe larger, which is undesirable in a stealthy platform.
"By the time you've embedded all of this underneath, it increases the diameter of the installation," says Banks.
So R-R is working on fusing magnets to the rotating shaft of the engine, which can reach 15,000 rotations per minute. The placement of wires outside this creates an electric motor. "If you have magnetics rotating, and you have wires, you generate electricity."
In addition to generating power for the aircraft, the system can also serve as a starter generator. By energising the shaft with, say, power from a battery or a ground power unit, the engine winds up. Magnets can be applied to both the high and low pressure shafts, effectively creating two separate power generators.
"I can take more energy from one shaft, rather than the other," says Banks. "If have a surge issue at high altitude because the air is rarefied, I can unload the fan and direct energy from one shaft to the other, providing more surge margin. You can intelligently siphon off energy."
"We can remove all of the gearbox underneath the gas turbine. Now we tell the airframer that all we need is a battery or electrical power source, and that starts your engine for you… it's a win-win. Everything is beneficial."
The company is also continuing to develop advanced composites that can reduce weight inside the engine.
Work is underway on these systems at Rolls-Royce's lab in Bristol. Banks stresses that the technologies are not being developed with a specific aircraft in mind, but for futuristic programmes in general. One of these, of course, is the prospective British-French Future Combat Air System (FCAS).
"If we can offer a slimline, power dense, propulsion system because we've cracked distortion tolerance, and with electrical energy that's generated at the heart of the gas turbine – and not generated via generators mounted on the pad like at the moment – then everyone wins and you have a much more efficient system," he says.