Buried deep inside the Rolls-Royce plant at Filton in Bristol is a bare brick room where the Olympus engines that powered Concorde and the Pegasus fans for the Harrier jump jet were put through their paces.
Now it is testing a generator the size of a washing machine drum designed to burn green fuels and produce enough electricity to power the passenger planes of the future.
The challenge is to radically reduce the amount of carbon dioxide produced by aviation. While carmakers and other industries can move at speed, the problem for planes is neatly summarised by Rolls-Royce's chief project engineer, Adam Newman. "If your hybrid Jaguar needs to pull over to the roadside, that's one thing, but things like that can't happen at altitude."
Rolls-Royce plans to get around these issues by introducing more efficient jet engines for the next generation of passenger planes to cut the amount of carbon released per passenger per mile.
Its next model, the UltraFan, will be 25 per cent more fuel efficient than its 1990s-era predecessor. It does so by being very big - its diameter is slightly smaller than that of a London Underground tunnel - and shifting more air through a larger fan, improving efficiency.
Theoretically another 20 per cent to 30 per cent of fuel efficiency can be squeezed out of the turbofan jet engine design, says Alan Newby, director of aerospace technology and future programmes, although size limits and high temperatures make that ever more challenging.
While aviation accounts for about 2.5 per cent of carbon emissions, flying is still limited to a relatively small proportion of the global population and the industry is under pressure to move towards zero emissions by 2050.
Step two is to move to greener fuels for existing engines. These will start as biofuels, but to make them greener synthetic fuels will be used.
"E-fuels" will be made using electricity from renewable sources to generate hydrogen, which will itself be bonded to carbon dioxide to produce aviation fuel. This fuel can be used in engines with a few modifications, mainly ensuring the seals still work, and Rolls hopes to have both existing and new engines using these latest fuels by 2030. As well as recycling carbon dioxide, the synthetic fuel produces fewer particles that are blamed for helping warm the Earth.
Along the way, airlines will seek to make shorter, more efficient routes rather than the dog-leg path they sometimes take today, and to reduce contrails, considered another warming factor as the soot combines with ice to retain heat from the planet.
The third step is the new technology the company is now developing, including its new generator. Strapped to a jet engine it can supply more than 800 amps of current at 3,000 volts AC, hence the inches-thick cables.
The idea is to act as a stepping stone technology, much like the hybrid of the car world. Rolls envisions a plug-in electric plane that can operate on battery power for some of the journey and then turn to fuel for the rest.
As with cars, the sheer weight of batteries poses a challenge for aviation. And for a machine aiming to defeat gravity, this is a problem neatly summed up by Newby. "If I take kerosene as my reference point, weight for weight, hydrogen is three times better, batteries are 60 times worse," he says. "That's the challenge for batteries."
To put that in context, a long-range plane such as an A380 might have about a third of its weight taken up by kerosene-based jet fuel.
"Fundamentally that's the challenge. So that's why you will see all electric solutions at a smaller, shorter range at the moment. But what you can do is enhance that with some kind of hybrid solution."
For smaller craft, including helicopter-style taxis, batteries are an option, especially as new chemical mixes are developed and the amount of power per pound of weight increases.
Rolls-Royce's single-seat battery-powered plane reached a top speed of 622 km/h in tests last month, making it the world's fastest all-electric vehicle. The Spirit of Innovation averaged 555 km/h over three kilometres, beating the existing record by 212 km/h.
The aircraft was powered by a 400kW motor with a battery pack that Rolls-Royce says has the best power density of any used in aviation. Breaking the record was very exciting, says Newby, "but for us, the attraction of that is learning how to integrate electrical motors and batteries, in particular, the thermal management of batteries" - that remains a big challenge.
The company is building on innovations like these to find quick solutions to each category of flight.
Back in the test centre at Bristol, the shiny metal generator is being put through its paces to ensure it meets the technical and safety demands of powering an aircraft carrying dozens of people.
Connected to an Allison 2100 turboprop engine, its performance is monitored from a room behind the test bed, gathering up to 2 million pieces of data per second as it scrutinises information on heat and vibrations.
Rolls made four of the machines as prototypes, taking three of them to pieces, one of which was tested "destructively", says Newman, in a bid to fully understand the generator. It plans for the hybrid to be made available for commuter and regional planes that seat up to 50 passengers.
As for hydrogen, storing the gas, keeping components cool and gathering enough of the stuff are all challenges that can be overcome in the long term.
The gas is seen as an attractive alternative because when burnt efficiently it produces only water as a by-product. Used in high-temperature environments like a jet engine, it can produce nitrogen dioxide, another gas policymakers want cut because it is linked to damage in lung health.
Companies such as ZeroAvia in Britain are developing hydrogen-driven smaller aircraft using fuel cells to generate electricity. But for Rolls-Royce, hybrids and greener fuels will be the stepping stone.
The biggest planes such as the Boeing 777 that have two aisles, will be a tougher nut to crack, says Newby. "Particularly in the near term, and almost certainly for the foreseeable future, for wide-bodies, you will need sustainable aviation fuel."
As well as acting as a range extender, the hybrid design gives aircraft designers many options. They can have more rotors driven by the same power plant, as splitting off cables for electric motors is far easier and lighter than installing gears to distribute mechanical power. The generator can also be used to give the plane an acceleration boost when needed.
The unit could even be used alone with no batteries if a fuel is developed that is zero-carbon
"We don't quite know how these markets are going to develop," adds Newby. "But what we're trying to make sure is we've got the technology building blocks in place, so that when the markets develop, we're ready to respond."
Renewables beginning to take off
- Aviation accounts for about 2.5 per cent of carbon emissions
- The industry is under pressure to reach zero emissions by 2050
- Rolls has made four prototype generators
- Strapped to a jet engine the generator can supply more than 800 amps of current at 3,000 volts AC
- It plans for the generator to be part of a hybrid solution for commuter and regional planes that seat up to 50 passengers
- Rolls-Royce's single-seat battery-powered plane recently reached a top speed of 387 miles an hour
The Telegraph, London