"Ultimately, one day I'm pretty confident that you'll be able to fly from here to San Francisco on an aircraft with something like a gas turbine burning hydrogen, but there's no way that we're going to be doing that in the next 15 years," Warren East told the Reuters IMPACT conference in London.

"So we need a transitional technology," said East, who will step down from the aero-engine maker at the end of the year.

Companies from oil majors to startups are making sustainable aviation fuels (SAF), which offer a reduction of up to 80% in carbon emissions over their lifecycle, using feedstocks like cooking oil.

SAF can be blended with traditional fuel and dropped straight into existing aircraft, but it is more expensive and makes up less than 1% of the fuel used by commercial aircraft today.

East said there would be an exponential rise in SAF as more capacity comes on stream.

But government intervention would be needed, for example in SAF mandates, to help the make the numbers stack up, he said.

Eventually, purely synthetic fuels made from carbon captured from the air combined with green-sourced hydrogen could offer a net zero solution, but the process is energy intensive.

One solution to the energy quandary is nuclear power, such as the small modular reactor Rolls is developing that could be supplying power to the grid by the end of the decade.

East said Britain's new government was supportive of the project. "The new government's quite busy at the moment, but my read is that they're very keen on it," he said.

He said aviation had to use technology to become net zero by 2050 - its stated goal - because people were not going to stop flying.

"People don't tend to go backwards," he said. "We learned about flying in the 20th century. It's now the 21st century and we're not going to go back."

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To view a livestream of Reuters Impact, please click here: https://www.reuters.com/business/reuters-impact/reuters-impact-global-virtual-broadcast-2022-09-29/

(Reporting by Paul Sandle and Sarah YoungEditing by Mark Potter)

By Paul Sandle and Sarah Young