Unveiling plans for a new technology research centre in the UK on 2 December, Safran chief executive Olivier Andries was clear: such a facility is vital as the next generation of engines will be hybridised.
In this embodiment of hybridisation – and depending on the size of the aircraft there are multiple ways to use electrical power – a motor-generator will complement the thermal function of an engine depending on powerplant use during different phases of flight.
By adding electric motors to a next-generation gas turbine engine, designers should be able to right-size the thermal core for better efficiency, using electric motors to cater for peak power requirements.
Most estimates suggest around 4-5% of the total power requirement for a future narrowbody engine will be provided by electricity.
Of course, there will be those advocating for the benefits of batteries who suggest that figure should be much higher.
But as the size of the aircraft grows, so does the power requirement: a 50-passenger aircraft capable of flying routes of 500-1,000nm (926-1,850km) will need about 4MW of peak power, whereas a 150-200-seater – a typical narrowbody jet – that can traverse Europe or fly across the continental USA needs more like 45MW, Jeremy Hughes, head of demonstrator programmes, Rolls-Royce, told the UK Aerospace Technology Institute’s (ATI’s) conference in November.
And for a long-haul 400-passenger widebody, peak power is something in the range of 80-100MW. While Hughes believes “hybrid can play its part”, it is clear it is about “how you augment rather than replace” the gas turbine.
And to hit an ambitious emission-reduction target of around 30% for the next generation of narrowbodies, engine makers will have to pull every lever available.
All of which is a fine ambition, but one that can mask the complexity of the challenge.
Besides demonstrating that a hybridised engine can work as a “harmonious system”, there is a real hurdle to overcome if electric motors are to be added to both spools of a typical two-shaft engine, said Neil Webster, head of UK research and technology at Safran, also speaking at the ATI conference.
While the low-speed spool is a relatively benign environment, adding an electric motor that can cope with the extreme heat at the rear of an engine is “the challenge our engineers are going to have to overcome”, he says.
On top of which, certification will be key challenge for this novel technology: while the data set for gas turbine engines goes back to the 1940s “electric machines as primary propulsion - the data set is pretty much zero”, says Webster.
“We are going to have to present a data set to the regulator – we are not going to be able to rely on grandfather rights.”
If those hurdles were not large enough on their own, the industry also faces a particularly tight schedule, with the technology needing to be at TRL6 by around 2028: “We really haven’t got that much time – we are going to end up with some very focussed engineers in the short term,” Webster adds.
Demonstrating the required levels of reliability will be another hurdle: performance gains cannot come at the expense of “reliability and robustness”, says Rolls-Royce’s director of research and technology Alan Newby.
And replicating the rigours of in-service performance will be another difficulty, says Todd Spierling, senior technical fellow at Collins Aerospace.
He cites the experience of developing a more-electric architecture that debuted on the Boeing 787. After the Dreamliner entered service, it became apparent “that what we should have done is tested an order of magnitude more equipment for an order of magnitude more time”, he says.
His advice? Test, test and then test again: “You are never going to get enough [data] to replicate it in service.”