Powering a net zero future
What will the airliners of the future be powered by? Electric, hybrid-electric, hydrogen, or even
ammonia? TIM ROBINSON FRAeS reports from the RAeS Alternative Propulsion Technologies – the
Challenges and Opportunities for Aircraft Design Conference, held on 1-2 December in a hybrid format
at both RAeS HQ and virtually online.
The conference took place using a hybrid live/virtual format.
Today aviation stands on the threshold of a whole new age as the challenge of climate change now
focuses some the best minds of the industry on how to decarbonise aerospace and allow for sustainable
growth. A key challenge then, is what will take the place of kerosene-derived Jet AI – a fuel that is
almost perfect for aircraft with its energy density and ease of storage. In early December, the RAeS
hosted a conference on ''alternative propulsion'' looking at this very question -and the challenges and
opportunities posed by electric, hybrid-electric and hydrogen propulsion.
While the conference had as its title ''alternative propulsion'', the event also saw significant time
devoted on configurations and airframes – a sign as to how closely integrated new powerplants and
airframes will have to be when designing new zero-carbon airliners. For example, if electric-distributed
propulsion is used, with multiple electric engines and propellers spaced out on the wings, what will be
the effect of these on aeroelasticity and flutter? Where is the best place to store heavy batteries for an
electric aircraft – spaced around the airframe or all in one place? And perhaps the biggest question of all
– how to incorporate hydrogen fuel tanks into a future airliner in a way that keeps drag penalties to a
minimum, reduces CoG changes and provides maximum safety to crew and passengers in the event of a
crash landing. There are also question around the airport infrastructure and the supply chains that will
need to potentially support multiple types of aviation fuel or energy in the future – for example the
standardisation of electric aircraft chargers.
The conference featured a variety of speakers from the big airframers such as Airbus and Boeing, to
engine companies and academics – and a wide range of viewpoints from electric aviation evangelists to
the hydrogen skeptics. The presentations also ranged from concept aircraft that exist only as CGI
renders to briefings on real green aircraft, such as Ampaire''s hybrid-electric EEL and the
Rolls-Royce/Electroflight record-breaking electric ''Spirit of Innovation'' racer. The hybrid format also
allowed for robust and lively Q&A sessions from the floor and from online delegates – adding to the
learned discussion and debate.
With two full days of presentations this invariably will be just a snapshot of the proceedings. Lets take a
look at some of the highlights,
ATI teases narrowbody concept
FlyZero'' teased at narrowbody liquid hydrogen powered concept in its presentation. (ATI)
Giving a update on UK ATI and its FlyZero green airliner design study were Mark Scully, Head of
Technology and Propulsion, ATI, Simon Webb, Chief Engineer – Propulsion, FlyZero and Anna Calder,
Flight Physics Lead, FlyZero. FlyZero has been a year-long design and engineering study (see ''Flying the
carbon-free way'' AEROSPACE, October 2021) involving around 100 experts, many seconded from
industry, to imagine the future of zero-carbon civil aviation, with a brief to narrow down over 20
concepts to three ''scout'' airliner designs covering regional, short-range and twin-aisle airliner sectors.
At the conference, FlyZero teased an image of their short-range A320-class ''scout'' design, which
features a V-tail, canards and read-mounted engines, possibly using Boundary Later Ingestion. This, like
a larger medium-range concept revealed on 6 December, would use liquid hydrogen to power turbofan
engines. This would carry 150 passengers with a range of more than 1,500nm. Having revealed two of
the three designs, a third regional ''scout'' aircraft concept is set to be unveiled in early 2022,
FlyZero''s Webb also gave a fascinating breakdown of the suitability, performance and economics of the
various alternative power sources being proposed, and concluding "We believe that the development of
liquid hydrogen and SAF is needed to accelerate netzero".
However, despite excitement of the potential of hydrogen, particularly green hydrogen made using
renewable energy, not all were convinced, with Prof. Dr.-Ing. Dieter Scholz, from Hamburg University of
Applied Sciences flashing up in his presentation a 1995 headline from Flight International, that reported
that DASA (forerunner of EADS/Airbus) was planning to fly a Dornier 328 with hydrogen in 1998 and
wryly noting that nothing happened. Will the same happen again?
Ammonia as a fuel of the future?
Reaction Engines explainer on how green ammonia could be a semi drop-in zero carbon aviation fuel.
(Reaction Engines)
While much discussion centred around electric, hybrid-electric, fuel cell and hydrogen as potential
power sources for future aircraft, a presentation by the UK''s Reaction Engines explored the feasibility of
ammonia as green aviation fuel. Reaction Engines, best known for its game-changing SABRE air-
breathing rocket outlined to delegates how its top secret precooler technology – the key to rapidly
cooling super-heated air moving at high Mach speeds, has now an increasing number of spin-off
applications for alternative green propulsion systems. For example, thermal management of high-
performance batteries, which require a specific operating temperature is a major challenge for electric
flight. To that end, Reaction Engines have now developed a spin-off family of pre-cooler heat exchangers
and radiators that can be incorporated into cooling and thermal management systems. The first
application of this spin-off precooler technology may be the Britten-Norman Islander being converted to
hydrogen fuel cell technology by Cranfield Aerospace under Project Fresson. In October, Reaction
Engines announced that it would be joining Project Fresson to provide a ultra low-drag heat rejection
system to meet the fuel cell cooling requirements.
Reaction Engines also believes that its heat exchanger technology could make ammonia-powered
airliners viable – a fuel source that would only produce nitrogen and water vapour as waste products.
Describing this as a ''semi drop-in'' fuel solution, the liquid ammonia would be stored in the wings, like
kerosene fuel today. Using heat from the jet engines exhaust. Reaction Engines'' heat exchanger would
pass this on to a cracking reactor which would separate the ammonia into hydrogen and nitrogen, with
the hydrogen being combusted in the engine. To demonstrate the feasibility of this concept, Reaction
Engines is now partnered with IP Group and the UK Government''s Science and Technology Facilities
Council. The benefits of this approach then, is that hydrogen is being burnt by the engine, yet the
aircraft does not need the vast cryogenic fuel tanks that require a completely new airframe
configuration.
Double-fuselage airliners?
A radical proposal - but a twin fuselage hydrogen-powered airliner offers the safest configuration,
argued Nangia and Hyde. (Nangia)
Meanwhile a presentation from Dr Raj Nangia and Les Hyde showed off a design for a twin-fuselage
twin-engined liquid-hydrogen-powered airliner, for around 160 seats. With a configuration similar to the
Twin Mustang or Rutan Boomerang, Nangia and Hyde explained their ''Gondola'' concept had been
designed for maximum safety, by locating the LH2 fuel tanks in a separate fuselage pod, which would
then ease certification and crashworthiness issues. After considering a range of other options (fuel tanks
above the cabin, pannier tanks, ''drop tanks'' under the wings) the team selected the twin-fuselage as
offering the best solution as to keeping liquid hydrogen as far away from passengers and crew as
possible. Interestingly they also noted that a twin-fuselage design would also simplify fuelling/de-
fuelling procedures in that the entire fuselage pod could be removed and swapped with a full tank, or
even removed overnight to be kept in a climate-controlled cold storage facility to minimise boil-off. With
an extended span wings (46-48m) this means that some sort of folding wingtips will be needed, to fit
into a standard airport type C box, but the increased wingspan does mean that despite the increased
wetted area compared to a single fuselage design, fuel efficiency and overall drag remains similar to a
traditional narrowbody.
New tools for design
Digital design tools can now speed up the optimisation process considerably. (Martins)
With the design space expanding out into radical configurations as engineers grapple with the challenge
of incorporating electric, fuel cell, hybrid and hydrogen propulsion systems into tomorrow''s airliners,
the conference heard how the latest in digital software tools can speed up the optimisation. Giving a
presentation on Multidisciplinary Design Optimisation (MDO), Dr Joaquim Martins, Professor of
Aerospace Engineering, University of Michigan showed the conference how given design parameters,
MDO software could turn a sphere into a supercritical aerofoil in a couple of hours on a laptop, vastly
speeding up the design process. In another example, a ''plank'' was optimised into a swept wing with
hundreds of interrelated design changes able to be changed simultaneously. Internal volume (for
example, for pilots/passengers, fuel/battery storage) could also be defined, with the software optimising
the perfect aerodynamic shapes around these. As well as aerodynamic optimisation, MDO also supports
electro-propulsive, aero-thermal, aero-acoustic, trajectory optimisation and aero-propulsive design
optimisation, said Martins, who gave another example where MDO had been used to refine NASA''s
STARC-ABL Boundary Layer Ingestion (BLI) airliner design. MDO and the wider Model Based Systems
Engineering (MBSE) thus has the potential to speed up aircraft design from the traditional, linear,
iterative process into an agile parallel process – helping accelerate the race to green airliners
Summary
A Boeing presentation showed off this elegant concept for a NASA X-plane which builds on the
company''s work on truss-braced airliners. (Boeing)
This conference then was a highly topical and extremely useful forum for those working in, or interested
in keeping up with the fast-moving field of alternative propulsion technology – which now seems to be
accelerating even faster as momentum builds towards a net zero future of flight.
