The future of aircraft propulsion

Date 4.1.2021

Every time I fly, I keep thinking to myself: what has changed in commercial flying over the last 50 years? Right, let me rephrase that: why do I have the feeling that nothing has changed in half a century of flying? Because nothing has changed, is the answer to that question. I mean, to your regular cargo self-loading passenger sitting in economy class, the seats are just as functionally uncomfortable, the food probably worse, though this really has more to do with a deteriorating food quality around the world than with the airlines themselves (though some airlines really do serve you rubbish food… to be polite) and most importantly: we still fly at M 0.83 @ FL330 (more or less). Too bad the Concord’s gone, at least that was something out of the box us humans could have bragged about. The most noticeable changes to your mortal common passenger are the inflight entertainment system as well as the no smoking policy and disappearance of those cool little ash trays lodged in the seat arm rest.
Of course, to the more expert eye (or if you are an aerospace engineer or aviation buff), we know where the changes have been made, and quite substantial ones: propulsion, materials, some advances in aerodynamics (mostly due to super smart IT people who have been developing super-fast computers, allowing for a massive use of CFD/Computational Fluid Dynamics – if you are reading, thank you John Vassberg @ Ex Mc Donnell Douglas), software programming allowing for mostly reliable fly-by-wire, and avionics. All of this… and we still fly at M 0.83 and fly @ FL330 and burn jet fuel.
So, before we get to cruising at M 3.5 @V FL1000 and start getting some decent food served on board (probably a lost cause), let’s see if we can add one more significant change, still invisible to the eye of passengers, by burning something different than jet fuel (3.2 tons of CO2 emission per ton of jet fuel burned) or avgas (3.6 tons of CO2 emissions per ton of avgas burned), which both by the way are responsible for more than 1.5% of global CO2 emissions. The story here is about green hydrogen, and potentially green ammonia; and forget about batteries and fuel cells for a moment, which would apply only to prop planes.
Hydrogen packs 3 times more energy than jet fuel, but the problem is that 1 litre of hydrogen is only cca 0.09g heavy at STP (Standard Pressure Temperature), whereas 1 litre of jet fuel is 800g heavy at STP, therefore making jet fuel 10,000 times “heavier” than hydrogen. By cooling and compressing the hydrogen to reach more reasonable numbers, which are now at 55kg/m3 of hydrogen at -25*C, which has the same energy as 165kg of jet fuel… that is only about a fifth of what we should carry. Hydrogen, on its own, is not really going to work (not even the cooling of hydrogen at -253*C, which is a big problem in itself), at least not in our lifetime. Let’s have a look at ammonia now: Ammonia (NH3 in gas form) has again a very low density when compared to jet fuel (0.73kg/m3 vs 800kg/m3, a ratio of 1 to 1,100), but once cooled to -33*C, its density jumps to 619kg/m3 (25% lighter than jet fuel). The specific energy density of liquid ammonia is low (15.6MJ/kg or less than 40% of that of jet fuel). This is already a more acceptable number. In theory, what we could agree on is to have a plane powered by ammonia, in a liquid state at -33*C, but its fuel tanks would have to be more than double the size of those on a jet fuel turbojet powered aeroplane. This is of course a simplified view, which does not address all of the other numerous technical aspects, though all workable (i.e.: higher flammable point, slow flame speed of ammonia, the required high ignition energy etc…). In the case of ammonia being used as a propellant, we would also need to address (bad) NOX emissions, which need to be reduced (through some filtration at the exhaust nozzle).
In all of this, there actually is a third way, which is being developed very seriously at Reaction Engines Ltd in the UK (does anyone remember HOTOL, SKYLON?), and which is to carry NH3 ammonia as a propellant, but crack it and separate H3 from N, and feed the turbofan with a mixture of NH3/H2 (say an 80/20 NH3/H2 blend). This is obviously an intermediate step, not an easy one but quite probably feasible. The combined energy density of the mix which you get is comparable with the burning jet fuel levels, which then becomes a big deal and clears a big hurdle. What this really is about, is a marvel of technology of a heat exchanger which takes heat/energy from a very exothermic process (combustion in the turbojet) and transfers it to the cracking unit of the NH3, turning it into H2, which in reverse is a totally endothermic process, thus closing the energy loop.
When will zero CO2 emissions commercial planes be flying near you? Very likely by 2030, possibly by 2025 (experimental). What is quite certain though is that by 2050 – a year that most countries have pledged to become carbon neutral by, China in 2060 – all aircrafts will have zero CO2 emissions.
So this is how the movie ends: for ‘small’ distances (<1.000nm), batteries and (ammonia or hydrogen) fuel cells propeller powered commercial flights; and for distances above that, a NH3/H2 propellant mix, burning it out in a turbofan jet engine. As to how we could fly faster and higher… as the saying goes: if you build it, they will come; an invention of a new propulsion system powered on a new carbon free fuel (NH3/H2) will be the key to a fast flight… and something tells me it will have a very British accent. Text by: Nouri Chahid Photo: autor

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