April 8, 2022, ©. Leeham News: Last week, we discussed the architecture of a liquid hydrogen fuel system. We now start looking at the propulsion system of a hydrogen aircraft.
Before discussing how a propulsion system is done, we must understand what power requirements different airliner types have and the importance of these types in the market.
Before we look at the power requirements of different airliner types, let’s understand which categories emit the bulk of the emissions. If we develop new, more efficient, and less pollutant airliners, we need to understand their pollution weight in the market.
The major airframe and engine OEMs all do market forecasts on how many new airliners will be required. Typically the projections are over the next 20 years. The OEM’s programs typically color the forecast buckets and their numbers. Ideally, we should use a forecast without such colors.
There is an independent forecast of high class (we have 20+ years of experience in airliner market forecasts), the Japan Aircraft Development Corporation (JADC) reports. One of its advantages is that it forecasts Jets (Figure 1) and Turboprop aircraft (Figure 2) in relevant seat categories.
We can see from its 2021 open report (there are also more detailed pay reports) that the main airliner market over the next 20 years is the 120 to 169 seat Jet market with 10,200 deliveries and the 170-229 seat Jet market with 11,500 deliveries.
Combined, these segments represent 1,085 deliveries per year, with an average of over 500 Jets per year for each segment. If we compare this with the Turboprop market, the 40 to 59 seat segment represents an average of 22 aircraft per year, and the 60 to 100 seats 80 new deliveries per year.
This data tells us where the heart of the market is and where sustainability changes will impact the world emissions of Greenhouse gases.
This data is for new production airliners. What about retrofits into, for instance, the 50 seat Turboprop market? As we shall see, the challenge to produce a viable retrofit for these markets is huge. Any such projects will serve more to mature technologies rather than change emission levels in the market.
We use GasTurb, an OEM grade engine preliminary design tool, to generate detailed propulsion data. It gives us 180 parameters for each engine we analyze. One of these is the drive power to the fan or propeller. It’s a good reference for what we need from our hydrogen propulsion systems as we can assume the same conversion efficiency from shaft power to forward thrust when we use similar propeller or fan/nacelle arrangements. Figure 3 gives the typical power levels we need for different airliner classes. The table lists the takeoff power we need to satisfy the One Engine Inoperative (OEI) case for the aircraft’s certification.
Combining Figures 1 and 2 data with Figure 3 shows that the key power bracket is 8 to 11 MW. Present electric engine development and certification is for 0.3 to 0.6 MW, with the following projects targeting 2MW engines.
As we go up in power for our propulsion systems, the currents involved become a problem. For the 100 seat Turboprop, an electric motor needs 4,500 Ampere if we use an 800V system. We can reduce this to 1,200 Ampere if we have a 3kV power system, but the higher Voltage level creates its own problems (it’s not proven in aeronautical applications).
The high currents involved in fuel cell plus electric motor propulsion alternatives and the problems around high Voltage levels and high cruise heights is the reason behind Airbus’ work on superconducting systems for this propulsion alternative.
We will discuss these problems more in the next Corners.