Bjorn’s Corner: Sustainable Air Transport. Part 14. Propulsion system requirements.

By Bjorn Fehrm

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.

Figure 1. The World Jet market forecast for the next 20 years. Source: JADC.

Airliner classes and their importance for our emission problem

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.

Figure 2. The World Turboprop market forecast for the next 20 years. Source: JADC.

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.

Power requirements for airliner classes

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.

Figure 3. Shaft power to the propeller/fan for different airliner types. Source: Leeham Co.

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.

29 Comments on “Bjorn’s Corner: Sustainable Air Transport. Part 14. Propulsion system requirements.

  1. The first picture confirms Boeing cannot leave alone the 120-160 seat segment. And an NMA’s aircraft would get trashed today by A220s.

    A big assumption for electric engines seems it will be twin engined, which doesn’t seem logical. Above 50 seats, 4 props seems more likely. How to general & store the electricity is another topic as discussed extensively.

    • The prop sizes depends on wing placement and height of landing gear in addition cost and performance of the power gearbox on its power capacity and gear ratio. Normally for a big new turboprop you want a big and 8 blade or more prop. Noise can be a problem too and for electrical powered aircrafts with fuel cell stacks in the same nacelle there is an optimal solution in location, size and number of props considering the mass of the fuel cell, mass of engine and prop system including de-ice than can benefit from fuel cell heat/steam. Certification issues can pop up regarding containment of separated prop blade limiting its energy.

    • With electric motors it will be easier to have multiple motors with the heavy penalties in efficiency and maintenance associated with multiple smaller thermal engines.
      There is no need to have the engines all under the wings. We may see a return of the trimotor or the use of the uncommon tail pusher. It’s not even neccesary to have the motors, fans and propellers of the same size.
      The electric ducted fan seem particularly suited to boundary layer ingestion propulsion if placed in an aircraft tail with an annular intake. That can potentially cut fuel burn by 8.5% according the NASA Glenn Research Centre.
      So maybe out first hydrogen cryojets will have 3 engines. Hydrogen fuelled gas turbine engines under the wings and a fuel cell powered fan in the tail to ingest the boundary layer.
      There are other possibilities such as placing the engines to blow the flaps or perhaps on top of the flaps to provide boundary layer suction like liliumjet.

      • I agree electrical motors are easier to place and there is a real effect of reenergizing the boundary layer. I remember the positive effect on the MD-95/B717 was bigger than aticipated. There has been big prop planes before like the Lockheed YO-3 Quiet star. I still lean on a ATR72 type of aircraft, slow, reliable and economical for half an A321 pax count that can burn LH2 placed in extended engine gondolas with a fuel cell APU between landing gear door wells. For a big 8 bladed Ratier Figeac prop with higher landing gears to make room for a bigger prop.

        • Good idea, Fuel Cell electric propulsion might allow aircraft to be free of noise curfews complexly. The ATR-42/72 Q400 sized market is a small one so such an aircraft is a test before preceding to larger aircraft.
          I am very curious about the drag reduction possible through boundary layer ingestion propulsion over the wing. Some barriers may be psychological perceptions of the public.

          • The reingenstion of the sizable fuselage boundary layer is in many new design proposals with a tail mounted prop that can be driven by the APU as it is pretty idle after start and can be used during climb/cruise. Wings are easier to keep laminar flow and a small boundary layer up to 25-65% of chord and the longer the wings and smaller the chord the higher % of total wing area with possible laminar flow. I expect soon there will be active flutter suspension of long slender wings by fly by wire control of small control areas to damp any flutter onset.

  2. Bjorn,

    Can you please clarify what is meant by “high tension”; this is not an expression I am familiar with.

    Also, does the takeoff power per engine in Figure 3 provide the required single engine out performance and what are the advantages of installing more than two electric engines (two being fine for only the comparatively small turboprops)? As suggested by keesje the required takeoff power for each engine would be lower and the loss of a single engine means each of the remaining engines only needs to compensate for a fraction of the lost thrust rather than the whole.


    • Hi RCAFBrat,

      I changed high tension to high voltage (which is what it is).

      The power level is the level needed for single-engine out, which in certification speak is called One Engine Inoperative (OEI). You lose one-fourth of the power instead of half if you go for four engines instead of two, but then your OEI climb angle increases after liftoff (V2 climb angle). Most trades show the two-engine aircraft to be beneficial. With electric engines, you can often gang them two and two in the nacelle instead of laying them out as four. It’s an alternative.

    • -For the average person High Tension can be used interchangeably with High Voltage. However Voltage is a unit of measure whereas Tension refers to a Electrical Stress across a material or space from a Electrical Potential Difference. One is the phenomena and one is the unit of measure.
      -I’m an electrical engineer and we even argue over it. Ive been told to change my documentation from referring to a HT Breaker to HV Breaker and vice versa.
      -Roughtly anything over 1000V would be referred to as high tension

      -There are standards depending on the governing authority.
      ELV Extra Low Voltage, below 70V
      LV Low Voltage, below 600VAC
      MV Medium Voltage 600VAC-33,000VAC
      HV High Voltage 33,000VAC to 220,000VAC
      And we can go on the Extra and Ultra High Voltage.
      Refer to ANSI/IEEE 1585-2002 IEEE Std 1623-2004 or national regulators.

      In the case of MV or voltages much above 1000V special measures are needed in practice. For instance if insulation was simply extruded around the cable the small voids in contact between the insulator and conductor would lead to arcing that will destroy the cable in maybe 5 years. Therefore a thin inner layer of semiconducting plastic laden with 20% or so of graphite is extruded over the conductor to equalise electrical stresses and then over this the actual insulation. On the outside another semiconducting screen to equalise earth potentials is added. These inner and outer screens then need to be correctly terminated.
      -There is a lot to joining and terminating cable. Corrugated insulting terminations may be needed, the outer screen earthed. I would expect to pay $3000 for a set of 6 insulator terminations from 3M for 11kV. It could cost tens of thousands to splice a 100,ooov cable.
      There is a lot to terminate a high current cable. Lugs are expensive, need proper compression tools, heat conducting grease needs to be used, bolts, washers, nuts of the correct material. They must be torqued correctly. All is documented and signed of. If you loose a tool or nut in a HV environment it is a serious issue.

      • Thank you for the explanation.

        My background is Mech Eng and while I don’t have any difficulties with system level schematic diagrams, as soon as you throw a capacitor into a circuit or talk about PF I get lost pretty quickly:)


        • My head gets done in when I see shafts being aligned. I had to learn about it to understand vibration types. One of those things where so much could be done to predict faults but not enough folks around to understand it.

  3. Tension is in this case about voltage, I believe, which is commonly described as ‘potential’ by electrical engineers etc. Essentially, it makes it easier to move electrons/current/energy quickly if the potential difference from A to B is greater, but, it also makes it likely that it could arc more easily, so you need a lot more insulation. Think about an electrical substation stepping down to residential voltage vs. a high voltage line. There’s a reason they are fenced/bricked away so people don’t just walk around in them.

    You can save weight/increase performance in one respect, but lose it in others, perhaps analogous to the high pressure pneumatic system in the V-22. It has trade offs.

  4. Very exciting with superconductivity research. Excellent opportunity for synergy on an aircraft equipped with a liquid hydrogen system.

    • -I think as hydrogen technology is rolled out many refinements and dramatic improvements will come in that will greatly improve efficiency and uses. There will be hydrogen delivered by pipeline for heating, electricity generation, iron ore smelting and as well to power automobiles and trucks. It will compliment electric trucks and cars.
      -Apart form large scale “utility” electrolysers we will seem small scale ones that store into 10,000psi/700bar polymer tanks.
      -As more devices become able to handle and use this fuel the extra effort for cryogenic storage becomes viable. Small scale cryogenic plant as small as 1 Litre per hour have been made.
      -As you say, with cryogenics comes synergy with electric motors. We may yet see cryogenic storage in vehicles. Sumitomo has build a superconducting motor for automobiles.

  5. Electronic switching devices, IGBJT, that can handle 6600V direct and even 11kV direct are now available so 6000V operation should be possible. The issue will of course be insulation standards in the low pressure air and that reductions in conductor cross section will required increased insulation thickness. These are big conductors.

    • Useable Single Fuel Cell or Accumulator Cell voltages go from 1V to about 4.0V. High system voltages increase the stacking demand. 6kV with 1V fuel cells is a stack of 6000 cells!

      • Good point.
        There are ways of boosting an individual battery pack’s or Fuel Cell’s output voltage to a higher voltage (Say 400V to 3000V) but they would add a certain amount of weight and energy losses. These losses would be in the order of 1% or so using the latest FET transistor technology. The weight might be compensated for by the reduced conductor size. I’m not sure since I’m not deeply involved in this side of the business.
        Superconductors require zero voltage to transmit. One of the first hoped for uses of superconductors was to replace the high voltage power lines inside a buildings centre services duct. These require a great deal of space due to thermal requirements and voltage spacing issues. The space saving in an aircraft are obvious.

  6. Bjorn,
    Great series. It is very educational in addition to steering one back to books and websites to revisit basics. I had suggested different size and dual-podded engines for the upgraded 737 back before it acquired its MAX model name. It wasn’t practical then, but now seems to be an enabler for electric and hydrogen power. Hoping that all works out and presuming a single engine test/trainer version comes first, leads to a question that is more economics than technical. Once the student is ready to leave the traffic pattern training area environment for cross country work, how is that supported? Are the destination and diversion airfields equipped like home base or is there a piston equivalent airframe or is the ab-initio student now to be dual qualified.
    I realize it’s a bit off topic but hope you will offer some insights.

  7. NB: This is an off-topic comment. 😉

    The new EU Hydrogen Strategy for a climate neutral Europe and the increased EU support to Ukraine’s overall economic and financial resilience could be applied in such a way as to allow for a merger between Airbus and Antonov StC.


    To manufacture a next generation hydrogen powered very large cargo transport aircraft that would be designed to replace the An-124 and An-225 (and the C-5 if the USAF is interested in the idea) and which would fill the recognised shortfall in European strategic airlift capabilities that led to a new EU programme being launched on 16 November 2021.

    Final Assembly Line: Antonov Airport

    Name of aircraft; A480-800XF 😉

    The A480-800XF design would be based on that of the An-225. However, the lower fuselage should be modified and internal height increased from 4.4 m to, at least 5.2 m in order to allow for a removable height-adjustable second cargo deck that would make the aircraft much more attractive to civilian operators — i.e. similar to the HTA “elephant” fuselage design (option 2) on page 3 in this paper from TsAGI in Russia:


    It is interesting to compare the characteristics of defined aircraft with modern cargo aircraft of similar capacity. The closest analogues of the HTA “Elephant” are the cargo aircraft An-124-100 and Boeing 747-8F. The Airbus A380- 800F project [10], which was frozen due to delays in the delivery of the original passenger version and the subsequent cancellation of orders for the cargo version, should be compared too. The main characteristics of the HTA “Elephant” and competitor aircraft are shown in Table 2. Figure 16 shows the “payload-range” diagram of the HTA “Elephant” (variant №1) in comparison with the An-124-100, Boeing 747-8F and Airbus А380-800F. The use of a removable height-adjustable second cargo deck in the HTA “Elephant” allows the airline to improve the cargo compartment volume filling with respect to the transportation task (fig. 17). At the same time, the possibility of transporting oversized cargo (within the dimensions of the cargo hold) is not available for the Boeing 747-8F and Airbus A380-800F. The transport capability of the HTA “Elephant” in the double deck cargo load option is shown in figure 18.

    A480-800XF Specifications:

    Fuel volume LH2 = 650,000 litres

    Number of LH2 tanks and location:

    3 x 15,000 litre LH2 tanks placed in the centre wing box
    ( )

    2 x 25,000 litre LH2 tanks placed in the upper forward fuselage (i.e. between the centre wing box and the cockpit + courier area.)

    2 x 35,000 litre LH2 tanks placed in the upper aft fuselage

    1 x 500,000 litre LH2 tank placed in the lower aft fuselage

    NB: For a good look at the location of the large LH2 tank in the lower aft fuselage, please do watch this video of the An-225:

    Empty weight: 240,000 kg
    Max payload: 160,000 kg
    LH2 fuel capacity: 46,000 kg (650,000 litres)
    Max take-off weight: 450,000 kg (i.e. 640,000 kg for the An-225)
    Powerplant: 6 x 35,000 pounds of thrust PW1100G engines modified to burn LH2 fuel. Thrust-to-weight ratio at max take-off weight: 4.72

    The cockpit section would have the same outer mold line as that of the An-124/An-225 cockpit sections. However, the section would be manufactured at Airbus’ Méaulte site, located in the north of France and it would be based on the A350 cockpit.

    The wings would be built at existing Airbus sites.

    The LH2 tanks would be built at new Airbus facilities (or Tier-1 OEMs) that would be especially designed for the manufacturing of large LH2 cryogenic tanks.

    However, the fuselage and the centre wing box (based on the large voluminous An-225 centre wing box) should be manufactured at the Antonov facilities in Ukraine — i.e. in rebuilt and in new production facilities.

    • Why would an EU consortium wish to manufacture strategic aircraft in a non-NATO country located right beside Russia?
      And, seeing as transporters are used so seldom, why bother with the investment to make it hydrogen-powered? Using SAF would be just as good, and would leave a lot more cargo capacity.
      Just plain old kerosine would also be fine, since transporters spend most of their time on the ground.

      • @Bryce

        A) As a future EU member state, Ukraine will have a lot to offer Airbus — or “the EU consortium” as you call it.

        B) Designing a hydrogen powered very large cargo transport aircraft, using existing technologies (fuselage, wing + engines) would significantly shorten development times.

        C) The aircraft would be designed to offer a sustainable LH2-powered alternative to all current civilian freighters (i.e. 777-F/XF, 747-8F and A350F) thanks to a removable height-adjustable second cargo deck (i.e. concept shown on page 11 in this paper: ). Hence, the market would be much bigger than just meeting the shortfall in European strategic airlift capabilities. If there would be a problem assembling the aircraft in a non-NATO country– which IMO would be a non-issue — the military version could be assembled at an existing Airbus facility (i.e Hamburg, Toulouse or Seville).

        D) Why hydrogen? Because green hydrogen is sustainable whereas kerosene is not.

        E) A payload of 160 metric tonnes is more than enough. If you want a bigger freighter, but still a sustainable one, you might want to check out this concept from the Delft University of Technology: Design of a Hydrogen-powered Unmanned Ultra Large Cargo aircraft

        • Ukraine has virtually zero chance of becoming an EU member. It doesn’t come anywhere near meeting the (mandatory) membership criteria, e.g because it is one of the most corrupt nations on the planet. At least 3 EU members have zero appetite to admit the country to the union. It’s misguided of Ursula von der Leyen to get the country’s hopes up.

          The same applies to NATO.

          Burning relatively small quantities of kerosene in a tiny fleet of very-low-use aircraft is not going to kill the planet. There are bigger priorities and better ways to spend money.

          Locating strategic facilities right beside an aggressive enemy is not a good idea. Here’s a hint: can you think of a certain country in the ME that has made a habit of making incursions into neighboring countries so as to carry out unprovoked attacks on facilities that it doesn’t like?

          • @Bryce

            Again, this aircraft would be designed primarily as a civilian freighter and certified as a civilian freighter. It would offer a sustainable LH2-powered alternative to all current civilian freighters. The size of the market should, therefore, be quite large (i.e. hundreds of units). The military derivative would be a niche market (i.e. tens of units) unless the USAF wants in on the action (i.e. with a FAL located in the U.S.).

          • Civilian freight companies are generally far more interested in transporting volume rather than weight: that’s why the An225 only served a very small niche market. Both the Boeing Dreamlifter and the Airbus BelugaXL have larger cargo volume than the An225–and they’re much lighter, and thus more economical to operate.

            Bjorn showed in a previous series that LH2 is a problem for longhaul…which is what most freighters are used for.
            A shorthaul LH2 freighter version of the A321 might make sense, thought significant cargo hold space would be lost to LH2 tanks. SAF would make much more sense.

          • @Bryce

            The A400-800XF would have:

            i) A much lower MTOW than the An-225 (i.e. 450,000 kg vs. 640,000 kg)

            ii) LH2 capacity would not be a problem for large long-haul freighters. There’s more than enough onboard LH2 fuel (i.e. fuel capacity of 650,000 litres of cryogenic liquid hydrogen) on the A400-800XF in order to fly 4,000 nm with 160,000 kg of payload.

            iii) A more voluminous lower fuselage than the An-225 (i.e. internal height increased to 5.2 m from 4.4 m).

            iv) Again, thanks to a removable height-adjustable second cargo deck (i.e. concept shown on page 11 in the linked paper below), the aircraft could carry the same number of 6-meter (20-foot) M2 containers on the main deck as that of the 747-400F (e.g. 14 M2 containers), but 52 LD-1/LD-3 containers on the second (removable) cargo deck on the A480-800XF vs. 32 LD-1/LD-3 containers on the two lower cargo decks on the 747-400F.

            Page 11:

          • I agree with your assessment about the use of SAF rather than Cryohydrogen.
            The Ukraine will need to be rebuilt after Putin’s Russia has shelled and destroyed its cities and economy. The EU will need to build a stable state on its borders, not leave one open to destabilisation and insurrection. It’s obviously must first offer a path to EU membership which includes the elimination of corruption and assistance to stabilises its finances. The Ukrainian people will pay a huge price in blood and they must be offered hope not just weapons. There will need to be a Truth and Justice commission for the Donbas. All states are corrupt and it must be a continuous fight against it.

            It is mad to attack an EU country anyway even without NATO membership. Lisbon Treaty Article 42.7
            Mutual defence clause (Article 42.7 TEU)
            “If a Member State is the victim of armed aggression on its territory, the other Member States shall have towards it an obligation of aid and assistance by all the means in their power, in accordance with Article 51 of the United Nations Charter. This shall not prejudice the specific character of the security and defence policy of certain Member States. Commitments and cooperation in this area shall be consistent with commitments under the North Atlantic Treaty Organisation, which, for those States which are members of it, remains the foundation of their collective defence and the forum for its implementation”

            In other words the Article 42.7 already protects EU countries, just not as much as there are no troops on the soil. (There are British troops & Challenger 2 tanks currently in Estonia protecting it under NATO)

          • @William
            If it doesn’t meet the membership criteria — which are set in stone — it won’t be let in…it’s as simple as that.
            The Balkans have been waiting for 20 years — they’ll have to keep waiting until their act is together.
            No exceptions.

  8. 3500 kW per side for a 100-seat twin engine electroprop seems on the low side.

    The 82-seat Q400 has 3800 shaft kW per side. Plus approx 5% of equivalent SHP coming from the exhaust net thrust.

    Assuming a directly proportional shaft power to pax ratio, the twin electroprop should get:

    3500 * 100/82 * 1.05 = 4500 shaft kW

    Now we’re talking.

    A quad electroprop would greatly benefit from losing only a quarter of total power, in an OEI situation during take-off. Instead of losing half of total power, as in a twin.

    FAR 25 climb gradient requirements are stricter for a quad. But keeping 3/4 of total power during an OEI situation more than compensates for the tougher take-off performance requirements.

  9. The Q400 is rather over powered because it attempts to be competitive with Jets in speed. The ATR72 is now more successful due to its focus on economy. If we use data from the 70 passenger ATR72 using your methodology is somewhat less. The ATR72 has only 1846kW per engines.

    1846 * 100/70 * 1.05 = 2769 shaft kW

    Such an aircraft would have smoother composites skin, possibly boundary layer ingestion propulsion, perhaps passive laminar flow, a higher aspect ratio wing with active flutter control so the speed might be quite a bit higher than An ATR72.

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