Podcast: 10 Minutes About SSTs

Scott Hamilton and Pontifications are taking the day off.

July 5, 2021, © Leeham News: Development of small supersonic transports (SSTs) seemed, for a time, a fad. Three companies announced plans to develop an SST business jet with two of them expanding the concepts to be small passenger airliners, each with less than 100 seats.

Today, only one company appears remaining. Aerion collapsed recently for lack of funding. Boom remains, announcing a “commercial agreement” with United Airlines early this month for 15 Overture SSTs and options for 35 more. The agreement, however, is highly conditional.

LNA discussed the agreement in a previous 10 Minutes About podcast. Today’s 10 Minutes About is a technical discussion about designing an SST.

Leeham News and Analysis
Podcast: 10 Minutes About SSTs

35 Comments on “Podcast: 10 Minutes About SSTs

  1. The examination of the technical challenges was fascinating but I still don’t see
    that the economic case for an aircraft slower than Concorde, even allowing for a much more efficient design, is good enough to generate the premium business required. As long as there is a ban on supersonic flight over land, there are just not sufficient city pairs and the green lobby would have a field day.

    • Agreed, dubious at best and even over land I see no case for it.

  2. I think the non-afterburning engines set the top speed for supercruise. NASA-Lockheed Martin will pretty soon test the X-59 to get data to allow supersonic flight over land within limitations. One would assume the USAF would be an early customer and pay for development like aerial refueling and some stealth features. Having Pilatues do a makeover so it becomes more practical and flexible might be a smart idea as the US Armed forces then could fly downed soldiers to trauma care at M 1.7 from dirt runways. The USMC and USN would then order a Navy version with big P&W engines….

    • The trauma care that counts is the front line stabilizing. Once stable while time is important, its not critical. Initial evacuation is the time critical element.

      I don’t see SST re-fueling or a need to speed to a location, the penalties in cost for something that can carry a load, fuel burn and maint are beyond fathomable.

      • We will see, the US has taken many steps to make SSBJ possible and the USAF would not like to be outraced in the skies.

  3. Nice one, clear explanation & illustration discussing the fundamentals by Richard, all in 10 minutes !

    Supersonic is exiting & brings exclusivity and status. But for travelling that’s hard to justify in businesses.

    As I understand to keep aerodynamics in hand & keep weight as low as possible you are going to stuff as many people as possible in a narrow tube, for high rates.

    On longer flight you are passing significant time zones. A good meal, bar, space and a nice bed for lower cost seems hard to beat. Plus you’ll arriving exhausted after relatively short flights at strange times. The few hours won probably aren’t that critical when well scheduled.


    Btw Richard : “Scott, that’s why the CFM Leap engine on the A320 is a little more fuel efficient than the one on the MAX, it has a wider fan”.. there are places where you get a warning, temporary ban for claiming that 😉

    • Nothing strange as bypass ratio is different and I assume burner pressure is the same. The 737MAX gains some fuel burn advantage by its smaller fuselage diameter for the same 3+3 seating and I assume lower empty mass. Its wing is one notch ahead of the neo and hence it should be an even match moneywise for $ seat/mile. There are other differences in systems, cabin space, cargo handling etc, but just shipping “self loading cargo” i.e. pax 1-4hrs it looks pretty even between the MAX8 and A320neo.

      • Yes. The CFM engine in the Boeing isnt really ‘exactly’ the same core as the Airbus version, they made the core a smaller diameter again to eke out some efficiencies that way including faster rotation of the stages and other material changes including ceramics
        Boeing has always claimed their plane in service ( not test stand numbers) is 7-8% more efficient , when ‘an airline’ says it would be
        more around 2%.
        ‘CFM tells us the 737’s lighter weight is the “secret” in this entire debate, albeit it’s hardly a state secret. Because Boeing has the lighter airplane, Boeing (and CFM) point out that lower thrust is needed.’


  4. I just can’t see this concept working. Take London/NY. The supersonic aircraft will need to fly out of Heathrow or one of the larger airports. Those take longer to get to, longer to clear formalities and are more subject to ATC delays (30 minutes circling to get into LHR is not uncommon). Instead you could fly an A220-100 in all business premium out of London City today. Door-door time will be about the same.

    And ranges for aircraft in the 70-120 seat range are only going up as engines get better. This will open more and more options for premium point-point travel.

    • Plenty of Britons living near Heathrow and/or further west of London would consider it a nuisance to have to travel to London City airport.
      The same applies to passengers flying into Heathrow — who now would have the hassle of having to transfer to the city center.

      Every airport has advantages and disadvantages relative to others.

      • The premium passengers are from the financial heart of London, and of course that same central core has highest real estate prices and the richest people in Europe.

        • Those same wealthy inner city Londoners are well able to find their way to Heathrow for today’s longhaul flights.
          Windsor — the wealthiest town outside London — is much closer to Heathrow than to the inner city. Not everyone with money lives in Belgravia and environs.

          • We are talking about the 1 million people who live in central London. Getting to the airport is one issue , mainly congestion….what happens when you get there is another planet.

      • Before Boom could enter service I suspect we are likely to see eVTOL services such as Lilium Jet or others in service. They will certainly be able to take travellers from inner city areas to major airports. If they can carry out some of the security and check in procedures (eg baggage check in and forwarding) and can position the passenger for easy clearance of security and immigration it may change things for small and regional airports. Some could benefit by becoming more accessible others become less relevant.

        • > Before Boom could enter service I suspect we are likely to see eVTOL services such as Lilium Jet or others in service. <

          I think we'll see both at about the same time.


          • I get the humour and it did make me smile but Lilium have access to about $500 million compared to Boom’s $186 million. Lilium have a flying test bed and don’t have potential environmental concerns. Certification is their challenge. Tom Enders is Chairman, some very experienced people in the area of propulsion, avionics and certification have come on board or are involved.

            CEO and cofounder Daniel Wiegund is pushing for service by 2014 of their new 7 seater. The aircraft is supposed to have a speed of 320km/h and a range of 250km plus reserves (presumably 30 minutes of VFR) and VTOL landing.

            Just say they fail to make 2014 and make 2016 instead. Just say they initially make a certified range of 150km range instead of 250km.

            Id argue they still have a very useful product. Such a shortfall in range would make an electric aircraft useless but in an eVTOL aircraft capable of landing in small roof top areas in cities even helicopters are restricted from without noise or blade strike or heavy maintenance concerns that is not the case. Transfers to of Londons 4 major airports are easy. A flight to Calais or Dunkirk is about 140km hence the 250km range would really be needed for this and would greatly increase the appeal of the aircraft.

            The concept of the aircraft is that although the use of Electric Ducted Fans requires 50% more power than the use of rotors for hover the additional efficiency gains of cruise flight more than make up for that. There is no problem obtaining batteries of sufficient power density and unlike a helicopter the aircraft will not spend time hovering.

          • This will make you smile even more
            “In January 2020, an article published by German aerospace magazine Aerokurier titled “Hoffnungsträger Oder Hochstapler?” — “Hope Bearer or Imposter?” — an anonymous aerospace engineer concluded it was impossible for the Lilium jet to reach its desired flight time and range.
            The author calculated that, using current battery technology of 240 watt-hours per kilogram, the jet would only be able to sustain a hover for 67.7 seconds. Allowing only 60 seconds of hover time without reserve — parameters that regulators will almost certainly not approve — the engineer found the jet would be able to fly for less than four minutes, or 11 miles (18 km).
            “Either Lilium has found solutions for technical problems no one else knows, or Lilium’s promises can’t be kept,” wrote four engineering professors in agreement with the anonymous engineer. “Lilium is using brilliant PR to create an illusory world to attract investors.”
            Hochstapler has a nice gutteral sound to it, means a beggar acting high and mighty.

          • @Dukeofurl.
            You might read CTO of Lilium Alastair McIntosh’s article which answers these claims of “Herr Professor Doktor Inginineurs” in a sensation seeking part of the German press.:

            Lilium is designed to require only 20 seconds of hover after transition during landing but flight planning will allow for 60 seconds as normal allowance during landing. That doesn’t mean it can’t hover for longer.

            As Alastair explains the Lilium uses variable area nozzle on the ducted fans to substantially improve hover performance compared to EDF with fixed nozzle.

            Also noteworthy is that the use of silicon in recent batteries allows sufficient power for hover when the SOC “State Of Charge” is only 15% compared to the 35% for the test bed’s batteries. Only 5% of battery energy will be used in a typical 100km range mission.

            And also Lilium’s own “Herr Docktor Ingineur”

            It’s worth a look at their newsroom.

            Liliums calculation sare based around a 300WHr battery in 2024 but we are seeing 400WHr cells starting to become available:

            It’s obvious that Liliums plans call for precision approaches and likely automatic takeoffs and landings to achieve the 20 second hover transitions to landing. The aircraft will not be used like a helicopter but instead will use established flight paths to ‘Lillipads’ with appropriate alternate diversion landing points. Jet airliners can’t land on random grass strips and also need tightly controlled flight paths, alternative airfields etc.

          • The hocstapler has even more rags, as they are caught between ‘unobtainium’ battery power density and low life cycles before they are kaput or more commonly …’Feuer aufgehen’

            Weight of batteries being tested in a lab doesnt yet account for the extra structural weight to the planes fuselage, plus the cooling system for recharging….unless they want ‘hot swap’ of fully charged batteries

    • > I just can’t see this concept working. Take London/NY. The supersonic aircraft will need to fly out of Heathrow or one of the larger airports. Those take longer to get to, longer to clear formalities and are more subject to ATC delays (30 minutes circling to get into LHR is not uncommon). Instead you could fly an A220-100 in all business premium out of London City today. Door-door time will be about the same. <

      Agreed; and I think SST-talk is misdirecting (from what ??)

  5. If a 70 passenger Boom was built and charged business class rates for it Mach 1.7 speed (twice the speed of subsonic jets) it would fund passengers and special cargoes. The seat Boom is promising look like a high end premium economy with plenty of legroom about 40 degree recline and all with direct aisle access. That’s fine for a 5 hour flight and good enough to nap.

    This concept however will rise or fall on environment or rather the opinions of the environment mob which is often quite ignorant and difficult to reason with once a thought gets hold in them. Boom will need a superb public relations campaign. Boom has to be shown saving children lives with rapidly transported transplants. That level of stuff that makes you cry.

    Furthermore a substantial source of carbon neutral PtL fuel is needs to be available before the aircraft flies. Boom promotes Prometheus fuels on its website. The fuel really needs to be in production and the public need to be aware of it before Boom takes to the air if it is not to face insuperable opposition.

    • You’re right about the environmental mob: they’ll kill this concept before it can ever fly.
      We’ll actually be lucky if they don’t curtail a large part of conventional aviation: it’s already starting to move tentatively in that direction in the EU.

    • Per the previous, there just is not that much time critical stuff that has an economic return.

      Concord had two government behind it and it failed.

      • I hear you..

        “Pay no attention to the man behing the curtain.”

      • The Concorde ‘B’ version, which was almost ready for production, would have eliminated the after burners and with changes to Olympus engine numbers of stages and the inlets to allow takeoff and push through the sound barrier without the AB. But the governments wouldnt authorize more production without paying customers. The plane would still be very noisy on takeoff (by modern standards ) but more comparable to jets of that era.

        • Those technical issues the “B” version solved should probably have been solved before the aircraft was put into service. Once the media and the mob had started talking about after burner noise its impossible to put the issue back in the box despite improvements to the aircraft. It would be foolhardy to undertake such a project without a professional media and PR team engaged.

          • The development took long enough anyway for the original version and of course the opposition orchestrated by US wasnt going to let Europeans get one up on them once their plane dropped out. Theres a phone call at the time with Kennedy giving a US airline head a hard time over ordering the Concorde, the delays meant the orders lapsed

          • William – me thinks it would be foolhardy to undertake such a project without the engineering and environmental challenges having been solved; until then a professional media and PR team will very much need to be engaged to keep the wool in position over the eyes of various myriad opposing groups.

          • @Pundit. Not all engineering and environmental challenges can be fully solved immediately but they can usually be solved sufficiently to make satisfactory operations at a modest scale acceptable. This gains experience and soon leads to improvements.

            I do agree some technologies need to be at a high level before deployment, nuclear for instance.

            I suspect the designers and people behind Concord expected people to be fair and reasonable. This is not the world we live in and they needed to be more careful. The reality is that those opposing Concord used hyperbolic scare stories and manipulated media and so the backers of Concord needed to engage them directly. Saul Alinsky’s rules for radicals springs to mind.

            It’s the same with any new technology. Unfortunately these days what technology you back is a political decision based around political cohesion and allegiances rather than the best strategy and that often comes from a wealthy individuals opinions.

            Our civilisation is loosing the logos once at its core, something predicted and analysed in detail by Nietzsche and Spengler.

  6. Once upon a time an SST was a SuperSonic Transport. Isn’t it a misleading use of this abbreviation for Small Supersonic Transport?

    • We can just call it an SSST

      As big as Concord was, 100 passengers?.

      • Boom was initially less than 50 seats not 100. A later plane went up to 205 ft and over 80 …its all fanciful anyway.

  7. Certain challenges to SSTs are frequently mentioned, including the sonic boom, takeoff noise, the need for new engines which are quiet at takeoff but efficient at supersonic speed, environmental concerns and economics.

    Rich Saffron mentions the structure weight penalty due to pressure going up with the square of velocity, which was news to me. While advances may hold some hope of solutions to some of these difficult problems, you can’t violate the laws of physics with regard to pressure. He also mentions the need for special materials above mach 2.3, which seems to ensure any future SST will be no faster than the Concorde.

    This further confirms there are too many serious problems for a commercially viable SST.

    • Thats easy to overcome , just fly at much higher altitudes at supersonic speed where the air density is much much smaller . This is the normal way even fighter type aircraft do it.

  8. Okay, please forgive my ignorance.

    But isn’t the A380 the fastest passenger airliner today? According to Wiki, it can travel at 903kmh. Wouldn’t NEOing it with GE9X or something, increase its speed?

    Supersonic is 1236kmh only, which is just above business jets. Cant Boom just ask RR to scale the Pearl up or something? So whats this talk about afterburners.

    Again, I know nothing. Apologies

Leave a Reply

Your email address will not be published. Required fields are marked *