Bjorn’s Corner: Supersonic transport revival, Part 15. Wrap-up.

November 16, 2018, ©. Leeham News: In the last 14 Corners we have discussed the challenges facing the new SuperSonic Transport (SST) projects, 50 years after the Concorde took flight. The challenges facing projects from Aerion Supersonic, Boom Supersonic or Spike Aerospace are the same as for the Concorde.

In the wrap-up of the series, we go through the challenges and talk about which has gotten easier to solve with modern technology and which has gotten harder because of changing environmental standards.

Figure 1. The new SST project which has come the furthest, the Arion AS2. Source: Aerion Supersonic.

Which SST problems are easier to solve today?

In the series, we have covered several areas of problems which needs solving when designing an SST, and which are not present when designing a normal airliner.

The first area we covered was two added drags which crop up when we pass the sound barrier, volume and lift wave drag. These drag components add to the normal drag components we have (like air friction and induced drag).

The tools to understand the effects of wave drags and their size have improved, especially the CFD tools (Computer Fluid Dynamics programs). SST aircraft can, therefore, be designed more precisely and efficiently than before. There will be fewer surprises once test-flying begins.

The problem of aerodynamic heating is easier to manage as heat resistant composite structures have been researched since the Concorde was restricted to Mach 2 because of heating concerns on its aluminum structure.

The lack of active SST projects has stopped this research to reach a mature state and to produce a composite material system which is accepted by the FAA/EASA. So there is work left which will cost time and money for projects which fly beyond Mach 1.6, where normal aluminum or composite materials works fine.

And which are harder to solve?

The biggest problem facing all modern SST projects is the change in environmental standards since the Concorde flew.

There hasn’t been any change in the acceptable level of supersonic booms hitting the ground from an SST. In fact, there is research and discussions to see if a more nuanced standard than today’s “no supersonic flying overland” can be agreed.

The changed standard causing major problems for SSTs is the more rigorous Noise standards around airports.

As we have explained in several Corners, efficient SST engines are inherently noisy, as these accelerate the air going through the engine to high exhaust speeds.

For today’s airliners flying at a maximum of Mach 0.85, it’s been possible to lower the engine’s air exhaust speed and increase the amount of air which passes through the engine. This has had the benefit of lowering engine noise while simultaneously increasing fuel efficiency.

But an SST engine cannot take in much air into the intake. It causes a high Ram drag, which is not bothering lower speed engines. A high Ram drag makes the engine inefficient. The faster the engine shall operate, the less air it shall take in and the faster it shall kick it out the back.

Such engines are called High Specific Thrust engines (Specific Thrust is engine speak for how fast the air goes out the back). But High Specific Thrust engines are noisy.

We could see when we designed a compromise engine which worked with acceptable efficiency at Mach 2.2, it was by far too noisy at takeoff and landing.

The Aerion AS2 project is the first SST project after the Concorde which has a solution to this problem. The first part is limiting the supersonic cruise speed to Mach 1.4 from originally planned 1.8. This allows a moderate Specific Thrust engine to be designed by GE; the Affinity.

The second part is not using full throttle for takeoff, by it limiting the noise producing jet speed of the engines to acceptable values.

The lower maximum cruise speed also helped with compressor temperature problems for SST engines, so an existing airliner engine core, the CFM56, could be used. This lowered the cost of Affinity development.

Summary

The dominant problem for a modern SST project is the engine efficiency versus noise problem. It grows larger the faster we fly. The Arion and Spike projects have, therefore, limited their cruise speed to Mach 1.4 and 1.6 respectively.

This leaves Boom Supersonic alone at Mach 2.2. How Boom is solving this major problem for an SST is not clear. The variable cycle engine the project needs has not left the research stage.

19 Comments on “Bjorn’s Corner: Supersonic transport revival, Part 15. Wrap-up.

  1. Really enjoyed this series. Thank you for sharing it with us.

    A tiny typo in the first sentence, so sharing it with you. “Faceing”

      • Thanks Bjorn. Sorry I was not clear. Faceing is the typo. Facing is the correct spelling. Now the typo appears twice 😳

          • After having seen how the ladies in school (the A+ type) were so good at language, I decided they invented it.

            We had one class where the biology class teacher had to teach English without an answer book (new school, needed bodies and he had a 2nd in English).

            He had a pool of 4 or 5 A+ ladies who worked out the answers for him in class.

            He would call a friend who was an English teach and had the book with the answers.

            They were never wrong.

            The rest of us were happy to pass with a C.

  2. Thnak you very much for the series. But now I feel like I missed that golden opportunity to fly in the Concorde, that will not be available. To cruise at Mach 2,2 is something that seems beyond our lifetime!

  3. Has anyone considered a 3 engine option with the third engine optimizes for take off. The other two supersonic engines would be idled for emergency take off thrust. I think an electric engine with just 5-10 minutes of run time would be light enough and keep the sound down.

    • The idea of having seperate engines for takeoff and cruise was tried with VTOL/STOVL designs and didn’t really catch on. I guess the concept on the F-35B using a fan driven by the main engine is sort of like this, and sort of like a variable-cycle engine. It reduces the dead weight compared to an entire engine. I wonder if that could work for an SST.

      • Almost universally all the engines are matched on airliners. The only example I can think of that differed was the the stretched Hawker Siddeley Trident 3B. It had 3 matched
        RR Spey engines and added a small turbo jet, a RB162 around 5K thrust, which shared the inlet duct for the centre engine and was used only for takeoff. It would have been a better idea to increase the TO thrust on all 3 by a small amount.

        • While not an Airliner, the C-123 had wing jets for takeoff.

          Pretty cool pod jobs and highly streamlined.

      • Oddly we called them JATO back in the day, when they really were RATO

        JATO is more cool, before there was such a thing as cool.

  4. Agreed, most interesting. I guess they will keep trying because its there.

    Concorde came to Anchorage one time, did sweeps out in the Gulf at Supersonic. Tickets were seriously nose bleed area.

    My salary was not, sigh.

    • The Concorde actually came to Columbus one time! Of course I was out of town that day…

  5. This is probably a ridiculous suggestion, but has any thought been given to rocket assisted take-off for a supersonic airliner – or would they be noisier than the Mach 2 engines? There would be minimum weight penalty as the fuel would be totally used up, only leaving the casings, which would have to be faired into the wings for minimal drag, as discarding them wouldn’t be environmentally acceptable.

    • I am sure thought of yes.

      And just as soon dismissed.

      Something about flaring flames, civilian airlines and as they say, the optics would not be good!

    • “would they be noisier than the Mach 2 engines” You bet, the ‘speed of the thrust’ could be multiple times that of the speed of sound.
      Check the Rocket thrust equation from Nasa
      https://www.grc.nasa.gov/www/k-12/airplane/rockth.html

      As was described in this series, the engines are sized for the push through the sound barrier at altitude not runway takeoff like most airliners, so maximum thrust isnt required for most takeoffs.
      Like the discussion around ‘catapult assisted takeoff’ , rocket thrust is now only used in military situations for a short runway/heavy load takeoff.
      There were some mixed jet/rocket powered prototypes in the 1950s for airframe designs with integral rocket motors for climb rate – SR53 from Saunders Roe
      https://en.wikipedia.org/wiki/Saunders-Roe_SR.53
      Another integral rocket system was the Bristol Siddeley BS605
      a pair of motors which were built into the rear fuselage of the Blackburn Buccaneer naval strike aircraft via a downward rotating door. South Africa needed it for takeoff on its hot and high altitude airfields.
      Plenty of images on the Web of it being used.

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