Bjorn’s Corner: Supersonic transport revival, Part 14

By Bjorn Fehrm

November 9, 2018, ©. Leeham News: In last week’s Corner we compared the GE Affinity, the Mach 1.4 engine for the Arion AS2, to the engine of the Concorde when both propel a Mach 2 Supersonic Transport.

We could see an engine must be designed for working at Mach 2. The Olympus, now a 50-year-old design, was more efficient in propelling a Mach 2 SST than the hypermodern Affinity. Now we design a custom Mach 2.2 engine.

Figure 1. A Mach 2.2 suitable SST turbofan modeled with GasTurb. Source: GasTurb.

Design requirements for a Mach 2.2 SST engine

We will design an engine for a Mach 2.2 SST, by it analyzing the problem areas for a Boom SST engine. We estimate the Boom SST as presently defined to need the following data from an engine (it has three engines like the Aeron AS2, Figure 2):

Figure 2. The Boom Mach 2.2 SST. Source: Boom Supersonic.

  1. Thrust per engine at Mach 2.2, 51,000ft and ISA temperatures: 4,600lbf
  2. Take-Off thrust at sea level and ISA temperatures: 19,000lbf
  3. A high thrust to weight ratio, ideally the same as the Olympus
  4. A minimum diameter to minimize volume wave drag.

As we could see in the previous Corner a ByPassRatio (BPR) 3 engine like the Affinity is a nonstarter. I have modeled an engine optimized for Mach 2.2 with GasTurb which has a BPR of 2. It has acceptable characteristics at Mach 2.2.

We will now go through the data for this engine and see how it fares against the Concorde’s Olympus 593 and if we can match its fuel consumption and handle the take-off noise problem.

A Mach 2.2 turbofan

The engine is designed to have optimal flow conditions and TSFC at Mach 2.2. To get to a TSFC which is equal to the Concorde engine’s, I have used state of the art technology in the engine.

The engine has compressor and turbine efficiencies of up to 93% Polytrophic at cruise (it converts the mechanical energy from the turbines to thermodynamic energy in the compressors with 93% efficiency). T41 is 1800K at cruise and 1900K at takeoff (it makes for a smallest possible engine). The intake and nozzle are modeled at the very high efficiencies of the Concorde nacelle. Its values beat the Mil standard intake we looked at in previous Corners.

The Pressure Ratio (PR) at cruise between the ambient air going into the intake and the pressure in the combustor is 1:134. The engine PR at Mach 2.2 is kept at 14.6, not to waste combustion energy on compression which is not needed. The intake delivers PR 9.2 at Mach 2.2.

End of compressor temperature T3 at cruise is 950K/677°C. As this is a sustained cruise value it requires state of the art materials in the compressor. Net thrust is 4,600lbf with Gross thrust at 19,200lbf (ratio 4.2). Specific thrust at cruise is 204m/s and the jet velocity is aircraft forward speed + Specific Thrust: 649+204=853m/s. The propulsive efficiency at cruise is 87% and the Core efficiency is 64%, both very high values.

TSFC is 1.29lb/lbf/h and this is the same value I get for the Olympus when modeling it with GasTurb. Bristol got to 1.21 and they had smarter people who could spend more time on it than I. The key message is; I could design a BPR 2 engine which could match the TSFC I could get for the Olympus with GasTurb, but I could not beat it. The reason is; its architecture for Mach 2.2 cruise is less optimal than the Concorde’s straight jet. Its Specific Thrust is too low, hence Ram drag is high, 14,600lbf.

The not so good news

Now the less good news. This engine has a weight of 1600kg according to GasTurb (GasTurb does a pretty good job of modeling all component dimensions and weights and then sums this to the total engine weight), Figure 3.

Figure 3. The engine dimensions and mass modeling part of GasTurb.

Its thrust to weight ratio at Mach 2.2 is 0.77. The Olympus develops 11,600lbf and weighs 7000lbm, its ratio is 1.66. The diameter for the engine is 1.2m, the same as the 250% stronger Olympus.

And the worst news, despite a worse thrust to weight ratio and the same diameter for 2.5 times lower thrust, we have a Specific Thrust at takeoff of 530m/s. We will not pass any Stage 5 noise standards with this engine.

We can throttle it back to an acceptable 350m/s, but then our thrust is 12,500lbf and the aircraft will need very long runways. If we design a larger engine which can be throttled back to 350m/s while developing 19,000lbf of takeoff thrust, we have a 45% larger engine and the diameter and thrust to weight ratio is already bad for our aircraft.


The above shows how difficult it is to design an engine for an SST which is flying at Mach 2 or more with today’s noise standards.

A solution is a variable cycle engine, but as said, these are at their infancy. Perhaps Boom can come up with some other smart way of solving the problem; it will be interesting to watch.

11 Comments on “Bjorn’s Corner: Supersonic transport revival, Part 14

  1. Hello Bjorn,

    I wonder if it’s optimal to use three engines of the same type.

    How would an asymetric concept with a low-noise take-off engine and two engines optimized for hypersonic cruise perform?

    The Low-Noise Engine could be placed at the end of the fuselage. During cruise flight, it could be shut down completly, with the inlets closed for low drag.

    The redundacy power for the take-off with the single engine would be given by the two hypersonic-engines, which only spool up to full power/noise in case of a failure in the Low-Noise engine.


    • Hi Jörg,

      you would need the high bypass engine to produce 54,000lbf which is the typical GE CF6 thrust class. Such an engine weighs 4,300kg/9,400lbm. Including installation, closable intake etc we would be at a dead weight for operation at cruise of 5t on an airframe which has an empty weight of 27t-30t. This is almost 20% of OEW and therefore not realistic. Aircraft designers fight for every kg to shed from the design, 5,000kg additional is not an option.

      Otherwise not a bad idea.

  2. Boom were doomed, a stillbirth, right from the very start by choosing Mach 2.2 as their cruise speed requirement. Presumably, Blake Scholl thought they needed to be better than Concorde (“+10%”) because technologies had improved so much since then. If he had done some more research (“I gave myself three weeks”) instead of choosing to start an admittedly brilliant PR wave and to sell that Mach 2.2 figure to investors, he could have learned that community noise regulations have moved on as well. Now they’re trying to change regulations because they cannot possibly step back from their promises.

  3. Thanks for your effort on this Bjorn.
    I was wondering if the Boom engine could be compared with
    a later Olympus version that was designed and part tested but not built, the 622. The main feature was to do away with the afterburner completely.
    As they said in Flight Global back in 1974

    Olympus Mk 622 has a higher mass flow
    thanks to a slightly bigger compressor—l in greater in
    diameter—and a slower jet velocity. Mass flow is about
    8 per cent higher and thrust between 7 and 8 per cent above
    the existing Mk 610 take-off rating. The demonstrator
    compressor has been built and is on test.
    For the demonstrator three new stages have been added
    to a “sawn-off” front of a standard Mk 602. Though only a
    minimum-cost improvisation, it is demonstrating the estimated
    increases in mass flow and thrust. “

    Its amusing how often and ‘extra inch’ diameter fixes things even now- the Leap 1B added an extra inch to the fan diameter late in the development.

    • Hi Dukeofurl,

      this is what I understood happened. As always the Concorde grew in weight while developed. Instead of scaling up the Olympus (which you typically do by increasing mass flow), which would have delayed the project, Snecma/Bristol added a mild afterburner to get the required thrust.

      As the Concorde was finally in EIS you know what you could improve. One area was doing the engine you needed in the first place, i.e. with higher mass flow and no afterburner which reduces takeoff Specific Thrust and by it noise, while improving fuel burn for the missions.

      Nothing much else was changed in the architecture (other than piped bleeds like for the SR71’s J58s) so it’s still a straight jet which is great for M2.02 and less so for takeoff and landing.

      • I’m glad that it did end up with afterburners. The sight, sound and feeling of Concorde getting airborne at night, 4 bright plumes throwing it into the sky, never, ever failed to raise the hairs on the back of one’s neck! Noisy? Yes. Too noisy? Well as it was so rare, I don’t think anyone really minded.

        BTW Stanley Hooker’s book “Not Much of and Engineer” is an illuminating read. The chapter on the Olympus shows the engine to have been tremendous from the very beginning. Apparently on its first ever run, once it was at idle Hooker slammed the throttle fully open. And there it was, running at the design thrust within less than 1 minute of being wetted by fuel for the first time. Scared everyone else to bits! This included a visitor from Curtis Wright who were looking to license the design, senior management, etc, all of whom were convinced that the engine was going to blow itself to smitereens and that Hooker was some kind of madman.

        By the time it had evolved all the way through to the engine for Concorde its thrust reached 40,000lbs. Not bad for something that started off with 10,000lbs. Hooker reckons that the Concorde engines reached a thermal efficiency of 42%, which is truly remarkable for any fuel burning engine of any sort. I mean, carefully tuned marine diesels in cargoships get better than that, but they can afford the weight of a lot of machinery to achieve that.

        Beating this performance (or the performance of the J58) is going to be remarkably difficult.

    • Assisted T-O really helps in the noice and fuel consumption calculation. The mass of any substantial electrical engine in the aircraft during Take-off gives a massive penalty, using it for taxi makes more sence especially if the light weight motor can be built into the brakes rotors and stators and get its electrical power from a 787 type of APU.

      • ‘Assisted takeoff ‘ doesn’t assist airport takeoff at all. Its not a short carrier deck, the plane needs to rotate at the end of takeoff run and continue the climb- all at high thrust settings. Even when thrust is reduced a bit it could be at 3000ft. Thats going to spread the noise over a wide area.
        Assisted TO for airports is just absurd .

        • I do not claim it can be done easliy today, but doing the numbers gives you a hint on the noice and perfomance benefit.

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