Bjorn’s Corner: Supersonic transport revival, Part 8

By Bjorn Fehrm

September 28, 2018, ©. Leeham News: In the last Corner we started looking at the biggest challenge for an SST, the powerplant. We first discussed the most fundamental problem of the engine, the level of Ram drag for an SST engine.

Now we continue with the challenges of the Nacelle. This week we talk intakes.

Figure 1. The Concorde nacelle with its Multi-Shock inlet. Source: Google images.

The Supersonic intake

The complexity of the SST intake is dependent on the maximum speed of the aircraft. As described last week an SST must invest considerable energy in accelerating the air which enters the intake to a speed close to the aircraft speed.

This energy investment can be used to raise the pressure of the air, in which case the investment in compression in the engine compressor will be less.

If the intake doesn’t manage to conserve the invested energy it will heat the air instead and some of the energy will be lost to spillage outside the intake. As air with a higher temperature is harder to compress, this is a double loss for the engine.

There has been quite some research into intake design. Figure 2 shows how different intake types recover the invested energy to pressure, based on flight speed.

Figure 2. Pressure recovery dependent on intake type and speed. The MIL-E curve is for a Multi-Shock inlet. Source: Cumpsty “Jet Propulsion”.

As long as the speed stays below Mach 1.5 a fixed inlet will work. Once we want to fly at or over Mach 2 we need a variable multi-shock inlet, which gradually accelerates the incoming air.

The most demanding control environment of the Concorde was the variable multi-shock inlet. It required such precise control so the analogue computer types which were OK for the Fly-by-Wire flight controls weren’t precise enough for the inlets. The first digital computer on the Concorde was designed to control the inlets.

The precise control of the multi-shock inlet is only half the solution of an SST inlet. If the inlet doesn’t work or the engine surges, the air must continue to flow into the inlet or the drag created by an intake which doesn’t swallow air can risk the stability of the aircraft, Figure 3.

Figure 3. The three main states of the Concorde Nacelle. Source: The Concorde SST site.

There is also the need for a larger intake area a take-off speed. Concorde had several additional movable inlet doors to cater for these and other functions.

It’s an old rule that each additional movable part in an inlet/nacelle creates its own problems and needs extensive testing to verify all the functions to the design team and to the Certification authorities.

The Boom Supersonic project will have to solve all the intake problems created by a Mach 2.2 inlet whereas the Arion AS2 can be designed with a fixed normal shock inlet as its cruise Mach is 1.4.

Next Corner we move to the back of the Nacelle to discuss the nozzle and thrust reverser problematic.

15 Comments on “Bjorn’s Corner: Supersonic transport revival, Part 8

  1. @Bjorn – in both of your articles about intakes so far you have stated that above M1.5 a variable intake is required.

    this is clearly not an absolute rule as the M2.0 F-16 has a fixed intake, the M2.3+ YF-23 had a fixed intake (other than I believe some spill doors to grab extra air at takeoff) and was able to supercruise at >M1.6 (specific numbers were classified)

    neither has/had variable intakes such as you see in the Concorde or F-15.

    so: are variable intakes actually required, or is it just that with the technology and understanding of aerodynamics available when the Concorde/F-15 were designed such that the sophisticated shaping required for fixed intakes wasn’t feasible?

    • Figure 2 shows that the F16 has higher pressure losses than the F15. It just matters less to the F16 because it was not designed as a air-superiority fighter / interceptor and so is not optimized for those mach numbers. Yes it can get there, but at the cost of burning more fuel.

    • Hi bilbo,

      you have to distinguish between “capable of” speed (Fighters) and the speed for hours and hours of cruise (SSTs).

      I flew a Mach 2 capable fighter (J35 Draken) but never passed 1.7 in practical use, and this was a high speed run without weapons.

      When LM sais the F16 can reach Mach 2 that’s a clean aircraft, flying at optimal altitude with full zone 3 afterburner, accelerating slowly until Mach 2 is shown on the Mach meter. By then you are out of fuel (no extra tanks to reach those speeds) and descend at idle to base. Check any discussion with real fighter pilots and you find this. The F15, Su27/30, Mig29 and Typhoon are real Mach 2 aircraft where this is a practical speed as well, all have multi-shock inlets. Still this is with afterburner lit, so they can’t fly at these speeds for long.

      The supercruise with YF23 at Mach 1.6 can’t be correct, here Wiki: “The YF-23 was tested to a top speed of Mach 1.8 with afterburner”, which is plausible given the config.

      To cruise for 3-4000nm (three to four hours) at supersonic speeds you need efficient intakes, engines and outlets at these speeds. This means multi-shock inlets and convergent-divergent outlets if you fly over M1.6-1.8 and with the efficient intakes and nozzles, you can run with non-afterburning engines.

      • Per contemporary articles I read at the time and Goodall’s book on stealth fighters, the YF-23 supercruised at M1.45 on the PW engines and M1.6+ on the GE engines, by the run rules of the competition, any supercruise capability above M1.6 would not be publicly released.

        YF-22 got to M1.43 and M1.58 on the respective engines, which makes one wonder why the PW was selected with the GE clearly outperformed it…

        I presume then that the fixed inlet design of the yf-23 was optimized for high mach.

        • JBEEKO: While I am nothing remotely into the intkaes and such, I do know the following is wrong.

          “It just matters less to the F16 because it was not designed as a air-superiority fighter / interceptor and so is not optimized for those mach numbers.”

          Boyd and his group intended it fully as a extremely capable dog fighter (and in order to dog fight you need to intercept!)

          Its only latter that hanging bombs etc all over it came into vogue.

          The pilots seat was reclined and side stick controller implemented for high G purposes.

          Having read some comparisons between the F-16 and the F-18, the F-16 came through as a pilots dream, the F-18 is almost a garbage truck in comparisons and not nearly as pilot friendly./

  2. Pulling some g’s at supersonic speeds forces an optimized variable intake to follow with geometry changes not to risk an inlet unstart or engine stall. Not a big problem for SSBJ’s , supersonic bombers or interceptors but fighters manuvering is another story where the shock waves can move around quickly and the drag explode. Even Dassault moving from variable inlets on almost every Mirage fighter changed to “looks like fixed” inlet on the Rafael.

    • The real value of a multi-shock inlet on a fighter is if you are in a tail chase to your target (classical Russian tactic) but even more, if you decide to exit and run. Then quickly accelerating to Mach 2 and staying there for a while is life-saving. The Mig 21 uses this as a lifesaver against fix inlet fighters, but then it was out of fuel, so needed a base nearby after the run.

      This requires the other guy is out of missiles (missiles accelerates to Mach 3-4 during the boost phase, then glides to the target), your countermeasures against his missiles are good or you get beyond his missiles tail chase range (the missile range in a tail chase against a Mach 2 runner is a fraction of the head on range).

      • There have been comments speed no longer matters but that is not true, both speed and acceleration have a quality all their own.

        The old P-47 was no dog fighter, but you were hard put to climb with it and you sure could not match its speed ( and it dove like the brick it was and all for the good)

      • If it enough to run away at M1.6-1.72 and you have 2ea 35 000 lbf class Engines and a pretty good intake you can do it while supercruising and benefit from the combination of high speed and low fuel consumption, The F-22 even with the F-119 Engines should be able to do it for quite some time, the F-120 with variable bypasss even better.
        The combination of high speed and a good inlet pressure recovery and beeing able to run pretty high turbine inlet temperatures gives you the numbers of a supercruiser with better than x5 the range of a fighter having lit afterburners.

  3. Bjorn, very good article. You touch on several design aspects that we went through during the F-14 D development and E/F studies. A Wind Tunnel with flow visualization capability is a tremendous asset in understanding the shock structure.
    And Aerion’s AS2 design with a fixed ramp will probably be optimized for supersonic cruise conditions.

    • Thanks Guz, I was part of the team which developed the SAAB Gripen and we had long discussions if a variable inlet vs. a fixed one should be used. The variable is good for your corner case of a tail chase or run but will impact almost all other use cases, so it’s a tough choice.

      A Mach 2 SST has no choice, it must go for maximum efficiency.

  4. Another feature , which is obvious from the Concorde SST graphic Fig 1, but what no ones mentioned is the engine by pass air or secondary airflow as its called. The engine used was the turbojet Olympus which doesnt have by- pass air passing its front compressor stage and yet we have some portion of the air taken in the duct and is compressed by the inlet ramps to slow it down then passing around the engine. That secondary airflow doesnt pass through the afterburner so cant add to the thrust that way when AB is lit- At takeoff the Concorde secondary flow is shut off.
    For designs without variable ramps but fixed inlets , Im wondering about a possible loss of thrust as the Concorde during supersonic cruise only 8% of the thrust comes from the engine, 29% is from the rear nozzles and an impressive 63% from the intake.
    The only military aircraft designed for sustained SS cruise was the SR-71 Blackbird and that used a movable spike in the engine inlet to control the airflow to the J58 turbojet engines. Along with secondary bypass doors the SR-71 used secondary airflow passing around the engine but unlike the Concorde this airflow could be used by the afterburner and the variable airflow inlets could provide respectable amounts of thrust at cruise conditions.

    During high-speed flight in the Blackbird, compression of air in the inlets generated most of the vehicle’s thrust.
    At Mach 2.2 the inlet produced 13 percent of the overall thrust with the engine and exhaust ejector accounting for 73 and 14 percent,respectively. At Mach 3 cruising speeds the inlet provided 54 percent of the thrust and the exhaust ejector 29 percent. At this point the turbojet continued to operate but provided only 17 percent of the total motive force. The inlet had a compression ratio of 40:1 at cruise conditions ..

    From American Institute of Aeronautics and Astronautics
    Design and Development of the Blackbird:Challenges and Lessons Learned .
    https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090007797.pdf

    • Hi Dukeofurl,

      I think both the text on the Concorde SST pages and the document you referred to are misleading us. Let’s start with the Blackbird nacelles and engines.

      If an intake, which reduces the speed of the air relative to the nacelle to Mach 0.4 (or accelerates the air to Mach 2.8 relative to the air’s original state, the freestream) can convert the lossy pressure gain in this process to 54% of the thrust and the exhaust ejector adding 29 percent, for a total of 83% of thrust, without adding fuel, we are talking about Perpetuum Mobile sciences.

      What really happens is, the engine thrust is going from a turbomachine dominated thrust below Mach 2.5 to a Ramjet dominated thrust above this speed. At the cruise Mach of 3.2 the jet engine is only contributing 17%, this is the only correct part of the above IMO. The rest is the Ramjet contribution.

      The intake compresses the air 40:1 at Mach 3.2 and bypasses it to the afterburner area at Mach 0.4, where fuel is added and then the increased energy combustion air all goes to the nozzle.

      A Ramjet (which Lockheed states is the dominant engine mode at Mach 3.2) can’t work without fuel being added to re-accelerate the air to around Mach 4 from Mach 0.4 past the flameholders (relative to the nacelle), and the fuel contributes the energy needed to overcome the airframe drag of the Blackbird. Not the intakes or nozzles. In fact, both have process losses, they are not Isentropic (state change without energy loss).

      The Concorde SST sites have the same misleading statements on its pages. The Concorde is running in turbojet mode all the time, its speed is to low for a Ramjet mode. But the intake, on a whole, delivers a huge momentum drag, the Ram drag. It also delivers compression, which can be used to burn fuel efficiently. But the jet fuel delivers the energy to produce thrust, neither intake nor nozzle does. They contribute to keeping the overall process loss down, to use the available energy in the fuel as efficiently as a turbojet process allows, that’s all.

      • Yes, it is the Engine that does the work, but the designer of the propulsion package decides which pressures/temperatures in which chamber of the Engine/Inlet, often to get acceptable Engine bearing axial loads. Then you get the effect of a pressure difference on the wall of a large chamber that generates axial loads (thrust).

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