As shown by this illustration in the SR-71 flight manual, its inlet cones (spikes) retract as its speed increases:

enter image description here (source)

Meanwhile, the Wikipedia page for the MiG-21 describes the opposite behavior for its intake cone:

On early model MiG-21s, the cone has three positions. For speeds up to Mach 1.5 the cone is fully retracted to the maximum aft position. For speeds between Mach 1.5 and Mach 1.9 the cone moves to the middle position. For speeds higher than Mach 1.9 the cone moves to the maximum forward position.

No source is provided for this part of the Wikipedia article. If the info about the MiG-21 is accurate, why does its intake cone behave differently than the SR-71's? Do the two aircraft's intake cones serve different aerodynamic purposes?

  • $\begingroup$ You need to look at the bypass flows to understand spike scheduling. The spike moves back above Mach 2.5 to reduce throat area and limit intake mass flow. The larger inlet area doesn't match that – the excess air at the inlet flows through the bypass vents aead of the throat straight back to the afterburner. More air is sucked through the boundary layer removal vents on the cone and flows through the struts to the outside of the nacelle. $\endgroup$ Commented Dec 15, 2019 at 8:01

2 Answers 2


This is only a guess, so please take this with a grain of salt.

An important characteristic of supersonic engine inlets is the total pressure loss (or also sometimes pressure recovery). The total pressure loss over a normal shockwave is a lot higher than if you would slow down the air to the same speed through several oblique shock waves.

If you want to understand why the sr-71 spike retracts I recommend you watch this video. The sr-71 had a quite complex inlet geometry to ensure that the air was slowed down over many oblique shocks. sr-71 engine inlet

On jet engines that don’t have such a complex inlet geometry as the sr-71, the best pressure recovery is mostly achieved by getting the primary oblique shockwave from the spike or inlet ramp, to hit the leading edge of the engine inlet, where it gets reflected as another shockwave. enter image description here

As speed increases the primary oblique shockwave that gets formed on the spike gets shallower meaning that if you wanted to keep the shockwave of the mig-21 spike at the leading edge of the engine inlet, you need the spike to move forward as speed increases.

  • $\begingroup$ I suspect the difference in engine types also plays a part: the MiG-21 uses an afterburning turbojet, while at high speeds, the SR-71 uses a ramjet, with virtually all the intake air bypassing the turbojet core and going directly to the afterburner. $\endgroup$
    – Mark
    Commented May 31, 2019 at 20:04
  • $\begingroup$ @Mark that shouldn't make any difference for the design of the inlet. Your two goals for both are to get the air subsonic so you can properly burn your fuel and to achieve a good pressure recovery. $\endgroup$
    – user33651
    Commented Jun 1, 2019 at 8:10

Unfortunately this answer is going to be speculative as I was not part of the design team for either aircraft. However I would have chosen similar behavior.

The first thing to note, is neither of their spikes are retracting before mach 2.5 (according to the info you provided).

The mig doesn't say what it does above mach 2 but up to mach 2 they are both extending.

Extended spikes are important when traveling at super sonic speeds as your air-craft gives the air in font of it no warning that you're about to hit it.

This means most "aerodynamic" features such as round curves become almost pointless as there is no way to "slowly" push the air out of the way.

For a more detailed explanation of this you can check out this video:

Now as to why they retract at even higher mach speeds you have to remember these engines are turbo jets. Which means ideally you want combustion to happen in-between the compressor and the turbine. This happens best at sub sonic airflow, faster than that it becomes difficult for the fuel and air to mix and ignite.

That means the engines need a way to slow down and regulate the airspeed and that's exactly what the bleeder values are doing behind the spikes. The less air you let into the engine the easier it is to slow down as there is less combined momentum.

If you find more information about exactly when the spikes are retracting in the SR-71 it'd be interesting to read about. However given the diagrams above this is what I think is happening.


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