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I was reading that aircraft like the F-15 and SR-71 have movable inlets. What's the purpose of this?

I was also reading aircraft like the F-22 doesn't have a movable inlet. Why?

F-15 engine inlet positions

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Jet engines require the air that flows into them to be subsonic. Supersonic air hitting the front fan would cause shockwaves inside the engine, with undesirable results.

So the job of the intake on a supersonic aircraft is to slow down the air to subsonic speeds. The intake is designed to set up a series of shock waves. The air will slow down to subsonic speed as it crosses these shockwaves.

The location of these shockwaves changes with speed: the higher the speed, the shallower the angle of the shockwave and the further the shockwave will reach down the intake duct. So if you have a fixed-geometry inlet, your maximum speed is limited: there is a speed at which the shockwaves will intersect the front fan.

Variable intakes avoid this problem: the movable vanes in the inlet aim the shockwave so it stays in front of the front fan at all speeds. This enable a higher top speed to be reached.

If you accept a lower top speed, you can avoid the extra complexity of a variable inlet. For the F-22, for example, a variable inlet would make it more difficult to maintain a low radar cross section, so the tradeoff was made to maintain stealth and lower the top speed.

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  • $\begingroup$ Might just want to clarify what you mean by "stays out of the engine at all speeds", as an internal shock system is still inside the engines inlet. $\endgroup$
    – Stuart Buckingham
    Aug 18, 2020 at 16:21
  • $\begingroup$ Also, some cool anecdotal evidence of what happens when the intake unstarts: roadrunnersinternationale.com/unstart.html $\endgroup$
    – Stuart Buckingham
    Aug 18, 2020 at 16:22
  • $\begingroup$ Does the shock wave responsible for slowing air down to subsonic speeds? $\endgroup$
    – Luke Justin
    Aug 19, 2020 at 12:58
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I will leave the more accurate answer to someone with deeper knowledge than me, but here is an intuition.

A jet engine needs a certain amount of air flowing through it in order to work. It can't be too much air, and it can't be too little air.

The faster you go, the more air is "rammed" into the engine, just by virtue of you moving faster through the surrounding air. A variable geometry inlet allows you to "shape" the flow of the incoming air such that always the right amount of air is flowing into the engine.

The SR-71 operated from standstill to almost 3.5 times the speed of sound. Now, the sound barrier is particularly interesting, because a normal jet engine requires that the air flowing through it is subsonic. So, the SR-71 in particular had the capability that, by moving the "spike" in the inlet, the engine could transition between a "normal" turbojet engine (subsonic airflow in the inlet and in the combustion chamber) and a supersonic turbojet engine (supersonic airflow in the inlet, but subsonic airflow in the combustion chamber).

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  • $\begingroup$ Ramjets use just the forward momentum to compress the air and have never been used on human carrying aircraft. The SR-71 (and all other jet powered aircraft) still use an bladed (in this case axial) compressor to compress the incoming airflow. They are just turbojets, but with inlet ramps to slow the incoming air. $\endgroup$
    – Stuart Buckingham
    Aug 18, 2020 at 15:19
  • $\begingroup$ This is really interesting. On Wikipedia, it says that the J58 is sometimes wrongly described as a turbo ramjet. And literally in the sentence before that sentence, in the same Wikipedia article, it is described as a turbo ramjet. :-D Well, at least the second sentence is definitely correct: it is sometimes described such, namely in the sentence directly before :-D $\endgroup$
    – Jörg W Mittag
    Aug 18, 2020 at 16:07
  • $\begingroup$ I guess it's just a matter of interpretation. The shock system will indeed cause a pressure increase, but the axial compressor is still needed to get the required pressure rise. $\endgroup$
    – Stuart Buckingham
    Aug 18, 2020 at 16:10
  • $\begingroup$ I just found it funny that the Wikipedia article first says it's a turbo ramjet and in the very next sentence says that it's sometimes called a turbo ramjet but that is wrong. $\endgroup$
    – Jörg W Mittag
    Aug 18, 2020 at 16:12
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    $\begingroup$ I've edited the answer to just call it a "supersonic turbojet". $\endgroup$
    – Jörg W Mittag
    Aug 18, 2020 at 16:14

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