I noticed that over the years (on fighter jets specifically) there have been many different methods of directing air into the engines. Primarily, the nose-intake, side intake, and underbody intakes have been very popular with many different designs.

Could somebody explain what the pros and cons behind each type are?

  • 1
    $\begingroup$ This may answer the question for you. And this is a possible dupe of at least part of it. $\endgroup$
    – Dave
    Commented Feb 24, 2018 at 4:47
  • $\begingroup$ Possible duplicate of Why is the nose of the Mig-21 shaped so differently compared to other fighter aircraft? $\endgroup$
    – user14897
    Commented Feb 24, 2018 at 10:21
  • $\begingroup$ What? @ymb1 that doesn't explain like 8/9's of my question. There are tens of different intake designs please if you want to call a dupe read my whole question and figure out the answer first. And thanks Dave that did answer my question if you could put that in an answer post and just link the URL I'll accept and upvote it. $\endgroup$
    – Jihyun
    Commented Feb 24, 2018 at 19:26
  • $\begingroup$ By reading all the answers, there is a lot more that is covered there IMO. Don't take it by the title or the first answer. $\endgroup$
    – user14897
    Commented Feb 24, 2018 at 19:50

1 Answer 1


It is mostly a tradeoff between complexity and speed.

High speed inlets not only fundamentally need to be more complex for efficient operation (more oblique shocks required before the normal shock), but they also must operate at all the speeds lower than their design speed (unless it is a boosted missile or something like that).

This figure is from 1980 and it illustrates this tradeoff.

I recall reading somewhere that a huge amount of the F-15 development costs went into the last 0.2 Mach number capability (i.e. 2.3 to 2.5) -- largely driven by added complexity in the inlet system.

Depiction of speed vs. complexity of inlets

The big thing that is different with modern military aircraft is the requirement for stealth. So modern supersonic inlet design is a tradeoff between complexity, speed, and stealth.

If you are unfamiliar with the need for more shocks for higher speed, here is the fundamental tradeoff. Total pressure ratio is the fundamental efficiency measure of an inlet. You want it to be as close to 1.0 as possible. This is easy for subsonic engines.

Once you are supersonic, you need to slow down the air before it reaches the front face of the fan. You want the flow to eventually be subsonic, so your final shock is a normal shock. A single normal shock at high Mach number is a very strong shock and is a disaster for your inlet pressure ratio.

Consequently, we use one or more oblique (or conical) shocks before the final normal shock to slow down the flow gradually. As it turns out, the optimal solution is to have each of the shocks have equal total pressure ratio.

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