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Why do reusable rockets (e.g. SpaceX Falcon 9) still utilize detachable fairings? SpaceX keeps trying to find a way to recover the $6M fairings for obvious cost saving reasons, why not have them simply open, eject the payload, then close again, via a hinge mechanism?

The precedent from the era of throwaway rockets was to let them fall into the ocean and be lost - is SpaceX/NASA being overly conservative by continuing with the detachable approach, or is there a fundamental engineering reason why it can't be done?

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It would only make sense to keep the fairing attached if the stage it was attached to was going to be recovered. The SpaceX falcon and falcon heavy are multi-stage rockets, only the first stage is reusable. The fairing is jettisoned once the aerodynamic pressure is low enough it isn't needed anymore, which is before orbital velocity is reached. Because it's not going that fast the fairing is recoverable, the rest of the rocket keeps going and is not recoverable.

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    $\begingroup$ @ToolUsr Recovery of the second stage is technically much harder, as it's going about 3x faster, which makes for a much, much hotter reentry. The problem of landing engine is relatively small compared to this. $\endgroup$
    – Russell Borogove
    Commented Jun 26, 2019 at 15:48
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    $\begingroup$ It's a tough nut to crack @RussellBorogove, you need a thermal protection system, downstream recovery, and enough fuel to land the stage. Lots of weight $\endgroup$
    – GdD
    Commented Jun 26, 2019 at 15:49
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    $\begingroup$ @GdD and adding hinges and motors to open and close a reusable fairing makes it even worse. $\endgroup$
    – jwenting
    Commented Jun 27, 2019 at 6:02
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    $\begingroup$ I'm not sold on that one @whitebeard, you'd be building a massive reusable spacecraft with only one purpose, the costs of doing that would probably outweigh any possible savings. It sounds way simpler to design a re-usable second stage which can get itself back to earth. $\endgroup$
    – GdD
    Commented Jun 27, 2019 at 8:15
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    $\begingroup$ @JohnDvorak that assumes reentry in such a way that the air flow won't rip them off, or the reentry plasma not leak in and damage the internals. $\endgroup$
    – jwenting
    Commented Jun 27, 2019 at 12:06
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The fairings are detached to prevent carrying their un-necessary mass to orbit. This typically happens during the second stage burn. See this question for information about the timing of fairing jettion: How strong and "hot" is the wind on the payload after the fairing is deployed at ~110km?

An image showing the fairing separation during the second stage burn: enter image description here Source: This question: Why does the SpaceX first stage booster boost up and back after separation not just back?

Failure to separate retains enough mass that the vehicle can fail to make it to orbit.

There have also been a few failures where the payload fairing failed to separate properly and that led to either an inability to separate the payload or the payload failing to attain orbit due to the added mass of the fairing.

Source

If a vehicle had sufficient extra performance to carry the added weight, there's no fundamental reason why the fairings couldn't stay attached. But "no fundamental reason" and "making the engineering work in the real world" are not the same thing.

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    $\begingroup$ @ToolUsr "microgravity" has nothing to do with it. And the fairings are typically jettisoned long before the 2nd stage burns out. The 2nd stage engines have to accelerate the mass of the second stage. Which would include the fairing if it isn't jettisoned. Edited "weight" to be "mass". $\endgroup$
    – Organic Marble
    Commented Jun 26, 2019 at 15:13
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    $\begingroup$ I think its an interesting idea nevertheless; if the 2nd stage would need a heatsheild to be recovered it seems worth looking at improving the one that's already there (the fairing) rather than add another $\endgroup$
    – Puffin
    Commented Jun 26, 2019 at 20:27
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    $\begingroup$ @Puffin sure, if you can afford the performance it's a splendid idea. Along with the openable fairing and heatshield, let's add a robot arm, wings, a crew cabin....nah, that's science fiction. $\endgroup$
    – Organic Marble
    Commented Jun 26, 2019 at 21:16
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    $\begingroup$ @OrganicMarble Well they have already thrown away a huge amount of performance in Falcon Heavy, basically they use three first stages in order to get the single F9 performance and yet keep the hardware. $\endgroup$
    – Puffin
    Commented Jun 29, 2019 at 21:56
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    $\begingroup$ @puffin I think the thing that's being missed is that the fairings don't really constitute a full reentry heatshield. And neither do the sides of the body tank. $\endgroup$
    – ikrase
    Commented Jan 25, 2020 at 9:02
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Apart from the mass penalty of carrying the fairing through the entire flight that Organic Marble mentioned, a hinged fairing and the mechanism to open and close it would be much more expensive, massive, and unreliable than the simple spring or explosive ejectors used on current rockets. You don't want to get the upper stage all the way into orbit and then have the fairing doors jam up on you.

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    $\begingroup$ just include some WD40 and a mallet in there to coax them open :) $\endgroup$
    – jwenting
    Commented Jun 27, 2019 at 6:03
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    $\begingroup$ and perhaps a robot that scurries about weilding the WD40 and mallet, cursing, "D@mm1t, open up you dad-blasted fairings!!" :D $\endgroup$
    – whitebeard
    Commented Jun 27, 2019 at 6:20
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    $\begingroup$ The explosives are not that simple either. Unfortunately I only have this video from Scott Manley on why the Taurus fairings failed due to manufacturing defects: youtube.com/watch?v=drSv1biVJZQ $\endgroup$
    – Michael
    Commented Jun 27, 2019 at 11:10
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why not have them simply open, eject the payload, then close again, via a hinge mechanism?

Because NOTHING is EVER simple when:

  1. accelerating to 17,000 mph,
  2. vibrating like mad, and
  3. performing an intricate dance among hundreds -- or even thousands -- of moving parts.

KISS and blow off what can be blown off.

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I suspect that the main reason is this:

The number one priority is to deliver the payload. Everything else is subordinate to this imperative.

Putting hinges and doors on the fairing adds a layer of complexity -- what if the doors don't open? You've lost the payload. Much better to lose the fairing! That's only $6 million!

So they do try to recover the fairing, but only after it has done its job, and not at the risk of endangering the primary mission.

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