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What are the disadvantages of pneumatic stage separation systems over systems like linear charges, frangible joints etc?

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    $\begingroup$ Apart from the excellent reliability of explosive bolts, I guess a pneumatic system is going to have a lot more mass and a lot more components (thus failure points). And if you need to separate at multiple points at the same time, I wonder whether a pneumatic system would be able to match the timing requirements. $\endgroup$
    – DarkDust
    Feb 22, 2022 at 10:00

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The separation between stages has to be hefty enough and quick enough so as to avoid recontact between the stages. This can happen (and has happened) if for example the stage separation occurs while the vehicle is still in the Earth's atmosphere or if stage separation occurs before the lower stage terminates thrust. Recontact has been the cause of multiple launch vehicle failures. Does your pneumatic stage separation pass muster in this regard?

Another issue is reliability. Stage separation is a single point of failure and it must work. Moreover, up until the moment of stage separation, the separation device is a "must not work" function. Devices that transition from "must not work" to "must work" (or vice versa) are tricky. Pyrotechnics are simple and reliable, but even they have failed on occasion. Does your pneumatic stage separation pass muster in this regard?

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  • $\begingroup$ I would also expect pneumatics to be heavy. First you need tanks hefty enough to hold that compressed air, then add valves and pipes etc. Pyrotechnics need only enough mass to contain the material as it blows. And the wires needed to carry the initial charge are very light in comparison to pipes and other assorted hardware associated with pneumatics (and the pneumatics likely need the wires anyway for sensors and control systems). $\endgroup$ Feb 22, 2022 at 17:32
  • $\begingroup$ @SoronelHaetir Compressed air? A pneumatic separation device more likely needs to be powered by an auxiliary power unit, at least for a larger rocket. That's a whole lot more mass than a compressed air tank. And a whole lot more plumbing, devices, valves, etc. $\endgroup$ Feb 22, 2022 at 17:38
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    $\begingroup$ @DavidHammen Falcon has nitrogen vessels for reaction control / stage separation and helium vessels for tank pressurization (the latter were responsible in some way for both catastrophic failures in Falcon's history.) With a spacecraft pneumatic system, you don't want auxiliary power - that implies recapturing the low pressure gas in order to recompress it. Also bear in mind that flight time is only a few minutes. The only closed circuit systems I'm aware of on Falcon are the pumped hydraulics for engine/grid fin steering. Starship will supposedly separate centrifugally by doing a backflip. $\endgroup$ Feb 23, 2022 at 0:19
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    $\begingroup$ @LevelRiverSt The Space Shuttle had an APU (multiple APUs, for redundancy) to power its hydraulic aerosurface actuators that were essential for reentry. $\endgroup$ Feb 23, 2022 at 0:27
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    $\begingroup$ @DavidHammen yes you definitely need powered actuators for control surfaces for reentry: hydraulic for shuttle & falcon 9, and direct electric drive for Starship. The falcon 9 separation pushrod is a tiny user by comparison and can be seen at 22:30 here youtube.com/watch?v=eiKOMCRymsw , all over in seconds. The nitrogen is also under about 100 bar pressure so you get more energy per volume than you would from a typical workshop pneumatic system. $\endgroup$ Feb 23, 2022 at 0:44

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