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I am only aware of one US rocket that used hot-staging for maintaining proper ullage during stage seperation--the Titan family. Otherwise, US rockets tend to use reaction control systems or small solid ullage motors to settle fuel. However, Soviet rockets & their derivatives (Russian, Indian, Chinese...) have been hot-staging since the very first Soviet upper stage (as should be apparent from the open interstage surrounding the Blok-E RD-0105 engine).

Why did this practice persist? Conversely, why did US rocket manufacturers never adopt the practice?

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    $\begingroup$ Rocket designers are very conservative; once they try something and it works they tend to keep doing the same thing. Soviet designers tried hot-staging, had success, and kept doing it; US designers tried ullage rockets, had success, and kept doing it. $\endgroup$ Commented Jan 30, 2020 at 4:29
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    $\begingroup$ The Titan II did hot stage as well so Americans did use the concept for a while as well. See also my related question "Why do the Russians use these fence-like interstage fairings?" $\endgroup$
    – DarkDust
    Commented Jan 30, 2020 at 7:46
  • $\begingroup$ @DarkDust Thank you for the information! I have already mentioned Titan hotstaging in my first sentence (and I mention open interstages in a later one). :) $\endgroup$ Commented Jan 30, 2020 at 8:59
  • $\begingroup$ D'uh… sorry for not paying enough attention. $\endgroup$
    – DarkDust
    Commented Jan 30, 2020 at 10:28
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    $\begingroup$ @CarlWitthoft Yes! Look into the Scout, Minotaur, ASLV, J-I, Taurus (and probably others). All full-solid OLVs. $\endgroup$ Commented Jan 30, 2020 at 17:23

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Paraphrasing parts of an answer to a different question, hot-staging has a few advantages:

  • It's less complex than staging using ullage motors since fewer parts are involved (whole rocket motors and their plumbing and tanks are missing, as well as the sensors and controllers to make them work correctly).
  • Reduced complexity often means improved reliability.
  • The previous stage is pushed away by the firing motor, thus no risk of the stages colliding.

Some downsides are:

  • More mass on the stage below the next rocket nozzle. That stage needs to be able to withstand the heat of the next stage firing. That might be offset by the not-needed ullage motors and their tanks, etc.
  • It makes the rocket taller (I'm not so sure about that, though).
  • The timing window is narrower. The next stage has to fire within a certain window (two seconds for Soyuz were quoted in a different answer to a related question).

I also read that timing with ullage motors needs to be more precise (even though the window is wider) but I haven't found a quote on why that is the case.

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    $\begingroup$ Were there differences in engine capabilities adjacent to performance between the US and USSR that informed to the initial decision to hot- or not- stage? $\endgroup$ Commented Jan 31, 2020 at 21:46
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    $\begingroup$ Interesting question that I'm not able to answer. Might be worth a dedicated question. $\endgroup$
    – DarkDust
    Commented Feb 1, 2020 at 9:23
  • $\begingroup$ This answer seems incomplete. Hot staging is the seemingly obvious way to do it, and the downsides cited are not convincing. Why would significantly more mass be required to protect a stage you are in the process of discarding? The difference in rocket height would be minimal if any. And you've acknowledged uncertainty about that and about timing margin differences. $\endgroup$ Commented Jul 24, 2023 at 12:32
  • $\begingroup$ @ScottMcPeak: It's not necessarily significantly more mass. Depending on the design, some kind of reinforcement may be needed so that rocket motor of the upper stage doesn't puncture the lower stage's tank (which usually still contains a bit of fuel and might explode; it's safer to shut down with a bit of fuel left instead of letting the motor run dry). The weight of the reinforcement might get offset by the more lightweight stage "connectors". I'm not an expert in this and read about it some time ago, so if I'm wrong on this feel free to provide additional infos. $\endgroup$
    – DarkDust
    Commented Jul 25, 2023 at 6:53
  • $\begingroup$ @ScottMcPeak: But I'm very sure about the timing window. With ullage motors, you potentially have a lot more time before you need to ignite the upper stage. Without ullage motors, you need to fire as soon as possible after lower stage shutdown since otherwise you risk having gases instead of liquids getting sucked into the upper stage's motor which can destroy it. See the linked question.. $\endgroup$
    – DarkDust
    Commented Jul 25, 2023 at 7:00

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