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I am wondering if it is possible to propulsively land a solid rocket booster after it detaches from the core booster. I know you could use thrust termination ports to stop thrust, but it would drop the pressure inside and the leftover propellant would burn away. Is there any safe way to shut off an SRB, and could you relight it afterward? Alternatively, could you somehow land an SRB without shutting it off? Thanks!

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    $\begingroup$ Guessing you mean propulsively land an SRB? The Shuttle regularly landed its solids :) $\endgroup$ Nov 22, 2021 at 6:26
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    $\begingroup$ Yeah, I mean propulsively land; didn't think about that, sorry! I'll edit my question. $\endgroup$
    – Duck
    Nov 22, 2021 at 6:46
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    $\begingroup$ There's also the Soyuz style "partially powered", mixed landing style. Have small, pretty low-power SRBs attached to the bottom with TWR slightly less than 1 (when carrying empty main booster) and a quite modest parachute on top, definitely insufficient for a soft landing but good enough to keep the terminal speed in check. Shortly before touchdown you light the small SRBs and they act as "weight reducers" of the main one, the parachute handling, say, 10% of its weight (though still 100% inertial mass) and able to brake it to a very safe descent rate. $\endgroup$
    – SF.
    Nov 22, 2021 at 12:03
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    $\begingroup$ I'm surprised nobody's linked to this guy yet. $\endgroup$ Nov 22, 2021 at 15:42
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    $\begingroup$ Lol, I was going to link to bpsspace as well. Is it POSSIBLE? Sure. Is it unreliable? Oh hell yeah. $\endgroup$ Nov 22, 2021 at 19:58

2 Answers 2

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Fundamentally soft landing a solid requires controlling the thrust. This is not totally impossible, as demonstrated by the Nulka system that hovers by adjusting away from the optimal nozzle shape to 'waste' thrust. The burn off of fuel makes the rocket lighter, so to do a hovering descent would involve a very complex grain structure that runs a tapering down thrust on ascent, then had some sort of slow burning layer for the descent and then a ramp up to high thrust again as that burned through shaped to produce a touchdown, assuming you could precisely predict your burn rates (which change with starting ambient temperature, humidity of the grain and pretty much everything other than moon phase).

Precise trimming of solid driven systems is possible by having large numbers of electrically fired chambers, used in the ASM-135 ASAT terminal guidance system , the question at this point is how useful this 'simple' re-usable solid rocket booster is now there is a complex (and probably custom per launch) grain main motor and a very large number of mini trimming rockets all needing reloading for reflight.

It is worth noting that Soyuz in uses solid rockets to cushion touchdown since the presence of the parachute reduces the needed hoverslam precision (steady and known descent rate, stable orientation). It would probably be possible to use a similar system to land a booster, though the lessons of complex solid firings on Credible Sport are probably relevant.

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  • $\begingroup$ +1 more info available in various links at When did soviet capsules begin using retropropulsion immediately before landing? though I'm not sure the solid rocket nature of the retropropulsion is spelled out anywhere there. $\endgroup$
    – uhoh
    Nov 22, 2021 at 9:17
  • $\begingroup$ Of course that trick on the Soyuz only works because it's landing in a dense atmosphere. You couldn't land a Soyuz on the Moon or probably even Mars because the parachute would be useless on the Moon and nearly useless on Mars without the dense atmosphere we have on Earth. $\endgroup$ Nov 22, 2021 at 16:53
  • $\begingroup$ Credible Sport was a failure of the human controlling the rockets; the rocket-braked propulsive landing system itself worked fine. $\endgroup$
    – Vikki
    Nov 23, 2021 at 5:59
  • $\begingroup$ @DarrelHoffman: The specific method used by Soyuz won't work. The basic idea is reusable: solid boosters for the ISP, and a secondary method for fine control $\endgroup$
    – MSalters
    Nov 23, 2021 at 12:56
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Yes disclaimers apply

Others addressed "classic" SRBs that fire continuously, and the near-impossibility of such an endeavor with them. But there is an experimental (though quite matured) technology of Electric Solid Propellant which allows extinguishing and relighting your SRB at will, and even fine-tuning the thrust mid-flight. You will likely lose some of specific impulse vs classic SRBs and you'll need to haul a pretty good battery (or other high-power electric power source) but with this technology your SRB is fully (or partially, if you wish) throttleable, relightable and so on.

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    $\begingroup$ @OrganicMarble If you're willing to call this pretty small... Generally they are experimental, I don't think anything comparable to the space shuttle SRB is planned before smaller ones are fully proven to be flight-worthy, and whatever the results of the spinsat test article were, they aren't published to this day. Additionally, the research could have gone down the military black hole and whatever is being developed is classified - it has a lot of potential for missiles, ability to throttle a missile in flight would be quite valuable. $\endgroup$
    – SF.
    Nov 22, 2021 at 15:18
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    $\begingroup$ @OrganicMarble I'd estimate the motor is comparable to what's found in the Sidewinder missile. Standing next to the assembly, it would reach to your thigh, maybe below waist. $\endgroup$
    – SF.
    Nov 22, 2021 at 16:26
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    $\begingroup$ Thanks. So order of magnitude, 10 kN thrust? $\endgroup$ Nov 22, 2021 at 16:27
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    $\begingroup$ @OrganicMarble Probably. Rather less than more, especially that this test article doesn't have a deLaval nozzle. $\endgroup$
    – SF.
    Nov 22, 2021 at 16:31
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    $\begingroup$ @OrganicMarble On rethinking the issue, it may be considerably lower. In classic SRBs and missiles the propellant burns along the whole missile simultaneously, flame front progressing inside towards outside. ESRBs requiring the constant contact with the electrodes and mostly constant distance between them would make it quite tricky. I can imagine triangular cross-section, length-wise "ribs" as electrodes, with rectangular cross-section bars of propellant between them, but that wastes a lot of volume. Otherwise the flame front would go back-to-front. $\endgroup$
    – SF.
    Nov 25, 2021 at 23:09

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