Sorry if this is the wrong place to ask this question, as its very much hypothetical.

In the computer game Kerbal Space Program , there are 3 main Staging Options that are used

  1. Conventional Staging where each stage separates, then reveals the engines above it, the same as used in the Saturn V Rocket.
  2. Onion Staging where an outer tank feeds into an inner tank and when empty is dropped, similar to the crossfeed between the Shuttle and Main Orange Tank
  3. Asparagus Staging where Engines and tanks surround the main rocket, which feed into the rocket beside it around the central core then to the next to the next and then to the central core which above it has the final stage, this setup in KSP allows lifting huge spacecraft into orbit.

Asparagus Staging Diagram

This means that the Fuel and Oxidizer Tanks in S4 actually feed all 7 Engines, until empty and dropped, then S3 feeds all 5 until empty, the dropped, Same with S2, and then S1 is now already very high in the atmosphere traveling very fast but is basically fully fueled and continues on into orbit

This is the meat of the question, Asparagus staging in KSP is easy, just done with magical fuel lines, but in reality, it would take huge pumps and fuel lines and structural supports. so all this added weight would reduce the effective delta-V you could gain from it. but I'm not sure how much

Now I know that without specific information in terms of Mass of the stages etc it's impossible to do the actual maths, but:

The Question

Is this even realistically possible in real life? or would the drawbacks outweigh the benefits

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    $\begingroup$ Not quite a duplicate, but very relevant: space.stackexchange.com/questions/18665/… $\endgroup$ – Lex Nov 6 '18 at 16:47
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    $\begingroup$ It was considered for Falcon Heavy byt eventually abandoned. en.wikipedia.org/wiki/Falcon_Heavy#Propellant_crossfeed $\endgroup$ – Steve Linton Nov 6 '18 at 16:57
  • $\begingroup$ @lex, Cheers, i looked through a couple of the how did the shuttle do it questions. Its genuinely interesting stuff but doesn't really look into if it is actually possible, people just seem to wave it off as not possible without explaining why. $\endgroup$ – Blade Wraith Nov 6 '18 at 16:59
  • $\begingroup$ @SteveLinton, again, cheers, i see options to it often but never any reasons behind why it wasn't done. just it won't be done $\endgroup$ – Blade Wraith Nov 6 '18 at 17:08
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    $\begingroup$ "Staging where an outer tank feeds into an inner tank and when empty is dropped, similar to the crossfeed between the Shuttle and Main Orange Tank" -- Is that true? I thought that the main engines weren't used after ET separation, and instead the OMS (with its separate fuel tanks) was used for orbital injection. $\endgroup$ – Wayne Conrad Nov 6 '18 at 19:18

It's possible, but not as easy in real life as it is in KSP.

To maintain proper tank pressurization, the crossfeeds have to be pump-driven; the fuel and oxidizer crossfeed lines have to be pretty large to move the required amount of propellant (i.e. on the order of the same power as the core section's engine turbopumps). All this increases weight and complexity.

In your proposed design, a minor issue is that the pumping direction of the outer tanks produces a rolling torque on the rocket which has to be countered (via gimbaled engines or other attitude-control mechanisms). No torque is produced by the S2->S1 feeds in your design, so a single-level N-to-1 crossfeed, like Falcon Heavy's proposed 2-to-1, doesn't have to worry about it.

It turns out to be possible to get some of the benefits of asparagus crossfeed by throttling the core engines down while the boosters are running -- the outer tanks thus empty first because they're consuming propellant faster. This is much easier to engineer -- no crossfeed plumbing, just throttlable engines. This is what Falcon Heavy actually does. Installing more or larger engines on the outer boosters would be generally equivalent, as well.

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    $\begingroup$ If you want to empty and drop off S4 first, its tubopumps have to have 4x as much power as the S1 turbopumps (S4 has to be emptied at 4x the rate of S1). $\endgroup$ – Hobbes Nov 6 '18 at 19:04
  • $\begingroup$ 3x for crossfeed, 1x for its own engines, but that's only if you need the core to stay 100% full -- crossfeed can still be useful at less than 100%. Your point is taken, though, and I've edited. $\endgroup$ – Russell Borogove Nov 6 '18 at 19:13
  • $\begingroup$ @RussellBorogove "To maintain proper tank pressurization, the crossfeeds have to be pump-driven" Are you sure this is true? If all the tanks are connected at their bottoms (with large enough connections as to not impede flow), and only the ones you are trying to empty are being fed ullage pressurant, it should cross-feed right?. $\endgroup$ – Lex Nov 6 '18 at 20:40
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    $\begingroup$ Ahhh, okay, the pumps need to move as much propellant, but not achieve the same pressure, as engine turbopumps. $\endgroup$ – Russell Borogove Nov 9 '18 at 12:58
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    $\begingroup$ Yes, pump power is proportional to massflow and pressure difference. You may not need the additional pump if the inducer of the main pump can handle the reduced inlet pressure. Several engines, such as SSME and several Russian engines have a boost pump spinning at a different speed. That allowes them to work down to almost a vacuum. $\endgroup$ – Rikki-Tikki-Tavi Nov 12 '18 at 1:38

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