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Let's say we have some kind of rockoon design where our rocket gets carried 20 kilometers up before it finally launches from its balloon/blimp platform. Would that maneuver save a significant amount of fuel when compared to sending it form sea level, when you account for its now very slightly lower escape velocity, not having to worry as much about air resistance since 90% of the mass of the atmosphere is behind it, the slightly reduced distance needed to travel and the amount of fuel saved by virtue of not needing to carry as much fuel, as well as other factors?

(Assuming that we are starting from earth and trying to leave orbit in order to reach another planet, but I am curious if only trying to reach orbit or if only trying to reach some kind of space elevator or skyhook would change the percentage of fuel saved and thus how worthwhile flying an extremely large airship up there would be.)

(Yes, I am aware that 20 kilometers is not very far when concerned with space and that most of these would probably only lead to very small changes in the amount of fuel needed, especially the only minor decrease in escape velocity.)

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    $\begingroup$ +1 because it's a great first question! I think there are several answers here already addressing high altitude balloon or aircraft launch savings. There's usually significant reduction in benefits because most schemes for large launch vehicles require the rocket to be horizontal and it uses a lot of propellant turning from horizontal speed to vertical speed, along with some more operational challenges. See posts containing "rockoon" $\endgroup$ – uhoh Feb 26 at 11:58
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    $\begingroup$ Surely one of the many "Why don't we launch rockets from balloons" questions on this site already answers this. $\endgroup$ – Organic Marble Feb 26 at 14:45
  • $\begingroup$ @stoby noting the previous comments, and depending what comes up in previous balloon launch Q&A, you could break the problem into two and concentrate on just the abstract propellant saving from that altitude/lower atmosphere/alternatives to vertical launch rather than worrying about whether it will break even - that can be a separate question. $\endgroup$ – Puffin Feb 27 at 17:21
  • $\begingroup$ Honestly, I think the question as posed is difficult to give a simple answer to. You can sort-of compare the energy for Earth Escape with C3 to Mars vs 20km Up Earth Potential Energy, act like a rocket is X Percent Efficient at adding energy to itself with fuel and get something. After that I think you need more details on the rocket and launch platform and the answer will look more like a trade study. $\endgroup$ – Erin Anne Feb 28 at 3:28

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