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There are lots of questions on here which ask "Why aren't rockets launched from railguns/mountains/balloons/etc." and lots of nice answers mainly dealing with the practical difficulties of doing so, which I fully appreciate.

I'd like to set that aside though, and just ask what benefits there would be for orbital launch if you could (for the sake of argument cost-free) start at various combinations of altitude up to 30km or so, and velocity up to 1 km/s or so (relative to the ground and in the direction of your choice)? Among the things that could be included in an answer if anyone can quantify them:

  1. Reduced Delta-V from altitude/gravity drag/iniitial velocity
  2. Lower initial thrust requirements (because you can choose to loose a little altitude if you like
  3. Possibility of lighter (or no) payload fairing
  4. Lower acoustical issues
  5. Possibility of using engines with something closer to vacuum nozzles
  6. Less complexity with engine ignition (no need for hold-down clamps etc.)
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    $\begingroup$ If you really want to, you can SSTO from current launch sites on earth. One planned SSTO spaceship is Skylon. It is not a question whether SSTO is possible but whether it's more cost-effective. $\endgroup$
    – DaGroove
    Commented Jul 24, 2018 at 11:36
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    $\begingroup$ Lots of people have planned SSTO. No one has yet delivered it. Skylon is still basically testing components of their engine. $\endgroup$
    – Steve Linton
    Commented Jul 24, 2018 at 11:55
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    $\begingroup$ @SteveLinton It's been done several times. Just not on Earth. space.stackexchange.com/questions/12552/… $\endgroup$
    – Organic Marble
    Commented Jul 24, 2018 at 12:37
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    $\begingroup$ @OrganicMarble granted. SSTO from say Phobos isn't too hard. $\endgroup$
    – Steve Linton
    Commented Jul 24, 2018 at 12:47
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    $\begingroup$ I think a better way would be to just ask for the differences between two hypothetical rockets. Payload is a 1 ton mid-sized communications satellite to 600 km altitude. One from Earth's surface, one from a hypothetical platform at 30 km altitude, both at the equator. With just those two cases, it would be easier to estimate velocities of each versus altitude, and make better educated guesses as to the differences in nozzles and fairings, etc. It needs structure, otherwise there's no way the correct answer can be up-voted and selected by users the way SE is supposed to be used. $\endgroup$
    – uhoh
    Commented Jul 24, 2018 at 14:21

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"You save a bit less than 2 minutes of rocket"

or "half a stage less"

or "You can cut the boosters"

One simple way to go about this is just to see how much of a rocket is spent reaching the initial conditions you are providing for free.

  • The Saturn V S-IC stage cuts off after 2.5 minutest at 67km and 2,300m/s
  • Space Shuttle SRB separation happens after 2 minutes at 44km
  • Soyuz booster separation happens after 2 minutes at 48km and 1,800m/s

As is evident, the Saturn V first stage overshoots your initial conditions by a factor of two, and the boosters, while not doing the full job by themselves, are still notably overshooting.

If we cut these initial parts of the systems, and released the remaining parts at the specified altitude and velocity, the remaining stages would reach space the normal way.

These cut-down systems should also be a pretty good equivalent for what's needed, since they are optimised to be deployed at that speed an altitude to begin with.

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