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View Diary: UPDATED: Quietly, SpaceX makes a revolutionary launch (67 comments)

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  •  ??? (2+ / 0-)
    Recommended by:
    buddabelly, FarWestGirl

    Wow. Not saying it isn't so, but the energetics have got to be crazy. The SpaceX web-site says "return to the launch pad for a vertical landing", so you're almost certainly right.

    Still, by the time the first stage burns out, it's a hundred miles or so downrange and moving very fast. Even though it's substantially lighter than the original stage and payload, the energetics of turning it around and getting it back are unreasonable. And then it would need to be slowed down (again!) and landed.

    It would be hugely easier to launch with downrange recovery targets, using the atmosphere to slow and orient the rocket, using minimal rocket thrust at higher altitudes to put the stage through altitude-velocity-distance windows, and more thrust at lower altitudes to drop it on the target.

    That would constrain the launch and recovery sites considerably, however, as the nation's traditional launch sites have been specifically chosen so that descending hardware can fall safely onto unpopulated areas.

    •  True enough, but (2+ / 0-)
      Recommended by:
      jrooth, Wino

      by that time the stage is almost entirely defueled, so it's very light, especially compared to the amount of thrust the engines can generate. Lack of weight can make up for a lot.

      We are all in the same boat on a stormy sea, and we owe each other a terrible loyalty. -- G.K. Chesterton

      by Keith Pickering on Sun Sep 29, 2013 at 11:53:19 PM PDT

      [ Parent ]

      •  Not to mention... (2+ / 0-)
        Recommended by:
        jrooth, FarWestGirl

        that at that point, it no longer has the weight a stack of stages sitting on top of it.   ;)

        Já þýðir já. Nei þýðir nei. Hvað er svona erfitt við það?

        by Rei on Mon Sep 30, 2013 at 07:16:45 AM PDT

        [ Parent ]

        •  Even so... (0+ / 0-)

          It's still going a goodly fraction of orbital velocity, and energy goes up w/ v^2.

          I did a little more looking, and it's both better and worse than I thought.

          To allow this, SpaceX had to make the second stage considerably more powerful, and have the first stage contribute somewhat less delta-v to the launch. That's not necessarily bad, as it means that more  propellant can be used more efficiently by the second stage. (Once you're out of the atmosphere, the larger, larger-expansion-nozzle engine is more efficient, and it doesn't matter nearly so much how quickly one burns it.)

          Secondly, one thing that SpaceX had to change to permit man-rating launches was to avoid the more-vertical, less-horizontal launches of Atlas and some other rockets. This, so that the ballistic trajectory at any point in the launch after a rocket failure can't plummet too quickly into dense atmosphere, where the g-forces would squash people flat. That makes the delta-v problems for the first stage worse, since it increases the downrange velocity, and it can't use the atmosphere for braking downrange motion.

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