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Space X falcon 9. Why return to the launch site?

  1. Jan 1, 2016 #1

    DTM

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    I don't know that many details about the latest Falcon 9 v1.1 launch and return landing, but I know the 1st stage returned and landed near the launch site. I can't understand why this makes any sense. I would think that would take a huge amount of fuel to completely change directions to get back to where they started. I would think simply deploying a parachute and letting a ship pick up the rocket down range would have saved a huge amount of fuel/weight. Unless the 1st stage made a complete orbit around the earth, which I don't believe is very likely for a 1st stage rocket.
     
  2. jcsd
  3. Jan 1, 2016 #2
    You have a good point, though it seems you've missed Space X's objective. They wanted to prove that a standing landing was possible. This is meant to reduce the cost of space travel for future missions. Traditionally, after ejection, the first stage would just land somewhere in the ocean to be recovered, which would damage the hardware and force the need of replacement parts. Piloting to some random strip of land could prove more trivial when you think about where that would be and the cost of buying out an area big enough. On top of that you would then need to send out a crew to retrieve it and transport it back for future missions further increasing the cost.
     
  4. Jan 1, 2016 #3

    DTM

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    OK, so if their main objective was to just prove they could land vertically, they did that very impressively. I'd think for the long term plan, this technique could make a lot of sense if the launch pad was somewhere where there was a good landing site the appropriate distance down range.
     
  5. Jan 2, 2016 #4
    It could make sense, but then assuming the launch is from US mainland, there are not very many handy islands to land on in the west Atlantic.
    (Well there is Cuba and neighbours hypothetically)
     
  6. Jan 2, 2016 #5

    DTM

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    I wonder if taking off from the southern coast of Texas and landing on the west coast of Florida has been considered...
     
  7. Jan 2, 2016 #6
    That's an interesting concept, but there's still retrieval and transportation.
     
  8. Jan 4, 2016 #7
    My understanding is that for light payloads, landing back at Cape Canaveral is the most practical solution. Heavier payloads can be achieved by landing on a barge in the Atlantic, but that requires calm seas at the time of launch, so launch conditions are more particular, and there is the extra expense of the barge, and an increased risk if the barge is moved or tilted by the ocean at the last second.
     
  9. Jan 4, 2016 #8
    Yes, it was just the first step. The next step is to proof that reusing the stage is safe and efficient.
     
  10. Jan 4, 2016 #9
    That's some 4000 to 5000 mile distance. The 1st stage would have to be aloft for some much greater time for such a scenario.

    Data on range-altitude is not all that easy to find for the launch. Perhaps on some of the videos it shows some velocities and times, range, altitude.

    First stage separation would be about Mach 6 to 10, altitude 60 miles and range perhaps between 50 to 100 miles ( I am assuming). At seperation, gravity and air drag would be the forces on the component. Gravity would have an affect upon the vertical velocity. Air drag would have an affect upon the vertical velocity AND the horizontal velocity. In the meantime, the earth is still rotating under the component. At some point, during descent most likely, where the air becomes more thick, the horizontal velocity will more match the rotational velocity of the earth, as a guess about 10 to 20 miles up.

    At that point, there is a fuel penalty to pay for horizontal motion of the stage and to re-cover the range distance to launch site.
    ( The range here is not the same range at seperation - it could be less than or greater than, depending upon flight criteria. Note that if one launches fully vertical, the drop point would be somewhere west of the launch pad. For the vehicle to remain above the lanch pad as it ascends, as seen from the launch pad, some horizontal motion must be applied to the vehicle to give it a "boosted" velocity in the eastward direction just to keep up with the earth at the vehicles higher altitude)

    Another fuel penalty to pay would be for the controlled vertical descent, where the component has to decelerate from some vertical velocity downward ( air resistance would take it down lower from the Mach 6 at seperation - not sure of the value when they start controlled descent with the engines) against the pull of gravity, to a zero velocity at the earth's surface.

    No doubt that more fuel is needed for recoverable components with controlled descent, than with throw away components.
    No doubt that the flight path to launch a payload most efficiently is different than for the flight path to most efficiently recover the component back at the launch site.
    Somewhere in between is a sweet spot for least amount of fuel to do both, when one trades one against the other.

    The extra cost of fuel per launch then has to be comparable to the cost savings of re-use over several launches to break even. If you can re-use multiple times than one could show a real cost savings over the lifetime of the re-usable component(s).
     
  11. Jan 4, 2016 #10

    sophiecentaur

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    Can someone tell me why they chose a vertical landing when they have already established that the shuttle could be flown back to the ground? I appreciate the drag of a wing could be a factor during takeoff and launch but landing vertically must use a vast amount of fuel under g. If you could glide / fly home, it would not be hard to fly westwards, back to the cape on a nearly empty fuel tank (would it?).
     
  12. Jan 4, 2016 #11
    1. Shuttles need a runway.
    2. The space shuttle wasn't really cheap after all.
     
  13. Jan 4, 2016 #12

    sophiecentaur

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    Why would a runway be a problem?
    Which part of the space shuttle mission was the costly bit? I didn't think it was the flying feature that made it cost so much. Only the main vehicle was reusable. The expensive launch system was wasted, ifaik. I know the economics can't be simple for a project like that.
     
  14. Jan 4, 2016 #13
  15. Jan 4, 2016 #14
    It limits the landing sites to existing runways with sufficient length. Vertical landing is possible almost everywhere.

    There were a lot of reasons for the costs and the flying feature was one of it. It makes the vehicle asymmetric, more complicate and increases air resistance.

    The boosters were reusable too.
     
  16. Jan 4, 2016 #15

    sophiecentaur

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    Oh yes, I remember. Did they always manage to recover them? I guess the refurb would have cost a lot - all that seawater in everywhere.

    A controlled landing, anywhere, would be an attractive feature but, from the above comments about recovery and transport back to the cape, there's a bit of swings and roundabouts at work.
     
  17. Jan 4, 2016 #16
  18. Jan 4, 2016 #17
    The features that made it possible for the Space Shuttle to land added a great deal of weight, all of which reduced the payload to orbit. Vertical landings eliminate that weight. An example: instead of a heat shield, the Falcon 9 uses exhaust; the landing struts, steering paddles, and hydraulic fluids mass much less that the wings of the Shuttle. Yes, there is a penalty paid in fuel, but it is modest; consider, less than 20% of what the Shuttle put into orbit was payload.
    Why return to land? As sophiecentuar mentions salt water is an extremely hostile fluid and would force much more extensive refurbishment. Why the launch site? All of the heavy handling gear is there, as is the assembly building.
    For the Falcon Heavy, launching from Vandenburg or Texas and landing the center core in Texas or Cape Canaveral makes sense. A CC launch would either require landing on one of the barges or writing off the core.
     
  19. Jan 4, 2016 #18

    mfb

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    Sort of. It was cheaper than building completely new boosters, but they needed months of repairs and exchange of components before they could be used again.

    Landing on a ship would save fuel, sure. But what would you gain? The Falcon 9 has a fixed size, SpaceX cannot launch a smaller rocket. Fuel costs are negligible (less than 1% of the total cost of a launch). Landing on land close to the launch site has a much higher success rate (stable, larger surface) and saves transportation costs. The relatively high target orbit of ~600 km and the oversized rocket (compared to payload mass) allowed to use a very steep launch profile, so the downrange distance of the first stage was quite limited.
    For heavier payloads, SpaceX still plans to use the drone ship, or even a fully expendable rocket if necessary.
     
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