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Wow I can't believe this finally launched. So far so good! And of course, the most exciting thing about this is the return attempt of the F9 first stage core. I

by cryptoz 12y ago

Wow I can't believe this finally launched. So far so good! And of course, the most exciting thing about this is the return attempt of the F9 first stage core. I don't think that'll be covered in the webcast but we'll certainly get video of it later. MECO successful. Edit: r/spacex has good updates regarding the landing attempt [T+27m]: Elon's jet and NASA plane circling a particular area... http://www.reddit.com/r/spacex/comments/2aany2/rspacex_orbcomm_og2_official_launch_discussion/ http://www.reddit.com/r/spacex/comments/2aany2/rspacex_orbco...

vonklaus 12y ago
I wonder if they have tracking planes or a spotter out on the water. After all this bad press and the violent ocean churn from the last attempt, it would be nice to get a money shot of the vehicle hovering over the water.
- JshWright 12y ago
  You mean like this one (from the last launch)? https://www.youtube.com/watch?v=CjZ33C9JZTM https://www.youtube.com/watch?v=CjZ33C9JZTM
- rst 12y ago
  It doesn't actually hover over the water. For that, the rocket's thrust would have to exactly balance the weight of the first stage, with the tanks nearly empty, and even a single Merlin engine can't actually throttle down that low. As long as the engine is firing, the rocket is accelerating up, so the actual landing maneuver is something they call a "hoverslam", which is designed to zero out vertical velocity close enough to the ground that after they cut off the engine, the legs can take up the difference. (Weight conditions at launch are obviously very different. In addition to the weight of the second stage and payload, there's also the weight of the first stage fuel -- and the fuel will be most of the weight of any rocket at launch.)
  - mrfusion 12y ago
    Hmm, but on the grasshopper videos it seems to be hovering in air and descending pretty agiley?
    - cdash 12y ago
      Grasshopper is a testing platform.
    - Crito 12y ago
      Grasshopper can hover for a while because, at least at that stage in its flight, it is heavier than the returning F9 core. As it burns more and more fuel it becomes lighter and lighter, and I believe that towards the end of Grasshopper flights it is too light to hover. The landing that Grasshopper has been doing recently, and which the F9 core will do, has been termed a "hover-slam" by SpaceX. Basically it decelerates rapidly, hitting 0m/s right when it also hits the ground. If it did not cut out the engine right at that point, it would begin going back up (until the fuel ran out).
    - rst 12y ago
      Grasshopper and F9R-1 (their newer test vehicle) are widely believed to fly with extra mass as ballast in order to make them easier to control -- most likely just as extra fuel in the tanks which they don't plan to burn on that flight.
  - trothamel 12y ago
    From what I remember, during the last landing, Elon announced that the Merlin engine can throttle down a lot farther than people though. (IIRC, it can go down to 30%, rather than the 70% figure bandied about in internet rumors.) This means it's possible for the Falcon to do a constant velocity descent, rather than having to do a hoverslam.
    - toomuchtodo 12y ago
      Merlin engines can throttle down to as low as 40%: https://twitter.com/elonmusk/status/462104679116050432 https://twitter.com/elonmusk/status/462104679116050432 http://www.reddit.com/r/spacex/comments/24j5dk/merlin_1d_can_throttle_down_to_40_elon_musk/ http://www.reddit.com/r/spacex/comments/24j5dk/merlin_1d_can...
ChuckMcM 12y ago
Awesome, I noticed they were tracking the landing leg attachment points at liftoff, so perhaps there is a new leg deployment system. Also the note that 'kaboom' after splashdown suggests that some part of the landing mechanism didn't deal well the transition from flight to sailing. One wonders what the temperature differential is between the landing first stage and the ocean, on the one hand it might be hot (burning retro) on the other it could be sub zero (cryogenic propellant usage). Questions then are: 1) What was the mechanism that instituted the rapid disassembly of the landing stage. 2) Would that mechanism come into play on a 'ground' based recovery? 3) What is the safe radius for ground recovery in the event of a structural failure?
- lutorm 12y ago
  Like trothamel commented below, the stage isn't designed for tipping over and slamming into the water.
  - ChuckMcM 12y ago
    My problem with that theory/scenario is that the stage is designed to land on its legs. When the test vehicles land they have very little angular momentum, presumably these tests would mimic that. And if the momementum was managed, then at engine shut off it would drop vertically into the water, and then fall over as the counter pressure of buouancy was not stabilized by the landing legs. If you've ever seen loggers drop tree trunks in a river you can imagine what that looks like, it starts out vertical, then reaches point where it displaces enough water that it is essentially floating (but unstably so) and then rotates to horizontal around the point on the trunk that is at the surface of the water. I totally understand that this isn't an intended landing mode :-). Thinking about how they manage fuel load, it made me wonder how I would do it, do you hover until you've exhausted all fuel? Or do you just shut off the engine? And at flameout are you out of volatiles or just some volatiles (like LOX)and do you have the 'gas tank' issue that auto mechanics deal with where residual vapors can be explosive. Would love to sit through the briefings on this stuff.
    - lutorm 12y ago

by cryptoz 12y ago

vonklaus 12y ago

I wonder if they have tracking planes or a spotter out on the water. After all this bad press and the violent ocean churn from the last attempt, it would be nice to get a money shot of the vehicle hovering over the water.

JshWright 12y ago
You mean like this one (from the last launch)? https://www.youtube.com/watch?v=CjZ33C9JZTM https://www.youtube.com/watch?v=CjZ33C9JZTM
rst 12y ago
It doesn't actually hover over the water. For that, the rocket's thrust would have to exactly balance the weight of the first stage, with the tanks nearly empty, and even a single Merlin engine can't actually throttle down that low. As long as the engine is firing, the rocket is accelerating up, so the actual landing maneuver is something they call a "hoverslam", which is designed to zero out vertical velocity close enough to the ground that after they cut off the engine, the legs can take up the difference. (Weight conditions at launch are obviously very different. In addition to the weight of the second stage and payload, there's also the weight of the first stage fuel -- and the fuel will be most of the weight of any rocket at launch.)
- mrfusion 12y ago
  Hmm, but on the grasshopper videos it seems to be hovering in air and descending pretty agiley?
  - cdash 12y ago
    Grasshopper is a testing platform.
  - Crito 12y ago
    Grasshopper can hover for a while because, at least at that stage in its flight, it is heavier than the returning F9 core. As it burns more and more fuel it becomes lighter and lighter, and I believe that towards the end of Grasshopper flights it is too light to hover. The landing that Grasshopper has been doing recently, and which the F9 core will do, has been termed a "hover-slam" by SpaceX. Basically it decelerates rapidly, hitting 0m/s right when it also hits the ground. If it did not cut out the engine right at that point, it would begin going back up (until the fuel ran out).
  - rst 12y ago
    Grasshopper and F9R-1 (their newer test vehicle) are widely believed to fly with extra mass as ballast in order to make them easier to control -- most likely just as extra fuel in the tanks which they don't plan to burn on that flight.
- trothamel 12y ago
  From what I remember, during the last landing, Elon announced that the Merlin engine can throttle down a lot farther than people though. (IIRC, it can go down to 30%, rather than the 70% figure bandied about in internet rumors.) This means it's possible for the Falcon to do a constant velocity descent, rather than having to do a hoverslam.
  - toomuchtodo 12y ago
    Merlin engines can throttle down to as low as 40%: https://twitter.com/elonmusk/status/462104679116050432 https://twitter.com/elonmusk/status/462104679116050432 http://www.reddit.com/r/spacex/comments/24j5dk/merlin_1d_can_throttle_down_to_40_elon_musk/ http://www.reddit.com/r/spacex/comments/24j5dk/merlin_1d_can...

ChuckMcM 12y ago

Awesome, I noticed they were tracking the landing leg attachment points at liftoff, so perhaps there is a new leg deployment system. Also the note that 'kaboom' after splashdown suggests that some part of the landing mechanism didn't deal well the transition from flight to sailing. One wonders what the temperature differential is between the landing first stage and the ocean, on the one hand it might be hot (burning retro) on the other it could be sub zero (cryogenic propellant usage). Questions then are: 1) What was the mechanism that instituted the rapid disassembly of the landing stage. 2) Would that mechanism come into play on a 'ground' based recovery? 3) What is the safe radius for ground recovery in the event of a structural failure?

lutorm 12y ago
Like trothamel commented below, the stage isn't designed for tipping over and slamming into the water.
- ChuckMcM 12y ago
  My problem with that theory/scenario is that the stage is designed to land on its legs. When the test vehicles land they have very little angular momentum, presumably these tests would mimic that. And if the momementum was managed, then at engine shut off it would drop vertically into the water, and then fall over as the counter pressure of buouancy was not stabilized by the landing legs. If you've ever seen loggers drop tree trunks in a river you can imagine what that looks like, it starts out vertical, then reaches point where it displaces enough water that it is essentially floating (but unstably so) and then rotates to horizontal around the point on the trunk that is at the surface of the water. I totally understand that this isn't an intended landing mode :-). Thinking about how they manage fuel load, it made me wonder how I would do it, do you hover until you've exhausted all fuel? Or do you just shut off the engine? And at flameout are you out of volatiles or just some volatiles (like LOX)and do you have the 'gas tank' issue that auto mechanics deal with where residual vapors can be explosive. Would love to sit through the briefings on this stuff.
  - lutorm 12y ago