4 ms·
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 empt
by 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 agoHmm, but on the grasshopper videos it seems to be hovering in air and descending pretty agiley?
- cdash 12y agoGrasshopper is a testing platform.
- Crito 12y agoGrasshopper 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 agoGrasshopper 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 agoFrom 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 agoMerlin 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...