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Can someone explain exactly what Bezos/Blue Origin did, and how it is different than what SpaceX did? EDIT: Here's a video that compares/contrasts: https://www

• TeMPOraL 11y ago
  This video ain't bad, but it skims over the two most important points and skips one completely: - SpaceX rocket goes really fast horizontally; it has to flip around and boost back, and then correct from horizontal to vertical orientation. - Falcon 9 is an actual space launch system; it has different (i.e. much stricter) engineering constraints than a suborbital rocket. In particular, this affects the engine design. - Most importantly, the first stage of Falcon 9 does what's known as "suicide burn" - it waits until the last possible second to fire back the engine, so that it reaches exactly zero velocity relative to the ground as it touches down. It does so, because it can't hover - even the single one one of the nine Merlin engines, throttled down to minimum, still generates more thrust than the now almost empty booster weights. On the other hand, Blue Origin's rocket can slow down to a hover, and then take its time to stabilize and land.
  • hyperpallium 11y ago
    Why don't they put a smaller engine in the centre for landings?
    • extrapickles 11y ago
      It would make the rocket too heavy. To add another engine, they would have to make the rocket larger in diameter, increasing the weight by a fair amount (they currently have the 9th engine in the middle, so it would have to get moved to the ring with the rest). Reducing the power of the center engine would also reduce the amount of payload to orbit. Rockets that can reach orbit have very bad weight to payload ratios (Falcon 9 is ~2.5% takeoff weight to LEO) so there isn't much room for dead weight.
      • RonaldRaygun 11y ago
        It's actually more about complexity. They could simply replace the center engine with a lower power design and still get plenty of mass to orbit. The issue then is that you have to design a completely new engine and have 2 different engines in your first stage. More complexity generally equals more risk, and historically SpaceX has sought to maximize simplicity for the sake of reliability and rapid iteration.
        • tempestn 11y ago
          And besides, it would be a waste. Landing with the engines they have is "just" a software problem. (Technically a control system problem.) Assuming they can make it work consistently, the save the added cost and inefficiency of a separate landing engine for all future launches.
        • jacquesm 11y ago
          And you suddenly introduce a single point of failure for the return part of the mission.
          • lmm 11y ago
            Can the first stage land with a failed center engine?
            • jacquesm 11y ago
              On a parachute maybe, but then it will still be damaged on landing.
              • lmm 11y ago
                What I meant was I don't think a different-model center engine would be any more of a single point of failure than it already is.
                • 11y ago
    • varjag 11y ago
      You won't be able to adjust thrust vector (and stabilize the landing booster) with just one engine.
      • ceejayoz 11y ago
        SpaceX's Falcon lands with only one engine. The other 8 are off.
      • snaily 11y ago
        The Falcon 9 does not rely on differential thrust - the center engine (and the others, I believe) can gimbal ("rotate").
    • stcredzero 11y ago
      The turbopumps that pump fuel into the main engines are basically mini rocket engines. They even contribute a small amount of thrust. So it's possible that the rocket could be redesigned so that the center return/hover engine could also be a fuel pump for the outer ring of main engines. However, this introduces a single point of failure for the main mission of launching payloads.
  • wereHamster 11y ago
    So, the falcon 9 could, in theory, do multiple attempts (given it has enough fuel)? If the first attempt doesn't work out, pull itself up a bit, correct, and then fall down again?
    • lmm 11y ago
      It doesn't have enough fuel, and keeping more in the booster would mean reducing the payload it could carry. I think most of the ways the landing could go wrong involve the rocket falling over, which wouldn't be recoverable (the fins do nothing at low speed, and the engine can gimbal but only a little bit).
      • mikeash 11y ago
        In addition to fuel, the engines also can't restart an unlimited number of times. Starting the engines requires a special chemical charge to get the fire going, and it only carries as many as it actually needs.
      • scrumper 11y ago
        On-pad airbags to cushion a toppling rocket that's had an otherwise near-perfect approach?
        • lmm 11y ago
          Sometimes the pictures lose the sense of scale - the stage they're landing is something like 20 stories high, and the engines alone weigh around 4 tonnes. Also anything you put on the pad has to survive being torched by a rocket plume as it comes in for landing - even the concrete is pretty scorched. So that kind of system would be pretty hard to implement - and even if it were workable, Musk is mainly interested in landing systems that will work on Mars too.