5 ms·

Where to start‽ (These are my recollections, perhaps wrong of course): - The wafer thin margin of actual payload that the booster put up now being shared with

Where to start‽ (These are my recollections, perhaps wrong of course): - The wafer thin margin of actual payload that the booster put up now being shared with landing fuel - Engine restart after launch - Re-entry forces on the engines - The fact (perhaps since addressed) that a single Merlin puts out more thrust at near minimum throttle than the dry mass of the booster itself. Think about that last one. If you start the burn too early you'll miss the ground .. for a while. The lesson here is trust calculus!

TeMPOraL 9y ago
> - The fact (perhaps since addressed) that a single Merlin puts out more thrust at near minimum throttle than the dry mass of the booster itself. It's addressed in the sense that they live with it. The rocket still does a suicide burn.
deleted 9y ago
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thaumaturgy 9y ago
I'm not sure I've ever thought it was impossible -- I've learned to be more circumspect about that kind of thing as I'm nearing 40 -- but I tried imagining the fine detail engineering that would be required to do it and was totally overwhelmed. I mean, take a yardstick or short rod or something, and try to balance it upright on the tip of your finger. That's basically what they're doing, but with a -rocket-. It's amazing.
- TeMPOraL 9y ago
  The rocket has its center of mass very low during landing, as it's mostly an empty shell by this point, with heavy engines at the bottom. So take a meter tape, pull it out a bit, lock and try to balance that. Much easier, right? The magic is in a) ensuring the whole construct survives reentry, and b) timing the landing burn - even a single Merlin engine is too powerful for the almost empty rocket to hover, so they need to zero out their velocity at precisely the moment they're hitting the ground. There's no second attempt.
  - thaumaturgy 9y ago
    I decided I had no way to guess on the mass distribution of fuel, engine, and airframe, plus wind shear, so I went with the hard scenario. If anybody knows the numbers for this, I'd love to hear it!
  - adventured 9y ago
    > So take a meter tape, pull it out a bit, lock and try to balance that. Much easier, right? While you drop your hand toward the ground at mach 5.
    - TeMPOraL 9y ago
      This actually helps stabilize the rocket. Lawn dart, and all that ;).
  - Diederich 9y ago
    > even a single Merlin engine is too powerful for the almost empty rocket to hover Indeed. For their last launch, they decided to try landing the booster with three engines rather than one, which brings the whole 'hoverslam' concept to the next level. This was considered so risky that they didn't try landing it on the barge, for fear of damaging it. Instead, they soft landed on the ocean. But much to everyone's surprise, it actually worked! There's a picture of the Falcon 9 booster floating awkwardly. Musk said they'll try to tow it to shore. And all that is really the core of why I love SpaceX: continuous improvement. It's a grand thing!

TeMPOraL 9y ago

> - The fact (perhaps since addressed) that a single Merlin puts out more thrust at near minimum throttle than the dry mass of the booster itself. It's addressed in the sense that they live with it. The rocket still does a suicide burn.

deleted 9y ago

[deleted]

thaumaturgy 9y ago

I'm not sure I've ever thought it was impossible -- I've learned to be more circumspect about that kind of thing as I'm nearing 40 -- but I tried imagining the fine detail engineering that would be required to do it and was totally overwhelmed. I mean, take a yardstick or short rod or something, and try to balance it upright on the tip of your finger. That's basically what they're doing, but with a -rocket-. It's amazing.

TeMPOraL 9y ago
The rocket has its center of mass very low during landing, as it's mostly an empty shell by this point, with heavy engines at the bottom. So take a meter tape, pull it out a bit, lock and try to balance that. Much easier, right? The magic is in a) ensuring the whole construct survives reentry, and b) timing the landing burn - even a single Merlin engine is too powerful for the almost empty rocket to hover, so they need to zero out their velocity at precisely the moment they're hitting the ground. There's no second attempt.
- thaumaturgy 9y ago
  I decided I had no way to guess on the mass distribution of fuel, engine, and airframe, plus wind shear, so I went with the hard scenario. If anybody knows the numbers for this, I'd love to hear it!
- adventured 9y ago
  > So take a meter tape, pull it out a bit, lock and try to balance that. Much easier, right? While you drop your hand toward the ground at mach 5.
  - TeMPOraL 9y ago
    This actually helps stabilize the rocket. Lawn dart, and all that ;).
- Diederich 9y ago
  > even a single Merlin engine is too powerful for the almost empty rocket to hover Indeed. For their last launch, they decided to try landing the booster with three engines rather than one, which brings the whole 'hoverslam' concept to the next level. This was considered so risky that they didn't try landing it on the barge, for fear of damaging it. Instead, they soft landed on the ocean. But much to everyone's surprise, it actually worked! There's a picture of the Falcon 9 booster floating awkwardly. Musk said they'll try to tow it to shore. And all that is really the core of why I love SpaceX: continuous improvement. It's a grand thing!