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>It turned out that parachutes are great for soft uncontrolled landings, but if you want a soft controlled landing you need some form of propulsion. They could

by gozur88 10y ago

>It turned out that parachutes are great for soft uncontrolled landings, but if you want a soft controlled landing you need some form of propulsion. They could do something like the Russians do with Soyuz - small solid rocket boosters that fire right at landing. But yeah, that's a lot more things that have to go right.

simonh 10y ago
Also solid rocket motors are a lot less weight efficient than liquid fuelled rockets and are not throttleable and are awkward to gimble. Also where would you put them without increasing air resistance on the way up? Also even with the rocket motors Soyuz hits the ground pretty hard.
- gozur88 10y ago
  You could put them around the rocket nozzles. They wouldn't have to be very big. And yes, Soyuz hits pretty hard. But Soyuz was developed over fifty years ago. I'll bet we could do better today.
  - simonh 10y ago
    Isn't that what SpaceX have already done? I'm not sure what advantage additional solid landing rockets would provide over just slightly bigger fuel tanks for the main engines.
    - gozur88 10y ago
      As I understand it, the F9 first stage can't actually hover because a single engine at the lowest setting gives the overall stage a TWR > 1. Thus the "hoverslam" - they calculate the landing burn such that the stage reaches 0 velocity and 0 altitude (from the pad) at the same instant. But this means the only margin for error is in the landing legs, and they've had some hard landings. I was originally thinking about solid rockets in context of a parachute system, but the more I think about the more I believe truly gentle landings aren't really a feature of the current design. I'm not sure how much weight it would add, but it seems like having the ability to fine-tune the thrust at the very end would allow them to land much more gently and feel better about not introducing metal fatigue or other damage. Of course the addition of a smaller LF engine probably makes the most sense.

by gozur88 10y ago

simonh 10y ago

Also solid rocket motors are a lot less weight efficient than liquid fuelled rockets and are not throttleable and are awkward to gimble. Also where would you put them without increasing air resistance on the way up? Also even with the rocket motors Soyuz hits the ground pretty hard.

gozur88 10y ago
You could put them around the rocket nozzles. They wouldn't have to be very big. And yes, Soyuz hits pretty hard. But Soyuz was developed over fifty years ago. I'll bet we could do better today.
- simonh 10y ago
  Isn't that what SpaceX have already done? I'm not sure what advantage additional solid landing rockets would provide over just slightly bigger fuel tanks for the main engines.
  - gozur88 10y ago
    As I understand it, the F9 first stage can't actually hover because a single engine at the lowest setting gives the overall stage a TWR > 1. Thus the "hoverslam" - they calculate the landing burn such that the stage reaches 0 velocity and 0 altitude (from the pad) at the same instant. But this means the only margin for error is in the landing legs, and they've had some hard landings. I was originally thinking about solid rockets in context of a parachute system, but the more I think about the more I believe truly gentle landings aren't really a feature of the current design. I'm not sure how much weight it would add, but it seems like having the ability to fine-tune the thrust at the very end would allow them to land much more gently and feel better about not introducing metal fatigue or other damage. Of course the addition of a smaller LF engine probably makes the most sense.