5 ms·

Good work! I still think that they should have some grippers on the barge, or a net, ready to spring and grasp the rocket on landing, instead of carrying flimsy

Good work! I still think that they should have some grippers on the barge, or a net, ready to spring and grasp the rocket on landing, instead of carrying flimsy legs all the way to space and back.

olivier_martel 10y ago
They are trying to nail it every time without those grippers because they won't be there on Mars.
ohitsdom 10y ago
The current legs aren't flimsy, and they seem way less complicated than a gripper system would have to be. How would a gripper system handle off-centered landings? It would have to catch the falling rocket very quickly because there's not much fuel left in the tank to hover. Your proposed system would add a lot of complexity, all for the small decrease in launch mass.
- rco8786 10y ago
  > Your proposed system would add a lot of complexity, all for the small decrease in launch mass. While I agree with your overall sentiment, I imagine that people building rockets make complexity tradeoffs to save tiny amounts of weight all the time.
  - JshWright 10y ago
    SpaceX, on the other hand, has been willing to sacrifice some weight in order to reduce complexity and improve reliability and reusability.
- SagelyGuru 10y ago
  Permanent decrease in launch mass is big money, whereas complications on the barge do not really matter. Like all good engineering problems, it only looks problematic at first sight and I am sure that SpaceX could rise up to the challenge. Use a big funnel mesh? However, I accept the comment that perhaps they are really practising for landings on Mars, where fair number of probes have overturned on landing.
  - JshWright 10y ago
    > Use a big funnel mesh? I'm not sure you realize just how fragile the stage is. The side walls are made of _very_ thin aluminum. Any landing support mechanism must put the impact force on the 'octaweb' structure that supports the engines. That's the advantage of the legs. They can be engineered to distribute the force exactly where they want it. A significant problem with any sort of "catching" mechanism is that the load on the stage is far less predictable.
    - andygates 10y ago
      ...or how big it is. That's a 12 storey empty beercan, on fire, hurtling in from space. Any catching hack would require the stage to slow down anyway.
      - Klathmon 10y ago
        [1] is the picture that finally gave me a sense of scale for this thing... [1] http://i.stack.imgur.com/FYvrC.jpg http://i.stack.imgur.com/FYvrC.jpg
- geon 10y ago
  > because there's not much fuel left in the tank to hover They can't hover at all, since the throttle doesn't go low enough. They need to do the landing perfectly, on a single try.
  - fixermark 10y ago
    Interesting. That's a change from the Grasshopper testing platform? If I recall correctly, it was able to hover.
    - doikor 10y ago
      They probably just added weight so that it can hover. For the things actually going into space you don't have that option.
    - david-given 10y ago
      I was all set to post something about how the Grasshopper used a different engine, but apparently both the Grasshopper and the F9 first stage use Merlin 1D engines. Wikipedia says that these produce 700kN at sea level and can be throttled down to 70%, which means at minimum power one engine could hover about 50 tonnes. I can't find out what Grasshopper massed. Does anyone know?
  - ohitsdom 10y ago
    Great clarification, thanks.
- taneq 10y ago
  I'm still surprised there's no active system on the barge to secure the rocket. Even now they seem to be able to stick the landing with some reliability, it's always nice to have belt and suspenders. I'm thinking a system with a pillar at each corner of the barge, pairs of cables between opposite pillars, and puller cables to open out the main cables into an iris-type aperture which can be rapidly closed on the rocket after it lands to keep it upright. Do that at 2-3 heights above the barge and it'll cradle the rocket even in heavy seas.
  - JshWright 10y ago
    The center of mass of the rocket is _very_ low. According to Elon, the CRS-8 core was very stable (it needed even less stabilization than they thought it did). They may add some means to secure it if it needs to travel through rough seas on the way home, but if the conditions are calm enough to land, they're calm enough for the stage to be stable. Watch the CRS-8 landing and look at how much the barge is rolling while the stage is parked on it...
    - omni 10y ago
      The response crew comes and secures the legs on the barge immediately after landing. They bolt them down somehow, I think.
      - Klathmon 10y ago
        they actually weld "shoes" over each of the legs, but Elon even tweeted that it was entirely unnecessary to do on the previous landing.
      - taneq 10y ago
        There's a response crew? Like, people? Are they on a boat nearby? I may have misinterpreted "drone ship". :/
        JshWright 10y ago
        Yes, the Go Quest is nearby with a support crew to deal with the stage after it lands.
JshWright 10y ago
The rocket can land anywhere in a area of a couple hundred square meters. How would you design the "grippers" to work anywhere on the barge? How would you "grab" the stage without punching a hole in it? The landing legs aren't all that heavy, are on the first stage (where the mass penalty isn't as bad), and can be engineered to distribute the force through the structure of the stage that can handle it.
justin66 10y ago
And land the rocket on the bell of the rocket motor? The legs are for more than keeping the rocket from tipping over.

Good work! I still think that they should have some grippers on the barge, or a net, ready to spring and grasp the rocket on landing, instead of carrying flimsy legs all the way to space and back.

olivier_martel 10y ago

They are trying to nail it every time without those grippers because they won't be there on Mars.

ohitsdom 10y ago

The current legs aren't flimsy, and they seem way less complicated than a gripper system would have to be. How would a gripper system handle off-centered landings? It would have to catch the falling rocket very quickly because there's not much fuel left in the tank to hover. Your proposed system would add a lot of complexity, all for the small decrease in launch mass.

rco8786 10y ago
> Your proposed system would add a lot of complexity, all for the small decrease in launch mass. While I agree with your overall sentiment, I imagine that people building rockets make complexity tradeoffs to save tiny amounts of weight all the time.
- JshWright 10y ago
  SpaceX, on the other hand, has been willing to sacrifice some weight in order to reduce complexity and improve reliability and reusability.
SagelyGuru 10y ago
Permanent decrease in launch mass is big money, whereas complications on the barge do not really matter. Like all good engineering problems, it only looks problematic at first sight and I am sure that SpaceX could rise up to the challenge. Use a big funnel mesh? However, I accept the comment that perhaps they are really practising for landings on Mars, where fair number of probes have overturned on landing.
- JshWright 10y ago
  > Use a big funnel mesh? I'm not sure you realize just how fragile the stage is. The side walls are made of _very_ thin aluminum. Any landing support mechanism must put the impact force on the 'octaweb' structure that supports the engines. That's the advantage of the legs. They can be engineered to distribute the force exactly where they want it. A significant problem with any sort of "catching" mechanism is that the load on the stage is far less predictable.
  - andygates 10y ago
    ...or how big it is. That's a 12 storey empty beercan, on fire, hurtling in from space. Any catching hack would require the stage to slow down anyway.
    - Klathmon 10y ago
      [1] is the picture that finally gave me a sense of scale for this thing... [1] http://i.stack.imgur.com/FYvrC.jpg http://i.stack.imgur.com/FYvrC.jpg
geon 10y ago
> because there's not much fuel left in the tank to hover They can't hover at all, since the throttle doesn't go low enough. They need to do the landing perfectly, on a single try.
- fixermark 10y ago
  Interesting. That's a change from the Grasshopper testing platform? If I recall correctly, it was able to hover.
  - doikor 10y ago
    They probably just added weight so that it can hover. For the things actually going into space you don't have that option.
  - david-given 10y ago
    I was all set to post something about how the Grasshopper used a different engine, but apparently both the Grasshopper and the F9 first stage use Merlin 1D engines. Wikipedia says that these produce 700kN at sea level and can be throttled down to 70%, which means at minimum power one engine could hover about 50 tonnes. I can't find out what Grasshopper massed. Does anyone know?
- ohitsdom 10y ago
  Great clarification, thanks.
taneq 10y ago
I'm still surprised there's no active system on the barge to secure the rocket. Even now they seem to be able to stick the landing with some reliability, it's always nice to have belt and suspenders. I'm thinking a system with a pillar at each corner of the barge, pairs of cables between opposite pillars, and puller cables to open out the main cables into an iris-type aperture which can be rapidly closed on the rocket after it lands to keep it upright. Do that at 2-3 heights above the barge and it'll cradle the rocket even in heavy seas.
- JshWright 10y ago
  The center of mass of the rocket is _very_ low. According to Elon, the CRS-8 core was very stable (it needed even less stabilization than they thought it did). They may add some means to secure it if it needs to travel through rough seas on the way home, but if the conditions are calm enough to land, they're calm enough for the stage to be stable. Watch the CRS-8 landing and look at how much the barge is rolling while the stage is parked on it...
  - omni 10y ago
    The response crew comes and secures the legs on the barge immediately after landing. They bolt them down somehow, I think.
    - Klathmon 10y ago
      they actually weld "shoes" over each of the legs, but Elon even tweeted that it was entirely unnecessary to do on the previous landing.
    - taneq 10y ago
      There's a response crew? Like, people? Are they on a boat nearby? I may have misinterpreted "drone ship". :/
      - JshWright 10y ago
        Yes, the Go Quest is nearby with a support crew to deal with the stage after it lands.

JshWright 10y ago

The rocket can land anywhere in a area of a couple hundred square meters. How would you design the "grippers" to work anywhere on the barge? How would you "grab" the stage without punching a hole in it? The landing legs aren't all that heavy, are on the first stage (where the mass penalty isn't as bad), and can be engineered to distribute the force through the structure of the stage that can handle it.

justin66 10y ago

And land the rocket on the bell of the rocket motor? The legs are for more than keeping the rocket from tipping over.