11 ms·

For those wondering, this isn't a simple repeat of CRS8 which landed on a drone ship about a month ago. CRS8 was a mission to low earth orbit which left the fir

by kirrent 10y ago

For those wondering, this isn't a simple repeat of CRS8 which landed on a drone ship about a month ago. CRS8 was a mission to low earth orbit which left the first stage with plenty of fuel to effect a landing. The landing was made as slow as possible and limited only by how low a single engine could be throttled. The re-entry burn which slows the rocket down before the landing burn was also more agressive. This mission was to launch JCSAT-14 to geosynchronous orbit which requires the falcon 9 to move the already heavy satellite into a geosynchronous transfer orbit. That meant the first stage had to do a lot more work and was left with a lot less fuel as a result. Therefore, the re-entry burn was less aggressive, the first stage came in with twice the speed, and SpaceX needed to do a far harder landing with three engines lit in the quicker and riskier suicide burn. Somehow, despite playing down expectations, they managed an even more precise landing than last time.

gpm 10y ago
To expand on this slightly the CRS8 (and OG2, the land landing) landing was done using only 1 engine, which even then was presumably throttled. This landing burn was done using 3 engines so they spend less time below terminal velocity, and thus use less fuel. The total deceleration should be ~40 m/s during the burn.
- deleted 10y ago
  [deleted]
- dsfuoi 10y ago
  So only about 4Gs? I somehow though it would be more and unsurvivable for humans.
  - mikeyouse 10y ago
    Check out the flight profile on Flightclub.io -- I'm not sure how accurate their instantaneous data is but they're showing ~12g acceleration upon landing the booster. Seems more like what I'd expect.
    - pi-err 10y ago
      Impatient to know if the 1st stage is reusable after a 12g phase. The entire concept hangs on this and it looks like we don't have a firm confirmation yet.
      - sandworm101 10y ago
        12g but empty. A structure meant to survive being filled with 10+ times (fuel+upper stage) it's own weight will survive 10+Gs without effort. In other words, the stress of a 12g acceleration isn't much different than the stress of the stage sitting on the pad a few minutes previously. The overall structure actually goes through all sorts of stresses, not all of which are measured in G. The combination of push from the rocket and air resistance compresses the rocket from both ends, something that isn't reflected in G numbers.
        Tarrosion 10y ago
        I am not a rocket scientist, but: I think this is only sort of true. Yes, the fueled rocket is much, much heavier. But not all parts of the rocket are load bearing as it sits on the pad waiting for launch. All parts of the rocket experience the 12g (or whatever it was) deceleration. Also, rocket skins are really, really thin (relative to their size, the forces they endure, etc.) I've read that without the fuel inside to reinforce the rocket, it is quite fragile - think of trying to crush an empty soda can vs a full one. This is part of why rockets which deviate too far from prograde on launch are ripped apart mid-air.
        mikeash 10y ago
        Regarding your last part, some rockets have balloon tanks, which means that the tank requires pressurization to keep its shape. The Falcon 9 has a sort of partial balloon tank. It's strong enough for ground handling without pressurization, but it requires pressurization to handle flight loads. Note however that it's pressure, not fuel, that provides the extra strength. These tanks are still pressurized even when almost empty.
    - 10y ago
- tigershark 10y ago
  40 m/s is a velocity, not an acceleration (deceleration). I guess that you mean 40 m/s^2
  - dkbrk 10y ago
    He did say total deceleration, so I believe he meant the landing burn starts at a velocity of 40 m/s and consequently the total ΔV of the landing burn is 40 m/s. That said, I have no idea where he got that number, whether 40 m/s or 40 m/s^2; both are incorrect. The best estimate available for the flight dynamics can be found at flightclub.io, specifically for JCSAT-14: https://flightclub.io/results/?id=eeaf889e-f3d8-4f9b-be98-b75ea987e8d2&code=JC14 https://flightclub.io/results/?id=eeaf889e-f3d8-4f9b-be98-b7... This shows the landing burn starting at a velocity of 269 m/s and experiencing a peak deceleration of 12.3g = 121 m/s^2 I don't know how accurate these numbers are, but some fans put quite a bit of effort into getting the flightclub.io profiles as close to reality as possible, so they tend to be pretty good.
  - gpm 10y ago
    Oops, you are right, I did miss a ^2. The approximation I had is almost certainly less reliable than flightclubs, not sure why it was so low, will have to look at the math I did and figure out what I screwed up.
downandout 10y ago
From the video it looks like it lands on a ship in rough seas and they do nothing to secure it. Won't it just fall over if the seas are rough enough, and if so, why don't they have some sort of robotic system that locks it down tight the moment it lands?
- robin_reala 10y ago
  All of the centre of gravity is low down with the engines. The rest is basically just a thin empty tube.
  - simonh 10y ago
    Plus the platform has four computer controlled engines on basically universal joints to stabilize it. A crew then boards the barge and puts 'shoes' over the landing legs then welds them to the deck.
  - bkor 10y ago
    To add to above: In the previous barge landing they did weld the landing legs to the barge using some kind of bracket. But they discovered that the centre of gravity is low enough to not need any welding. During the previous attempt the sea was heavier than this time. Maybe they still weld it this time, but that is done by having someone go to the barge and do it.
    - vermontdevil 10y ago
      Actually no they did not weld anything to the legs. Elon said that they realize it wasn't needed. They did use octojack though to support the weight off the legs.
      - tlrobinson 10y ago
        What's octojack? Can't seem to find any other references to that.
        10y ago
vermontdevil 10y ago
And they did this in the dark! ;) (yes I know it does not matter)

by kirrent 10y ago

gpm 10y ago

To expand on this slightly the CRS8 (and OG2, the land landing) landing was done using only 1 engine, which even then was presumably throttled. This landing burn was done using 3 engines so they spend less time below terminal velocity, and thus use less fuel. The total deceleration should be ~40 m/s during the burn.

deleted 10y ago
[deleted]
dsfuoi 10y ago
So only about 4Gs? I somehow though it would be more and unsurvivable for humans.
- mikeyouse 10y ago
  Check out the flight profile on Flightclub.io -- I'm not sure how accurate their instantaneous data is but they're showing ~12g acceleration upon landing the booster. Seems more like what I'd expect.
  - pi-err 10y ago
    Impatient to know if the 1st stage is reusable after a 12g phase. The entire concept hangs on this and it looks like we don't have a firm confirmation yet.
    - sandworm101 10y ago
      12g but empty. A structure meant to survive being filled with 10+ times (fuel+upper stage) it's own weight will survive 10+Gs without effort. In other words, the stress of a 12g acceleration isn't much different than the stress of the stage sitting on the pad a few minutes previously. The overall structure actually goes through all sorts of stresses, not all of which are measured in G. The combination of push from the rocket and air resistance compresses the rocket from both ends, something that isn't reflected in G numbers.
      - Tarrosion 10y ago
        I am not a rocket scientist, but: I think this is only sort of true. Yes, the fueled rocket is much, much heavier. But not all parts of the rocket are load bearing as it sits on the pad waiting for launch. All parts of the rocket experience the 12g (or whatever it was) deceleration. Also, rocket skins are really, really thin (relative to their size, the forces they endure, etc.) I've read that without the fuel inside to reinforce the rocket, it is quite fragile - think of trying to crush an empty soda can vs a full one. This is part of why rockets which deviate too far from prograde on launch are ripped apart mid-air.
        mikeash 10y ago
        Regarding your last part, some rockets have balloon tanks, which means that the tank requires pressurization to keep its shape. The Falcon 9 has a sort of partial balloon tank. It's strong enough for ground handling without pressurization, but it requires pressurization to handle flight loads. Note however that it's pressure, not fuel, that provides the extra strength. These tanks are still pressurized even when almost empty.
  - 10y ago
tigershark 10y ago
40 m/s is a velocity, not an acceleration (deceleration). I guess that you mean 40 m/s^2
- dkbrk 10y ago
  He did say total deceleration, so I believe he meant the landing burn starts at a velocity of 40 m/s and consequently the total ΔV of the landing burn is 40 m/s. That said, I have no idea where he got that number, whether 40 m/s or 40 m/s^2; both are incorrect. The best estimate available for the flight dynamics can be found at flightclub.io, specifically for JCSAT-14: https://flightclub.io/results/?id=eeaf889e-f3d8-4f9b-be98-b75ea987e8d2&code=JC14 https://flightclub.io/results/?id=eeaf889e-f3d8-4f9b-be98-b7... This shows the landing burn starting at a velocity of 269 m/s and experiencing a peak deceleration of 12.3g = 121 m/s^2 I don't know how accurate these numbers are, but some fans put quite a bit of effort into getting the flightclub.io profiles as close to reality as possible, so they tend to be pretty good.
- gpm 10y ago
  Oops, you are right, I did miss a ^2. The approximation I had is almost certainly less reliable than flightclubs, not sure why it was so low, will have to look at the math I did and figure out what I screwed up.

downandout 10y ago

From the video it looks like it lands on a ship in rough seas and they do nothing to secure it. Won't it just fall over if the seas are rough enough, and if so, why don't they have some sort of robotic system that locks it down tight the moment it lands?

robin_reala 10y ago
All of the centre of gravity is low down with the engines. The rest is basically just a thin empty tube.
- simonh 10y ago
  Plus the platform has four computer controlled engines on basically universal joints to stabilize it. A crew then boards the barge and puts 'shoes' over the landing legs then welds them to the deck.
- bkor 10y ago
  To add to above: In the previous barge landing they did weld the landing legs to the barge using some kind of bracket. But they discovered that the centre of gravity is low enough to not need any welding. During the previous attempt the sea was heavier than this time. Maybe they still weld it this time, but that is done by having someone go to the barge and do it.
  - vermontdevil 10y ago
    Actually no they did not weld anything to the legs. Elon said that they realize it wasn't needed. They did use octojack though to support the weight off the legs.
    - tlrobinson 10y ago
      What's octojack? Can't seem to find any other references to that.
      - 10y ago

vermontdevil 10y ago

And they did this in the dark! ;) (yes I know it does not matter)