6 ms·

How high did it get? Does it burn the whole way down (I assume it uses those little X-wings to 'fly down?) When does it start the descent burn? How fast is it g

How high did it get? Does it burn the whole way down (I assume it uses those little X-wings to 'fly down?) When does it start the descent burn? How fast is it going at this point? How much does it weigh, what sort of forces are we talking about? How hard is landing this puppy in terms of human achievement? Top 10% of tricky things humans have so far achieved? [edit] wouldn't it be easier to hover it a few meters above land and grab it with some big robot arms?

sfeng 12y ago
It burns three times after the second stage detaches. The first is to get the landing zone closer to the takeoff, the second slows it down from 1300 m/s to about 250 m/s, the third slows it to (hopefully) 0 m/s on the landing pad. Here's a diagram: http://media2.s-nbcnews.com/i/newscms/2014_51/812491/141217-launchprofile_f4fcb1ed474952c078a75c07a96cae6f.jpg http://media2.s-nbcnews.com/i/newscms/2014_51/812491/141217-... If the 'x-wings' are what I think you're referring to, they're the landing legs. They are shaped like that to have as minimal impact on the air resistance of the ascending rocket as is possible. They open just before landing. It can't hover. Even just using a single one of the nine engines, the engine can't be throttled down enough for the rocket to hover. The landing method relies on what they call a 'hover-slam', where they fire the engine during landing to slow it down just before it touches. Landing this is extremely impressive. It is going four times faster than the speed of sound, and they are landing it on a pad smaller than a football field. Vertical landing of a returning rocket has never been done. It's also the key step in the process of dramatically lowering the cost of spaceflight which could usher in a new era of technology around earth, and on other planets.
- monk_e_boy 12y ago
  Aah, grid fins were the things I was thinking of. That first burn must have some complex maths behind it. Do we know anything of their simulation software?
  - deleted 12y ago
    [deleted]
- 69_years_and 12y ago
  What's the difference between the tests on earth (as seen on youtube etc) where the engine runs the whole time and seems to display good throttle control and this type of landing?
  - Two9A 12y ago
    The Grasshopper tests had a full tank of fuel and (if I recall) additional ballast, to counteract the thrust of the engines. That's what allowed them to hover and perform the various low-velocity translation tests. Of course, these barge landings are the first high-velocity tests, so there'll be kinks to work out.
    - 69_years_and 12y ago
      Thanks, that would explain that. I was impressed by how small the barge looked compared with the F9 it's clever that the F9 can autonomously target such a small area. If they keep targeting 'Just Read the Instructions' by the time land based landings come to be it will be nailed. Awesome stuff!
- swamp40 12y ago
  Maybe it needs to hover? There must be something that can be squeezed to throttle it down some more. Call the NASCAR guys with their restrictor plate designs.
  - TeMPOraL 12y ago
    IMO that landing clearly proved suicide burning is working perfectly fine - they just need to fix something to get rid of the remaining lateral velocity.
teraflop 12y ago
According to http://www.spaceflight101.com/falcon-9-v11.html http://www.spaceflight101.com/falcon-9-v11.html, the first stage has an empty weight of about 20 metric tons.
jccooper 12y ago
Stage separation is somewhere around 100km and Mach 6 or more. The exact figures will depend on the mission profile. There's a "boostback burn" at altitude to slow the horizontal velocity right after separation, an entry burn to put it into the atmosphere, and a landing burn just before it hits the deck. The guidance in between those last two burns is aerodynamic from the grid fins. The first stage mass is ~18,000kg empty (which it nearly is on landing), and it's coming in at its terminal velocity, which is, as a WAG, maybe 70m/s. What's more, it cannot hover, as even one engine has a greater thrust at minimum throttle than the rocket weighs. So it must do what SpaceX calls a "hover-slam", and reduce its velocity vectors to 0 at the same moment its altitude gets to 0 in a single burn. Burn too short, you crash, burn too long, you go back up (and then crash.) And the horizontal vectors also need to be very small, and you can't end up too tilted either, or you fall over. And explode, because you're a thin-shelled tube with plenty of fuel and oxidizer still left. Oh, and you have to hit a very small target within a couple meters. So it's a pretty tricky task. Plenty of rockets can hover these days, but landing a full-sized launch stage coming in from the edge of space is pretty tricky. And they almost got it. Looks like it overreacted just a hair to a gust of wind or something. Your solution has two problems: (1) it cannot hover and (2) you'd have to design and build and test big robot arms. And put them everywhere you want to land. It's probably easier to make your big robot spacecraft, which you already have, do the landing itself on a plain concrete pad.
- will_hughes 12y ago
  According to a (now deleted?) tweet from Elon, it was stiction in a control valve that caused a lag in response time, causing repeated over corrections.
lutorm 12y ago
In general, burning while you descend is a terrible waste of fuel, because you have to generate 1G of acceleration just to keep your speed constant. (See http://en.wikipedia.org/wiki/Gravity_drag. http://en.wikipedia.org/wiki/Gravity_drag.) That's fine if you're not fuel limited (like the grasshopper tests) but if you want to land with minimal use of propellants, you want to use as high acceleration as possible. So the idea is to let the rocket drop freely and then light the engines just early enough that you can stop before hitting the ground.
- deleted 12y ago
  [deleted]

sfeng 12y ago

It burns three times after the second stage detaches. The first is to get the landing zone closer to the takeoff, the second slows it down from 1300 m/s to about 250 m/s, the third slows it to (hopefully) 0 m/s on the landing pad. Here's a diagram: http://media2.s-nbcnews.com/i/newscms/2014_51/812491/141217-launchprofile_f4fcb1ed474952c078a75c07a96cae6f.jpg http://media2.s-nbcnews.com/i/newscms/2014_51/812491/141217-... If the 'x-wings' are what I think you're referring to, they're the landing legs. They are shaped like that to have as minimal impact on the air resistance of the ascending rocket as is possible. They open just before landing. It can't hover. Even just using a single one of the nine engines, the engine can't be throttled down enough for the rocket to hover. The landing method relies on what they call a 'hover-slam', where they fire the engine during landing to slow it down just before it touches. Landing this is extremely impressive. It is going four times faster than the speed of sound, and they are landing it on a pad smaller than a football field. Vertical landing of a returning rocket has never been done. It's also the key step in the process of dramatically lowering the cost of spaceflight which could usher in a new era of technology around earth, and on other planets.

monk_e_boy 12y ago
Aah, grid fins were the things I was thinking of. That first burn must have some complex maths behind it. Do we know anything of their simulation software?
- deleted 12y ago
  [deleted]
69_years_and 12y ago
What's the difference between the tests on earth (as seen on youtube etc) where the engine runs the whole time and seems to display good throttle control and this type of landing?
- Two9A 12y ago
  The Grasshopper tests had a full tank of fuel and (if I recall) additional ballast, to counteract the thrust of the engines. That's what allowed them to hover and perform the various low-velocity translation tests. Of course, these barge landings are the first high-velocity tests, so there'll be kinks to work out.
  - 69_years_and 12y ago
    Thanks, that would explain that. I was impressed by how small the barge looked compared with the F9 it's clever that the F9 can autonomously target such a small area. If they keep targeting 'Just Read the Instructions' by the time land based landings come to be it will be nailed. Awesome stuff!
swamp40 12y ago
Maybe it needs to hover? There must be something that can be squeezed to throttle it down some more. Call the NASCAR guys with their restrictor plate designs.
- TeMPOraL 12y ago
  IMO that landing clearly proved suicide burning is working perfectly fine - they just need to fix something to get rid of the remaining lateral velocity.

teraflop 12y ago

According to http://www.spaceflight101.com/falcon-9-v11.html http://www.spaceflight101.com/falcon-9-v11.html, the first stage has an empty weight of about 20 metric tons.

jccooper 12y ago

Stage separation is somewhere around 100km and Mach 6 or more. The exact figures will depend on the mission profile. There's a "boostback burn" at altitude to slow the horizontal velocity right after separation, an entry burn to put it into the atmosphere, and a landing burn just before it hits the deck. The guidance in between those last two burns is aerodynamic from the grid fins. The first stage mass is ~18,000kg empty (which it nearly is on landing), and it's coming in at its terminal velocity, which is, as a WAG, maybe 70m/s. What's more, it cannot hover, as even one engine has a greater thrust at minimum throttle than the rocket weighs. So it must do what SpaceX calls a "hover-slam", and reduce its velocity vectors to 0 at the same moment its altitude gets to 0 in a single burn. Burn too short, you crash, burn too long, you go back up (and then crash.) And the horizontal vectors also need to be very small, and you can't end up too tilted either, or you fall over. And explode, because you're a thin-shelled tube with plenty of fuel and oxidizer still left. Oh, and you have to hit a very small target within a couple meters. So it's a pretty tricky task. Plenty of rockets can hover these days, but landing a full-sized launch stage coming in from the edge of space is pretty tricky. And they almost got it. Looks like it overreacted just a hair to a gust of wind or something. Your solution has two problems: (1) it cannot hover and (2) you'd have to design and build and test big robot arms. And put them everywhere you want to land. It's probably easier to make your big robot spacecraft, which you already have, do the landing itself on a plain concrete pad.

will_hughes 12y ago
According to a (now deleted?) tweet from Elon, it was stiction in a control valve that caused a lag in response time, causing repeated over corrections.

lutorm 12y ago

In general, burning while you descend is a terrible waste of fuel, because you have to generate 1G of acceleration just to keep your speed constant. (See http://en.wikipedia.org/wiki/Gravity_drag. http://en.wikipedia.org/wiki/Gravity_drag.) That's fine if you're not fuel limited (like the grasshopper tests) but if you want to land with minimal use of propellants, you want to use as high acceleration as possible. So the idea is to let the rocket drop freely and then light the engines just early enough that you can stop before hitting the ground.

deleted 12y ago
[deleted]