# Continuing discussion of SpaceX launches \[edited title\] **URL:** \ **Category:** Miscellaneous and Personal Stuff I Must Share\ **Created:** [December 22, 2015, 12:37am UTC](https://boards.straightdope.com/t/continuing-discussion-of-spacex-launches-edited-title/741048 "2015-12-22T00:37:01Z")\ **Posts on this page:** 20\ **Page:** 9
**Author:** ![N9IWP](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/n9iwp/32/3154_2.png) [@N9IWP](https://boards.straightdope.com/u/N9IWP)\ **Post date:** [April 8, 2016, 8:54pm UTC](https://boards.straightdope.com/t/continuing-discussion-of-spacex-launches-edited-title/741048/161 "2016-04-08T20:54:40Z")
Successful landing! Watched at live from T-15min or so Brian ---
**Author:** ![Shadez](https://avatars.discourse-cdn.com/v4/letter/s/90ced4/32.png) [@Shadez](https://boards.straightdope.com/u/Shadez)\ **Post date:** [April 8, 2016, 8:55pm UTC](https://boards.straightdope.com/t/continuing-discussion-of-spacex-launches-edited-title/741048/162 "2016-04-08T20:55:38Z")
That was awesome 🙂 ---
**Author:** ![Sarabellum](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sarabellum/32/3492_2.png) [@Sarabellum](https://boards.straightdope.com/u/Sarabellum)\ **Post date:** [April 8, 2016, 8:57pm UTC](https://boards.straightdope.com/t/continuing-discussion-of-spacex-launches-edited-title/741048/163 "2016-04-08T20:57:20Z")
Nailed it!!! That was a thing of beauty! ---
**Author:** ![Pork\_Rind](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pork_rind/32/111_2.png) [@Pork\_Rind](https://boards.straightdope.com/u/Pork_Rind)\ **Post date:** [April 8, 2016, 8:58pm UTC](https://boards.straightdope.com/t/continuing-discussion-of-spacex-launches-edited-title/741048/164 "2016-04-08T20:58:02Z")
That was so slick I thought I was watching an animation at first. ---
**Author:** ![Chisquirrel](https://avatars.discourse-cdn.com/v4/letter/c/6bbea6/32.png) [@Chisquirrel](https://boards.straightdope.com/u/Chisquirrel)\ **Post date:** [April 8, 2016, 9:02pm UTC](https://boards.straightdope.com/t/continuing-discussion-of-spacex-launches-edited-title/741048/165 "2016-04-08T21:02:15Z")
This was the first SpaceX launched I’ve actually had the chance to watch. That was awesome! ---
**Author:** ![usedtobe](https://avatars.discourse-cdn.com/v4/letter/u/ec9cab/32.png) [@usedtobe](https://boards.straightdope.com/u/usedtobe)\ **Post date:** [April 8, 2016, 9:07pm UTC](https://boards.straightdope.com/t/continuing-discussion-of-spacex-launches-edited-title/741048/166 "2016-04-08T21:07:16Z")
Phhhhht! They couldn’t even hit the silly bulls-eye. What a bunch of losers! I take it the barge is now on course back to land, and it is the motion which was creating the waves fore and aft. ---
**Author:** ![MEBuckner](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mebuckner/32/2896_2.png) [@MEBuckner](https://boards.straightdope.com/u/MEBuckner)\ **Post date:** [April 8, 2016, 9:08pm UTC](https://boards.straightdope.com/t/continuing-discussion-of-spacex-launches-edited-title/741048/167 "2016-04-08T21:08:21Z")
Man–that was a beautiful landing! > [@Sarabellum1976](#): > > I’ve got a good feeling about today’s barge landing. I think they’ll nail it. Any tips on the next Powerball numbers? ---
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [April 8, 2016, 9:10pm UTC](https://boards.straightdope.com/t/continuing-discussion-of-spacex-launches-edited-title/741048/168 "2016-04-08T21:10:53Z")
Awesome! ---
**Author:** ![Baron\_Greenback](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/baron_greenback/32/48_2.png) [@Baron\_Greenback](https://boards.straightdope.com/u/Baron_Greenback)\ **Post date:** [April 8, 2016, 9:12pm UTC](https://boards.straightdope.com/t/continuing-discussion-of-spacex-launches-edited-title/741048/169 "2016-04-08T21:12:04Z")
Beautiful, just beautiful. Well done SpaceX. ---
**Author:** ![Folacin](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/folacin/32/3195_2.png) [@Folacin](https://boards.straightdope.com/u/Folacin)\ **Post date:** [April 8, 2016, 9:15pm UTC](https://boards.straightdope.com/t/continuing-discussion-of-spacex-launches-edited-title/741048/170 "2016-04-08T21:15:47Z")
> [@Elon Musk](#): > > Everyone said I was daft to build to land a rocket on a barge, but I did it all the same, just to show them. It blew up. So I tried a second one. That one blew up. So I built a third. That one hit the barge, fell over, then blew up. But the fourth one stayed up. Ok - the count is off one . . ---
**Author:** ![Novelty\_Bobble](https://avatars.discourse-cdn.com/v4/letter/n/a3d4f5/32.png) [@Novelty\_Bobble](https://boards.straightdope.com/u/Novelty_Bobble)\ **Post date:** [April 8, 2016, 9:31pm UTC](https://boards.straightdope.com/t/continuing-discussion-of-spacex-launches-edited-title/741048/171 "2016-04-08T21:31:52Z")
Watched the live stream, we are actually on holiday in Florida but couldn’t get to see the launch. No matter. The live stream actually gives you a more complete view of what is going on. And yes, that barge landing is so incredibly cool. It looked really choppy out there but at first glance it seemed textbook. I’m glad we have a world where eccentric billionaires see fit to live out their fantasies. ---
**Author:** ![Baron\_Greenback](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/baron_greenback/32/48_2.png) [@Baron\_Greenback](https://boards.straightdope.com/u/Baron_Greenback)\ **Post date:** [April 8, 2016, 9:44pm UTC](https://boards.straightdope.com/t/continuing-discussion-of-spacex-launches-edited-title/741048/172 "2016-04-08T21:44:55Z")
> [@Novelty\_Bobble](#): > > I’m glad we have a world where eccentric billionaires see fit to live out their fantasies. Well, until one of them builds a secret undersea lair and starts capturing nuclear submarines ---
**Author:** ![AndrewL](https://avatars.discourse-cdn.com/v4/letter/a/3d9bf3/32.png) [@AndrewL](https://boards.straightdope.com/u/AndrewL)\ **Post date:** [April 9, 2016, 12:56am UTC](https://boards.straightdope.com/t/continuing-discussion-of-spacex-launches-edited-title/741048/173 "2016-04-09T00:56:28Z")
There was a lot of fire visible between the engine bells and even licking up one side of the rocket at one point in this launch. I know it’s just fuel-rich turbopump exhaust recirculating in the wake behind the rocket, but it’s still alarming to see. I thought it was going to crash when I saw how far off vertical it was during the landing, but they pulled it out. Apparently it was coming down in a pretty strong crosswind, which just makes this all the more impressive. ---
**Author:** ![2gigch1](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/2gigch1/32/3313_2.png) [@2gigch1](https://boards.straightdope.com/u/2gigch1)\ **Post date:** [April 9, 2016, 12:57am UTC](https://boards.straightdope.com/t/continuing-discussion-of-spacex-launches-edited-title/741048/174 "2016-04-09T00:57:08Z")
It’s nice to see space advancement - in this case SpaceX’s incremental engineering advances - in real time and with the excitement and joy of others. I barely remember something neat happened in 1969 (I was 3) and through the Shuttle era I still had faith in something wonderful was ahead, but the last few years were rough. I’m excited again for a lot of reasons. I recommend watching the weekly webcast TMRO which looks at the advancement in space science and exploration. It’s one of the highlights of my week. https://www.youtube.com/embed/videoseries?list=PLkyWSQYV0A5Mya7MnpbrIBrKUNVx87VrZ&wmode=transparent&rel=0&autohide=1&showinfo=1&enablejsapi=1 ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [April 9, 2016, 1:09am UTC](https://boards.straightdope.com/t/continuing-discussion-of-spacex-launches-edited-title/741048/175 "2016-04-09T01:09:03Z")
> [@AndrewL](#): > > There was a lot of fire visible between the engine bells and even licking up one side of the rocket at one point in this launch. I know it’s just fuel-rich turbopump exhaust recirculating in the wake behind the rocket, but it’s still alarming to see. > > I thought it was going to crash when I saw how far off vertical it was during the landing, but they pulled it out. Apparently it was coming down in a pretty strong crosswind, which just makes this all the more impressive. Fire on the back end of the rocket stage is pretty common; on the cryogenic Delta IV it is common to see the entire back end consumed as hydrogen leaks out and combusts with some of the protective insulation. It’s not really a big deal, however, as everything on the aft end is either designed to withstand that heating or is protected by well-designed heat shields and thermal blankets. It is only a problem when those protective measures go wrong, as the have sometimes done, resulting in a burnthrough on a GNC cable or loss of hydraulic line on an actuator, and then it’s a bad day. I was actually more impressed that they were able to remain stable on the barge after landing in what was clearly WMO Sea State 5-6 conditions. You can see large scale whitecapping and the barge visibly rocking. I’ve been concerned about the integrity and reliability of the landing legs, especially after some of the problems they’ve had with them, but this bird appeared very steady so they seemed to have worked out any issues. It’s not the way I would design a landing leg system, but it seems to work now. Stranger ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [April 9, 2016, 1:13am UTC](https://boards.straightdope.com/t/continuing-discussion-of-spacex-launches-edited-title/741048/176 "2016-04-09T01:13:10Z")
> [@Stranger\_On\_A\_Train](#): > > The bigger challenge is navigation. The position of a land-based landing site is known to high precision and can be arbitrary in size. A barge, on the other hand (or rather, the water it sits in) is moving and its position needs to be constantly transmitted to the landing vehicle so that it can navigate to the landing site. To add a bit–during most launches you can hear a phrase along the lines of “recovery ship has AOS”. AOS is “acquisition of signal” and means that the drone ship has successfully set up a telemetry link to transmit its coordinates and other information to the landing vehicle. You generally hear this just a bit before stage separation. Although it is tricky, SpaceX has made this part of the landing look easy. In every instance (aside from the first ones where they didn’t have the grid fins), they have hit their target within several meters. In most cases this meant impacting the drone ship itself, but in others it meant a “soft landing” over water, but still in the expected position. This is quite astonishing because of the rather harsh conditions along the way. The most difficult part is likely the reentry burn, since the vehicle is entering the atmosphere hypersonically and doing a precision burn. _No one_ has done this before. There have been some supersonic retropropulsion tests in wind tunnels before, but only at low thrust levels and none on a real vehicle. SpaceX has been successful on every one of these; I’ve lost count, but it’s 8 or 10 at this point. > [@Stranger\_On\_A\_Train](#): > > they do have ACS thrusters up top, but they’re really intented to reorient the stage during initial reentry and don’t have enough thrust or control authority to correct for a gross failure upon final approach, and in fact if the vehicle had a significant yaw or pitch rate they would likely limit cycle and use up all of the gas before landing was complete. As you can see [from one of their previous attempts](https://www.youtube.com/watch?v=4cvGGxTsQx0). The RCS tries, but it’s no match for gravity. Wouldn’t have been enough even if there were infinite RCS propellant. If the stage hasn’t nulled displacement, velocity, rotation, and angular velocity at touchdown, there’s not much they can do. ---
**Author:** ![AndrewL](https://avatars.discourse-cdn.com/v4/letter/a/3d9bf3/32.png) [@AndrewL](https://boards.straightdope.com/u/AndrewL)\ **Post date:** [April 9, 2016, 1:13am UTC](https://boards.straightdope.com/t/continuing-discussion-of-spacex-launches-edited-title/741048/177 "2016-04-09T01:13:31Z")
They’ve already released the on-board video of the landing: > No kidding about that wind, you can see the entire rocket actually slid sideways after touching down and shutting off the engine. ---
**Author:** ![The\_Niply\_Elder](https://avatars.discourse-cdn.com/v4/letter/t/8e8cbc/32.png) [@The\_Niply\_Elder](https://boards.straightdope.com/u/The_Niply_Elder)\ **Post date:** [April 9, 2016, 1:49am UTC](https://boards.straightdope.com/t/continuing-discussion-of-spacex-launches-edited-title/741048/178 "2016-04-09T01:49:17Z")
I’m just gonna leave this here (may include a little crude rapping) ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [April 9, 2016, 7:51am UTC](https://boards.straightdope.com/t/continuing-discussion-of-spacex-launches-edited-title/741048/179 "2016-04-09T07:51:59Z")
[Some 4k video of the landing](https://www.youtube.com/watch?v=sYmQQn_ZSys). I did a bit of pixel math on the video. The rocket enters the top of the frame at 5.2 s and touches down at 13.4. The rocket is 48 meters from base to tip, and the frame is 3.46x the rocket height. From this, we use d=0.5at[sup]2[/sup] to infer an acceleration of 4.94 m/s[sup]2[/sup]. Done a slightly different way, we can see that the rocket takes about 1 second to pass through the top plane, and passes halfway through at 5.73 s. Therefore it takes (13.4-5.73)=7.67 s to decelerate from 48 m/s, for 6.25 m/s[sup]2[/sup] Obviously these numbers are fairly crude, but they do seem to indicate that the vehicle was decelerating at about half a gee (1.5 gees if you include gravity) on average. I’d suppose that the peak is somewhat higher. ---
**Author:** ![Fear\_Itself](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fear_itself/32/19637_2.png) [@Fear\_Itself](https://boards.straightdope.com/u/Fear_Itself)\ **Post date:** [April 9, 2016, 2:44pm UTC](https://boards.straightdope.com/t/continuing-discussion-of-spacex-launches-edited-title/741048/180 "2016-04-09T14:44:37Z")
So what happens next? How is the first stage recovered from the droneship? Does it autopilot back to Cape Canaveral, as the term “droneship” would imply? Or is there some kind of recovery ship with a crane or gantry to move the first stage to the ship? [Previous page](https://boards.straightdope.com/t/continuing-discussion-of-spacex-launches-edited-title/741048.md?page=8) [Next page](https://boards.straightdope.com/t/continuing-discussion-of-spacex-launches-edited-title/741048.md?page=10)