How does the SpaceX Falcon 9 first-stage straighten for landing?

46

7

I just saw this video of supposed "SpaceX Rocket Camera Landing Footage Fakery" with over a million views.

The narrator repeatedly says it's complete garbage because he doesn't understand and would like to know:

Now, you got a rocket at the bottom, how does this pencil, basically a flying pencil, regain itself and land straight down like this? ...

[How does it] go from that position, I understand the rocket [at the bottom] but what is the mechanism to straighten it out after being basically parallel, and coming straight down? Someone wanna explain that?

Hugo

Posted 2016-05-31T07:59:45.580

Reputation: 436

4Additional factor: the center of gravity is not in the middle, it's right at the bottom where the engine is. That gives a certain amount of drag stabilisation like a shuttlecock.pjc50 2016-05-31T10:45:27.147

24That video is hillariously bad.Journeyman Geek 2016-05-31T11:52:47.870

37That video is a classical case of I don't understand how they do it, so it can't be done. Instead of asking (here) he/she chose to make a video.Jan Doggen 2016-05-31T12:25:35.793

10All you need to know about that video is in the comments. Commenter: "How did you come to believe that the earth is flat?" Video maker: "Watch my other videos. I explain it."ceejayoz 2016-05-31T13:39:55.543

22As someone who played Kerbal Space Program, I can say that the videomaker knows nothing about space or spaceships.Ave 2016-05-31T14:26:22.733

so that guy has no problem with the fact that that "pencil" can be pushed to space vertically (try to do that with a pencil), but can't understand how it comes back?njzk2 2016-05-31T14:39:21.303

3

For what it's worth, the official webcast footage is here.

WBT 2016-05-31T15:11:54.397

1

@ardaozkal: To be fair, in this particular case the fact that it works in KSP doesn't say much, since KSP rockets have magic super-powered reaction wheels that you can do crazy, crazy, crazy, crazy stuff with.

Ilmari Karonen 2016-05-31T17:43:14.660

@IlmariKaronen yeah. KSP ones are indeed crazy, but you can also use cold gas thrusters to stabilize the ship even more.Ave 2016-05-31T17:49:14.460

1I find it quite a mental leap to go from "I don't see how XYZ" to "XYZ is garbage".Kuba Ober 2016-05-31T21:45:06.207

@IlmariKaronen They aren't completely magic, you could get that sort of behavior with a very light structure and heavy reaction wheels. I made a toy RC ball with three reaction wheels that could spin up to their structural limits inside and it worked more-or-less like in KSP. It probably could launch itself off Deimos :)Kuba Ober 2016-05-31T21:55:00.013

10

@JanDoggen: This fallacy has a name: "Argument from personal incredulity". I mention it because it is my favorite fallacy name. http://rationalwiki.org/wiki/Argument_from_incredulity

James 2016-06-01T01:22:10.730

20:30 - "no audio" - LMFAO!Digital Trauma 2016-06-01T15:46:46.263

1

PZ Myers once wrote There are no marching morons. The creator of the cited video makes me think that Myers might be wrong.

David Hammen 2016-06-01T21:16:50.297

Problem: this question is now generating excess views for that video... :)Wilf 2016-06-02T18:16:36.607

Solution: Leave and live comments linking back here!Hugo 2016-06-02T18:22:01.677

Answers

88

Ironically, the answer is in his own (or rather SpaceX's) video.

enter image description here Still from 0:49 of the video showing cold gas thruster firing

The first stage of the Falcon 9 uses a set of nitrogen cold gas thrusters to perform its flip after separation, and you can see them repeatedly firing in the video. As the compressed gas leaves the thruster its pressure drops very quickly, it condenses, and shows up as white puffs as the thrusters fire.

The thrusters are mounted on the interstage at the top of the first stage to provide the highest torque to orient the stage.

enter image description here Thruster group highlighted with red circle

Newton's Third Law is one of those physical laws that is so blatantly obvious that everyone knows its result, but many people can't name it. If you've ever used a CO2 fire extinguisher, you're familiar with the kick as the contents are released, this force can even propel a tricycle or office chair. It is exactly that force from releasing compressed gas that allows the thrusters to operate and flip the stage.

enter image description here

OSHA violation

The first stage has an apogee of around 100km (depending on flight profile). At that altitude there is essentially no atmosphere (0.00003% of ground level ambient pressure), so aerodynamic control surfaces are completely useless for quickly flipping the stage. However, after the entry burn (about 0:53 in the video) the stage rapidly encounters enough atmosphere to start using the grid fins for control.

enter image description here

Grid fins highlighted with red circles

The grid fins operate like any other aerodynamic control surface; they push air one direction, and good ol' Newton provides a force the other direction. For a practical demonstration get hold of a children's swing tennis set.

enter image description here

Future rocket scientists

Often the rackets are a good 20-30mm thick, so if you angle one as you swing you feel a force as the racket moves through the air.

Finally, as the stage performs the landing burn (about 1:00 in the video) we add in the main engines. The stage now becomes essentially an inverted pendulum supported by the firing engine/s. The inverted pendulum is a control problem that humanity has Very. Much. Solved.

(see also How is a rocket stabilized during the initial, slow speed, portion of launch?)

enter image description here Stooping Falcon

During the landing burn all three systems are in use, the engine/s are gimbaling, grid fins rotating, and thrusters firing (not visible in this video, but very obvious in the CRS-6 landing video).

ForgeMonkey

Posted 2016-05-31T07:59:45.580

Reputation: 5 324

Thanks, you can see the thrusters periodically going off during the whole descent. That video was sped up a bit, they're more evident in this normal-speed clip, and especially this slowed-down one.

Hugo 2016-05-31T09:31:29.083

3

Here's a GIF of the thrusters I think the torque is fairly small and is used to adjust the attitude before it hit's the atmosphere. Once that happens, I'd be surprised if those thrusters could have any influence at all compared to the aerodynamics of a cross-sectional area of almost 150 $m^2$ moving at mach speed through the atmosphere!

uhoh 2016-05-31T09:56:22.787

12Thanks for the "Future rocket scientists" caption on one of the pictures. The best way to counter internet silliness is through humour!Andy 2016-05-31T12:58:35.393

2"thrusters firing (not visible in this video...)" What do you call that big orange thing under the pointy thing?Aron 2016-06-01T05:24:06.663

7@Aron I call it a main engine. It is an interesting point of nomenclature where we draw the line between thruster and engine. Usually thrusters are pressure-fed and control rotational or lateral velocity, whereas main engines are pump-fed and provide impulse in only one direction. There are many exceptions, so it's not really worth sticking to any one definition exclusively.ForgeMonkey 2016-06-01T07:37:41.663

4This is an outstanding answer - well written and illustrated.user316117 2016-06-01T20:37:45.840

2

Sadly if the original YouTube poster sees this answer it will only confirm for him that it is fake - because "thrusters don't work in a vacuum because there's nothing to push against". Various flat-earth affiliates try to demonstrate this through woefully misconceived demonstrations, e.g. the balloon car with built-in reverser buckets here https://www.youtube.com/watch?v=CuEXH8YV2R8

Tom Goodfellow 2016-06-02T08:10:48.847

1

@TomGoodfellow I give up.

ForgeMonkey 2016-06-02T08:48:57.287

More like "racket scientists"valepu 2016-06-05T18:00:21.380

28

It's actually a combination of several systems:

  • Nitrogen gas thrusters for attitude control in vacuum (one can see them firing in the video)
  • 4 grid fins (two of them visible in the video) for attitude control during the atmospheric descent
  • Gimbaled engines for attitude control during powered flight

Reference: Falcon 9 User's Guide

oefe

Posted 2016-05-31T07:59:45.580

Reputation: 1 726

While the YouTube Video of SpaceX Thaicom-8 does indeed call the "white puffs" cold nitrogen (a few seconds after 27:44), I'm not sure why they would be visible here in space. Are they becoming liquid nitrogen droplets? (@ForgeMonkey calls them $CO_2$ making "dry ice" crystals). Here is a GIF of it. Do the grid fins do much at "final approach" to the drone ship? I thought they were only effective at fairly high velocity.

uhoh 2016-05-31T09:36:08.163

3Yes. In the cold of space, the nitrogen quickly condenses again, I.e.oefe 2016-05-31T09:43:04.187

Regarding the grid fins, you can see them moving few seconds before the touchdown (even during the final burn). Most clearly seen at around 1:10 in the slow-mo video https://youtu.be/b-yWTH7SJDA

oefe 2016-05-31T09:46:24.927

...plus the temperature drop associated with the expansion itself. Thanks - good to know!uhoh 2016-05-31T09:46:58.620

I think that's like the Tyrannosaurus Rex's front arms moving - they're hooked up and getting signals, but it doesn't mean they are actually doing anything significant. Without high velocity, I don't think those tiny little paddles - that are mostly holes anyway - can generate enough torque to make a difference. I have a feeling that near the end, any significant attitude change comes from the thrust vectoring.uhoh 2016-05-31T09:50:00.557

6

According to https://m.reddit.com/r/spacex/comments/421js4/analysis_of_a_hoverslam_spacex_falcon_9_stage_1/ it decelerates at ~0.7 g. Given that, it would still be ~100 km/h at three seconds before touchdown. So the fins might still contribute to the overall torque until a few seconds before touchdown.

oefe 2016-05-31T16:31:48.507

Instead of speculating, let's find out!! - I've asked the question here.

uhoh 2016-06-01T03:50:38.533