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

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?

• That video is hillariously bad. – Journeyman Geek May 31 '16 at 11:52
• That 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 May 31 '16 at 12:25
• All 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 May 31 '16 at 13:39
• As someone who played Kerbal Space Program, I can say that the videomaker knows nothing about space or spaceships. – Ave May 31 '16 at 14:26
• @JanDoggen: This fallacy has a name: "Argument from personal incredulity". I mention it because it is my favorite fallacy name. rationalwiki.org/wiki/Argument_from_incredulity – James Jun 1 '16 at 1:22

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

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.

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.

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.

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.

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.

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).

• 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 May 31 '16 at 9:56
• Thanks for the "Future rocket scientists" caption on one of the pictures. The best way to counter internet silliness is through humour! – Andy May 31 '16 at 12:58
• "thrusters firing (not visible in this video...)" What do you call that big orange thing under the pointy thing? – Aron Jun 1 '16 at 5:24
• @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 Jun 1 '16 at 7:37
• This is an outstanding answer - well written and illustrated. – user316117 Jun 1 '16 at 20:37

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

• 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 May 31 '16 at 9:36
• Yes. In the cold of space, the nitrogen quickly condenses again, I.e. – oefe May 31 '16 at 9:43
• 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 youtu.be/b-yWTH7SJDA – oefe May 31 '16 at 9:46
• ...plus the temperature drop associated with the expansion itself. Thanks - good to know! – uhoh May 31 '16 at 9:46
• According to m.reddit.com/r/spacex/comments/421js4/… 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 May 31 '16 at 16:31

## protected by ForgeMonkeyApr 8 '17 at 15:41

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