191 votes

What computer and software is used by the Falcon 9?

SpaceX uses an Actor-Judge system to provide triple redundancy to its rockets and spacecraft. The Falcon 9 has 3 dual core x86 processors running an instance of linux on each core. The flight software ...
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114 votes
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What computer and software is used by the Falcon 9?

In this AMA by the SpaceX software development team, they wrote: We've been getting a lot of questions about how C#/MVC/etc have to do with rockets. They don't. About their development they said: ...
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104 votes
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What happens to the Falcon 9 second stage after payload separation?

There's actually a few outcomes of the second stage that can occur (and some interesting tales to go along with them), but as geoffc has mentioned, second stage reuse is no longer planned for Falcon ...
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  • 8,715
93 votes

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

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 ...
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  • 7,089
90 votes
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Difference between BlueOrigin and SpaceX rocket landings?

You see that tiny thing on the far left? That's the Falcon 1. It's a comparable size to Blue Origin's New Shepard spacecraft and SpaceX's Grasshopper (which accomplished a similar feat 6 times, around ...
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  • 1,174
90 votes
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Why doesn't the Falcon-9 first stage use three legs to land?

I can't speak for why SpaceX made the decision. However, while three legs won't wobble, four legs are less likely to tip over. SpaceX has demonstrated tipping over is a major problem. Dr Peterson of ...
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  • 7,816
65 votes
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Why would SpaceX not use parachutes for the final descent of the first stage?

There are many key points to this, probably none on their own sufficient to ditch the parachutes approach (except economics, those are good enough on their own), but together they make for a ...
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  • 75.6k
65 votes
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Why doesn't SpaceX land boosters in Africa?

The boosters do not have the range to get to Africa because they aren't going fast enough. If you look at the graphic below it shows a Falcon Heavy mission. The side boosters do not get very far ...
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  • 18.9k
64 votes
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Why does the SpaceX reusable launcher use vertical rather than horizontal landing?

In two words: Precision landings. Underlying all of SpaceX's decisions is the desire to go to other planets, especially Mars. For exploring the solar system, Elon Musk feels that precision landings ...
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  • 756
59 votes
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Why can't Falcon 9 fairings touch the water?

The fairings are not boats. While they appear to float (at least for some time), there will also be water on the inside of the fairing. That results in some issues. Inside the fairing, there are ...
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  • 1,324
58 votes
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How does the Falcon 9 first stage avoid burning up on re-entry?

The heat of re-entry is highly dependent on speed. The second stage of the rocket is responsible for providing most of the speed needed for orbit, after the first stage lifts it out of dense ...
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57 votes
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Why is Falcon 9's fairing so big?

One of the keys to SpaceX's cost advantage is standardization on common parts. One example is the Merlin engine, which is used on both the first and second stages of the Falcon 9 and Falcon Heavy (...
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55 votes
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Falcon 9 design: Why is the cargo fairing so much wider than the lower parts?

The diameter of the stages is the largest size that can be transported by road without extensive "outsize load" issues (permits, having to move traffic lights and signs out of the way etc.). This ...
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  • 122k
50 votes
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Is a SpaceX launch completely automated?

The rocket is autonomous, it flies itself. The navigational math, engine, and flight dynamics of a Earth-based orbital class rocket in operation are far too complex for manual operation, especially ...
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  • 6,377
49 votes

Difference between BlueOrigin and SpaceX rocket landings?

Here is an image of the two trajectories. (From Reddit) Here is a nice infographic explaining the differences between the two. Kudos for both images above to Jon Ross of ZLSA Design. And here is ...
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  • 76.7k
49 votes
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Why does the SpaceX Falcon 9 rocket do a 180 flip for reentry?

Hobbes has already showed you a diagram of the Falcon 9 launch profile, so I won't repeat that. Note: This answer is not intended to be a complete, scientific treatment of the subject. I knowingly ...
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  • 7,204
48 votes
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Falcon 9: engines stabilize spin

The other answers are great demonstrations of F9's capabilities, but I'll be the contrarian here and say they're all wrong and perhaps Elon oversimplified things for a tweet. This was a one engine ...
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  • 6,377
48 votes
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Why was there only a single Falcon 9 landing on ground-pad in 2021?

Whenever possible, SpaceX prefers Return To Launch Site (RTLS) landings over Autonomous Spaceport Droneship (ASDS) landings, since they get the booster back immediately instead of having to wait for a ...
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47 votes
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Why would NOAA restrict the ability of SpaceX to live stream the second stage?

Because beaming down images of the earth from space is restricted, and Companies and Universities require a Commercial Remote Sensing Licence (weirdly these are issued by NOAA) to do so. SpaceX has ...
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  • 5,078
46 votes

Why does the SpaceX Falcon 9 rocket do a 180 flip for reentry?

Trajectory of the Falcon 9 first stage: Graphic courtesy ZLSA Design (zlsa.github.io) As you can see, before the boostback burn, the stage flips so the engines point in the direction of travel. When ...
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  • 122k
45 votes

What holds SpaceX’s rocket in place in this launch?

The Falcon 9 (and Falcon Heavy) use launch clamps at the base of the vehicle to hold it steady pre-launch. In fact, the gantry and umbilicals don't actually provide much support even before they ...
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  • 9,854
45 votes

Falcon 9 launch process - perception of launch speed

Aside from thrust/weight ratio, I suspect at least part of this conditioning came from a peculiarity with the Space Shuttle and its launch process. The Space Shuttle's launch process was different ...
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  • 1,569
44 votes
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Why does the Falcon 9 have 9 engines?

SpaceX initially was looking to buy an engine but could not find one on the market that would allow them to meet their goals. Once they decided to develop their own, they had to consider their goals: ...
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  • 76.7k
42 votes
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What are the droplets/particles falling off rockets at launch?

Ice. All these rockets use oxygen as the oxidizer component of their propellant. The Saturn 5 also used hydrogen in some of its engines (upper stages). They are stored in liquid state, which requires ...
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  • 17.1k
42 votes
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How are the SpaceX Falcon 9 Mod 3 and Mod 4 grid fins different?

Ok, I asked so I could use these awesome photos in an answer... I confess. The material has changed from Aluminium with an ablative paint to bare titanium. The specific shape, size, and mount points ...
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  • 76.7k
40 votes

Why can't Falcon 9 fairings touch the water?

In addition to the water damage is the impact damage: the fairings wouldn't "touch" the water they'd hit the water, even with parachute retardation. A net slows deceleration down and spreads it more ...
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  • 18.9k
40 votes
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What is the primary reason for SpaceX motion to have astronauts board Dragon before fueling up the rocket?

The Merlin-1D engines are now tuned to use the super cooled fuel and oxidizer. Thus you would be running the engines in an out of normal state, if not using it the same as all other launches with ...
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  • 76.7k
39 votes
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Why was AMOS-6 mounted on the rocket for a test fire?

The static test wants to be done relatively close to launch day, to minimize the likelihood of anything happening to the engines between the test and the launch. The more conservative procedure is ...
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39 votes
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Why won't SpaceX launch a rocket for a month?

The Eastern Range run by the Air Force has a 2 or 3 week maintenance window. Thus no one is launching from either Cape Canaveral (Where LC-39A is located), nor the CCAFS (Cape Caneveral Air Force ...
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  • 76.7k
39 votes
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Why did SpaceX retire Falcon 1?

The Falcon 1 was less profitable to maintain, and it didn't have the customer base to support using it. A Falcon 1 launch cost around $10 million, of which about 10% was profit. They also considered a ...
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