91

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 ...


87

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 The Math Forum explains... There are different kinds of stability! A three-legged stool is guaranteed not to wobble, because the ends of its legs always ...


78

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 as Musk thinks the resources to develop it are better spent elsewhere. It's not an insurmountable technical challenge. Intentional Deorbit This is done for ...


65

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 downrange at all so they return to the cape. The drone ship for the core booster was located 1236km downrange, Africa is over 6000km downrange. The graphic came ...


59

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 compelling case against it; Descent control: As already mentioned, there's a significant guidance uncertainty with the use of a parachute system. Some of it comes from ...


58

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 electronics and other corrodable materials. Now the fairing is designed to be as light as possible. Therefore, SpaceX probably doesn't want to make the entire ...


56

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 (though a slightly different vacum optimized version on the second stages). This is also done for fairings. Fairings are very expensive to make (around $6M). ...


53

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 atmosphere. Falcon 9 separates its first and second stages at relatively low speed, so its reentry starts off drastically slower than a reentry from orbit -- about ...


53

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 makes the rocket much cheaper to transport. The fairing size (5.4 m) is dictated by the standard satellite diameter set by the Shuttle and Ariane 5.


50

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 remote manual operation. Even simpler rockets (like Apollo LEM) that could be flown manually have still attempted to offer automatic operation in the interests ...


49

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 and deliberately simplify, gloss over and ignore things in several places, in order to explain this in a way that hopefully makes sense to the OP while still ...


48

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 landing burn. A single engine lacks the ability to control roll on its own, unless it has a vectoring turbopump exhaust. Merlin 1C had this feature for roll ...


47

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 not received one for streaming second stage imagery. You can see the list of issued licenses here (a fun read). This was confirmed in this tweet by Eric Berger:


46

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 the engines fire, this slows down the stage. This trajectory is used when the stage returns to the launch site (and for some early experiments where the ASDS ...


43

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: Reliability, and reduced costs. Nine engines means at most any time in the flight profile, you can handle an engine out event. The Saturn V with 5 F-1 ...


43

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 retract - they're there primarily for power and fuel loading. The launch clamps release soon after the engines reach nominal thrust and can support the vehicle's ...


40

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 to attach the payload after the static test, but they'd have to lower the rocket, move it back indoors, stack the payload, take it back out and reelevate it. ...


40

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 have changed as well. Let's start with a nice shot of the Mod 3 design on a Falcon 1.1 Full Thrust. You can see the size, shape, and design differences in ...


39

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 Station, where LC-40 is located) locations. SpaceX is using the time the range is down to modify the RSS (Rotating Support Structure) that is left over from ...


39

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 Falcon 1e design, which would carry slightly more to orbit, but only 1100 kg tops. There were a few people who are known to have booked a Falcon 1 launch, ...


39

Elon Musk made reference to the "Holy Mouse Click" that happens right before they start loading fuel. From that point forward, the rocket will launch itself at the planned time, about 2 hours in the future, unless something happens that stops it prematurely. I'm not sure if there are other prompts that are required, but things are pretty automatic. In ...


38

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 very low temperatures (below -183c for oxygen, below -253c for hydrogen). Despite insulation, some of the outside surfaces can get cold enough to condense and ...


38

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 is written in C/C++ and runs in the x86 environment. For each calculation/decision, the "flight string" compares the results from both cores. If there is a ...


38

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 evenly across the structure, which is a lot friendlier to the materials and the supporting electronics.


36

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 warmer propellant. It would imply different software to handle the different performance levels. Last thing you want to do is run things differently, if you can ...


35

Normal hydraulic systems are closed. When hydraulic fluid is squeezed out of a cylinder, it returns to a holding tank ready to be pressurized again by the pump. SpaceX have confirmed the Falcon 9 uses an open system instead: Hydraulics are usually closed, but that adds mass vs short acting open systems. F9 fins only work for 4 mins. We were ~10% off. In ...


35

No, on the contrary. The larger the engines get, the more expensive they get. Just look at the massive amount of full-scale tests they needed to get the F-1 engines of the Saturn V main stage running smoothly. Now imagine, you spend all this money to design a huge engine, and you want to manufacture it. You will need a top-notch machine park to do that. ...


34

Earlier on Musk indicated it may be possible to fly a block 3/4 booster more than twice but it would depend on the missions flown, with LEO missions like this Commercial Cargo mission being easier than GTO missions. I suspect this booster and the previous one that did two cargo missions could be reused if SpaceX wanted to but they are choosing to move ...


33

There are many contributing factors to this problem. The Shuttle was designed in the 1970's and technology has matured since then. Additionally, the issues the Shuttle ran into (While in truth may have been predictable at the time) are now more obvious and a new design can try to avoid them. Consider the simple case of the heat shield on the Shuttle. It ...


33

At the moment no one knows, since no one has done quite this task before. SpaceX is learning alot with each recovered core. In this image of three of the recovered cores in the LC-39A HIF building you can see different levels of wear on each stage. You can see that some of the engines have been removed for likely testing on the initial Orbcomm mission ...


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