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100

During the Pathfinder landing the airbag system hit the ground at about 20m/s. This seems fast but compared to other space speeds, it's very slow. When the pathfinder rover arrived to land at Mars, it was cruising at around 7300m/s. To get this velocity to zero, the parachutes, heat-shield, and single-fire rocket engine did their work to kill the velocity ...


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 ...


89

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 3 years ago, but didn't *technically* enter space). You see those 3 in the middle? That's what SpaceX landed today. Grasshopper / Blue Origin were ...


89

Because it's at the end of a 6 month cruise and there's no turning back. InSight will not enter closed orbit around Mars - its trajectory is hyperbolic so either it misses Mars entirely or it enters the atmosphere. There were six planned course corrections during the cruise phase, the final one of which - TCM 6 - occurred on the day of the landing. This ...


63

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 are extremely important. The precision landing requirement means that you want to start slowing the spacecraft before you reach the surface, which points ...


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

The reason is delta-v, which is a crucial concept in Spaceflight. It means change in velocity, and is the primary 'currency' that space mission have to expend in order to reach places in the solar system. On earth, if you want to go anywhere, you can get there at any speed, it just takes longer. Unfortunately, that is not how it works in space, because the ...


56

SpaceX's demonstrated booster-landing ability isn't the result of a breakthrough but rather a bunch of small incremental improvements. The major limitation has been funding and the will to make it happen. In 1966, unmanned spacecraft landed on the Moon under rocket power in the Surveyor program. It used (IIRC) three fixed-position thrusters, pulsed, to ...


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 ...


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 ...


46

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 another fun size comparison (source unknown): This video especially where I cued that link, should also show an aspect of the discussion.


45

It is not practical to use this approach from orbital (1.6km/s) or escape velocity (2.4km/s), for two major reasons. The first is the acceleration reason. The kinematic for bringing objects to a stop under constant acceleration is $$d = \frac{v^2}{2a}$$ from which we can quickly solve for the acceleration to be $$a = \frac{v^2}{2d}$$ Even with 10 meters of ...


44

It was charred by the center core after separation: (Source: SpaceX FH launch webcast) Looking at it I would expect one side to be charred too but it may not be - the nose cone is afaik composite (same as the interstage) and not metal.


42

Look at the STS-122 video. How many astronauts do you see? I see six. Seven astronauts landed with STS-122. The six you see were the crew of STS-122 who spent twelve days in space. They could walk because twelve days in zero g isn't enough time to take a significant toll on musculature, bones, and blood. The seventh returning astronaut, Daniel Tani, who you ...


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 ...


40

Some pictures from Apollo 11 of the landing gear – struts, footpads and contact probes. Three Apollo 12 images: Two Apollo 14 footpad images: An Apollo 16 image: The two probes bend straight up on the left of the left and right footpad. I found no Apollo 17 images with visible contact probes. So yes, some contact probes did bend and stick out sideways. ...


39

The minimum-fuel (Hohmann transfer) travel time to Mars is about 8 months each way. It's possible to shave some off that time by using more fuel, but fuel-to-payload ratio is among the biggest engineering factors in ambitious space missions. However, the alignment of the planets has to be just right to get that fuel-efficient course, and after reaching Mars ...


39

Actually there’s no reason that an airbag system could not be used on the Moon, nor that it could not be a good design decision in some circumstances. The MPF and MER airbags took out the last 10 to 26 m/s of velocity. The reason that there was that much velocity left was the accuracy, or inaccuracy of the solid rocket motors’ total impulse, along with ...


38

The core has been lost, according to the technical stream: If you wait until ~38 min and 30 seconds, the announcer says "We lost the center core". Update: Elon Musk has confirmed that the center core didn't make it. To paraphrase from the SpaceX post-launch news conference The center core obviously didn't land on the ...


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.


35

We didn't know how hostile Venus's surface was, until we had landed there. The atmosphere of Venus makes it easier to land there than Mars. From Wikipedia, we learn: Before radio observations in the 1960s, many believed that Venus contained a lush, Earth-like environment. While there was some concept that Venus was hot, and had a high pressure, the exact ...


35

It is correct that the probe on the forward footpad was omitted to avoid interfering with the ladder: The probe located on the forward landing gear was deleted because of a concern that the failed probe could interfere with crewmen descending the LM ladder. [p. 8] There was more than one probe for redundancy. In particular, there was no electrical ...


34

Blue Origin's flight was straight-up, straight-down, with a fairly small rocket that can't carry much useful payload. It's a great demonstration of technology, but it's only practical for space tourism. Today's Falcon flight was a paid orbital payload mission. In order to get the payload to orbit, the first stage has to not only bring the second stage to ...


33

It is unnecessary to use anything but the rockets. For the same reason a helicopter uses its engine and rotor to land. It would be a bit ridiculous to land a helicopter with parachutes and airbags, wouldn't it? Boeing CST-100 will use parachutes and airbags to land, and Elon Musk made fun of it in an interview, saying that its drop test landing looked like ...


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 ...


33

The "three second" item is incorrect. In general (and this can be confirmed by watching multiple landings), the rockets fire approximately half a second before landing. Maybe they meant to say "three feet"? As an aside, astronauts who have ridden the Soyuz home invariably describe it as a fairly wild ride. Terms I've personally heard include "car ...


32

STS-3 landed at White Sands runway 17. In addition to Kennedy, Edwards, and White Sands, several sites were selected as targets for a transoceanic abort landing (TAL), but no launch ever had to perform such an abort. If something went bad during launch before the vehicle had enough energy to get to a TAL site, a mission might have had to do something even ...


32

Page 331 in the Shuttle Crew Operations Manual, an official NASA astronaut training document, confirms that The deorbit burn usually decreases the vehicle's orbital velocity anywhere from 200 to 550 fps, depending on orbital altitude. The deorbit burn was not intended to reduce the Orbiter's velocity to a small value, but rather to change its orbital ...


32

Here's an image of the bottom of the stage before launch. As you can see, the entire bottom is covered in white panels. I suspect those panels are a heat shield. This SpaceX press release on the introduction of the Falcon 9 v1.1 refers to a heat shield. The reference is a bit oblique, but I think this refers to the first stage. Here's the same area ...


32

InSight doesn't enter Martian orbit before EDL; it plows straight into Mars' atmosphere from interplanetary space. Thus, the time of landing is pretty much un-alterable after its final midcourse correction maneuvers; it cannot wait for perfect weather conditions to land.


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