# Tag Info

44

There are several compelling engineering and design reasons why a bigger spaceship makes sense and several reasons why making a mini-starship does not make sense for SpaceX specifically (and their vision). First and foremost, Elon Musk has made it clear that his goal for the company and the future isn't to provide cheap satellite launch capabilities, it's ...

40

Sorry for the length of this, but it brings up some interesting facts and possibilities. The moons you mention, Titan, Europa, and Enceladus, are three very different places. Titan has a relatively large surface gravitational acceleration (as far as satellites go) and a very thick atmosphere; Europa has a relatively large surface gravitational acceleration ...

39

Neither has much financial purpose without the other. A BFR cannot perform any useful function without an upper stage, and that is the BFS. Since the whole platform is a major investment in a new architecture, they are starting with the smaller piece - the BFS. Since it uses some of the same engines as the BFR, it can act as a testbed for both BFR and BFS ...

38

In my experience, I've noticed that SpaceX uses a lot of temporary components until such temporary components need to actually be tested. Since the legs of MK1, MK2, and most probably MK3 will never see uneven soils rather than concrete landing pads, they will most likely just have simple legs. After a few successes with MK3, we should start seeing much more ...

31

In the question you mention about why Falcon 9 uses four legs to land instead of three, there are multiple great answers. One of them by @David Hammen states that: "Another factor is that the Falcon has nine engines, one in the center and the other eight around it in an octagonal or square-like arrangement. The bottom part of the thruster reflects ...

30

F9 can enter engine first because it isn't returning from orbital speeds. While fast, it's a fraction of the speeds something returning from orbit (or further) comes in at. So the engines are out as an entry surface, you need more protection. One way to achieve this is Dragon-style: put a heat shield on the bottom, and engines in the sidewalls. Great for ...

29

Elon Musk stated in a news conference after the Falcon Heavy launch that the BFS will be the focus because they think they understand designing booster rockets pretty well, and thus they decided to focus on the more difficult piece first. He answers this in response to a question that starts at 20 minutes 52 seconds here, and speaks specifically to starting ...

28

Anyone know what the story is? The main driver for the large number of engines on the BFR first stage is the desire to use a common engine design (albeit with different optimized nozzles) for both the booster stage and the interplanetary spacecraft stage. Building and maintaining only one type of engine makes things more efficient, and is a strategy that ...

20

SSTO on Earth requires about 9400 m/s of ∆v depending on the exact acceleration profile and other factors. Ascent from Mars' surface to low Mars orbit requires only about 3800 m/s, and from there it's another 2500 m/s to break out of Mars orbit and get on a trajectory to intercept Earth -- a total of 6300 m/s. Allocate another 300 m/s or so for touchdown on ...

19

It's probably going to be less of a concern than you'd guess. The icy worlds of our solar system have essentially no atmosphere, so the surface materials will sublimate directly to vapor and be dispersed rather than melting and freezing the landing pads into place. Fairly little of the surface will be disturbed to begin with. The gas expansion which ...

18

tl;dr: at an apparent magnitude of about +18.5 you need a several meter telescope and a dark sky. Hubble can do it too. So by reflected sunlight, definitely not by eye. The exhaust from a Methalox (CH4 + LOX) engine barely makes any light in the visible, so no help there. Starting with the math from this answer: I'm going to characterize the 55 x 9 meter ...

18

By the time Starship launches for Mars, SpaceX may have changed the number of legs several times. They redesign as quickly as the Haggunenons of Vicissitus Three evolve during lunchtime. Plausible guesses for factors favoring three legs, at the moment: lighter less aerodynamic drag, if the legs also act as fins send a scout beforehand to choose a boulder-...

17

Russell Borogove's answer gets to the root of the issue. They wanted minimum 6 engines on the BFS, which meant that the BFR had to have several times more, and they picked the number 31. 31 nozzles, however, is not a record. The Soyuz has 5 cores. The centre core has 4 main and 4 vernier nozzles, and the side cores have 4 main and 2 vernier nozzles each, ...

16

Falcon 9 reentry is only designed for first stages, with a reentry burn that is pretty minor - on the order of a few hundred meters/second. A second stage, reentering with far more velocity than a first stage, can't shed the extra velocity with a cheap burn, because the total delta-v necessary for that would be about as much as the second stage was capable ...

15

If you want an easy way to think about it, imagine how bright it might be in low Earth orbit, 240 miles up (which is just a bit lower than the International Space Station). However bright that is, it will be only a millionth as bright when it’s near the Moon, 240,000 miles away (and so a thousand times the distance). Is it likely that it will be bright ...

13

There are no published/proposed abort modes for ascent. We can speculate that some incidents can be survived by letting Starship tumble free and light it's own engines to burn to a low orbit or land depending on altitude and velocity, but that capability has not been proposed by SpaceX. Landing redundancy is provided by carrying 7 deep throttling, sea level ...

12

Pretty much certain it's a flyby. Grey Dragon mission was a flyby. To send a mission to the surface of the Moon would require many refueling missions, and I can't imagine that would be the case. Also, the tweet that started everything says so. SpaceX has signed the world’s first private passenger to fly around the Moon aboard our BFR launch vehicle

12

Quick answer is all I have time for now -- if someone wants to do a more detailed and referenced answer, please feel free and I'll delete this: While it's very big, its mass is not that high, and gravity is low. On Mars, for instance, it needs about 150 tons of fuel, plus 100 or so tons of vehicle (mass) to get back to orbit. Under Mars gravity thats only ...

11

SpaceX has been developing a methane/LOX engine called Raptor which is about 2 times more powerful than the Merlin engine. (This is the first Raptor iteration at around 400Klbs thrust. The first Merlin (1A) was 75Klbs, current version is closer to 200Klbs. So expect performance growth). 31 engines at about 400Klbs thrust gives about 12.4 million lbs of ...

11

According to tweets from Elon, at least part of the decision is due to their design for dealing with the reentry heat: instead of adding ablators to cool the craft on reentry, Starship is going to actively cool the hot side with liquid methane. Steel is better at coping with this than carbon fiber. Tweet 1: Usable strength/weight of full hard stainless ...

10

This is a really good question and the answer is probably not 100% known, even by SpaceX at this moment. No doubt they will have some active cooling to minimize boil off. Structurally there are tricks they can play. For example the landing fuel is stored in a smaller tank, which is submerged in the main tank. Thus the surface area to warm up over is ...

9

Possibly to the Moon, not to Mars. The one possibility is the two people who paid SpaceX for the "Grey Dragon" mission, where a Falcon Heavy was going to lift a Dragon 2 with 2 tourist astronauts to a flight around the Moon. Elon has said that Falcon Heavy will not be man rated, with no mention of the Grey Dragon mission. It seems likely they might be flown ...

9

SpaceX has implied that there will be a crew/cargo lift system similar to how skyscraper window washing platforms work (a crane swings the platform out from inside and lowers it to the ground): SpaceX teased it at the end of this video from last year:

8

Most foods cooked in space are not brought up to a complete boil because dealing with the steam would be an issue. Currently they inject hot water into vaccuum sealed pouches containing food. I would expect that instead of a zero G rice cooker, spaceX would send par-cooked rice or instant rice that only requires hot water to be added. Instant rice is pre-...

8

Gwynne Shotwell was speaking at a conference and said that at least initially they will be leasing land in Los Angeles near the harbour to build the first BFR units. She specified that the cost to move power lines/lights/trees and so on would be too high to build it at the current factory. It has been noted by fans that the doors on the factory at ...

8

There are two parts to this question. (1) does a refueled BFS have the delta-V budget to get from LEO to the surface of the moon and back to Earth intercept (2) can the control systems, landing legs, etc. handle a soft landing on the Moon. Re (1) the answer is (as usual) implict in the diagram in this question. The delta-V required is about 8.7 km/s. This ...

8

Musk's IAC2017 presentation included a "chomper" variant for large cargos: Source: http://spaceflight101.com/spx/wp-content/uploads/sites/113/2017/09/IAC2017-Musk-26.jpg His 2018 announcement included aft cargo holds, around the engines: There's also at least one side hatch. If you're launching a group of small satellites, you could use a carousel or rail ...

8

Capturing into Mars orbit would consume a fair amount of fuel. You can see this with Mars probes. They usually do one of three things. Propulsive capture into orbit Minimal propulsive capture, but aero breaking for the rest. Direct descent, no real breaking into orbit. Every pound of fuel carried to Mars means lifting it from Earth, so minimizing fuel ...

8

You can find most of the information you need on the picture below which comes from Reddit. Crucially, if you add up the relevant lines, delta-V from Mars to Earth-Mars transfer is about 5.92 km/s. Assuming they produce the vacuum raptor (not planned for the very first starship missions) Starship will have an $I_{sp}$ around 380s, so by the rocket equation,...

8

An answer might be, that three legs will always be able to land on a rough surface with all three legs touching the ground, while with four legs there is the possibility that only three of them will touch the ground. In order for the four legs version to touch the ground with all four legs, those legs should be made telescopic with some sort of dampers, ...

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