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posted by martyb on Saturday September 15 2018, @03:49AM   Printer-friendly
from the queue-up-some-Pink-Floyd-for-the-journey dept.

After a previously planned flight around the Moon using a Falcon Heavy fizzled out, SpaceX has announced that it will send a private passenger around the Moon using a BFR launch vehicle. More details will be announced on Monday:

On Thursday evening, without any advance notice, SpaceX tweeted that is had signed the world's "first private passenger to fly around the Moon aboard our BFR launch vehicle." Moreover, the company promised to reveal "who's flying and why" on Monday, September 17. The announcement will take place at the company's headquarters in Hawthorne, Calif.

There were only two other clues—tweets from Elon Musk himself. Was the rendering of the Big Falcon Spaceship in SpaceX's tweet new? Yes, Musk said. And was he the passenger? In response to this, the founder of SpaceX simply tweeted a Japanese flag emoji. This would seem to be a strong clue that the passenger is from Japan. Or maybe Musk was enjoying the epic Seven Samurai movie at that moment.

By announcing this on Thursday, and waiting four days to provide more details, the company has set off a big guessing game as to who will fly. Of course that is an interesting question, but we have many other questions that we'd like to see answered before that. We've included some of those questions below, along with some wild and (slightly) informed guesses. Musk even answered one of them for us.

The design of the BFS has apparently changed to include three prominent fins and an underside heat shield.

Related: How to Get Back to the Moon in 4 Years, Permanently
SpaceX to Launch Five Times in April, Test BFR by 2019
SpaceX to Begin BFR Production at the Port of Los Angeles
2020s to Become the Decade of Lunar Re-Exploration


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  • (Score: 2) by takyon on Saturday September 15 2018, @11:28AM (4 children)

    by takyon (881) <reversethis-{gro ... s} {ta} {noykat}> on Saturday September 15 2018, @11:28AM (#735272) Journal

    The design has undergone some big changes. 2 of the fins may be able to rotate, and the 7 engines on BFS all appear to be the same sea level version now instead of the vacuum version. The circular thing around them may be able to adjust based on whether it is in space or landing.

    Last time around the 2 passengers for the Moon flight were not revealed. Are they going to go on BFR or have they given up? For this new passenger, how compelling is this "why" going to be? Is it a Japanese billionaire trying to stimulate Japanese interest in space? The good news is that we should actually get some answers this time since there will be an event on Monday.

    I'm also eager to know whether the previously claimed 150 tons to LEO figure has gone up or down.

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  • (Score: 2) by turgid on Saturday September 15 2018, @11:35AM (1 child)

    by turgid (4318) Subscriber Badge on Saturday September 15 2018, @11:35AM (#735275) Journal

    150 tonnes to LEO would be good, but I also think that once on-orbit assembly becomes properly developed it'll be less of an issue. Reliability and safety are probably more important. Just my gut feeling.

    • (Score: 3, Informative) by Immerman on Tuesday September 18 2018, @04:46PM

      by Immerman (3985) on Tuesday September 18 2018, @04:46PM (#736592)

      You might be right - but orbital construction hasn't even taken any baby-steps yet, the closest we've come is the ISS, which was really just fastening completed modules together using standardized quick-connect joints. And from what I recall, even that is exhausting, dangerous work in space.

      Such relatively easily assembled modules will probably be the mainstay of orbital "construction" for the foreseeable future, probably throughout the life of the BFR - and that means means that payload masses and volumes will be a major limiting factor on what can be done. Inflatable modules like Bigelow's habitats can push the limits some, but when you get right down to it the square-cube law declares that bigger is better. You want something bigger than can be launched by the BFR or its successor, then it's going to have to go up in pieces, which means we'll need serious orbital construction capabilities.

      Welding underwater is one of the most dangerous jobs in the world, how much worse would welding in space be? There's no hiding from the radiation while working in a space suit, and any suit breach is likely to kill you, or at least cost you a limb. I suspect that we won't see major space industry until we've mastered either (mostly)autonomous construction robots, or good industrial telepresence robots. Devices that can function in the harshness of space while the operator is safe within a nearby habitat (trying to operate from Earth introduces a lot of lag, especially since you keep having to route the signal halfway around the planet every hour or so.

  • (Score: 4, Interesting) by Immerman on Saturday September 15 2018, @02:36PM (1 child)

    by Immerman (3985) on Saturday September 15 2018, @02:36PM (#735297)

    >The circular thing around them may be able to adjust based on whether it is in space or landing.

    You're right - I had missed that detail. That does look rather robust and complicated for a backsplash plate, and all those segments make it look like it could close into a much tighter cone. Probably not as good performance as a huge, smooth vacuum bell - but if they could capture 50-80% of the incremental benefit while allowing all the engines to be used in space? Inter-orbital maneuvers are most efficiently done in short, powerful bursts - the gains from more power might largely offset the bell losses.

    Actually though - look at the size of those bells compared to those in the earlier 4+2 designs. I think those are seven vacuum bells, not atmospheric ones. At the very least, near-vacuum. Which makes perfect sense for a second-stage launch vehicle, but means the exhaust flow will separate from the bell during landing, causing chaotic thrust instability and accelerated bell wear.

    Hmm, or will it? I'm trying to remember, and I think that every description of rocket bell dynamics I've heard operates on the assumption that the bell is behind the rocket, accelerating it, rather than flying bell-first at supersonic speeds as it decelerates. It's a natural assumption given the history of rocketry, but the fluid dynamics of the situations would be very, very different. Heck, the leading pressure wave alone would mean that even atmospheric engines would be operating in heavily over-pressure mode by traditional assumptions. Perhaps bell size simply isn't that important in retro-firing conditions. It'd still likely be an issue in the moments before landing - but the engines will be throttled way down for that anyway - I'm not sure, but I think that would also mean you effectively have a too-large bell for ambient pressure - so at this point they may have lots of experience just dealing with the associated problems.

    • (Score: 0) by Anonymous Coward on Saturday September 15 2018, @05:40PM

      by Anonymous Coward on Saturday September 15 2018, @05:40PM (#735349)

      every description of rocket bell dynamics I've heard operates on the assumption that the bell is behind the rocket

      I assume actual rocket scientists look into it with more detail, but the undergraduate-/hobbyist-level studies I've seen don't consider aerodynamics either way -- in that way, they're only directly applicable to a test-stand firing, or to a first stage at the moment of liftoff. These simple analyses would be wrong in both directions, given any significant airspeed -- the lower pressure behind a 2nd stage would make a vacuum engine suitable earlier during launch than one would expect just from altitude, while as you say much of a tail-first landing trajectory would be over-pressure for even sea-level engines.