16 thoughts on “Lunar Elon”

  1. Aerospace question:

    Elon is landing the lunar starships on their tails, then setting up a system of anchors and winches to lower them into a horizontal orientation. Is there any way a vehicle in space could rotate its attitude by 90 degrees for landing, or is that only possible in science fiction?

    [/sarcasm]

      1. The idea being to avoid engine damage from kicked up debris. Assuming take-off will use same nose rockets. Which sort of begs the question for lunar starship why bother with tail mounted engines unless necessary to get to LEO the first time?

        1. Lots more thrust and efficiency I’d guess, plus they already have them to get into LEO as you say. Might as well use them.

        2. More thrust in losing lunar orbital velocity {deorbiting} has less gravity loss. Though the Moon doesn’t have much gravity loss.

      2. But even with the engines high, the same mass is going to impact the surface at the same velocity, likely kicking up the same amount of dust unless he detunes the exhaust nozzles for a wider exit cone angle.. He’ll just be making wider, shallower exhaust crafters.

        I would recommend he drop is descent engine ISP down into the 290 range near the surface, as that’s lower than the velocity required to get into lunar orbit, so no dust will escape the moon’s gravity to cause future issues in cis-lunar space.

        As an aside, another simple fix to the dust problem is to deploy a hooped petticoat that makes contact with the surface prior to touchdown. Dust grains will impact the skirt instead of launching off to who knows where on ballistic trajectories. It might even pay off in landing mass if they can take advantage of an air-cushion effect in the final landing phase. But most importantly, deploying the skirt will create a critical female oriented crew position.

        But more seriously, we’ve been seeing probe after probe tip over on touchdown because lunar gravity only provides a sixth of the vertical force, so any slight horizontal momentum on touchdown results in a tip., and a tall tower that’s inevitably going to land on a slight slope is not a great idea.

        I did some quick calculations on the exhaust issue.

        Assuming they use four engines mounted about 120 feet up (just below the top of Starship cylindrical section, below the base of the ogive), with a 10 degree exit half angle, and cant them out 15 degrees so the inner edge of the exhaust is going outwards at 5 degrees. If the base of the rocket sits 5 feet above the surface, then the engines are 125 feet up. Then each exhaust plume has an elliptical footprint that’s 47 feet long and 46 feet wide, for 6750 square feet per print, and a total footprint of about 27,000 square feet.

        If the ship was horizontal instead of vertical, with the bottom clearing the surface by about 10 feet, and you mounted the engines along each side, halfway up, the engines would be about 25 feet above the surface. With the same cant and exit half angle, each plume would be 12.2 x 11.8 feet, for an area of 454 square feet. You could fit 10 engines per side along the 120 foot cylindrical section without overlapping the footprints, for a total area of about 9,000 square feet, which is only a third of the footprint of the vertical configuration.

        But the Apollo LEM had an exhaust footprint at touchdown of 21.64 square feet, pretty much digging the bell into the surface. When you figure up the descent mass near touchdown, and allow for lunar gravity, the area under the LEM’s bell should’ve been at about 2.3 psi.

        If you do the same for a vertically landed one-way Starship at 600,000 lbs (272 tonnes), with a 27,000 sq foot exhaust footprint, you get 0.026 psi, about 1.1% as much pressure as the Apollo footprint, and for a 700,000 tonne Starship you get 0.066 psi, which is 2.8% as much as Apollo.

        If you land horizontally you’ve got a third the footprint, so the surface pressure would be 3.3% of Apollo’s for the light one-way Starship, and 8.4% of Apollo’s for the heavy Starship.

        Obviously 100% of Apollo’s exhaust pressure didn’t seem to hurt anything, so I’m not sure that would be an issue. Lots of dust is going to go flying at pretty close to the engine’s exhaust velocity no matter what you do (except for something like the hoop skirt fix).

  2. But most importantly, deploying the skirt will create a critical female oriented crew position.

    Call it a kilt.

      1. It is sad that the Scots remain so poorly understood. At least the situation is improving, one well-cleaved corpse at a time!

  3. Hopefully he will be at Luna South Pole, and name the settlement Muskopolisburgtonville….

  4. A vertical landing orientation, if the final position is supposed to be horizontal, also has the drawback of ship layout. Until it gets tipped over (possibly weeks later in some of the sketched plans) “up” is toward the nose, but once it’s tipped “up” is rotated 90 degrees. So the internal cabin or cargo area has to accommodate two different “ups” for the different phases of operation. That’s an unnecessary complication to have to design around. Walls have to double as floors, and vice versa, and ladders either have to be moved around or become redundant.

    If the crew is supposed to go out on the surface while the ship is still vertical, to rig equipment to lower it down, then you have to add all the complexity of a giant elevator, which then becomes useless once you lower the ship to the horizontal orientation. If you just land it like you’re going to use it, everything is ready to go from the moment of touchdown.

    1. There’s already a giant elevator in development (also a crane). Tippy Starship is unlikely to happen. If it’s up to Musk in the end, I’d bet on an inflatable base,

  5. Look up DTAL (dual thrust-axis lander). LSS is repurposed from a vertical Earth-Mars lander and not well suited for further repurposing as a moonbase.

    As for the upper thrusters, doesn’t thrust in a vacuum expand spherically once it’s clear of the bell? So the angle of the exit cone doesn’t matter? Musk has said he doesn’t believe they’re necessary but is adding them as insurance.

  6. He will never do any of this.
    Wouldn’t be surprised if some SpaceX interns threw this presentation together in a weekend or something.

Comments are closed.