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I was wondering how a Starship or Falcon-9 booster could land on an uneven surface, like that of the Moon or Mars. Is the landing gear similar to the Boeing hydraulic landing gear, which cushions and converts the kinetic energy into heat? Without GPS system, how can it accurately land on a pre-planned site.

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There are two possible answers to this, although you probably won't like either of them.

Answer 1: They don't. Falcon 9 only lands on prepared landing sites and Starship doesn't exist yet.

Answer 2: We don't know. Falcon 9 will never land on non-prepared landing sites and Starship iterates so fast that it is impossible to predict what it will look like in 4 years. In every presentation that Elon Musk has done so far, Starship had different landing legs. Every Starship that has flown so far (Starhopper, SN5, and SN6) had different landing legs (and neither of the three had landing legs that matched any of the presentations, although Hopper was at least somewhat close), and Elon Musk has just tweeted last week, that they will be completely changing them again.

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  • $\begingroup$ It's also worth pointing out that both SN5 and SN6 stood noticeably leaning to one side after just one landing on a perfectly flat surface, suggesting that the current landing legs are not working very well. $\endgroup$ – plasticinsect Sep 9 '20 at 23:06
  • $\begingroup$ @plasticinsect That is a result of the current one engine offset testing configuration. The prototypes have to fly slightly tilted to keep the center of mass balanced. So one of the legs touches down first and is crushed under the weight. You can find pictures and videos of this via google/youtube. $\endgroup$ – Daniel Sep 10 '20 at 11:39
  • $\begingroup$ @Daniel I know they had to fly tilted because of the offset engine, but it seems like any adequate landing gear meant to be reused should be able to handle that kind of stress without being permanently crushed. These vehicles will (as pointed out in the main question) need to land on uneven surfaces. It seems likely the crushing of the landing legs is the main reason for the decision to change them. $\endgroup$ – plasticinsect Sep 10 '20 at 17:04
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    $\begingroup$ @plasticinsect: Considering that they are still working on the tanks, I always assumed that the current iteration of the landing legs was more a cobbled together contraption on the quality level of "as long as the thing doesn't fall over, they're more than good enough". SpaceX believes heavily in not designing anything until it needs to be designed. Remember, a year ago, Starship didn't have landing legs at all, it was landing on the fins. $\endgroup$ – Jörg W Mittag Sep 10 '20 at 17:26
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Falcon 9 likely could land on a somewhat uneven surface because its leg-base is rather wide and the legs contain one-time-use crush-able aluminum honeycomb impact absorbers. Still, this surface would need to be rather hard and flat to avoid being excavated by the landing engine's exhaust plume and the massive weight of F9 sinking it into the ground too much. Additionally, I'm reasonably sure that the landing sites for F9 have some sort of locator beacon/radar reflector/similar device that helps the F9 zero in on the exact landing spot which would need to be placed (or software adjustments made to F9). I'd say that with prep including software modifications, the F9 could probably land in a salt-flats desert but not on any unprepared Lunar or Martian surfaces.

Starship, on the other hand, is different considering that "Lunar Starship" which is being developed for NASA is specifically capable of landing on unprepared Lunar surfaces. The Lunar Starship variant will be able to land on the Moon and would probably be able to land on Mars too. This is because it presumably has some terrain-adaptive landing legs along with landing jets high on the ship's body to prevent the rocket exhaust from excavating the Lunar surface as it lands.

Here you can see a landed Lunar Starship, notice the engine ports on the sides:

Landed Starship

And here you can see a Lunar Starship making a landing and firing the landing engines:

Landing Starship

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  • $\begingroup$ "the legs contain one-time-use crush-able aluminum honeycomb impact absorbers" – That's actually only the emergency backup system. They also contain normal hydraulic shock absorbers. In a nominal landing, the crush cores should not be needed, otherwise you'd have to replace the landing leg, which they don't want to do. $\endgroup$ – Jörg W Mittag Sep 9 '20 at 17:36
  • $\begingroup$ "I'm reasonably sure that the landing sites for F9 have some sort of […] device" – Not according to public information. So far, they've been saying that the booster is simply programmed before the flight with the coordinates of the LZ, and it simply goes where it's programmed. In case of an ASDS landing, it's the drone ship's job to be at that precise location and stay there (hence the 4 powerful 360° maneuvering thrusters), there is no communication, not even passive, from the ASDS to the booster. The land LZs obviously aren't moving anyway. (They probably have radar altimeter, though.) $\endgroup$ – Jörg W Mittag Sep 9 '20 at 17:40
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    $\begingroup$ I am sure that my statement about SpaceX's public statements is correct. I make no claims about the correctness and/or completeness of SpaceX's or Elon Musk's public statements. For all we know, the booster is tethered to the ASDS by a nano carbon rope and there is a winch on deck that just reels the booster in, and all the grid fins and engine gimbaling is just a smoke screen. But that is not what SpaceX are publicly disclosing. $\endgroup$ – Jörg W Mittag Sep 9 '20 at 18:02
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    $\begingroup$ It makes sense actually. The ASDS is holding a GPS position, and the F9 is trying to navigate to that GPS position. The GPS errors cancel out, no RTK necessary. $\endgroup$ – Erin Anne Sep 9 '20 at 19:39
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    $\begingroup$ @JörgWMittag - There was a tweet saying: we painted the target area with radio reflective paint $\endgroup$ – paj28 Sep 10 '20 at 14:12

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