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#1 2026-10-02 03:18:08
- Oldfart1939
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Landing Legs for Lunar Starship
This may not be where this should be posted, but it is a decent YouTube presentation that purports to announce/review the landing legs required for Starship Lunar Landings.
I've been quite skeptical about some of the hare brained ideas posted across the Internet and find that this one has some merit in concept:
https://www.youtube.com/watch?v=NqhTsAP … WL&index=1
I'd definitely be interested in GW's take on this presentation!
I truly believe that and lunar lander needs a wide base and low center of gravity to be successful. There's a lot of wild Bravo Sierra on the Internet but this seems to be addressing the problem realistically.
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#2 2026-10-02 06:08:00
- tahanson43206
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Re: Landing Legs for Lunar Starship
This post is reserved for an index to posts that may be contributed by NewMars members:
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#3 2026-10-02 10:52:29
- GW Johnson
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Re: Landing Legs for Lunar Starship
Oldfart1939:
If I understood the video correctly, they are claiming that Starship HLS's cg will be low enough to be stable on fairly short legs at 8 degrees effective slope. The numbers I ran for Starship on guessed short landing legs showed tip-over static instability at 5 degrees, so there is that issue. At least, the video indicates they (SpaceX) are aware of the issue.
There is the separate issue of rocket blast kicking up dust and rocks. I thought that SpaceX was planning not to use the Raptors for the touchdown (for one thing they do NOT throttle down to the 5-8% thrust the video talked about). Instead, there were to be a ring of things resembling Super Draco's, as canted tractor rockets way up near the nose.
I was a little taken aback by the video talking about "high wind landings" on the airless moon! It may be more AI slop than anything generated by a knowledgeable human. That is one thing to be aware of. There is no policing for truth on Youtube. Or pretty much any other site.
There is another serious issue for landing leg design that the video seemed to mention too briefly and then quit addressing. That would be unwanted/unintended horizontal speed at touchdown. Which is mostly what tipped over 2 commercial landers so far.
As the vehicle rotates about the effective contact point on the leading side of the leg pattern, its cg will rotate a bit upward in an arc about that contact point. That's a rise in potential energy. If the horizontal kinetic energy exceeds that potential energy gain, you WILL tip over due to the unwanted horizontal speed! Plus, if the leading leg (or legs) digs in, the contact point rotation center is pushed below grade, reducing the PE gain that you need, for overcoming the unwanted horizontal KE.
That brings up two things about the pads at the ends of the landing legs: they have to be big enough not to sink into soft regolith, and they need to be tipped inward off of horizontal, so that the lead pad(s) leading edge(s) cannot dig in and "trip" the vehicle as it touches down. It needs the ability to "skid" on the leading side, while digging-in ONLY on the trailing side. The video ignored these issues entirely.
There is also a serious experience problem to consider here: SpaceX has ZERO experience landing anything on soft dirt. The only things they have ever landed on were smooth-and-level, hard-surfaced, reinforced-concrete landing pads. Landing in the soft dirt is entirely a different experience!
The robot that makes bricks put of lunar regolith is not the solution for landing large things on landing pads it constructs, unless you can bond the bricks together into a single piece. The landing pads will just push the loose bricks down into the soft regolith, locally, right under the pad. The pads still sink in, just maybe not quite as deeply.
GW
Last edited by GW Johnson (2026-10-02 11:00:18)
GW Johnson
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"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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#4 2026-10-02 11:08:34
- Oldfart1939
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Re: Landing Legs for Lunar Starship
GW-This is why I threw the topic in your direction. The winds issue made me almost laugh watching the video. My thought about the bricks made from regolith was to double or triple the depth and excavate the regolith from under the touchdown point and "cement" them together by some means.
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#5 2026-10-02 13:42:44
- Oldfart1939
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Re: Landing Legs for Lunar Starship
At some point in development process, they will need to expend a ship (or several!) trying to land on some sand or dirt somewhere!
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#6 2026-10-02 15:58:29
- GW Johnson
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Re: Landing Legs for Lunar Starship
That "soft dirt" here is dry, fine, sand dune sand. The min allowable stress for that material is 0.1 MPa, or in US units, one US ton per square foot, which corresponds to rounded sand grains. The max is only twice that at 0.2 MPa = 2 US tons per square foot, corresponding to angular sharp grains. Those are allowables for long-term resistance to settling, the actual immediate failure pressures are usually factor 2 to 2.5 lower. EDIT 10-3-2026: meant to say higher! Not "lower".
I successfully used criteria like that for designing the thrust-blocking and dead-manning of underground fire water mains, compliant to the fire codes, which must be unburied for the fire marshall to witness. Test pressure is usually specified as 175 psi, unburied in any way, and absolutely NOTHING can move! If he wants the pressure held like that for days, you do it! Most of that stuff is 8-inch or larger slip-joint pipe. There is essentially ZERO strength in slip joints! The water supply people get away with burial, and only about 60 psi. That's why water mains blow out and get repaired all the time, while fire mains work without failure for a century or more.
Both Mars and lunar regolith are similar to dry, fine, Earthly sand dune sand, with lunar the sharp particles, and Mars the rounded. The presence of rocks and gravel in that regolith does NOT help with strength, until and unless there is so much rock and gravel that all the stones touch each other, everywhere within it. Most places they do not!
I got this understanding from the civil engineering texts and references that deal with foundation design, where the load is the static weight, to be supported upon a certain area. I originally trained in aerodynamics, thermodynamics, and propulsion, but building fire mains was something I had to do, and professionally, after the big drawdown layoffs after the fall of the Soviet Union. So I learned it.
I also learned how to build slab foundations for high-expansion clay soils, and I did that very successfully. It is very rare when a customer is willing to spend for the concrete and steel to do that job right! But about a quarter century ago, one did and built what I designed, and those two houses have NEVER, EVER moved and cracked in that quarter century! That's for a clay plasticity index near (or above) 30, when the federal loan rules say that a plasticity index of 15 is "extremely expansive"! Worst I have seen around here is 60 to 70. Most of the clay around here is Bentonite, which is what everybody else around the country uses to keep the water in their ponds from sinking into the Earth. That delays the effects of rain under a foundation by about 6 months to a year or so. That kind of clay can swell when wet by almost 10%.
From both mechanical and civil engineering comes the practice of multiplying the static load by certain factors to account for low-speed dynamic effects, and certain other factors to account for the unevenness of load application across the intended supporting area. I typically use factors of 2 for both effects, for a factor of 4 applied to the static load. Works well enough, and without the need to run other people's computer codes. Fits right in with my pencil-and-paper engineering abilities.
Unless SpaceX comes and talks with somebody like me, their first attempt at a soft-dirt landing is going to be really, really disappointing. That kind of stuff is NOT taught in aero or mechanical engineering schools. It's civil engineering. I did not originally train in civil, but I learned some of it to continue having a way to make a living after aerospace-mechanical engineering effectively died about the mid 1990's. I retired before it ever really came back. And it came back in the form of kids underpaid to run other people's computer codes, without any ability to do the pencil-and-paper stuff that I do.
Some of the hazardous material experience from aerospace-mechanical stood me in good stead, though. Once of the things I had to do was dispose properly of over-age science lab materials from public schools. I did have to recognize and properly treat both explosives and deadly poisons doing that work.
GW
Last edited by GW Johnson (2026-10-03 09:17:06)
GW Johnson
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#7 2026-10-02 17:11:56
- SpaceNut
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Re: Landing Legs for Lunar Starship
One issue for the starship is the mass balancing from ground upward to the tip of its nose since the fuel is mostly gone the rocket is light at the bottom but heavy at the top. Height from ground is some where at 52 m and its just 9 m wide at the base. The Center of mass during lunar surface operations: approximately 15-20 m above the lunar surface if the mass of each section stays true. But if the mass of cargo and habitat goes up its no longer low.
A possible mission surface payload: roughly 5-25+ metric tons depending on mission objectives. Based in part on a small 4 man crew which is now where near the dream 100+ tons of payload capacity. The larger challenge is usually not carrying the consumables, but safely landing the mass, generating power through the 14-Earth-day lunar daylight period, and climbing the 20-meter-plus elevation from the lunar surface up into a 52-meter-tall vehicle. Which makes it unstable as mass is lowered to the surface.
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#8 2026-10-03 09:22:46
- GW Johnson
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Re: Landing Legs for Lunar Starship
Spacenut:
I quite agree. They have not thought all this through properly yet. The flight tests should include landings on desert sands and soft muds here on Earth. The desert sands should include terrain with big rocks and creek beds or washes, plus surface slopes.
GW
GW Johnson
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#9 2026-10-04 04:05:31
- Oldfart1939
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Re: Landing Legs for Lunar Starship
My thoughts are they should simply expend a non-orbital starship with legs coming in from maybe 25 km and then land it on a beach or a patch of desert with boulders. Or as GW suggested in mud. See how stable (instability testing!) it is (isn't). This is how I would approach the problem. Don't worry about heat shield installation. Do it "on the cheap," and learn from the experience.
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#10 2026-10-04 05:56:52
- tahanson43206
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Re: Landing Legs for Lunar Starship
The Starship that would land on the Moon does not need to worry about aerodynamic shape after it reaches LEO on the way to the Moon. Huge landing legs can be added to the vessel in LEO. The landing legs can be in the form of a right triangle with sides equal to the length of the Starship. Only three are needed.
I'll try to come up with an image to go with this concept.
Gemini offered to try to create an image to go with the concept above:
[img]blob:https://gemini.google.com/140d7b6c-025a-44e8-8887-c090d661912e[/img]
That didn't work.... Gemini thought the URL above would work here but obviously I'd does not.
Here is a version served from the NewMars lifeboat:
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#11 2026-10-04 07:11:33
- SpaceNut
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Re: Landing Legs for Lunar Starship
current image on wikipedia is vary similar which say what is launched from earth is not the current starship which is not a returnable vehicle back to earth only to its orbit or the halo station. Its a heavily modified for lunar use vehicle and not what we are currently launching.
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#12 2026-10-04 08:22:49
- Oldfart1939
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Re: Landing Legs for Lunar Starship
Well, if we are doing orbital refueling, why not adding the landing legs while in orbit? It's never coming back through the atmosphere and will be a candidate for reuse afterwards. The mass of the heat shield can be replaced by landing legs.
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#13 2026-10-04 10:27:31
- tahanson43206
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Re: Landing Legs for Lunar Starship
following on Oldfart1939's post #12, I asked Gemini to change from Falcon 9 to Starship, and to add external tanks and storage attachments for shipment since in space there is no atmosphere to worry about. All the load master has to worry about is symmetry of the mass distributed around the ship, and also keeping the center of gravity as low as possible.
I note that Gemini increased legs count from 3 to 4 with the larger ship.
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#14 2026-10-04 14:51:20
- SpaceNut
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Re: Landing Legs for Lunar Starship
High Tip-Over Risks on a Shifting Lunar Base
A primary rule of building a safe landing zone is surface stability. Starship’s skyscraper-like height-to-width ratio creates a dangerously high center of gravity.
• The NASA OIG audit officially flagged that Starship has an unacceptably high risk of tipping over if it attempts to land on slopes exceeding an 8-degree tilt tolerance.
• Finding or constructing a perfectly flat landing pad on the cratered, rugged terrain of the lunar south pole without tipping over a 160-foot-tall rocket is an extraordinary risk. If the ship tilts past its narrow tolerance on touchdown, the crew has no way to launch or survive.
NASA audit raises concerns about astronaut safety on SpaceX, Blue Origin’s moon landers
Nasa has no lunar lander to make use of it for the next mission to the moon and that is a problem.
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#15 2026-10-04 19:30:40
- tahanson43206
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Re: Landing Legs for Lunar Starship
As a follow up to SpaceNut's Post #14, here is a link to GW Johnson's paper on Moon Lander using Starship as a model:
https://www.dropbox.com/scl/fi/nbna7k4w … cz4vu&dl=0
This paper contains mathematics for those who are so inclined.
1) CG Position For CG Height, Plus Some Basics
2) Doing Basic Statics Plus Some Geometry for Leg Reaction Forces
3) Summary of Statics Results for Leg Member Forces
4) Other Related Results (Too-Short An Reff Was Used)
5) All of the Rough Field Lander Design Criteria in One Place
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