Three legs or four?

[boriz]

I have often wondered why the Apollo 11 LEM had four legs and not three. I'm sure that an extra unnecessary leg would represent extra unnecessary mass, and three legs would be better when landing on uneven terrain. So why the extra leg?

[Randazzo]

20 minutes ago, boriz said:

I have often wondered why the Apollo 11 LEM had four legs and not three. I'm sure that an extra unnecessary leg would represent extra unnecessary mass, and three legs would be better when landing on uneven terrain. So why the extra leg?

Stability and safety. I think they started with five.

[Beamer]

Although 3 fixed legs are inherently stable on an uneven surface, springs/dampeners can adjust for that, and 4 legs give you a greater distance between center of gravity and turning point which means you can stand on steeper inclines without tipping over - and perhaps crucially, land with a greater horizontal speed without tipping over.

To prevent tipping, assuming a circular limited space (like a rocket fairing) the more legs the better, but there are diminishing returns w.r.t. tipping the more legs you add, and of course there are weight considerations. Even though more legs mean each leg can be lighter, presumably there is some overhead that adds up (although being able to design legs to specification rather than having to choose between a couple of prefab models like in KSP helps ). The tipping point difference between 3 and 4 is quite big though so I guess 4 was the acceptable trade-off they came up with between those considerations.

 

[Snark]

Moving to Science &  Spaceflight, since this is about IRL spaceflight rather than KSP.

(Though you could verify this issue yourself in KSP, if you wanted to, by trying some landings on the Mun, on rugged terrain, and see the difference it makes in terms of how easy it is to tip over...)

[kerbiloid]

Four is too few.

Smart guys have six.

Spoiler

If one fails, five stand.

[mikegarrison]

That is a consideration. Four legs is one more point of failure, and yet adds no redundancy because the remaining three legs do not create a stable base.

[magnemoe]

1 hour ago, mikegarrison said:

That is a consideration. Four legs is one more point of failure, and yet adds no redundancy because the remaining three legs do not create a stable base.

An point however a leg main fail point is the suspension or folding mechanism. If the leg snaps off at the mounting points you are likely to have more serious problems. 
So if you design this a bit smart you still has an semi usable leg.
I assume the main reason is an wider base,as Snark said above. 3 legs are self stabilizing but the base is not very big compared to distance between feet. 4 is much better and its an dimishing return with 5 or 6 legs. 

 

[Aerodynamic Kerbal]

9 hours ago, kerbiloid said:

Four is too few.

Smart guys have six.

  Hide contents

If one fails, five stand.

Smort guys have infinite

[kerbiloid]

21 minutes ago, Aerodynamic Kerbal said:

Smort guys have infinite

Smart guys use legs instead of catching a rocket by armpits.

[Nazalassa]

Very smart guys think landing legs are overrated and use engine noozles instead to save weight and part count in Career.

[kerbiloid]

5 minutes ago, Nazalassa said:

Very smart guys think landing legs are overrated and use engine noozles instead to save weight and part count in Career.

On landing any nozzle is unburnt metal fuel.

[Nazalassa]

Just now, kerbiloid said:

On landing any nozzle is unburnt metal fuel.

Hahaha yes

[StrandedonEarth]

If three legs was “stable enough,” then 3-wheel ATVs would still be for sale new

[magnemoe]

4 minutes ago, StrandedonEarth said:

If three legs was “stable enough,” then 3-wheel ATVs would still be for sale new

Story is a bit more complex I think as lots of them was used by kids,  obvious benefit of 3 wheels is  that you can use an simple bicycle handlebar rather than the more complex control needed for four wheels.  
https://www.motorbiscuit.com/theres-a-very-good-reason-you-dont-see-3-wheeled-atvs-anymore/

As an bonus I got an add from the site, the ultimate fake meme tank is now an Lego model. 
https://theblockzone.com/products/missile-carrier-tank-3665pcs?variant=8103616315489&currency=GBP&utm_medium=product_sync&utm_source=google&utm_content=sag_organic&utm_campaign=sag_organic&gclid=EAIaIQobChMI7-bsj9769QIVbdIRCB3cnw6EEAEYASACEgIzjPD_BwE
No the original design had an Katyusha rocket launcher rather than the two huge ones but still had the feature that the large gun would hit the small one firing straight forward. 
It don't look like its an model you can drive, other crafts come with an remote control and listing of motors like tanks or cranes. 

[kerbiloid]

It works in KSP, why shouldn't irl.

https://spacedock.info/mod/1905/Reliant Robin

Spoiler

 

 

[StrandedonEarth]

36 minutes ago, magnemoe said:

Story is a bit more complex I think as lots of them was used by kids,  obvious benefit of 3 wheels is  that you can use an simple bicycle handlebar rather than the more complex control needed for four wheels.  
https://www.motorbiscuit.com/theres-a-very-good-reason-you-dont-see-3-wheeled-atvs-anymore/

Thanks for that, I should have realized the dynamics of a ATC would be different than a non-wheeled lander. Still, if redundancy is valued over mass, then five would probably be best, with plenty of suspension travel. 

[wumpus]

4 hours ago, magnemoe said:

Story is a bit more complex I think as lots of them was used by kids,  obvious benefit of 3 wheels is  that you can use an simple bicycle handlebar rather than the more complex control needed for four wheels.  
https://www.motorbiscuit.com/theres-a-very-good-reason-you-dont-see-3-wheeled-atvs-anymore/

There's also the issue that you get much better stability on a 3-wheeled vehicle if you put 2 wheels up front and 1 behind.  The Polaris Slingshot comes to mind as an example, as well as Buckminster Fuller's Dymaxion car (I thought Polaris made a much higher "motocyclish" 2-wheels in front trike, but can't find it).  You are vastly more likely to be decelerating while turning than accelerating while turning, so you want your wheels ahead and to the sides.

[SunlitZelkova]

On 2/11/2022 at 1:36 PM, boriz said:

I have often wondered why the Apollo 11 LEM had four legs and not three. I'm sure that an extra unnecessary leg would represent extra unnecessary mass, and three legs would be better when landing on uneven terrain. So why the extra leg?

https://space.stackexchange.com/questions/13501/why-did-the-apollo-lunar-module-have-four-landing-legs#:~:text=In particular%2C chapter 6.4 notes,during 1963%2C decided on four.

Two quotes from this that answer the question-

Quote

Grumman had first considered five legs but, during 1963, decided on four. The change was dictated by the weight-versus-strength tradeoff that had produced the cruciform descent stage, with its four obvious attachment points. The revised gear pattern also greatly simplified the structural mounting of the vehicle within the adapter. Four legs set on the orthogonal axes of the lander (forward, aft, left, and right) mated ideally with the pattern of four reaction control "quads" (the basic four-engine package). The quads were rotated 45 degrees so the downward-thrusting attitude control engine fired between the two nearest gear legs, overcoming a severe thermal problem of the five-leg arrangement.

Quote

Also, a three-legged design would've had a significantly smaller stability margin when landing on a slope, and/or if any residual horizontal velocity remained while landing.

 

Edited February 13, 2022 by SunlitZelkova

[NFUN]

three legs good, four legs better

8 hours ago, kerbiloid said:

It works in KSP, why shouldn't irl.

https://spacedock.info/mod/1905/Reliant Robin

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uh, do you know why they modded that into ksp?

[kerbiloid]

3 hours ago, NFUN said:

do you know why they modded that into ksp?

They wanted something three-wheeled?

3 hours ago, NFUN said:

three legs good, four legs better

The Pierson's Puppeteers aren't agreed.

Spoiler

 

[NFUN]

11 hours ago, kerbiloid said:

They wanted something three-wheeled?

The Pierson's Puppeteers aren't agreed.

  Reveal hidden contents

 

because somebody already did it IRL

https://youtube.com/watch?v=pJdrlWR-yFM

Edited February 13, 2022 by NFUN

[kerbiloid]

47 minutes ago, NFUN said:

because somebody already did it IRL

And not once.

https://en.wikipedia.org/wiki/Three-wheeler

[Beamer]

On 2/13/2022 at 7:31 AM, kerbiloid said:

They wanted something three-wheeled?

The Pierson's Puppeteers aren't agreed.

  Hide contents

 

Maybe the lack of a 4th leg is why they were such scaredy-cats though . I love Niven, but as useful as a 3-legged design is for a picnic table, it is a pretty bad layout for an ambulatory creature, and assuming growth by cell division probably extremely unlikely to evolve without 'help' by some designer of sorts. It was on Earth anyway. Even the artist of that picture seems to have had trouble with it, as that hind leg looks like it has a double femur, suggesting it started off as a 4-legged creature where the hind legs fused together during its development.

 

[kerbiloid]

I guess, he was writing them from a home theater for children, with sweater on the head and a stool as the rear leg.

[magnemoe]

On 2/13/2022 at 12:25 AM, wumpus said:

There's also the issue that you get much better stability on a 3-wheeled vehicle if you put 2 wheels up front and 1 behind.  The Polaris Slingshot comes to mind as an example, as well as Buckminster Fuller's Dymaxion car (I thought Polaris made a much higher "motocyclish" 2-wheels in front trike, but can't find it).  You are vastly more likely to be decelerating while turning than accelerating while turning, so you want your wheels ahead and to the sides.

True not only braking and turning but just turning sharply off road and its an risk the front wheel hit something or dig in. Downside is that  2 front wheels kills the simple bicycle steering cost saving,