Newsgroups: sci.space.history
From: henry@spsystems.net (Henry Spencer)
Subject: Re: Apollo Lunar Module: Why four landing gear instead of three?
Date: Wed, 30 Dec 1998 17:21:42 GMT

In article <368a450c.3036396@news.corp.hp.com>,
rusty barton <rusty_barton@hp.com> wrote:
>I know that early designs of the Apollo Lunar Module had five landing
>gear.
>This was later changed to four to save weight. But why stop there?
>The Surveyor and Viking landers worked well with three.

The problem with three legs is stability in the presence of side velocity:
if the spacecraft touches down moving away from one leg, it's easy for it
to pivot on the other two and fall over, especially if the surface happens
to slope down in that direction.  The very wide spread of the legs puts
the pivot line joining two feet overly close to the center of mass.  This
is especially a problem when configuration constraints call for a tall
spacecraft with a high center of mass, like the LM.

Four legs improve the situation markedly.  Five doesn't add much more, and
since leg structure is strongly driven by the one-leg-hits-first case, you
can't make the individual legs much lighter as their number increases...
so the smallest number with reasonable stability (4) is the clear choice.

A *lot* of work was done on stability for lunar landing, and some fairly
worst-case assumptions had to be made because the design was frozen before
much was known about the lunar surface.  Surveyor and Viking had lower
centers of mass and greater tolerance for risk.

>It would seem that three landing legs is a more stable platform than
>four on an uneven surface.

Not when all four legs include crushable shock-absorbing material, which
will simply collapse as necessary to put all four feet on the surface.
--
The good old days                   |  Henry Spencer   henry@spsystems.net
weren't.                            |      (aka henry@zoo.toronto.edu)