In sci.space.policy message c41f5d96-cace-4d51-942d-9f61f8d34c01@x4g200
0pre.googlegroups.com, Sat, 6 Nov 2010 11:35:25, Alan Anderson
posted:

On Nov 5, 7:38*pm, Dr J R Stockton
wrote:
It should have six legs, so that while walking it stands stably on three
feet while moving the other three in the direction of travel.

Six legs simplifies the task of balancing greatly, to the point where
you don't need any special feedback or fancy motion to remain stable
on relatively level ground. However, given that you still need that
feedback and fine control to deal with unpredictable terrain and
obstacles, four legs are sufficient. Goats do just fine without six
legs, for example.

A system with six legs is not obliged to use all of them all of the time
if it can be designed to do better sometimes with fewer in use and the
others raised.

It should be able to manage quite well with one leg failed, with two
failed unless unlucky, and with three failed if lucky. Clearly there
should be a means whereby a failed leg can be raised or cut off.

--
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