Walking on other planets: legged rovers

[Loren Pechtel]

No legged vehicles, at least not yet. Legged robots are still very new, and they'd need a lot of testing before they are sent on a mission to another celestial body.

Walking requires either a flexible outer cover (and everything can only flex a limited number of times before it fails--we would fall apart if we didn't have self-repair) or it has a piece which is sometimes exposed and sometimes not. Every rover to date has gone into dusty environments.

While I believe it would be possible to construct a leg that used purely magnetic coupling and had no problematic surfaces that's heavy, power hungry and causes a big problem in getting power to the piece that moves. Somehow the sci-fi walkers always manage to ignore the dust issue.

 

[Spacetime Inhabitant]

I think the solution is to retain the wheels for normal utility, and have enclosed legs that can be deployed if needed.

 

[robnisch]

I think the solution is to retain the wheels for normal utility, and have enclosed legs that can be deployed if needed.

I don't think we will or should spent much more on Mars than more landers. Better wheeled robots.
There is no good reason to spend a trillion dollars to put humans there.
Look for water.
We will never get out of the solar system for a long time.

 

[bilby]

Somehow the sci-fi walkers always manage to ignore the dust issue.

The AT-AT also had an issue with rebels looping hawsers around their legs, that the designers really should have considered more carefully.

Not much dust on Hoth though, so there's that.

 

[bilby]

..

 

[Spacetime Inhabitant]

I think the solution is to retain the wheels for normal utility, and have enclosed legs that can be deployed if needed.

I don't think we will or should spent much more on Mars than more landers. Better wheeled robots.
There is no good reason to spend a trillion dollars to put humans there.
Look for water.
We will never get out of the solar system for a long time.

What does giving rovers both wheels and legs have to do with having humans on Mars?
It wouldn't cost a trillion dollars to get humans to Mars temporarily (especially if we apply what Robert Zubrin has taught).
Of course it is true that there is not a good reason in the foreseeable future to put humans on Mars, the main problem being Mars low gravity, which also applies to the idea of putting humans on the moon.

 

[steve_bank]

The surface layer on Mars is toxic to humans. One of the limits on solar rovers is dust build up on the panels.

It takes a lot just to keep the ISS going and there have been problems.

The idea of even a short term stay on Mars is unrealistic

Robots are the way to go.

 

[Bomb#20]

Two-legged robots are very challenging to control, even when at rest.

Four-legged robots are much easier to control

Four Legs Good - Two Legs Bad!

All robots are equal, but some are more equal than others.

What the heck are all these NASA-types thinking? Everybody knows how many legs a robot for walking on somebody else's planet needs!

 

[Bomb#20]

Speaking of even-number blinders...

Two-legged walking has emerged only a few times in our planet's biota, all from four-legged walking.
...
In kangaroos, likely around 30 million years ago if not earlier in the Cenozoic.

Kangaroos are pentapedal.

 

[Bomb#20]

Centipede gait: forward-to-backward wave
Millipede gait: backward-to-forward wave
...
 Metachronal rhythm - "A metachronal rhythm or metachronal wave refers to wavy movements produced by the sequential action (as opposed to synchronized) of structures such as cilia, segments of worms, or legs. These movements produce the appearance of a travelling wave."

An interviewer once asked a centipede how it decided which leg to move next.

Show

It never walked again.

 

[Loren Pechtel]

Somehow the sci-fi walkers always manage to ignore the dust issue.

The AT-AT also had an issue with rebels looping hawsers around their legs, that the designers really should have considered more carefully.

Not much dust on Hoth though, so there's that.

Military walkers make almost no sense at all. They have far too much surface for what they contain and thus their armor (and shields if you have a universe that permits them) is going to be vastly inferior to what the same weapons would have in a more compact configuration. The only case I can see for them is exoskeleton systems to permit armored soldiers to function in systems designed for humans (say, ladders.) Other than that, though, it's always about retaining the human component. Look at Ukraine and you'll get a much better picture of what will really happen.

And the AT-ATs were basically walking tanks--with the same tremendous vulnerability to close-in infantry that has been the bane of tanks since the development of the RPG. There isn't a tank in the world that can survive close contact with peer-level infantry. Even if you don't have the fancy warheads needed to defeat tank armor a plain RPG will give a mobility kill if you can hit the tracks with it.

 

[Spacetime Inhabitant]

Somehow the sci-fi walkers always manage to ignore the dust issue.

The AT-AT also had an issue with rebels looping hawsers around their legs, that the designers really should have considered more carefully.

Not much dust on Hoth though, so there's that.

Military walkers make almost no sense at all. They have far too much surface for what they contain and thus their armor (and shields if you have a universe that permits them) is going to be vastly inferior to what the same weapons would have in a more compact configuration. The only case I can see for them is exoskeleton systems to permit armored soldiers to function in systems designed for humans (say, ladders.) Other than that, though, it's always about retaining the human component. Look at Ukraine and you'll get a much better picture of what will really happen.

And the AT-ATs were basically walking tanks--with the same tremendous vulnerability to close-in infantry that has been the bane of tanks since the development of the RPG. There isn't a tank in the world that can survive close contact with peer-level infantry. Even if you don't have the fancy warheads needed to defeat tank armor a plain RPG will give a mobility kill if you can hit the tracks with it.

The walkers in 'The Empire Strikes Back' operated on the 'rule of cool', yet even so were shown to be quite vulnerable, and easily defeatable.

 

[steve_bank]

Important to know if we end up battling Martians. Don't use Star Wars walkers.

There are only so many options for mechanical locomotion.

 

[lpetrich]

There is a big problem with large walking machines: the energy needed to move the legs. I remember an illustration in a book on biomechanics an illustration of a kangaroo rat and a horse jumping upward about one meter. That distance is several times the rat's body size, and less than the horse's body size.

Why that same absolute value? Let us look at Newtonian mechanics.

\( \displaystyle{ E = m g h } \)
E = increase in potential energy, m = the object's mass, g = acceleration, h is the height that the object was lifted.

Let us solve for h:

\( \displaystyle{ h = \frac{1}{g} \frac{E}{m} }\)

The second term is the energy density of the object, if the energy comes from inside that object. This suggests that kangaroo rats and horses have the same muscle energy density, something plausible.

This also means that it is not so impressive that fleas can jump many times their body size -- in absolute numbers, they are not as good as kangaroo rats.

An additional consequence is that giant walking vehicles and robots are impractical. The largest present-day walking animals are elephants, and the largest that have ever lived are sauropod dinosaurs, those with elephantlike bodies with long necks, small heads, and long tails. The greatest recorded shoulder height of an elephant is 4 meters, and some sauropods are estimated to have had shoulder heights of 6 to 7 meters. Elephants and sauropods are/were quadrupeds, and the greatest known hip height of a biped is that of a large theropod, like Tyrannosaurus rex: 4 meters.

The Jaegers of "Pacific Rim" are 250 ft / 76 m tall, much larger, and thus much less practical.

 

[lpetrich]

A simple way to avoid expending a lot of energy in moving the legs is to let them swing freely. One can then use equations developed for pendulums. Swing period:

\( \displaystyle{ T = (2\pi) \sqrt{ \frac{l}{g} } } \)

for length l and acceleration of gravity g. The walk speed is roughly \( (2 \sin (a/2) ) \sqrt{ g l } \) for leg-motion angle a. For a <~ 1 in radians (~ 60d), the first term is approximately a.

With greater size, one can walk faster, but one needs more energy per unit mass to move one's legs.

 

[steve_bank]

Yep, energy is always a boundary.

Radioisotope thermoelectric generator - Wikipedia

en.wikipedia.org

https:A radioisotope thermoelectric generator (RTG, RITEG), sometimes referred to as a radioisotope power system (RPS), is a type of nuclear battery that uses an array of thermocouples to convert the heat released by the decay of a suitable radioactive material into electricity by the Seebeck effect. This type of generator has no moving parts and is ideal for deployment in remote and harsh environments for extended periods with no risk of parts wearing out or malfunctioning. //en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator

RTGs are usually the most desirable power source for unmaintained situations that need a few hundred watts (or less) of power for durations too long for fuel cells, batteries, or generators to provide economically, and in places where solar cells are not practical. RTGs have been used as power sources in satellites, space probes, and uncrewed remote facilities such as a series of lighthouses built by the Soviet Union inside the Arctic Circle. However, the Western Bloc did not use RTGs in this way due to worries about their risk to humans in a radiological accident.[1]

Build a large rover with a fission reactor or a large RTG that carries small crawlers with RTGs.

 

[Loren Pechtel]

An additional consequence is that giant walking vehicles and robots are impractical. The largest present-day walking animals are elephants, and the largest that have ever lived are sauropod dinosaurs, those with elephantlike bodies with long necks, small heads, and long tails. The greatest recorded shoulder height of an elephant is 4 meters, and some sauropods are estimated to have had shoulder heights of 6 to 7 meters. Elephants and sauropods are/were quadrupeds, and the greatest known hip height of a biped is that of a large theropod, like Tyrannosaurus rex: 4 meters.

The Jaegers of "Pacific Rim" are 250 ft / 76 m tall, much larger, and thus much less practical.

Disagree--you can make large walkers. Spider things where each leg is a counterweight to an opposing leg. Might not be good but you don't have the horrendous cost of lifting a leg.

 

[Bomb#20]

An additional consequence is that giant walking vehicles and robots are impractical. The largest present-day walking animals are elephants, and the largest that have ever lived are sauropod dinosaurs, those with elephantlike bodies with long necks, small heads, and long tails. The greatest recorded shoulder height of an elephant is 4 meters, and some sauropods are estimated to have had shoulder heights of 6 to 7 meters. Elephants and sauropods are/were quadrupeds, and the greatest known hip height of a biped is that of a large theropod, like Tyrannosaurus rex: 4 meters.

The Jaegers of "Pacific Rim" are 250 ft / 76 m tall, much larger, and thus much less practical.

Disagree--you can make large walkers. Spider things where each leg is a counterweight to an opposing leg. Might not be good but you don't have the horrendous cost of lifting a leg.