Landing legs

I think this is pretty cool!

Interview transcript:
http://www.hobbyspace.com/AAdmin/archiv ... atani.html
http://www.spacefuture.com/archive/flig ... cles.shtml

Supershuki: That doesn't change the fact that a machine with a computer and a lot of moving parts and retractable landing gear is still less reliable than a machine with a computer and a lot of moving parts .

Whether a seized up leg is a problem depends on how far the legs splay and how high up the center of gravity is. Let's assume that the vehicle has four landing legs, and that the two of them on the left splay, and the two on the right get stuck. If the center of gravity is high enough to go beyond the end of the splayed legs, the vehicle will topple.

The farther the legs splay, the farther the vehicle will lean over, but also the farther the CG will have to go to go beyond them. Unfortunately, when the legs are attached close to the ground, the CG moves outward more than the legs when the splaying angle is increased. Another way of looking at this is that when the legs are attached close to the ground, they become a lot shorter vertically when you move their tips outward a useful distance, thus causing a big difference between the splayed and the stuck legs.

Attaching the legs up higher would make the whole thing more stable, because the splaying angle to reach a certain "wheel base" is less, thus causing a smaller leaning angle when a leg fails to splay. A small splaying angle also has the advantage of making the mechanism I drew above thinner, and because of less sideways forces, less likely to jam. The disadvantage is obviously the extra weight of the longer legs.

How heavy would a metre of aluminium tube strong enough to carry a quarter rocket be?

That is WAY COOL! A Japanese Armadillo! No, it uses LIQUID HYDROGEN, it is a Japanese DC-X! Next step, Kankoh-Maru!

Maybe it's a glass half full/empty kind of thing, but I'd hardly say it has a lot of moving parts. Let's see, two ball valves, four solenoids, two gimbals... I think that's it.

Well if the vehicle doesn't move, what's the point?

Yeah and what about the telemetry bits?

Anyway, I've just looked up some data about the coefficient of friction between metal and concrete, and while I couldn't find any definitive numbers, and it's become clear to me that you can't say anything definitive without doing experiments, it seems that if you want your legs to splay because of the weight of the vehicle pushing down on them, you'd better have a wheel on the bottom of the leg so it can ride rather than slide outwards.Or make a teflon landing pad .

The static coefficient of friction of wood on concrete is about 0.62. Metal is a bit harder than wood, but let's take a value of 0.6 for the moment. If there is 2.5kN of weight on the leg, you'd need a sideways force of 1.5kN to slide the leg outwards along the concrete. I'm afraid that would overload my little aluminium rail, especially because that 1.5kN has to be delivered to the end of the leg. The sideways forces on the rail, which is near the centre of rotation, would be much larger, perhaps ten times as large.

Other materials might help, carbon legs on a steel landing plate would give a friction coefficient of about 0.14, requiring a force of 350N at the end of the legs, but that would still put 3.5kN on the rail. Carbon legs could be lighter though.

Having a wheel on the leg would reduce the coefficient of friction by a lot, probably down to about 0.001. This would lower the required force to 2.5N (25N on the rail), and allow lighter construction of the rail. Of course, the wheel would be extra weight, about 12 lbs per wheel or so (from here).

Maybe it would be a better idea to have a simple fixed gear as they have now, and just not land in muddy fields?

The problem is that no matter where you land, those stubby little verticle legs are unstable. What happens when a passenger decides to vomit, and throws his/her weight all over the place? What if weight shifts for some other unforseen reason?
I really don't see why the landing legs have to be so complicated. Take a bunch of hollow, metal tubes a little bit shorter than the length of the vehicle. Put a hinge on one end, weld the hinge to the top of the vehicle; attach a rod to each leg, attach the other end of the rod to a rotating cylinder in the vehicle [e.g. like a bank lock, or an airlock], and presto! We have liftoff!
What's the worst that can happen?

...

The problem with the La Cruces landing was primarily due to the control code not handling landing very gracefully with a gimballed engine. That could be fixed in a number of ways without involving the legs themselves E.g. using differentially throttled engines, or more marginally by gimballing the nozzle about its far extremity rather than above the chamber. Reducing the thrust much more rapidly and diminishing or eliminating the purge thrust spike.

However there isn't something fundamentally wrong with the vehicle itself, it only just managed to fall over and it was obviously doing something wrong - it just needs to learn from the experience and behave a bit differently. Having the control code recognise when part of the vehicle is touching the ground and responding accordingly should not be overly-hard.

Interesting thought; but keep in mind that the RCS is not designed to fight gravity.

I agree. However, I also don't think the landing legs are fundamental to the vehicle. Sure, the vehicle could land the way it is - but wouldn't it be better to land with those extra legs for a safety measure? Not only that, but it increases the variety of landing areas available. Again, what's the loss?

About the RCS: Besides gravity, they also cost fuel.

...

Why bother with the complexities of RCS to aid landing, when just having wide legs for landing prevents all possibilitiies of falling over?

In any case, RCS would not have helped with the fall at XPC, as when they turned off it would have fallen over anyway, because the CoG was outside the leg/ground contact point.

A Lunar hopper would need wide legs as you cannot predict the surface you will land on, and you dont want to keep engines running just the stay upright.

James

And a Lunar hopper will not encounter any air drag. Air drag is the reason Armadillo has made all their legs narrow.

...