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Landing legs

Posted by: JamesHughes - Mon Oct 24, 2005 12:47 pm
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Landing legs 
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Post    Posted on: Thu Nov 03, 2005 2:43 pm
JamesHughes wrote:
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.
It would too! Were it powerful enough, which is to say much more powerful than anything needed for in-orbit manoeuvres. The CoG projection only got outside of the legs because the control after the legs touched the ground did the wrong thing. I don't believe the landing spot was so bad it was a physical inevitability.

JamesHughes wrote:
A Lunar hopper would need wide legs as you cannot predict the surface you will land on, and you don't want to keep engines running just the stay upright.
A lunar hopper can potentially see the area it is about to land on and adjust its legs accordingly, furthermore it can see candidate landing spots and decide between them. It can even let a human pick one of them. I don't mean to say that is a better way to do things or the only sensible way, but having very wide legs is not remotely close to being a necessity - it is simply one reasonable choice. This would not have been possible on the last lunar visits but some things are much different now.


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Post Landing legs illustration   Posted on: Thu Nov 03, 2005 10:24 pm
good idea about paint. Here's what I'm thinking:

Image

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Post    Posted on: Fri Nov 04, 2005 6:56 pm
Having both types of landing gear is a good idea: The bottom types settle the thing, and just in case it tips the top legs will themselves fall toward the direction of the tip and contact the ground, allowing the craft to come back to level.


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Post    Posted on: Fri Nov 04, 2005 7:17 pm
I agree, that is a good idea. Maybe: The normal legs could be attatched to a spring system , which, upon touchdown, force out the upper, long legs. Or, they could be loosed from a locking mechanism due to the touchdown impact, causing them to hang freely. If the craft tips one way or another, the proper leg would fold out due to gravity.


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Post    Posted on: Wed Nov 09, 2005 7:04 pm
The spring would have to release slowly to keep from over compensating. Perhaps some kind of piston can keep it from snapping back violently.

It shouldn't fall over with two sets of landing legs.


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Post    Posted on: Wed Nov 09, 2005 8:22 pm
This is OT but I need some info before I suggest anything.

How many Seconds of fuel will the craft have when it returns from a full flight profile at hover thurst levels, 20 feet off ground? Time dose not include safety margins.


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Post    Posted on: Fri Nov 03, 2006 3:01 pm
SO i have been thinking about the leg problem for a while.

I use a 3D engineering program at my company so i whipped up a quick design for some very sturdy and very stable legs.

4" Square x 1/4" wall aluminum tubing. 2 1/2" holes drilled in both sides the full length. Welded across the existing 4" square frame and on a 60 degree angle from those cross pieces. It adds a total of 80 lbs to the ship though. Not sure if there is enough margin for that. But it sure looks like they have plenty of thrust!!

Here are a few renders of them.

http://www.rcgroups.com/forums/attachment.php?attachmentid=1048172


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Post    Posted on: Fri Nov 03, 2006 3:29 pm
Very nice renders SGTalon!
I was wondering when this thread would come back to life...


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Post    Posted on: Fri Nov 03, 2006 11:19 pm
The design is squat enough so that you don't need two sets of landing legs in this case. Perhaps some whip like extensions to make sure it settles properly.


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Post    Posted on: Sat Nov 04, 2006 12:54 am
Very pretty, but you haven't included shock absorbers, so the landing might be a bit hard, even at low velocity. A 12% increase in dry weight, more with added shocks, might drop performance below level 2.

The current 'legs' (shocks) on the the quad are quite spread out already, they just weren't strong enough. The tanks supporting them seem to be very strong though, so why waste mass adding more structure?

Maybe flat pads with a u-joint on the bottom of the shocks? Modify the top of the leg to 'cup' more of the tank and spread the load?


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Post    Posted on: Sat Nov 04, 2006 8:22 am
What about attaching a short length of 12" diameter tube on the base of each sphere to spread the load? You could insert a damper inside each tube to allow a slightly smaller diameter tube fitted inside with a foot on the end to cushion the landing.

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Post    Posted on: Sat Nov 04, 2006 11:04 am
It might be worth trying to avoid any extra welding to the tank closer to region of maximum thinning. There's still the possibility of having something slightly compliant engaging the tanks at a larger radius to help reduce the stress due to leverage (though things are more difficult with LOX) Something that may have been ominous in the updates was that the spring appeared capable of lifting the unfuelled vehicle off the shocks.
This leaves the vehicle more prone to bouncing. Hopefully with the 3" piston, it's heavily damped both ways, maybe the new piston is a hydraulic shock with a built in pneumatic accumulator??
Also in view of keeping the leverage at the base of the tanks low, it might be better to have a skid with a radius of curvature approaching the length of the leg, this way pressure applied by whatever is contacting the skid points more nearly through the very base of the tank, minimising the leverage there. You'd still want the skid to be made from something like a hard stainless, though it need not be particularly thick - especially toward the periphery.


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Post    Posted on: Sat Nov 04, 2006 12:13 pm
With a view to strengthening the base of the spheres, does the internal dip tube go all the way from the top to the bottom of the sphere and if so is it welded at both ends with a side entry for the fuel or oxidiser? This would obviously provide an internal support strut if it doesn't already.

To clarify a bit more about what I was describing in my last post as a landing leg, I was imagining something like a light weight version of the buffer you see on the font or a railway carriage. The foot itself could be fitted with a ball joint to enable it to sit flat on uneven ground.

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Post    Posted on: Sat Nov 04, 2006 2:41 pm
If I remember correctly the parts that broke were the phenolic insulators. Those are the parts that need strengthened if anything does.

A simple solution is needed, not a complex one. How about replacing the feet with a curved piece of spring steel, angled down and out not extending past the edge of the tanks. This would widen the stance and provide for shock absorption without the complexity of gas shocks. Use material similar to what is used in leaf springs on cars.

It was an impressive showing at the X Prize Cup in any case. While I watched it all on the webcast this year, I hope to attend in person in 2007.


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Post    Posted on: Sat Nov 04, 2006 10:14 pm
Sump: It would be odd if the dip tube were joined to the sump, it would diverge from the quad cutaway picture and be a highly stressed (tension) element.

"If I remember correctly the parts that broke were the phenolic insulators. Those are the parts that need strengthened if anything does."
And the shock absorbers... which I think were bottoming out when they hit one leg first??

Complexity of gas shocks: Hopefully gas shocks would be something you could buy rather than build.

Replacing the feet with a curved piece of spring steel: Springs on their own are heavy and may induce fairly evil problems with bounce. Sheets of spring steel in particular still leave you with a leverage problem and becuase of their close proximity to the ground might lose temper. They're also not uniformly stressed unless tapered so weigh more than an equivalent helical spring.

Armadillo are doing swing and drop tests this weekend (maybe right now). Not sure about the details of that.


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