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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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Launch Director
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Post    Posted on: Mon Nov 06, 2006 3:13 pm
Using the Fuel Tanks as your actual structure is not a safe practice. Any hard landing has the possibility of rupturing them and having a very large explosion. The way i designed it was to remove the fuel tanks from the structure completely.

The fact that the current landing legs are directly under the fuel tanks is a problem from a CG standpoint. 1/3 of the mass is outside of the legs. And un-even landing surface will give the potential to tip over.

As to the lack of shocks.... I didn't put them in for simplicity. But i have been thinking about it all weekend. So i did a few changes

They only needed phenolic spacers because the shocks were freezing up from the fuel. But i think any shock absorber on the feet is not going to be enough for a regular landing. An Ideal solution would be something like this. They add about 120 lbs instead of 80 lbs. But they are hinged legs with Large Air Shocks or Cylinders with Coil Over Springs.

http://www.rcgroups.com/forums/attachment.php?attachmentid=1052284


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Post    Posted on: Mon Nov 06, 2006 6:49 pm
Again, great renders. I kind of agree with you that it isn't necessarily safe to mount legs directly onto the tanks, (I'd also feel far better if the pressurization could be done remotely) but the tanks are going to bear some structural loads whether you want them to be or not. There are also pretty big stresses on the tanks due to how they are supported, because the structure around them has to be forced to move when they are pressurized or thermally contract... Tanks are very strong at resisting forces tangential to the surface, but overhanging loads are potentially worse unless they are applied around a proportionately large patch of the tank.

For something like a short leg, you might limit those kind of loads with a large rubber bushing (though I'm half expecting the new piston they are testing to be just something simple like a phenolic rod surrounding the old shock, being pushed into a tightly fitting cylinder). At the moment, the quad vehicles need at least a kilojoule of energy absorption per leg, which is approximately what you'd get with a 2.5 ms-1 landing just on one leg with only some residuals left in the tank. Free of bending loads, you could achieve that with under two pounds of shock absorber...


I don't think there'd be anything wrong with mounting to the tanks were it was just spherical joints connected to shocks, or maybe even hinged structures that naturally apply leverage over a much larger span. (but fairly stubby legs that didn't bottom out and with nice large radius skids wouldn't be too bad either). Your solution might be far better for landing on a level 2 surface, but as you suggest, is just too heavy as it stands. What you are trying to achieve would be much simpler if the support frame sat beneath the tanks - that might favour differential throttling a bit, though I expect that would also make the vehicle much more complicated.


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Post    Posted on: Mon Nov 06, 2006 6:54 pm
Note that the lander never fell over. It just landed too hard and broke the legs. The old vehicle from last year's X cup was tall and narrow and it did tip over, but that is not the problem with the current lander.

It seems to me that Armadillo was pressed for time and concentrated all their efforts on propulsion and control and just stuck any old simple legs under it. With a year until the next cup I am sure they will have some time to work on proper legs, including drop tests, tethered flights to landing and so on.


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Post    Posted on: Mon Nov 06, 2006 7:20 pm
I was thinking a lot about putting something below the tanks but the fact that the engine is mounted above that AND between the tanks it pretty much eliminates any potential for putting stuff below the tanks.

I am also thinking that the part of the reason their legs failed is the same reason. THey have this extremely hot rocket exhaust bathing those legs in fire for who knows how long. It has to have been a factor in the failure of those legs.

We shall see though.


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Post    Posted on: Mon Nov 06, 2006 7:54 pm
Maybe heat had an effect, but I really can't see why it has to have. From the way the phenolic was supported, it looked every bit as if you could kick the legs out from under Pixel if you wanted to.


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Post    Posted on: Fri Dec 22, 2006 10:25 pm
The legs need to be attached to the frame.


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Post    Posted on: Thu May 03, 2007 12:00 pm
Hello, John Carmack, Matthew Ross and the whole team,

might you imagine to later develop legs that can handle rocky surroundings that normally would be of danger for landers?

The thought I have in mind is a set of sensors that can recognize earlier that one or more particular legs will be placed on a rock while at least one other will not - or more generally - that at least one leg will contact ground at a bit larger height than others.

The vehicle would tilt in such a situation and might be destroyed. Then it would be good if according informations would be sent to a computer to handle the situation. The legs could be made telescope legs and the angle between each leg and the remainder of the vehicle could be made controllable instead of being fixed.

Then rocks or boulders might be a bit less dangerous.

What about it?



Dipl.-Volkswirt (bdvb) Augustin (Political Economist)


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Post    Posted on: Fri May 04, 2007 7:27 pm
The question is whether the extra failure modes the additional complexity brings are worth it. But if the terrain you were landing on was so bad that simple legs would mean certain failure, then you would have to live with it.

The simplest solution I can think of is telescopic legs that drop down under the force of gravity, a sensor that can tell whether the leg is fully extended or not, and some kind of mechanism that can jam the legs and keep them at their current length.

While descending, the legs simply hang down. When the vehicle closes in on the ground, the legs will touch down and be pushed inwards. When all four legs are no longer fully extended, you block all of them, then kill the engine.

Effectively what happens is that the vehicle will keep hovering until it's found its footing, and then transfer its weight from the rocket engine onto its legs.


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Post    Posted on: Sat May 05, 2007 4:25 am
Lourens wrote:
The question is whether the extra failure modes the additional complexity brings are worth it. But if the terrain you were landing on was so bad that simple legs would mean certain failure, then you would have to live with it.

The simplest solution I can think of is telescopic legs that drop down under the force of gravity, a sensor that can tell whether the leg is fully extended or not, and some kind of mechanism that can jam the legs and keep them at their current length.

While descending, the legs simply hang down. When the vehicle closes in on the ground, the legs will touch down and be pushed inwards. When all four legs are no longer fully extended, you block all of them, then kill the engine.

Effectively what happens is that the vehicle will keep hovering until it's found its footing, and then transfer its weight from the rocket engine onto its legs.


That's actually a pretty good idea; you could have the telescoping legs acting as pneumatic cylinders with small solenoid valves and some sort of gas flow sensor on the output of each one. As the leg starts to get pushed in, gas escaping through the open solenoid valve triggers the sensor so you know which leg has touched down. After all the legs have reached the ground you slam the solenoid valves shut and all the legs remain at their current length. Using an air-jamming system in the legs would give you some damping too to help cushion harder landings.

Some problems I can see with such a system would be added complexity with the valving of the legs (if the valving fails you either land in 4 very long legs or 4 very short legs). Also the legs would have to be fairly beefy, if for example you use 1.5ft long legs at their full extension, any lateral force on landing will create significant bending moments about their attachment points.


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Post Adding Wires To Strengthen Legs   Posted on: Mon Jun 04, 2007 6:17 pm
Just saw the new photos of the new lander legs today. The legs look a little unstable. Perhaps they could add some wires between the feet of the legs to add strength.

Accoording to my http://www.chroniclogic.com/ degree in bridge building, wire isn't as heavy yet provides much more pull strength than beams.


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Post    Posted on: Fri Jul 20, 2007 2:34 pm
> I’m sure most people are appalled at us using rubber bumpers for “landing
> gearâ€


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Post Re: Adding Wires To Strengthen Legs   Posted on: Sat Jul 21, 2007 3:19 am
Lone Coder wrote:
Just saw the new photos of the new lander legs today. The legs look a little unstable. Perhaps they could add some wires between the feet of the legs to add strength.

Accoording to my http://www.chroniclogic.com/ degree in bridge building, wire isn't as heavy yet provides much more pull strength than beams.


The legs are mostly going to be in compression though and wire is only good in tension. The legs look fine to me, much stronger than they have to be really, the only loadcase that would stress them is if the module landed while rotating rapidly, but in that case the maximum side load that could be transmitted to the legs is restricted by the friction coefficient of the rubber pads (also it's an unlikely load case).


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Post Landing legs/donut   Posted on: Wed Jul 25, 2007 8:13 pm
Of course we are still tinkering with the possibility of a module that has the engine side mounted, and eliminating and protruding landing leg and placing a donut, or ring on the bottom of each tank that would be a shock absorption point of itself.

When we did drop testing with heavy duty shocks from 12", we were seeing over 15G's of shock in the sensors. When we went to the 2" thick rubber, we saw less than a 10G shock, and dropped about 5lbs of weight per corner.

If we eliminated all of the structure at the bottom of the tank and went with a slightly softer rubber, we would shed a few more pounds, and have the absolute simplest shock available.

While landing on the pressure vessel is not preferred, we have found that as overbuilt as the Quads are, there is plenty of structure there to handle the force.

We saw the worst case survivable impact on our last flight attempt at the X-Prize Cup last year. The fully loaded and pressurized vehicle went into an abort mode 15 feet off the gound and landed with a thud on the side of one tank.

We thought the vehicle was damaged beyond use because of LOX leaking at that time, but it ended up being a broken fitting on the crushed roll thruster that was streaming out LOX because the vehicle was on its side.

If we added springs, they lose their temper as soon as the engine fires up. The rubber just ablates a small amount of the surface away, but keeps its shock absorbing properties without being too bouncy.

We are going to try one more type of landing leg/pad that is simpler still. You may see this in an update in the future, but I promise you will not be impressed with any new technology... unless of course you would be amazed by simplicity.

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Post    Posted on: Thu Jul 26, 2007 2:50 am
Since the modules weigh far more on take off then on landing. are you considering some unattached stands for takeoff? A kind of bolt together grid that mirrors the modules themselves and simply provides support while the tanks are full and sitting on the ground.

No need to take legs with you that you don't need later.


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Post    Posted on: Thu Jul 26, 2007 5:20 pm
That would be OK as long as it isn't used for the lunar lander challenge. That challeng allows them to refuel the vehicle after it lands and before making the return trip, but does not allow putting a stand under the vehicle to do it.


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