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Jestersage

Talk, lanky... and landable???

Question

In a sense, it's a continuation of my "landing legs settings" question.

There are many "tall-and-lanky using landing legs" landers, such as this guy (not mine). However, they are able to land easily (I think) without the bouncing and falling over syndrome during my test.

So why are they able to do it, while my attempt failed until I gone with this?

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On 5/25/2018 at 2:50 AM, FireKerb said:

If you do want to land a tall lander, then be sure to land on somewhere flat. Even a slight tilt can be bad for your lander if it's tall. Fat landers are better in every aspect, though. Except for atmospheres (If you want to return). Duna's is usually not strong enough to disrupt ascent. If you are landed on Jool, you are having a bad day and will not go to Kerbin today (or anywhere else). The only times it matters in the stock system is on Eve and Laythe, but you can use parachutes to land and keep your craft upside up. And if you're landing on Kerbin... why?

TL;DR: Fat landers are the way to go.

 

Pretty much. The only real downside is getting them up in the first place -- cargo diameter constraints can be a real hassle on ascent.

 

One useful thing for taller landers not already mentioned, if you find you need them, is to pack reaction wheels to fight rotational torque. It's a kludge solution, though.

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As Rizzo said, basically: in the final seconds you need to make sure it's a slow, perfectly vertical landing on flat ground.

If you look at that first pic again, the ship appears to be resting on its engine. The landing legs are a bit weak for that size of ship, so they won't provide much protection for a heavy landing and won't be able to right the ship if it starts to lean.

So I'd guess that (a) it was landed quite delicately, to avoid damage to the engine, and (b) SAS is still engaged to keep it upright, but (c) gimbal is probably turned off for that engine, so that SAS is only using reaction wheels and not moving the bell of the engine around.

 

Incidentally, turning engine gimbal off for landing is often a good idea since you want a smooth transition from horizontal to vertical motion in those final seconds before landing, irrespective of how much you're throttling the engine. Engine gimbal is fine if your throttle position is constant, but it's easy to spin out of control if you need to throttle up just as you touch down... and if you do end up facing upside down (like Rizzo mentions), you definitely can't use the engine gimbal to right yourself without accelerating right into the ground.
Engine gimbal can therefore put you quickly in a position that you can't get out of quickly without using the engine... but where using the engine would be suicidal.

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Some of it is down to the centre of mass. If it is low then the craft will be more stable on the ground. 

You also want to maximize the spread of the feet. Sometimes helped by angling out the landing struts. 

Fixing the basic problem of having a tall lander is the real solution though. Radially mounted tanks and engines are your friend here. 

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If you do want to land a tall lander, then be sure to land on somewhere flat. Even a slight tilt can be bad for your lander if it's tall. Fat landers are better in every aspect, though. Except for atmospheres (If you want to return). Duna's is usually not strong enough to disrupt ascent. If you are landed on Jool, you are having a bad day and will not go to Kerbin today (or anywhere else). The only times it matters in the stock system is on Eve and Laythe, but you can use parachutes to land and keep your craft upside up. And if you're landing on Kerbin... why?

TL;DR: Fat landers are the way to go.

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Some posts have been removed. Stick to the topic, please. 

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Flat landing zone, slow descent speed and make sure you've killed any horizontal velocity.

Also if you're using SAS set it to Radial Out before you hit the ground.  If it's set to Retrograde and you bounce at all your reaction wheels will try to turn the ship upside down. 

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A good reaction wheel will also help a lot.

Really though, you're taking the path of maximum resistance.  It's not unreasonable for a top heavy lander to be unstable.  Building wider, or just smaller in general, will make it more stable.

Edited by Corona688

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You're really not alone in this - Here's my Duna Lander.   You'll notice that the legs are out on girders to spread them wide.  This is because I found on testing even a shallow slope was tipping the centre of mass outside the circle of the landing legs.  The capsule up top and the fuel tank being that shape and on top of the lighter science package meant it was an upside down pendulumn.  Of course the girders meant more drag at launch so fairings were the tool for the job.

ahSXSMo.png
 

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Nice design. It looks like an AIDS virus.

I would have put the science stuff on top of the cockpit, thus lowreing the CoM, and maybe changed the 100 fuel tank for 2 or 4 oscar tanks radial.

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2 hours ago, Rauko said:

I would have put the science stuff on top of the cockpit...

Woulda made it harder to dump when finished with. 

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Oh, is there a stack separator? I cant see it, but it makes sense. Still, a science jr is only 0.2t, not much weight to carry comparing with the rest of that ship, and placing it on top drops the CoM a fair amount. Trading dv for landing stability, so it depends on your priorities and weaknesses.

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