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Wcmille

Engineering Landing Gear

Question

When selecting (non-wheeled) landing gear, how are you solving this problem:

Given a landing mass, gravity, and anticipated impact speed:

What is the correct gear count, type, spring strength, and dampening strength?

 

I am using experience/guesswork to make my choices now, so answers in that ballpark won't help much. Looking for the mathematics behind the parts, or experimental results someone has collected.
 

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2 hours ago, Wcmille said:

Given a landing mass, gravity, and anticipated impact speed:
What is the correct gear count, type, spring strength, and dampening strength?

Well, for starters, I don't use lander legs in 1.1.3 because of serious bugs.  Instead, I'm using box girders and will continue to do so at least until 1.2 if not longer, depending on how long it takes to squash the bugs.

But prior to 1.0 and hopefully after 1.2, my experience with lander legs is as follows:

We have 3 sizes of lander legs, which are scaled to the 3 original stock sizes of parts.  That is, use the smallest legs on 0.625m parts, the medium legs on 1.25m parts, and the biggest legs on 2.5m parts.  In most cases, 3 or 4 of the appropriate size of leg will do you.  If your lander is really heavy (such as a big ISRU thing), then add more legs.  And if your lander is still too heavy for that, use box girders.

You can test your lander design on the launchpad because Kerbin has more gravity than anything you can land on except Eve.  If the legs can survive a hard drop on Kerbin, then they'll work most places.  So, if your lander has enough thrust to get off the ground on Kerbin, launch it up a dozen meters or so, cut the power, and let it drop back onto the pad.  If you don't have enough thrust for that, use launch clamps to drop it from whatever height desired.

I have never tweaked any of the leg settings off their default valures.

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Think about how they are used on cars.  A heavier vehicle needs stiffer springs.  And the dampener helps prevent the oscillation of the spring, i.e. the bouncing.  I generally increase the dampened to reduce the side to side sway in planes and reduce the bounce back on legs.  Stiffness will be determined by how heavy the load is.  

 

By adjusting these and setting up the gear correctly, I haven't had t to many issues with them.

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1 hour ago, ForScience6686 said:

Think about how they are used on cars.  A heavier vehicle needs stiffer springs.  And the dampener helps prevent the oscillation of the spring, i.e. the bouncing.  I generally increase the dampened to reduce the side to side sway in planes and reduce the bounce back on legs.  Stiffness will be determined by how heavy the load is.  

 

By adjusting these and setting up the gear correctly, I haven't had t to many issues with them.

I believe stiffer springs/dampers on heavier payloads is wrong in KSP, because you hit the stress limit faster. Agree wth everything you say with regard to real life. I first noticed this building an Eve lander. By dropping the spring and dampers to 0.85, I was able to hit the surface at 10m/s, which was destroying my legs at the default setting.

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Don't.

All landing legs in KSP are crap and always were crap - only now they are more crappy than ever.

Put the engines radially and use the MK3 to 3.5m adapter. 70m/s impact durability and so flat it would take a miracle to flip it over. Or use girders. Or use wheels. For early Mun landing I really can't emphasize how good the plain long basic wheels of the airplane are. Or get some good, strong RCS or reaction wheels, with enough authority to set you upwards, and land on the side. For bigger craft, MK3 cargo bays and cargo ramps make cool legs. Or pretty much think up anything else than the landing legs.

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What is the current issue with landing legs?  I've only recently returned to KSP  and haven't encountered the bug you mentioned yet.

Cheers

Clipper

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14 minutes ago, Clipperride said:

What is the current issue with landing legs?  I've only recently returned to KSP  and haven't encountered the bug you mentioned yet.

Cheers

Clipper

Landing legs have a couple problems:

if your Kerbals touch them, they can get flung at high speed or killed.

legs explode instead of breaking. This can happen as the results of going on or off rails with the physics engine.

 

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11 hours ago, Geschosskopf said:

and let it drop back onto the pad.

Just don't blow up the pad in career, though. Some landers can get pretty huge.

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1 hour ago, Wcmille said:

Landing legs have a couple problems:

if your Kerbals touch them, they can get flung at high speed or killed.

legs explode instead of breaking. This can happen as the results of going on or off rails with the physics engine.

 

In that case I have encountered the bug - although I didn't know it at the time.

Whilst my scientist was on Minmus using his jetpack to get to a ladder to retrieving data, he suddenly shot up into the command module and exploded.  He had just clipped the landing gear on his way past.  Now I know why - thank you.

 

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5 hours ago, Wcmille said:

I believe stiffer springs/dampers on heavier payloads is wrong in KSP, because you hit the stress limit faster. Agree wth everything you say with regard to real life. I first noticed this building an Eve lander. By dropping the spring and dampers to 0.85, I was able to hit the surface at 10m/s, which was destroying my legs at the default setting.

Hmm good to note. I have not dealt with very large loads so far in this version.  I would guess at that point you may be better off with larger gear though.  As far as using girders and other parts, I just don't see that as acceptable.  I like some bit of realism here, and the bugs haven't cause much problems to consider that route.

Edited by ForScience6686

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3 hours ago, Clipperride said:

What is the current issue with landing legs?  I've only recently returned to KSP  and haven't encountered the bug you mentioned yet.

There are many separate issues:

1.  They are catapults for Kerbals who touch them.  The bigger the leg, the further and faster the Kerbal is flung.

2.  Legs explode instead of breaking.  The smaller legs also frequently explode when a Kerbal touches them.

3.  None of the legs have any friction at all.  Ships landed on legs will slide on the ground even on the most gentle of slopes.  In fact, they often slide UPhill.  The steeper the slope, the faster they slide.

 

2 hours ago, Ultimate Steve said:

Just don't blow up the pad in career, though. Some landers can get pretty huge.

That's why you revert the flight and call it a simulation :)

 

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2 hours ago, ForScience6686 said:

Hmm good to note. I have not dealt with very large loads so far in this version.  I would guess at that point you may be better off with larger gear though.  As far as using girders and other parts, I just don't see that as acceptable.  I like some bit of realism here, and the bugs haven't cause much problems to consider that route.

Yes, but I would like to "run" the numbers as it were. I am deliberate about my TWR and dV (I don't just "eyeball it"); I want to be the same about all the aspects of the craft.

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For me, I determine what the fastest acceptable landing speed is for a given body (I usually do 5-7m for munar landings, but higher if I don't trust my parachutes for the return trip). Then I pick a part or set of parts that are the designated impactors. Often I'll use wings for this role because, one, you can attach swept wings radially and they'll go below the part they're attached to. Two, they can act as stabilizers and drag sources during re-entry (which means you want them pointed backwards when you're going 1600 m/s or you risk flipping and blowing up). Three, they typically have impact speeds well above my tolerance threshold -- for example, tail fin has a 12 m/s tolerance, which is almost twice my 7 m/s target velocity. Even if a parachute is severely overloaded I don't expect it to go much higher than 10 m/s, so tail fin is a good small-vehicle landing candidate.

For larger landers, I like to attach my radial boosters to type A wings (the stubby, narrow wings) and this both decreases the likelihood of somethig blowing up on separation, and increases the base size of my lander if I land on my engines. If my lander is very tall I'll add swept wings to the outside of my radial boosters configured so that the swept wing tips coincide with the level of the engines. This gives me quite a large landing base that tends to be reasonably stable even on moderate slopes, even with pretty large craft.

For a twin boar+2 Jumbo-64 tank booster, I'll upgrade the type A wing connecting it to the main stack, to type a B wing which increases the distance to the booster and also increases my wheel base even more.

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On 22-8-2016 at 5:13 AM, Wcmille said:

When selecting (non-wheeled) landing gear, how are you solving this problem:

Given a landing mass, gravity, and anticipated impact speed:

What is the correct gear count, type, spring strength, and dampening strength?

 

I am using experience/guesswork to make my choices now, so answers in that ballpark won't help much. Looking for the mathematics behind the parts, or experimental results someone has collected.
 

My approach is not exactly rocket science:

  1. Tiny probes get 4 of the LT-05 (the tiny one)
  2. Pretty much everything I build gets 4 LT-2s (the big one)
  3. Exceptionally huge landers get 8 LT-2s.

I also experienced the legs blowing up when Jeb walked into them, so my advice would be: don't do that.

I am against McGyvering my landers. Girders are not landing legs, so I shall not use them for that purpose unless it is absolutely unavoidable. (Note, I am not saying others shouldn't do this - just my playing style...)

Edited by Magzimum

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I've just dropped my lander several times, trying different spring and damper settings. Here are some of my first conclusions:

  • Increasing both the lander spring AND damper is foolish. You increase the stress on the leg, and you're more likely to break something.
  • An ideal spring bears the tension between low enough that the lander won't bounce off the surface on impact, but high enough that the bottom of the lander will not strike the surface.
  • An ideal damper is high enough to prevent the lander's underside from striking the surface, but as low as possible to reduce leg stress and prevent overdamping. If the damping is too low, the lander will feel "bouncy" (underdamped).
  • While I believe its possible to design lander legs that could handle fast (>12 m/s) impacts, it appears the leg will fail at these speeds no matter what.

For Gilly, I am going to try a spring of 0.6 and a damper of 2.0. This should help my landers from bouncing off the surface on landing. It may be that Gilly could support extremely low spring value.

I am now inclined to believe that bases, in places like Mun, generally desire lower spring values in the interior portions of the base, so that the base best conforms to the terrain. It would appear that higher spring values cause some legs to lose contact with the surface, and its pretty annoying for legs to explode on the bases.

I wish that Engineers could not only repair landing legs, but also change their spring and damper values in the field.

 

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