sudden instability 100m from Tylo landing

[Quasispecies]

I'm testing a new heavy lander on Tylo. My ship is based around a mark 2 lander can on a large central fuel tank and a Mammoth engine and 6 side fuel tanks with terrier engines. I'm also carrying two detachable side stages with ore tanks and ISRUs to refuel after landing. I'm using RCS and Vernor engines to hold me in retrograde and keep me stable. I get down to 10m/s or less and a slow vertical descent, with everything looking great. At around 100m from the ground - with no inputs from me - the ship suddenly becomes unstable and starts swinging wildly, making landing very difficult, usually resulting in a crash. I've messed around with gimbal limits on the Mammoth, but can't seem to resolve this satisfactorily. Any ideas?

[AHHans]

Hi @Quasispecies, welcome to the forums.

Are you using SAS retrograde hold while landing? In that case it could be that it tries to keep up with the retorgrade direction that starts to wildly change when you slow down to a near stop. SAS is supposed to switch off retrograde hold when your speed goes below 1 m/s, but if the speed changes too fast then it can get confused and keep retrograde hold when it shouldn't.

Or are you using another SAS mode? If so, which one?

[Quasispecies]

35 minutes ago, AHHans said:

Hi @Quasispecies, welcome to the forums.

Are you using SAS retrograde hold while landing? In that case it could be that it tries to keep up with the retorgrade direction that starts to wildly change when you slow down to a near stop. SAS is supposed to switch off retrograde hold when your speed goes below 1 m/s, but if the speed changes too fast then it can get confused and keep retrograde hold when it shouldn't.

Or are you using another SAS mode? If so, which one?

Yes, I'm using SAS retrograde hold. And I think you're right, I just landed safely on Tylo with SAS turned off, I was slowly dropping vertically and gently touched down with hardly any instability. So going forward I'll get everything under control using SAS and then toggle it off. Hopefully that should do it. Thanks.

[GuyWithGlasses]

1 hour ago, Quasispecies said:

Yes, I'm using SAS retrograde hold. And I think you're right, I just landed safely on Tylo with SAS turned off, I was slowly dropping vertically and gently touched down with hardly any instability. So going forward I'll get everything under control using SAS and then toggle it off. Hopefully that should do it. Thanks.

This way is fine, also you can try with SAS On and "mantain stability" mode (can help in landing on slopes and unstables landings in general)

 

 

[Vanamonde]

If SAS is switching from orbit to surface mode, it may decide it's a few degrees off and initiate a sudden correction, which, on a ship that's already large, might be too much for it to get back under control. Perhaps manually set to surface mode higher up? 

[Quasispecies]

9 minutes ago, Vanamonde said:

If SAS is switching from orbit to surface mode, it may decide it's a few degrees off and initiate a sudden correction, which, on a ship that's already large, might be too much for it to get back under control. Perhaps manually set to surface mode higher up? 

This is a good point but in my case I'm in surface mode all the way down from 35km.

[AHHans]

4 hours ago, Quasispecies said:

So going forward I'll get everything under control using SAS and then toggle it off.

That's one way. Switching to "mantain stability" as @GuyWithGlasses recommended is another way. (And one way to quickly do this is to tab the <F> key!) What I usually do is to switch to "radial out". With the navball in surface mode this means pointing straight up.