Mun landing tutorial; out of control pitchover.

[Rmack]

I was doing another "To the Mun II" tutorial to see If I could get smoother.  Everything was looking good, then it looked like my vertical speed was getting too low, so I throttled back. I think I killed the engine, or was very close, when the ship started pitching over.  It does a whole flip back to upright, and I hit the throttle, which seemed to activated the SAS again, which I had set on 'target'.  I wound up landing on my side, though.

I fixed the problem next time by just setting the SAS to 'hold position' once I was almost vertical.

Is it because my decent got too slow, and I killed the throttle that confused the SAS?

[bewing]

No. When you are targeting retrograde, the retrograde marker can move very fast. Much faster than your ship can rotate.

This can happen when the display changes from "orbit" to "surface" mode, for example.

This can also happen when you change targeting modes (from hold to retrograde, for example) when you are near the ground. The ship tries to swing around, but while it is pointing in a funny direction, if you fire the engines, that makes the retrograde marker move -- and that makes your SAS try to rotate your ship into an even funnier direction.

So SAS is not "confused" -- it's just trying really hard to swing your ship around to a new heading. And it doesn't rotate your ship in the most efficient way.

[Venusgate]

I've never done any tutorials and decided to load it up to see what you were talking about. Craft didnt load and when I hit the maneuvering node, my non-craft exploded. I'd say the scenario is bugged.

[Rmack]

30 minutes ago, bewing said:

No. When you are targeting retrograde, the retrograde marker can move very fast. Much faster than your ship can rotate.

This can happen when the display changes from "orbit" to "surface" mode, for example.

This can also happen when you change targeting modes (from hold to retrograde, for example) when you are near the ground. The ship tries to swing around, but while it is pointing in a funny direction, if you fire the engines, that makes the retrograde marker move -- and that makes your SAS try to rotate your ship into an even funnier direction.

So SAS is not "confused" -- it's just trying really hard to swing your ship around to a new heading. And it doesn't rotate your ship in the most efficient way.

I think I understand how the 'retrograde' setting can move, as it's based on your direction of movement.  However, I had the SAS set on 'target'.  I assumed that this would be the surface.  Is the 'target' icon moving for some reason?

I set the SAS on stabilize and just came in vertical.  I probably could have made RCS adjustments if I'd needed them. 

Edited May 14, 2016 by Rmack

[Plusck]

37 minutes ago, Rmack said:

I think I understand how the 'retrograde' setting can move, as it's based on your direction of movement.  However, I had the SAS set on 'target'.  I assumed that this would be the surface.  Is the 'target' icon moving for some reason?

I set the SAS on stabilize and just came in vertical.  I probably could have made RCS adjustments if I'd needed them. 

"Target" may be a planet or moon, but that target will cease to be a target as soon as you enter the planet or moon's SoI. So if you targeted the Mun from Kerbin orbit, the Mun will no longer be a target when you get there.

Otherwise, "target" is another ship.

So unless you are actually trying to land right next to another landed ship, you most definitely do NOT want to be in "target" mode when landing. You absolutely and most definitely want to be on "surface" mode.

SAS will stop trying to hold retrograde (or any other specified direction) when your velocity goes under 1 m/s. It reverts to stability assist.
However, if you fudge the last part of the landing it's possible for retrograde to swing around so fast that it never gets under 1 m/s, so you can easily find your ship trying to point head-first towards the ground if you overdo your braking in the last few seconds.

The only real solution is: be careful with thrust as you come down to the last few metres. Overcorrecting for a touch too much speed in the very final approach can be fatal.

 

[Rmack]

21 hours ago, Plusck said:

"Target" may be a planet or moon, but that target will cease to be a target as soon as you enter the planet or moon's SoI. So if you targeted the Mun from Kerbin orbit, the Mun will no longer be a target when you get there.

Otherwise, "target" is another ship.

So unless you are actually trying to land right next to another landed ship, you most definitely do NOT want to be in "target" mode when landing. You absolutely and most definitely want to be on "surface" mode.

SAS will stop trying to hold retrograde (or any other specified direction) when your velocity goes under 1 m/s. It reverts to stability assist.
However, if you fudge the last part of the landing it's possible for retrograde to swing around so fast that it never gets under 1 m/s, so you can easily find your ship trying to point head-first towards the ground if you overdo your braking in the last few seconds.

The only real solution is: be careful with thrust as you come down to the last few metres. Overcorrecting for a touch too much speed in the very final approach can be fatal.

 

So, my solution of setting the SAS on stability assist will not hold if I go under 1 m/s?

[bewing]

SAS "Stability" is the only one that does work under 1 m/s.

[Snark]

Here's the recipe for a happy landing:

1. Put your navball in surface mode, and nothing but. @Plusck's explanation is dead right.  Listen to him. 
2. Set it to "hold retrograde."
3. Thrust as appropriate until you land.

Of course, there's a lot of technique lurking in those casual words "as appropriate" that I just tossed in there ... but the point is, as long as your speed doesn't fall really low (i.e. to a near-stop) during descent, you're not going to have any out-of-control tumbling.

And you really, really shouldn't be going that slow on the descent-- it would be wasting scads of fuel.  The ideal landing is a fast one, using a "suicide burn".  Techniques for that are a whole other topic in their own right, but anyway, even if you're not doing an ideal suicide burn, just don't go really slow and you'll be fine.

[bewing]

Except that when you are landing in a place you've never been before, then you don't know the altitude of the surface. So in that case, you have to keep your speed low enough that you can nearly stop when you see the ground is getting too close.

[Snark]

7 hours ago, bewing said:

Except that when you are landing in a place you've never been before, then you don't know the altitude of the surface. So in that case, you have to keep your speed low enough that you can nearly stop when you see the ground is getting too close.

You don't know the exact altitude, but you can see the ground.  With a bit of practice, that's all you need.  Broadly and very approximately speaking, you want your speed of descent to be somewhat proportional to the distance from the ground, at least in the final stages.  And the ground's apparent visual speed-of-approach happens to be directly proportional, too (just due to perspective effect-- i.e. the fact that the ground seems to drift very slowly by when you're flying in an airliner, despite the fact that you're zipping along at hundreds of kilometers per hour).  So as long as you train your eyeballs that "I want the ground to look like it's closing on me about this visual speed", it works pretty well.  Just takes a little practice.

Mounting a couple of downward-facing spotlights helps a lot there, too (even for daytime landings):  really helps to judge distance by eyeball, and also makes it a lot easier to tell what the slope is beneath you.

And aside from just training your eyeballs to judge the closing distance and speed, there are additional tricks you can do, too.  If you've got maneuver nodes unlocked, there's a simple trick that works in stock (requires no mod) that does a pretty good job  of giving a reasonably efficient suicide burn.  While you're on your descent, drop a maneuver node right at the spot where your trajectory intersects the surface.  Then grab the retrograde handle of the node and pull on it until your post-maneuver projected trajectory collapses to a point at the node and the prograde/retrograde handles start flipping back and forth.  There!  You now have your perfect guide.  The time-until-node is your time-until-impact, and the estimated-burn-time is how long you need to burn to kill your velocity at the surface.  For various math and physics reasons I won't go into at the moment, take that estimated burn time and multiply by roughly 0.7 (that's a rule of thumb that works for most TWR and descent-angle combinations, with a reasonable safety margin).  Start your full-throttle retrograde burn when the time-until-node equals that number, and you'll brake to a slow speed when you're pretty close to the surface, then you can descend at a couple of dozen m/s until you're practically right on the surface and can do a last-moment burst of thrust to slow for touchdown.

Or, if you don't mind installing a mod, may I recommend BetterBurnTime, which basically saves you the hassle of the maneuver node.  It will show you an estimated-burn-time and time-until-impact when you're on an impact trajectory over a vacuum world:

 

[Plusck]

And don't forget IVA - you have a radar altimeter inside your command pod, which works really well if you have a healthy respect for the dangers of coming in far too fast.

[wumpus]

Doesn't hitting the "T" button twice simply take you out of "retrograde targeting mode" and into "basic stability mode"?  This seems like a good idea once your velocity gets low enough.  My present system involves three landing burns.  A non-landing burn to lower munappsis to as low as I can get it without hitting any mountains.  

1: A sideways burn near munappsis to kill sideways velocity and wind up as low as convenient (I'll have to check out better-burn-time to see if it helps here).  Keeping SAS on "retro guide" is a no-brainer here.

2: A suicide burn to some predetermined "soft floor".  If you want SAS on "retro-guide" you will need to calculate the retro-floor in terms of velocity.  Note that as far as I can tell, Kerbal engineer's "suicide burn" only works for the vertical component of landing.  Don't try this without KE or similar.

3: A manual landing from a relatively short distance up.  You will probably have to turn SAS on "manual" or have your finger hovering over the "T" button to take over at the last minute.  Basically everything down to here was pretty efficient, from here on down you should be concerned with safety and keeping the impact velocity as low as possible.

Ideally, you should have some way of combining steps 1-2 into a single step, and minimize step 3.  Overdoing the minimization of step 3 is just asking to plough kerbals into the Mun, so be careful, especially on landers with less TWR than you are used to.

[Plusck]

If you use KER, the suicide burn distance isn't accurate if you're going mostly horizontal, but I haven't yet worked out whether it is wholly wrong or just partially wrong.

However, it is still very useful because if you have, say, 1000m to impact according to KER, and you apply full retrograde thrust, then you can tell exactly how much KER is out by whether it stays around 1000m or continues falling significantly below that. From the rate of decrease in KER's estimate, you can then judge quite easily how much further you can let that distance drop before you have to worry, which means that you should end up with the most efficient possible burn.

In other words, seeing how far KER is wrong is the perfect tool to perfect your perfect landing burn!