Bad bad piloting skills for landing

[MPDerksen]

I am using WAY more dV to land (anywhere) than the dV map would indicate.  Let's use Mun as an example.  I'm in a 25K orbit.  I pick a spot to land, a biome I haven't been to, in this case.  I put a maneuver node at about 90 degrees from it in my orbit, and burn retrograde to get my path to fall just beyond it.  I'm travelling a few hundred m/s, surface relative.  At about 10k up, I kill 100% of my velocity (probably the problem right there), and then drift down, pointing retrograde.  I never let my speed go back over 100m/s, and as I get close, I just burn slightly to narrow the margin. 

I can touch down about anywhere I want, but it's EXPENSIVE and I few times I have been left with too little to get home.  Or, I completely OVER plan, and I'm back, wasting a few hundred m/s of fuel.  Maybe I need a mod to give me a signal when to trigger my suicide burn?  Maybe I need to get braver and come in hotter instead of being so gentle from 10K up?

Pretty much done at Mun and Minmus, to I'm off to Gilly and Ike.  But the skills I need will be even MORE important out there, since rescue vessels will be really far away and time consuming.

[Vanamonde]

The most efficient landing would be just like a launch run in reverse. Wait until the last moment and burn full thrust so that you shed the last m/s exactly as you touch down. But that's just about impossible to do without crashing, which is why it's nicknamed a "suicide burn." But you can get closer to it with practice. And yes, the slower you come down the more fuel you waste in the process. 

[MPDerksen]

Practice?  MechJeb?  Many, many Quicksaves?  

(My only current mod is the Alarm Clock)

Edited September 14, 2019 by MPDerksen

[bewing]

The higher you fall from, the more fuel you waste. Also, the higher your Ap, the harder it is, because you are coming in faster.

It's better to be in a low orbit to start with. 10K to 12K and mostly circular -- or even a little lower than that, but then you can't timewarp.

Start by lowering your Pe on the other side of the Mun to 6km over your approximate landing site. Once you get there, watch the terrain on the horizon to look for a good landing spot. When you see one, burn like hell retrograde. Yes, it's somewhat inefficient to kill all your orbital velocity while you are still a km or more above the ground, but picking a high quality landing point is worth more than a bit of fuel. If it looks like you will end up hovering over a crater as you retroburn to kill your orbital velocity, then throttle back so you pass the crater.

But yes, it's dangerous to let your fall rate go too much above 100m/s. And as you start to see the terrain approach, you want to be even slower than that, because when you get down there, you have to stop -- and sometimes stopping isn't easy at all.

But partly it depends on whether you want to do this by eye, or whether you want to use a maneuver node. If you place a maneuver node at the surface with a retrograde burn then you will be able to see how much dV you need, and you will get an approximate burn time, and a countdown. But setting up the node takes time, and if it takes you too much time then you crash and die.

 

[AHHans]

Well @MPDerksen, you already manage the most important part: landing in one piece! So you "only" need to refine your technique to get more fuel efficient. And as @Vanamonde said: a suicide burn is the most fuel efficient. So get more fuel efficient you need to change your technique to be more like a suicide burn. And, yes, that takes practice. (I think there is also at least one mod, that computes when you need to start a suicide burn (and survive) but I never used those.)

On tip is to not stop  during the descend, but only burn retrograde (to surface, not orbit!). And only burn enough so that you are sure that you'll be able to stop before you hit the ground. I.e. start burning at a fairly high (= safe) altitude, and stop burning when you notice that you'll come to a stop before you reach the surface. Repeat once you have descended enough that you reach the limit what you consider safe. One thing I do is to estimate my "time to impact at my current velocity" and my "time to kill my current velocity". If they become equal, then I start burning, which increases my "time to impact" and reduces my "time to kill my current velocity".

One way to do a suicide burn is indeed with quicksaves (well, one save and many reloads): get into a suborbital trajectory, save, and then start burning retrograde at some altitude. If you crash: try again at a higher altitude. If you come to a stop way above ground, then try again at a lower altitude. Until you are happy with the result.

Finally: the dV in the dV map is the absolute minimum. So you shouldn't expect to be able to do with only that.

[BudgetHedgehog]

One thing I'm fond of doing is putting a manoeuvre node right where the orbit intersects the land, then pulling retrograde until it starts wiggling out and the velocity has been zeroed out. You then have a rough idea of how much speed you'll need to kill and a rough time when you'll hit the surface - it's no suicide burn and it's far from the most accurate method (the time to impact node will change as you burn, the terrain on the map isn't the same as the terrain in flight view), but it's a decent enough approximation. One that's worked for me for ages, anyway.

[Geschosskopf]

4 hours ago, MPDerksen said:

I am using WAY more dV to land (anywhere) than the dV map would indicate. 

NEVER believe the dV map.  ALWAYS add some some percentage of extra fuel, from 10-50% depending on how lucky you're feeling and how much you follow @bewing's excellent advice.

But I myself don't trust Mun at all.  Its terrain in most places is far from level and everywhere is covered with ship-killing scatters and ROCs.  So, when I'm just trying to land in a certain biome for Science!, I always use the very expensive stop-and-drop method.  I get in about a 12km orbit, kill my horizontal velocity completely so I stop over that biome, then ride on down controlling vertical velocity.  When the terrain becomes more visible, I slow down nearly to a stop and make sure I'm coming down on a safe spot, adjusting horizontally as needed.  I make my munar transfer stage big enough to do most of the landing, then ditch it once I'm about 1000m above the ground and lined up on a good spot.  Thus, my lander usually has only a tad bit more fuel than needed to get home and I get a nice explosion from the "crasher stage" in the final moments of descent.  Sometimes, parts of the crasher stage survive and I EVA Kerbals to go examine the wreckage.

OTOH, if I'm building a base and require landing a bunch of modules in the same (previously selected) place, I use MJ.  Every real spacecraft ever launched as been controlled by its guidance system either completely or nearly so, so I see nothing wrong with this  

 

4 hours ago, MPDerksen said:

Pretty much done at Mun and Minmus, to I'm off to Gilly and Ike.  But the skills I need will be even MORE important out there, since rescue vessels will be really far away and time consuming.

Gilly is different.  At that cursed place, velocities are so low that stop-and-drop doesn't cost you noticeably more than any other method.  And it's very important to land on as flat a place as you can find, due to the low gravity.  I also recommend having some up-pointing RCS thrusters to speed the damnably long descents.  It's either that or point towards the ground and run the main engine.  A lander totally powered by monopropellant is one of the simplest ways to build for Gilly.

[moogoob]

I'd recommend a readout mod that gives you a Suicide Burn countdown - Mechjeb is what I use. That's ALL I use, actually. My deorbit burn puts the trajectory where I need to go, and I start burning when the SB countdown reaches 4 or 5 seconds, easing off when I get slow enough to land. If I have low TWR, I aim shallower, if I'm trying to be picky about landing site I go steeper. It's is about as efficient as I've been able to make it so far, and it works on any atmosphere-less body. The key is that if your countdown ever falls below 0, you can tilt the craft upwards to remove more vertical velocity, causing your countdown to start going up again. The opposite is true, if your fall is slowing too fast you can aim more at the horizon and start losing horizontal speed faster, but at the cost of your countdown going down at an accelerated rate. It's actually hod Apollo did it with the LM only having so much throttle control, most of the fine tuning of the landing was done through attitude control.

[Superfluous J]

48 minutes ago, moogoob said:

I'd recommend a readout mod that gives you a Suicide Burn countdown - Mechjeb is what I use.

Hear, hear. I use Better Burn Time for this - and everything else it gives like rendezvous burns.

Then I do a combo of low-altitude stop-and-drop, on and angle suicide burns, and after doing it so many times just having a decent feel for it. I can pretty reliably land close enough to anything to knock the solar panels off if I'm not careful, without using more than 10-20% more fuel than the dV charts.

[magnemoe]

One major issue then landing on Mun or Kerbin for that matter is that the planet or moon rotate below you. 
For accurate landings I recommend setting up an 400 m/s burn from 20 km up, this will kill most of your orbital velocity and you will impact pretty close to the point your trajectory intercept ground. 
Update your node util you are close as the Mun rotates. 

Yes, you can use mechjeb landing to to tell landing location with the current burn. 
It does have to use mechjeb to land, I do this at Mun and higher gravity bodies without atmosphere. 
But on Minmus it tend to mess up not to talk about Pol, Bop and LOL Gilly who also rotate fast. 

[Snark]

On 9/14/2019 at 1:35 PM, MPDerksen said:

Practice?  MechJeb?  Many, many Quicksaves?

There are mods that can help with this-- my own BetterBurnTime is one-- but if you prefer a mod-free solution, here's a technique that works entirely in stock and can help take away some of the guesswork.

1. Start in circular Mun orbit, ideally fairly low.
2. Do your burn to lower your Pe so you're on a suborbital trajectory.
3. Drop a maneuver node right exactly precisely at the spot where your trajectory intersects the Mun's surface.
4. Start dragging the handle on the maneuver node.  Note that this causes your post-maneuver dotted-line projected trajectory to start collapsing towards the Mun's surface.
5. Keep that up until your projected post-maneuver Ap collapses right down to the maneuver node on the surface itself, then stop.
6. There you go, you're all ready!

At this point, the countdown-until-maneuver tells you how long until you're projected to hit the surface, and the estimated burn time tells you how long you need to burn.  So all you do is, switch your navball to surface-relative mode, set SAS to "hold ", and wait.  When the time-until-maneuver gets down to about 60-70% of the estimated burn time, max out your throttle.  (So, for example, if the burn indicator is telling you that the maneuver will require a 10-second burn, you'd wait until you're about 6-7 seconds before the maneuver node time.)

That won't take you perfectly down to the surface (there's a bit of safety margin built in), but it ought to do considerably better than you've got now.  Don't slow down all the way to a hover-- that's very wasteful of dV, you want to be falling fairly fast.  But this is a good start.

[Zhetaan]

On 9/14/2019 at 6:46 PM, Geschosskopf said:

Gilly is different.  At that cursed place, velocities are so low that stop-and-drop doesn't cost you noticeably more than any other method.  And it's very important to land on as flat a place as you can find, due to the low gravity.

Agreed.  At Gilly, 'flat' becomes an exercise in perspective; there's no easy way to tell.

Aside from that:

On 9/14/2019 at 2:11 PM, MPDerksen said:

At about 10k up, I kill 100% of my velocity (probably the problem right there), and then drift down, pointing retrograde.

[snip]

I can touch down about anywhere I want, but it's EXPENSIVE and I few times I have been left with too little to get home.

[snip]

Maybe I need to get braver and come in hotter instead of being so gentle from 10K up?

There is another way to do this.  @Vanamonde had the hint in the first reply:  treat it as the reverse of a launch.  When you come to a stop at 10 km over the landing site and then burn to reduce your velocity to something survivable before hitting the ground, that's the reverse of launching straight up to an apoapsis of 10 km, then turning sideways and burning for orbit.  If you know what a gravity turn is and how it works, then you know that what I just described is an inefficient way to launch.  For the exact same reason, it's an inefficient way to land.

Your intuition is absolutely correct in that respect, so you're enough of a pilot to identify the problem--since you need to do that before you can fix it, I'd say that you're not bad at all.

Anyway, here's the alternative.  This is an old, old, old video, but the principles are still sound:

Note that the lander in this video has extremely low thrust-to-weight.  Low-TWR vehicles are inherently inefficient on airless bodies (the low thrust means you need to fight longer against gravity), but the principle of a constant-altitude landing is a sound one.  The best part is that starting your burn too late won't destroy your vehicle; you can simply go back to orbit.

I will caution that this is not precisely beginner-level technique.  Suicide burns and what I will call 'stop-and-turn' burns have an advantage in that you figure out the right direction to point your rocket and then you burn in that direction until the burn is done; you don't need to change the direction except for corrections.  Constant-altitude landings require you to understand that you can control velocity not only with the magnitude of the burn (meaning the throttle), but also with the direction of the burn (which you do with the pitch angle), and that you can control both of these things at the same time.

If landing this way is not your style, then you will still enjoy better results by following @bewing's or @Snark's advice: start from a low orbit and bring your periapsis even lower.  Minimise the amount that you need to drop vertically, because that's where you spend the fuel inefficiently.

Good luck!

Edited September 16, 2019 by Zhetaan

[MPDerksen]

All excellent.  The example of comparing a gravity turn to a reverse landing is an excellent visual.  The biggest help has been lowering my Pe at the Ap, and then dropping it again about 90 degrees from target.  I'm not using as much, also because I'm getting braver and starting my burn later.  Practice does equal improvement.  Quicksaves give me a few shots too, which is nice.  Compensation for Minmus rotation is still a challenge.  I was able, last night, to land 100m from my science station to add a component.  And my dV was about 150 over the map minimum.  I'll take it!!

[Zhetaan]

16 hours ago, MPDerksen said:

Compensation for Minmus rotation is still a challenge.

It may help to treat it as a rendezvous.  Minmus's surface is in an 'orbit' that sees it move by approximately half of a degree per minute.  That's extremely slow--but all surface rotations must be slower than orbital velocity, or else the surface would fly into orbit.  A 10-km Minmus orbit has a period of about 46 minutes, so each orbit at that altitude will see your landing site (assuming you choose to observe it at the same point in your orbit) move by 23 degrees (relative to the axis, of course).  This works most easily when approaching equatorial landing sites from equatorial orbit, but you can adapt it to inclined orbits and high-latitude landing sites.

[Scarecrow]

The video theat @Zhetaan linked is, despite it's age, still a very good example of how to land economically.  The biggest difference now is that the Mun isn't as flat as it used to be, so you have to make allowances to make sure that you don't run in to the side of a hill.

It basically expounds that you should drop your peri to fairly close to the surface so that when you start reducing your velocity you don't end up fighting gravity from a gazillion kilometres up down to the ground.

[Starhawk]

Lowering my periapsis as near to the surface as possible from an already low orbit is always how I do a Mun landing.

The Mun's topography usually limits how close to the surface I can set the periapsis, though.

Since I often want to land in one of the larger craters, I often can't go lower than 8 km or so without danger of crashing into the crater rim.

Happy landings!

[k00b]

TWR.

i also, regretably... learned that "you need to research the biome you are intending on investigating if you want "to do things properly".

(OR "lots of fuel x high TWR" (i like to use multiples of twitch, thud etc (and that white one) etc to keep the "centre of mass" (read "height") down, in addition to f9 LOTS AND LOTS).