Trying to brush up the old powered descent and landing

[AlexinTokyo]

I recently started getting back into KSP after a break since basically 1.0 days.
I've pretty much got my head around the changes and new stuff, and find the old skills starting to come back.

However, I'm having a real job getting my powered landing (Mun, especially) back up to scratch; I seem to either crash (killing my Kerbalnauts) or end up with a ridiculous hang time and burn too much of my reserve fuel (stranding them).

Thus - Can anyone recommend a good tutorial (video or text, not fussed) from a (reasonably) recent version of the game, that focuses on powered landing?
I have no trouble with orbital mechanics, rendezvous, docking, building ships to a target Δv, etc., so I'm not looking for a "Here's how to get to the moon" type thing.

Something that says "Here's four different ways of doing a powered landing on a non-atmospheric body and these are the pros and cons of each," would be perfect.

Any and all recommendations appreciated. 

[ClamBoy]

This old thread has been most helpful to me. Sadly the pictures are gone, but the method really works for me. I also run Snark's Better Burn Time which shows the time to impact and burn time necessary. It gives a good sense of how close your cutting it in terms of descent rate.

 

 

[VoidSquid]

I'd think this is pretty good:

Spoiler

 

 

What I do usually:

Start with a (more or less) circular orbit of 20-25 km. Burn retro for some 50-60m/s until the trajectory is a bit behind the spot where I want to land. Maneuver node directly above the planned landing spot (6-8 km above ground), reducing the horizontal velocity to zero. Falling slowly down, I have enough time for fine corrections where exactly I want to land. Retrograde, keeping the vertical velocity roughly 1/10th of the numerical value of the altitude above ground. At about 20-30 m above ground, I switch to Radial Out and then soon have a smooth touch down at approx. 1m/s.

[vossiewulf]

14 hours ago, VoidSquid said:

What I do usually:

Start with a (more or less) circular orbit of 20-25 km. Burn retro for some 50-60m/s until the trajectory is a bit behind the spot where I want to land. Maneuver node directly above the planned landing spot (6-8 km above ground), reducing the horizontal velocity to zero. Falling slowly down, I have enough time for fine corrections where exactly I want to land. Retrograde, keeping the vertical velocity roughly 1/10th of the numerical value of the altitude above ground. At about 20-30 m above ground, I switch to Radial Out and then soon have a smooth touch down at approx. 1m/s.

Pretty much this, but you can save fuel by letting yourself fall all the way to 1000m AGL or so and then max-burning down to a manageable 20m/sec or so for the last 500m before one final slow-down burn that puts you in the <+5m/sec range for touchdown. This method gets much trickier the lower the TWR of your vehicle is, however. If the TWR is much less than 2 then you might need to practice a couple times, seeing where you have to start that first big burn to leave yourself at 500m alt before you get it right, with the closer you come to crashing the better

 

 

[VoidSquid]

30 minutes ago, vossiewulf said:

but you can save fuel

Very true. My crafts tough always have more than enough fuel, (I start with some 7400+ m/s from KSC for a Mun or Minmus landing), I've no need to fly that much efficient. Always better safe than sorry, and the extra fuel gives me that  

[AlexinTokyo]

23 hours ago, ClamBoy said:

This old thread has been most helpful to me. Sadly the pictures are gone, but the method really works for me. I also run Snark's Better Burn Time which shows the time to impact and burn time necessary. It gives a good sense of how close your cutting it in terms of descent rate.

 

 

Thanks for reminding me of that (I 'Liked' that post in 2015.  Have to practice this tech a bit, but I recall it worked really well.

[HansAcker]

I never got the hang of a gracious reverse gravity turn.

My method is to come to a (mostly) complete horizontal stop a few kilometers above the surface, then point retrograde and attempt a controlled suicide burn. I can land within a few hundred meters of a target, not exactly close to it

My typical early Mun mission looks like this:

Spoiler

 

 

Edited July 15, 2020 by HansAcker

[MrSystems]

My method relies on some readouts from Kerbal Engineer. Among other information, it can give the time to impact, the current acceleration of your craft, the surface gravity of the body, and display a red circle where your craft will impact the surface.

1. From a circular orbit, I decelerate about 1/4 of the way around the body so that the red impact circle is just past my intended landing point.
2. I check my craft's acceleration (for this example let's say it's 10.0 m/s²) and the body's delta-v from surface to low orbit listed on the Subway Map (for this example, we'll use the Mun's value of 580 m/s). 580 m/s÷10.0 m/s² is 58 seconds, so I estimate that's how long it would take to slow to a complete stop.
3. With a bit of calculus, you find that 1/2 of 58 seconds, 29 seconds, is how many seconds before impact you would need to begin max deceleration to reach 0 m/s as you hit the surface, assuming 0 gravity. (That number is always 1/2, no matter the body, so you can skip the calculus if desired.)
4. During the time you're decelerating, 58 seconds in this case, you're also being accelerated at the surface gravity of the body. For the Mun that's 1.628 m/s². So you'll also need to burn off (58 × 1.628) m/s = 94.4 m/s of speed you'll pick up while you're decelerating. That's an additional 9.44 seconds of deceleration time in our estimate (during which time you'll accelerate further, so we'll need to be generous in rounding up) (Yes, I know some of that acceleration is angular around the body and doesn't get added to the speed of your craft, but we're rounding up to be safe so we'll ignore that for purposes of an easy approximation)
5. We now have calculated we need to decelerate 29 + 10 = 39 seconds before impact, and you should add a fudge factor of maybe 20% on top of that to be safe, so 48 seconds.
6. Orient your craft retrograde to Surface (not Orbit!) and time warp to the calculated time.
7. Initiate max retrograde burn at the calculated time.
8. Watch the Time to Impact read-out from Kerbal Engineer. It updates to reflect both your current speed and your current engine thrust, so once you've decelerated nearly completely the number will start to climb. This is when you know to reduce the throttle--if the number is climbing you're using too much fuel.
9. Massage the throttle up and down to come to a smooth, slow landing. You should be low enough and close enough to your target that it won't take much fuel at this point.
10. If you accidentally hit 0.0m/s speed, the retrograde SAS hold gets unlocked, so point Radial Out (again, relative to Surface not Orbit) when your horizontal speed gets close to 0.

To do the math more precisely requires calculus, because both gravitational acceleration and your craft's acceleration change as you descend, but the approximation above works well enough for video game rocket science. There's also a tutorial in the game, but it doesn't use Kerbal Engineer and therefore incorporates a lot more guesswork and uncertainty.