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septemberWaves

Advanced Atmospheric Landing

Question

Though I am now proficient enough at spaceflight that i can even dock spacecraft with the UI hidden, there is one thing that has always eluded me, yet I see some players accomplish the task with apparent ease.

What is the best way to go about landing a pod in a specific location following re-entry from a moon, or from an interplanetary trajectory? For example, if I want to land at the KSC having left Minmus, how do I ensure that I consistently land in the right place without needing to circularize at a low parking orbit beforehand? I am mostly referring to cases where, much like in the Apollo lunar missions, the re-entry vehicle is a simple pod with extremely limited control and which lands with a parachute. I am sure there must be a better method than trial-and-error with a lot of quicksaves.

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There is a mod called trajectories that shows an approximation of the point of impact when your trajectory goes through an atmosphere. This is however in my experience rarely precise enough to the point that you could land at the KSC for example and not in the mountains nearby. I don't know how the mod calculates the trajectory, but as mentioned by OHara, your incidence etc factors GREATLY in where you'll land

I'd say try the mod to get a rough estimate and use it as a starting point from which you fine tune your approach using your incidence.

Another issue for what you ask is that you have to predict when you'll be falling also since you are coming from far away, and Kerbin rotates during your approach. A starting point would be to practice in LEO beforehand to minimize this drift

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4 hours ago, septemberWaves said:

It will be useful for vehicles which do not decouple anything from before the de-orbit burn all the way to landing

My answer for that is to use RCS on the pod/re-entry vehicle to tune the trajectory after decoupling. A linear RCS port or three on the bottom of the pod and offset to be below the heatshield works nicely for this.

With trajectories and a little practice I can land, if not at the KSC, then at least on the flat "lawn" around it. I usually aim for the water between the KSC and island though.

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I honestly think that "trial-and-error with a lot of quicksaves" is a big part of how "some players accomplish the task with apparent ease".

I think you know the bottom of capsules, with or without heat-shields, give significant lift, so you can use the magic reaction wheels to hold an orientation to steer your path.

A very shallow approach, periapsis above 35km altitude, maybe even a few skips before final re-entry, gives you more opportunity to change your path

Just one or two aero control surfaces can do a lot to let you control drag on the way down.

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The use of Trajectories is quite limited in my experience. It takes into account the shape of your craft, so the prediction is only accurate after you decouple that last stage, and thus don't have much control to actually modify your trajectory anymore... There is an option to set your incidence factor, by the way, but the trick is to actually maintain it thorough your flight...

That said, you can indeed use it to practice and figure out empirical rules.

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I know about the mod Trajectories. The reason I started this thread is because the mod is less precise than I would ideally like.

I did not know about the option to set an incidence angle (that must be new since I last used the mod) but it will not be useful for the scenario I described. It will be useful for vehicles which do not decouple anything from before the de-orbit burn all the way to landing, but such spacecraft are not exactly common (and most have wings, which makes precise landing significantly easier).

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MechJeb's landing guidance works pretty good, at least the "show predicted landing" feature. If you use landing guidance for your initial de-orbit burn to "KSP", the blue "X" will be right on KSP. If you leave it on when it hits the atmosphere it goes berserk and you won't get anywhere close, but if you turn it off it will land a little short when your parachutes open. So just turn on the "show landing prediction", and push the X a little out to sea, you can get very close.

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I use both Trajectories and MechJeb landing guidance prediction, and cross reference them. Then, I use RCS to adjust my entry as early as possible for rough corrections, and continue to fly the vehicle via RCS until around 650-700 m/s. After that, you're pretty much committed.

I also set the parachutes to open fairly late, to minimize the displacement from the target point they generate.

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