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How do I aim for an ocean for splashdown?


Go to solution Solved by Superfluous J,

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Okay,
I have googled, and I have searched the forum, yet found no answer.

Here's what it is: I play only sandbox, and I build funny little ships that I fly to the Mun, mostly.
When I set up my return trajectory, I can see where it will hit Kerbin. But Kerbin will, of course, keep rotating. So I often end up burning huge amounts of ∆v in a last-minute course alteration just so that I can hit an ocean.
If I don't use such huge amounts to first circularize and then set up a nice reentry. I can build my craft so that I'll be lugging quite enough ∆v for all of that. But I don't wanna.
I don't need to hit an ocean. I know I can land on land, no worries. I know it was once undesirable to hit an ocean, if memory serves.
But I am of the firm conviction that spacecraft that don't have landing gear go in the water. That's how I roll. Or splash and bob, as the case may be.

So. What part of the game have I totally missed learning about? What mod did I not find? What is the trick?
From what I understand, the Apollo spacecraft didn't exactly first circularize and then set up reentry. But were put on the right trajectory, right from the Moon. And even came down near the right aircraft carrier.
If those pinkskins could do it, I'm sure the Kerbals should be able to, also. Problem is how to get me able to do it, also.

I'm sure it's elemental, trivial, everybody knows about it.
I suspect there might be something in the game I just have failed to learn about, inexcusable as that may be.
So, as humiliating as this may be for me. Please somebody explain to me how I can aim for an ocean from the Mun. Thanks.

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7 hours ago, DerGolgo said:

If nobody else here has any ideas, I might post a mod-request. Something that will project Kerbin's orientation, with the landmasses etc., for the time of some future point along the trajectory. So that one could click a point on the trajectory, and under "Add Maneuver" and "Warp here", there'd be "Show orientation of target at this time" and get an overlay map. Or spin the whole rendering of the planet to match that point on the trajectory.

This is almost literally what the Trajectories mod does. It doesn't allow you to put maneuver nodes down but it will show you with pretty good accuracy where you'll land, taking into account the atmosphere if necesary.

 

Edited by 5thHorseman
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What I do for a return from Mun or Minmus is to set the periapsis at Kerbin at about 22-24 km.  At that height I find that I usually land at a point that is just about directly beneath the periapsis.  Of course it depends on the aerodynamic characteristics of the reentry vehicle, but if we're using anything like a Mk1-2 command pod, we'll be pretty close.

After setting up the maneuver node for the transfer back to Kerbin, I check the time to periapsis.  Let's say the time to periapsis is 1 day, 1 hour, 45 minutes.

Although we expect to land directly beneath the current periapsis, the terrain beneath the periapsis at the time of landing is not what it is now.  We have to take into account Kerbin's rotation.  In one day Kerbin will return to its exact current position, so we can ignore the number days.  What is important is the hours and minutes, because that represents how much Kerbin will change from current.  In this example we have 1:45, or 105 minutes.  Kerbin rotates one degree per minute, so in 105 minutes it will rotate 105 degrees.  Therefore the landing site is going to be 105 degrees west of the spot that is currently beneath the periapsis.

You can estimate the 105 degrees by eyeballing it, or you can use a map like that below.  The white vertical lines are spaced 15 minutes (15 degrees) apart, with the longer lines being 1 hour (60 degrees) apart.  Just find the current spot under the periapsis and move the appropriate distance to the left to find the predicted landing site.

kerbinmap1.gif

The time to periapsis that you read with the maneuver node may not be the same as you get after completing the burn (there's always going to be some error).  Therefore it pays to double check the landing site after you've completed the burn to make sure it hasn't change by a lot.

If you don't like the looks of the predicted landing site, then just delay performing the transfer burn for a later orbit.  Let's say you're orbiting Mun once every 45 minutes.  That means waiting one orbit to perform the burn will shift the landing site about 45 degrees to the west.  This way you can plan ahead and schedule the burn for the time that will place the landing site right where you want it.
 

Edited by OhioBob
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8 minutes ago, DerGolgo said:

If those pinkskins could do it, I'm sure the Kerbals should be able to, also. Problem is how to get me able to do it, also.

 

9 minutes ago, DerGolgo said:

I'm sure it's elemental, trivial, everybody knows about it.

NASA had teams of mathematicians crunching numbers I wouldn't want to watch someone else mess with.

Honestly I've had the exact opposite problem.  I keep hitting water when I want land.

What I do, is get my return trajectory so it's coming back to Kerbin just above its atmosphere.  Gives me a good amount of Kerbin's circumference to just wing it.  I burn retro when I'm around 80-90km, so my trajectory is putting me a good amount past where I want to land, since the atmosphere and the planet's rotation is going to muck that up a bit.

You really can't compare how it was done in real life versus this game, especially if you're playing in sandbox.  Just plan for enough dV left over for your re-entry. 

15 minutes ago, DerGolgo said:

So. What part of the game have I totally missed learning about?

AFAIK there was nothing in the tutorials on how to land where you want.

15 minutes ago, DerGolgo said:

What mod did I not find?

I wanna say MechJeb can do this, but can't guarantee it as I've never messed with any autopiloting mods.

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Or, you can look at OhioBob's map and notice that starting about 30 degrees south of the equator, Kerbin is more than 50% water. Starting about 60 degrees S, it's more than 75% water. So when you have left the Mun, you do a tiny antinormal burn to set your Pe to the right latitude. When you get to Kerbin you use your eyeballs (mk1) and see if there's a chance you're going to hit land anyway. If so, you burn to raise or lower your Pe a bit. Once you are below about 1.5 Mm, Kerbin will be rotating with you, so you can just look straight down.

Edited by bewing
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8 hours ago, DerGolgo said:

So. What part of the game have I totally missed learning about? What mod did I not find? What is the trick?

The surest way to hit the ocean is to capture into a low Kerbin 80-150km) orbit when you get home, instead of re-entering directly straight from Mun or wherever you were.  Once you're in LKO, there's no time pressure so you can take careful aim, come down on the daylight side so you can see your target, etc.

As to picking a spot, there are 2 ways, eyeball and the Trajectories mod.  The mod is very accurate and shows you exactly where you'll lad, but oceans are big enough to hit with the eyeball method.  You only need precision if you're trying to land at or near a very specific thing on the ground, such as KSC, an anomaly, another ship, etc.  Either way, the general approach I recommend is to create a maneuver node about 1/4 of Kerbin's circumference (90^ of longitude) ahead of where you want to land.  This gives a good re-entry path, not too hot and you slow down enough to open chutes with plenty of altitude to spare.  It's best to create this node only a few minutes before you reach it so Kerbin doesn't rotate much before the burn.

For the eyeball method, just pull the node's retro handle to drop your trajectory to the surface ~90^ of longitude downrange from the node.  Due to air drag and eventually chutes, you'll actually land about a finger's width (when looking at the map) short of where the trajectory hits the surface.  With a bit of practice, you can hit KSC (or the water just offshore) very reliably this way.

With Trajectories, start out the same way when placing the node but as you pull the retro burn handle on the node, eventually you'll see a red dotted line appear that dives down below your normal trajectory and ends in a big red X on the surface.  That's where you'll really come down.  Make sure the X is in the water and do the burn.

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Thanks for the info!
My self esteem thanks you, also!

I think I may have thought about a work-around I may try soon-ish, though.
I wanna see if I can't just dump something floatable in some ocean, set it as target, and let Astrogator figure out a maneuver.

Hardly elegant, aiming for a buoy.

If nobody else here has any ideas, I might post a mod-request. Something that will project Kerbin's orientation, with the landmasses etc., for the time of some future point along the trajectory. So that one could click a point on the trajectory, and under "Add Maneuver" and "Warp here", there'd be "Show orientation of target at this time" and get an overlay map. Or spin the whole rendering of the planet to match that point on the trajectory.

But before I bother the magnificent and magnanimous makers of mods, I'm hoping someone will have another thing up their sleeve.

Edited by DerGolgo
thoughtlessness
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The method I described does have one potentially big drawback.  If it happens to be nighttime where the periapsis is, then we can't see the terrain beneath it.  When that happens I just try to estimate the position of the antipode and work it out from that.

 

Edited by OhioBob
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I did now try out making myself a buoy. A scaled-down Sea Dragon ballast tank, stuck to a big, shiny ball, with a bunch of lights and the electric things for those. I'm German, we don't do "simple".

Took a craft to munar orbit, set the buoy as target, and called up Astrogator.
No data for a trajectory to rendezvous with my buoy, only dashes. Went on a trajectory to Kerbin, and tried Astrogator again. This time, all I got was the "escape trajectory" warning, no data at all.
So that didn't work.
Damn.
And I made such a nice buoy.

buoy.jpg

Edited by DerGolgo
used an adjectve where I should have used a particle
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19 hours ago, linuxgurugamer said:

I always aim for a return orbit first and then reentry.

I don't think I've ever entered into an orbit on a return from anywhere.  I've always just done a direct reentry.  The method I described works just as well for predicting the landing site for a return from deep space.  Only in that case there's much less control over where the vessel is going to land (for instance, we can't loiter in munar orbit waiting for a better landing site to rotate into position).  On a return from deep space I'm just trying to avoid landing on a steep mountainside that will result in somebody dying.  If the landing site looks dicey, then I do what @bewing suggested and just raise or lower the periapsis to change the range over which the vessel will travel during reentry.  Lowering the periapsis will cause the vessel to land short of the prediction, but with limited effect (generally can't shorten up the range by a whole lot).  Raising the periapsis is more effective, causing the vessel to land long.  It is even possible to produce a "skip" and land a good distance downrange of the initial periapsis.  However, both raising and lowering the periapsis too much can have undesirable consequences.  Coming in low means higher peak temperatures and g-forces.  If the critical part spikes above it's max temperature, kaboom.  On the other hand, coming in high means the vessel will soak up more total heat because it spends more time skimming through the upper atmosphere at high speed.  This results in the vessel burning off more ablator.
 

Edited by OhioBob
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Cool guys, thanks y'all for the advice!

I will try out the trajectory mod, and I'll try out @OhioBob's method, also.

And I'll just post a mod request. This is something NASA, and presumably the Russians (particularly after Voskhod 2) do as a matter of course. And I can't imagine the Chinese aren't doing it with bells on. So that you need the old Mk1 Eyeball™ and a good bit of Guesstimation© to just hit either water or land. While NASA could back into a parking spot right next to an aircraft carrier in the middle of an otherwise featureless seascape. That honestly bewilders me a bit. That's like ... not having launch-escape-systems, or something.

Cheers y'all!

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2 hours ago, DerGolgo said:

I will try out the trajectory mod

And I'll just post a mod request. 

Why? Trajectories literally does exactly what you are asking for.

Not that you'll need it after a bit, you'd be surprised how good you get at guessing after a while. I can bring a space plane down within visual distance of the KSC by Mk1 eyeball alone. Wish I could give you more practical advice but all my "aimed" landings are space planes, pods don't care where they land so I never bother.

Practice makes perfect.

Edited by Rocket In My Pocket
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4 hours ago, DerGolgo said:

This is something NASA, and presumably the Russians (particularly after Voskhod 2) do as a matter of course. And I can't imagine the Chinese aren't doing it with bells on. So that you need the old Mk1 Eyeball™ and a good bit of Guesstimation© to just hit either water or land. While NASA could back into a parking spot right next to an aircraft carrier in the middle of an otherwise featureless seascape. That honestly bewilders me a bit. That's like ... not having launch-escape-systems, or something.

 

I agree with Rocket In My Pocket. We are giving advice to someone new to the process. When you are new, you may have trouble hitting water, if you want to hit water. Stock KSP is about teaching you how to fly by the seat of your pants. All I use is my Mk1 Eyeball™ and a tiny bit of Guesstimation© to land every single one of my missions in the exact center of the runway for full funds credit. Or you could just use mods and never bother to figure out how to train your own Mk1 Eyeball™.

 

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I wouldn't call myself "new", I've been playing on and off since I believe it was 1.1.
I just have not been able to figure out how to do this without either first circularizing or, when I'm close enough to Kerbin and see that I'd hit land, using a lot of ∆v for a last minute (or, rather, last hour or so) correction . Estimating Kerbin's relative orientation upon reentry while I'm still orbiting Mun, I've just not been able to figure that out.

I have tried out the Trajectory mod, though. Thanks, @5thHorseman, and all the others who recommended it. It is infinitely helpful!
But it can't predict the trajectory that'd result from a maneuver node yet to be executed. While it solves most of my problems, it's not what I had hoped to find. But with enough F5, I should be able to get some fun stuff done.

Thanks, y'all!

Edited by DerGolgo
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Here's a way without mod/map - need a little bit more fuel, though. Since you're just coming back from Mun - can't you just set up maneuver node so that your (future) periapsis of Kerbin is exactly 1 day (+- several minutes don't hurt)? That way what you see in map view is pretty much the same as what Kerbin will be when you return. Of course, this works best if you start with a Mun orbit, since you have a lot of flexibility about the ejection angle. A mere flyby might be tricker, but idea is the same.

And if you're high up (like close to SoI edge), you can instead warp a little while until the periapsis is a whole number of days away, and then you start tweaking (this time mostly inclination change to adjust periapsis location).

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7 hours ago, DerGolgo said:

I have tried out the Trajectory mod, though. Thanks, @5thHorseman, and all the others who recommended it. It is infinitely helpful!
But it can't predict the trajectory that'd result from a maneuver node yet to be executed

It doesn't? I've not played in quite some time but I feel like it did.

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Oh it’ll show you a trajectory (though the last I used it in the dark days of the semi-abandoned 1.2 version, it wasn’t much good), the problem is that you can’t tell what part of Kerbin will be under that trajectory when the trajectory occurs

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Ah. Great. Marvelous.
I wasted about twenty minutes playing around with that mod yesterday. Saw nothing when I was setting up my maneuver node. Assumed this was what was meant by the reference to future stages in the FAQ.
Turns out I should have zoomed map view out a view times further than I normally do, and a bit further than I had to after executing my burn, when the white line had appeared.
Right. I got all I need here. Thanks everyone!

Edited by DerGolgo
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9 hours ago, fourfa said:

the problem is that you can’t tell what part of Kerbin will be under that trajectory when the trajectory occurs

I have now narrowed the problem down to a) zoooming the way out in map view to see the trajectory and b) selecting "body-fixed-mode" to see the spaghetti in relation to Kerbin. Then, it does all I want.

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How accurate is the Trajectories mod these days?  I tried it out about a year ago, maybe more, and I was disappointed in its accuracy.  For a return from Mun/Minmus, I got much better results using my method described earlier in this thread.  Since I could do a better job without it, I uninstalled Trajectories and haven't tried it since.  Has it gotten better since then?

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51 minutes ago, OhioBob said:

How accurate is the Trajectories mod these days?  I tried it out about a year ago, maybe more, and I was disappointed in its accuracy.  For a return from Mun/Minmus, I got much better results using my method described earlier in this thread.  Since I could do a better job without it, I uninstalled Trajectories and haven't tried it since.  Has it gotten better since then?

I've not used it in 6+ months, maybe a year, but it was always spot-on for airless worlds and chronically overshot on aired worlds. But it was predictable and more accurate than the orbit line, so I was able to do it just fine aiming for 1/8 to 1/4 of Kerbin beyond what I wanted.

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9 minutes ago, 5thHorseman said:

I've not used it in 6+ months, maybe a year, but it was always spot-on for airless worlds and chronically overshot on aired worlds. But it was predictable and more accurate than the orbit line, so I was able to do it just fine aiming for 1/8 to 1/4 of Kerbin beyond what I wanted.

Yes, for airless worlds it was correct.  Where I had a problem was with atmospheres.  I don't remember, however, if it overestimated or underestimated drag, but the results were always consistent.  My recollection is that it underestimated drag, which seems to agree with your experience.  As I recall, my landings always came up short of the predicted position, and when aerobraking I always got an apoapsis lower than predicted.

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On 10/21/2017 at 12:25 PM, OhioBob said:

Yes, for airless worlds it was correct.  Where I had a problem was with atmospheres.  I don't remember, however, if it overestimated or underestimated drag, but the results were always consistent.  My recollection is that it underestimated drag, which seems to agree with your experience.  As I recall, my landings always came up short of the predicted position, and when aerobraking I always got an apoapsis lower than predicted.

I'm using it...there are a couple of challenges.

  • There's no way to tell Trajectories to only account for your descent stage. If you have a classic pod + service module layout you're going to be dropping your service module before re-entry. Trajectories' calculation is based on the vehicle at the moment though, so you're getting calculation errors both because of mass and aerodynamic factors
  • After you've dropped your service module it becomes much more accurate, but still tends to overshoot a bit.

It's still useful - I especially like using it to calculate multi-pass aerobraking maneuvers. It does a really good job for those.

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On 10/19/2017 at 3:17 PM, DerGolgo said:

I wanna see if I can't just dump something floatable in some ocean, set it as target, and let Astrogator figure out a maneuver.

On 10/19/2017 at 6:27 PM, DerGolgo said:

I did now try out making myself a buoy. A scaled-down Sea Dragon ballast tank, stuck to a big, shiny ball, with a bunch of lights and the electric things for those. I'm German, we don't do "simple".

Took a craft to munar orbit, set the buoy as target, and called up Astrogator.
No data for a trajectory to rendezvous with my buoy, only dashes. Went on a trajectory to Kerbin, and tried Astrogator again. This time, all I got was the "escape trajectory" warning, no data at all.

Wow! I never would have thought of trying that. I'm kind of flattered that you thought Astrogator might be able to handle it! :) I don't think it could do targeted surface landings even on an airless world, let alone account for re-entry, and from a satellite body no less. Though now that you've pointed this out, maybe some improvements could be made for surface targets...

Please stop by the main thread if you have any more crazy ideas like this! (Link in my forum signature)

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