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How to create a synchronous orbit over Minmus with the Pe skimming the flats?


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So I saw this gif:

And I wanted to put a station in a skimming orbit like this.  I found out the guy who did it put his station in a Minmus Synchronous orbit with his Pe on the surface and his Ap high enough to give him an orbital period equal to Minmus' rotation.  I have mine in a Minmus synchronous orbit but it has a high Pe.  The problem is if I lower it to get my Pe where I want it so I can extend my Ap to keep it synchronous, the lower Pe is no longer Synchronous and the planet moves from where it was under my Pe before and I crash into the highlands before I can extend my Ap to keep it synchronous.  

How do I time that so that I can survive till I get my Pe down to where I want it?  Also how can I tweak it so that my Pe is in the exact spot I want without taking my orbit out of sync?

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You can't start from a synchronous orbit. You have to set your Pe last (except for tiny tweaks of the orbital period to get it perfect). Which means you have to set your Ap first. So you have to know in advance approximately what your Ap needs to be. Make a nice circular orbit at that altitude. And when the Greater Flats is directly beneath you, then you burn to set your Pe at a few hundred meters on the other side of Minmus -- which will happen exactly half a rotation later.

However, it is my understanding that what this GIF shows is impossible. So I'm at a bit of a loss. My understanding: every CB has an autodeletion altitude. If a craft is unfocused, and goes below this altitude, then the game assumes the craft "crashed into terrain" and deletes it. Period. And that altitude on Minmus is something like 5km.

 

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6 minutes ago, bewing said:

You can't start from a synchronous orbit. You have to set your Pe last (except for tiny tweaks of the orbital period to get it perfect). Which means you have to set your Ap first. So you have to know in advance approximately what your Ap needs to be. Make a nice circular orbit at that altitude. And when the Greater Flats is directly beneath you, then you burn to set your Pe at a few hundred meters on the other side of Minmus -- which will happen exactly half a rotation later.

However, it is my understanding that what this GIF shows is impossible. So I'm at a bit of a loss. My understanding: every CB has an autodeletion altitude. If a craft is unfocused, and goes below this altitude, then the game assumes the craft "crashed into terrain" and deletes it. Period. And that altitude on Minmus is something like 5km.

 

Yeah I'm not sure if it's stable in that way, I'll have to see.  Thanks for the help.

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Orbital synchronicity depends on the period, which depends on the semi-major axis, so the semi-major axis must remain constant.  For Minmus, the required SMA is 417.94 km (from the wiki).  The only way to keep that value when starting from an already synchronous orbit is to change both the Pe and the Ap at the same time, which means performing an extremely inefficient burn--if such a burn is even possible.  My suggestions are to set up some nodes and see what you can find; try burns midway between the Pe and Ap, and try radial burns at the Pe and Ap.  I don't think you're going to find what you seek, however.

Your best option, I think, is to set up a semi-synchronous orbit with the Pe near, but not quite what you want (just high enough to clear the hilltops on the off-side pass).  Then, while you're setting it up, make the Pe higher than needed by the same amount that you will need to drop it later to skim the ground--in seconds, not metres.  In other words, if you need 500 metres to clear the hills, but you want a final Pe of 50, you would first set up a semi-synchronous orbit with a Pe of 500 metres, then calculate in seconds how much shorter the period would be if the Pe were at 50 metres.  Finally, you would raise the Pe from 500 metres to the point that the period is the same number of seconds longer than true semi-synchronicity as the 50 metre Pe is shorter.  This is done to add time to the orbital period in a predictable way so that you can both clear the hills on the first orbit and then correct the error later in such a way as to pass over the same point as you started on the second orbit--in other words, you want to find a set of orbits that are semi-synchronous on average.  After the first pass, lower the Pe the few hundred metres to where you want it, which will take off the few extra seconds (and take up an equal amount of time extra) and give you an overall semi-synchronous path.  At the desired Pe on the second pass, burn to raise the Ap to full synchronicity.

Please do bear in mind that this is all theoretical; I don't have my KSP machine handy, so I cannot test it out.  But it should work.

Edited by Zhetaan
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3 hours ago, Zhetaan said:

Orbital synchronicity depends on the period, which depends on the semi-major axis, so the semi-major axis must remain constant.  For Minmus, the required SMA is 417.94 km (from the wiki).  The only way to keep that value when starting from an already synchronous orbit is to change both the Pe and the Ap at the same time, which means performing an extremely inefficient burn--if such a burn is even possible.  My suggestions are to set up some nodes and see what you can find; try burns midway between the Pe and Ap, and try radial burns at the Pe and Ap.  I don't think you're going to find what you seek, however.

Your best option, I think, is to set up a semi-synchronous orbit with the Pe near, but not quite what you want (just high enough to clear the hilltops on the off-side pass).  Then, while you're setting it up, make the Pe higher than needed by the same amount that you will need to drop it later to skim the ground--in seconds, not metres.  In other words, if you need 500 metres to clear the hills, but you want a final Pe of 50, you would first set up a semi-synchronous orbit with a Pe of 500 metres, then calculate in seconds how much shorter the period would be if the Pe were at 50 metres.  Finally, you would raise the Pe from 500 metres to the point that the period is the same number of seconds longer than true semi-synchronicity as the 50 metre Pe is shorter.  This is done to add time to the orbital period in a predictable way so that you can both clear the hills on the first orbit and then correct the error later in such a way as to pass over the same point as you started on the second orbit--in other words, you want to find a set of orbits that are semi-synchronous on average.  After the first pass, lower the Pe the few hundred metres to where you want it, which will take off the few extra seconds (and take up an equal amount of time extra) and give you an overall semi-synchronous path.  At the desired Pe on the second pass, burn to raise the Ap to full synchronicity.

Please do bear in mind that this is all theoretical; I don't have my KSP machine handy, so I cannot test it out.  But it should work.

That sounds super complicated and I'm having a hard time putting that together in my head.  Here's what I found on the reddit comment thread, tell me if it'd be easier.

"step 1: capture by Minmus.

step 2: kill eccentricity and circularize.

step 3: wait until above the greater flats (and optionally until the flats are in the sun, since we'll be passing by there about the same time every day).

step 4: burn prograde until your SMA is 417.941km or close to that value.

step 5: wait about an hour, then correct your periapse to about 0m with a radial out burn. Clean up your SMA/orbital period with pro/retro burns if necessary.

Step 6: as you get close to the peri again, adjust your approach with radial in/out burns. RCS is highly recommended.

Pure radial burns will keep your SMA and thus your orbital period the same. Since your radial/anti-radial vectors move as you perform the burns and your vessel will track these nodes imperfectly as they do so, some small deviations will be introduced. As long as you correct your SMA directly after performing the burn these deviations will not have a big effect on your orbit or true anomaly."

Edited by SlabGizor117
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@SlabGizor117:

The reason my method is so complicated is because it allows you to 'run up' to the final orbit with a minimum of radial in/out nonsense.  This is important because while it is true that a pure radial burn does not change your semi-major axis or inclination, it does change most everything else of the orbit, such as the longitude of periapsis--in other words, it will spin your orbit around the planet (not unlike a cosmic hula hoop) and so while your Pe will remain at the same altitude, it will not remain in the same location--which of course means you splatter into the Highlands if you move it too much.  If you are already close, however, the reddit method is the easier way.

The simpler explanation of my method is this:  get into a position where your orbital Pe is both located and elevated so it is exactly where you want it, and then while at that Pe, burn to make the orbit synchronous.  The extra stuff I gave to you was necessary because you're already in a synchronous orbit and do not want to end up over a different place on the map.  Switching to a temporary (almost) semi-synchronous orbit solves that, but if you're willing to scrap your current orbit entirely and wait until you're back over your desired location, you can do that, too.

@bewing:  I don't think it's impossible; I think there's a trick.  When the video starts, Bob and sat are already at 1.3 km distance.  I think the person focused on the satellite until the rover was in physics range, switched vessels, and moved as quickly as possible.

Edited by Zhetaan
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Just now, Zhetaan said:

@SlabGizor117:

The reason my method is so complicated is because it allows you to 'run up' to the final orbit with a minimum of radial in/out nonsense.  This is important because while it is true that a pure radial burn does not change your semi-major axis or inclination, it does change most everything else of the orbit, such as the longitude of periapsis--in other words, it will spin your orbit around the planet (not unlike a cosmic hula hoop) and so while your Pe will remain at the same altitude, it will not remain in the same location--which of course means you splatter into the Highlands if you move it too much.  If you are already close, this is the easier way.

The simpler explanation of my method is this:  get into a position where your orbital Pe is both located and elevated so it is exactly where you want it, and then while at that Pe, burn to make the orbit synchronous.  The extra stuff I gave to you was necessary because you're already in a synchronous orbit and do not want to end up over a different place on the map.  Switching to a temporary (almost) semi-synchronous orbit solves that, but if you're willing to scrap your current orbit entirely and wait until you're back over your desired location, you can do that, too.

@bewing:  I don't think it's impossible; I think there's a trick.  When the video starts, Bob and sat are already at 1.3 km distance.  I think the person focused on the satellite until the rover was in physics range, switched vessels, and moved as quickly as possible.

I have no way to get my Pe where it needs to be and keep it in the same spot over Minmus because if I move my Pe the orbit will no longer be synchronous and I'll crash into the highlands before I can get to my Pe to synchronize it again.

 

As for what you said to @bewing, I saw no implication that there was any trickery involved with keeping the station in orbit as far as game mechanics are concerned, it seemed like it was stable in any case to me.

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How low is your current Pe?  You may be able to make adjustments by raising it; it just means that you'll then have to wait a few days before making further adjustments.  I don't think there is any simple way to make this happen:  you will need to have multiple burns to fine-tune your orbit, and none of those burns can be allowed to lithobrake you, which adds some complexity to the planning process.  Synchronous orbits are finicky enough without adding an obstacle course at orbital speed to the mix.

That being said, finicky is not the same as difficult.  Once upon a time, there was a challenge posted about making a gauntlet run on the Mun through the canyon on the equator at the East Farside Crater.  Then people found out that once you got your ship into the canyon, going through it involved the occasional RCS burst to keep from drifting into the walls and otherwise was completely uneventful--the hard part was getting there in the first place.

In your case, I have nothing but confidence that you can do it--I just don't think you're going to be able to do it without moving your Pe to avoid terrain.  But that's okay; if that really is necessary, then accept it and work that fact into the flight plan.  So long as you know how you change your orbital period, you can change it back.  The idea is to be sure that you balance the changes so that you end up in the right place in the end.

@bewing is right, though; unless they changed the deletion altitude, there should be no way for an unfocused vessel to get that close to the surface.  Therefore, it must not be an unfocused vessel.  Therefore, it is a focused vessel instead.  It is not impossible for a focused vessel to approach that closely to the surface--otherwise, one cannot land--but I think that this case requires periodic attention to work correctly:  you'd have to look at the vessel every Minmus day to keep it from being deleted.  You'll have to experiment to find out how closely you can shave the orbit to make it work while you do other things--and do remember that what we think we know about the deletion altitude may be outdated.

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given that his "Station" had an engine strapped to the bottom and a huge fuel tank, I'm thinking he did something like this:

1. launches the station from the surface and flies it so that it skims the top of the minimus flats without making it actual orbit (he might of gone on an a super low escape trajectory).

2. waits till the station is in physics range

3. does what he shows inside the gif.

 

Given that the station was moving so fast and so low, and he was able to match the speed perfectly, I'm thinking it wasn't a space station, but infact it was a rocket that took off form the surface of minimus with a low altitude and high speed. I've done similar things with rovers and biome hoppers looking for easter eggs from about 500m up. I have not done anything that size though, and I never thought of even trying to match my speed with another moving vehicle.

I agree though that it seems impossible to make an orbit with that much precision with game physics. (unless he modded his game with korpernius to make it possible ??)

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14 hours ago, nascarlaser1 said:

given that his "Station" had an engine strapped to the bottom and a huge fuel tank, I'm thinking he did something like this:

1. launches the station from the surface and flies it so that it skims the top of the minimus flats without making it actual orbit (he might of gone on an a super low escape trajectory).

2. waits till the station is in physics range

3. does what he shows inside the gif.

 

Given that the station was moving so fast and so low, and he was able to match the speed perfectly, I'm thinking it wasn't a space station, but infact it was a rocket that took off form the surface of minimus with a low altitude and high speed. I've done similar things with rovers and biome hoppers looking for easter eggs from about 500m up. I have not done anything that size though, and I never thought of even trying to match my speed with another moving vehicle.

I agree though that it seems impossible to make an orbit with that much precision with game physics. (unless he modded his game with korpernius to make it possible ??)

No, he has an orbit:

 

 

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7 hours ago, SlabGizor117 said:

No, he has an orbit:

 

 

oh ok. I changed my idea.

 

1. he takes the station thingy off from minimus (or from somewhere else), and flies it into a really low orbit (but it might*  not have been synchronous).

2. waits till it is in physics range

3. does the gif :).

 

* emphasis on the word might

Edited by nascarlaser1
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9 minutes ago, nascarlaser1 said:

oh ok. I changed my idea.

 

1. he takes the station thingy off from minimus (or from somewhere else), and flies it into a really low orbit (but it might*  not have been synchronous).

2. waits till it is in physics range

3. does the gif :).

 

* emphasis on the word might

Watch the video and you'll see that it is synchronous.  What made you think it wasn't?

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Just now, SlabGizor117 said:

I think so.

oooh ok. I redact all that has been said then :P. I thought it was just a random video showing that the orbit was possible. (I'm still new to all the terminology, so I don't exactly know what a synchronous orbit is, but since that was the main point of the discussion I put in what little I knew, which was obviously not enough lol. Google time!).

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Just now, nascarlaser1 said:

oooh ok. I redact all that has been said then :P. I thought it was just a random video showing that the orbit was possible. (I'm still new to all the terminology, so I don't exactly know what a synchronous orbit is, but since that was the main point of the discussion I put in what little I knew, which was obviously not enough lol. Google time!).

Redact what I said and look at the edit, video made after the gif I think.

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Are you not free to change Pe at will without messing with your orbital period?

While I have not tried it I would think that if you set your Pe over the target and your Ap to the correct height and then gently lower your Pe with burns at Ap you would get what you want.  Beware that burns aren't perfect so you will have to correct your orbit as you go.

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